Modern Electronics CHAPTER 24 PHYSICS IN ACTION

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2 CHAPTER 24 Modern Electronics PHYSICS IN ACTION The wafer shown here is made of purified polycrystalline silicon derived from sand. Although silicon is usually a semiconductor, the addition of a few atoms of an impurity can alter its conductivity dramatically. Careful addition of impurities, along with precise etching, can create tiny integrated circuits with specific electrical characteristics on the wafer s surface. If no flaws are found during the testing process, shown here, the wafer will be cut up into individual chips with a diamond saw and packaged for use in electronic devices. What determines whether a material is a conductor, semiconductor, or insulator? How can components made of semiconductors be used to control current? CONCEPT REVIEW Work (Section 5-1) Conductors, insulators, and semiconductors (Section 17-1) Electric current (Section 19-1) Modern Electronics 865

3 24-1 Conduction in the solid state 24-1 SECTION OBJECTIVES Distinguish between conductors, insulators, and semiconductors. Identify valence electrons. Describe the role of energy bands in electrical conductivity. CLASSIFICATION OF SOLIDS As you learned in Chapter 17, materials can be classified according to their ability to conduct electricity. A good conductor has a large number of free charge carriers that can move easily through the material, whereas an insulator has a small number of free charge carriers that are relatively immobile. Semiconductors exhibit electronic properties between those of insulators and those of conductors. There is a large variation in electrical conductivity of conductors, insulators, and semiconductors. That variation may be due to energy bands. This section will help explain those energy bands. One of the accomplishments of solid-state physics is the development of a theory that uses basic physical principles to explain some of the properties of these three categories of materials. A more sophisticated model of atoms and solids is needed Figure 24-1 These are approximate shapes and sizes for the regions of space containing electrons of certain energy levels. The models that we have discussed up to this point have been of solids represented as a collection of positively charged nuclei surrounded by their associated electrons. This simple model is not very accurate. For example, it does not explain why electrons are sometimes bound and sometimes free. Further, it does not explain why the ability to conduct electricity differs between conductors and insulators as groups. The model also lacks an explanation of the differences among individual conductors and insulators themselves. Better models are needed both for the atom and for a solid. We will attempt to develop better models in this section. Positively charged protons and neutral neutrons are contained in a small, dense nucleus at the center of an atom. The nucleus is surrounded by negatively charged electrons in a series of shells, such as those shown in Figure As a whole, the atom is uncharged. Ground state of atom Second series of shells Third series of shells 866 Chapter 24

4 Valence electrons determine an atom s chemical properties The number of electrons in an atom can range from 1 to more than 100, depending on the type of atom. The complicated arrangement of electrons around the positively charged nucleus can be simplified. To do so, use a model that groups the electrons into sets, or shells, each within a space having a specific shape, such as that of a sphere or a dumbbell (see Figure 24-1). As the distance of the electron from the positively charged nucleus increases, the electrostatic force between the electron and the nucleus decreases and the electron becomes more loosely bound to the atom. This result explains in part why the valence electrons (the outer shell of the atoms) are more loosely bound than the inner shells. Because the electrons in the outermost shell, called the valence electrons, are the most loosely bound, they are the ones that can interact more strongly with other atoms. As a result, the behavior of an atom s valence electrons determines the chemical properties of the atom. The inner electron shells and the nucleus can be thought of as a single point-charge surrounded by valence electrons. Electrons occupy energy levels As we saw in our discussion of the Bohr atom, the electrons in an atom can possess only certain amounts of energy. For this reason, the electrons are often said to occupy specific energy levels. Electrons in a shell sometimes form a set of closely spaced energy levels. Normally, electrons are in the lowest energy level available to them. The specific arrangement of electrons in which all are in the lowest possible energy levels of an atom is called the atom s ground state. As described in Chapter 23, an atom can sometimes absorb energy from the environment. If the available energy is sufficient, one of the atom s electrons can move to a higher energy level. When this happens, the atom is said to be in an excited state. The electron may also absorb so much energy that it is no longer bound to the atom. The electron is then called a free electron. CONCEPT PREVIEW The structure within the nucleus will be studied in greater detail in Chapter 25. valence electron an electron in the outermost shell of an atom ground state the lowest energy state of a quantized system excited state the state of an atom that is no longer in its ground state BAND THEORY One model that can be used to help understand why solids fall into the three categories of conductor, insulator, and semiconductor is called band theory. Band theory can explain the mechanisms of conduction in many solids and the large variation in the electrical conductivity of these materials. When identical atoms are far apart, they have identical energy-level diagrams and identical wave functions. As the atoms are brought closer together, their wave functions overlap. Because no two electrons in the same system can occupy the same state, the energy level in an atom is altered by the influence of the electric field of another atom. In the case of two atoms, each energy level Modern Electronics 867

5 Two atoms Four atoms Many atoms Allowed energy band Figure 24-2 Energy levels split when two atoms are close together (a). Adding a few more nearby atoms causes further splitting (b). When many atoms interact, the energy levels are so closely spaced that they can be represented as energy bands (c). Energy Energy Energy Forbidden energy gap Allowed energy band Atomic separation Atomic separation Atomic separation (a) (b) (c) Energy Conduction band Band gap Valence band splits into two different energy levels. Figure 24-2 shows the splitting of two energy levels as two atoms get closer. Notice that the energy difference between two new energy levels depends on the distance between the atoms. When more atoms are brought close together, each energy level splits into more levels. The number of splittings depends on the number of interacting particles. If there are many atoms, the energy level splits so many times and the new energy levels are so closely spaced that they may be regarded as a continuous band of energies. When atoms are close to each other, some energy levels become energy bands When atoms are bound together in a solid, the clearly defined energy levels of a single atom widen and blur into energy bands, like those shown in Figure The most important energy band is the highest band containing occupied energy levels, known as the valence band. In solids that are semiconductors and insulators, the valence electrons do not conduct electricity. On the other hand, in solids that are conductors, the valence electrons are able to conduct electricity. There can be many more bands of lower energy than the valence band. These bands are completely filled. They are of little consequence in determining the electrical properties of the solid. There are additional unoccupied bands of higher energy besides the valence band. In semiconductors and insulators, the band immediately above the valence band is called the conduction band. If an atom is excited, valence electrons can sometimes move to this higher energy level. Then they can contribute to the conduction of electricity, as we will see. Figure 24-3 Energy levels of atoms become energy bands in solids. The valence band is the highest occupied band. band gap the minimum energy separation between the highest occupied state and the lowest empty state There is often an energy gap between bands The range of energies lying between the valence band and the conduction band is called the band gap, or energy gap, as shown in Figure An electron in an insulator or semiconductor cannot have a value for its energy that would lie within the band gap. Because these energies are not available for an electron in the insulator or semiconductor, they are often referred to collectively as the forbidden energy gap. The size of the band gap differs with different materials, resulting in different properties. In an insulator, the gap is so large that it is unlikely that an electron 868 Chapter 24

6 could ever gain enough energy to move from the valence band to the conduction band. In a semiconductor, the gap is somewhat smaller, making it more likely that an electron could move to the conduction band. In a conductor, the valence band is only partially filled, so there is essentially no gap between filled levels and available unoccupied levels. A band can be full or partially full Electrons in a solid normally occupy the lowest available energy levels within an atom. As electrons fill the energy levels of an atom, they occupy the lowest levels first. Normally there are no electrons in higher energy levels of an atom unless all of the lower energy levels are completely filled. In a solid, if there are more electrons than there are energy levels in the lowest energy band, then the lowest energy band is full. Because no more electrons can move into an already full band, any additional electrons must occupy energy levels in the next higher band. In some materials, there are more energy levels in a band than there are electrons to fill them. In such cases, the band remains partially full. Whether the highest energy band is completely full or only partially full is very important in determining the electrical properties of a material. In other words, partially full energy bands have different electrical properties than energy bands which are completely full. Electrons can move between energy levels in a solid Recall from Chapter 23 that when an electron in an atom absorbs energy, it can be excited to a higher energy level. However, the electron can absorb only certain amounts of energy that correspond to the differences in energy between the energy levels in the atom. A similar process can occur in a solid. Electrons that absorb energy can be excited to higher energy levels. An electron must be given sufficient energy to occupy some higher available energy level. Figure 24-4 is a schematic diagram showing the three highest bands in a solid with an energy gap between each band. Remember that even though these bands are pictured as one solid color, they are actually made up of closely spaced energy levels for electrons. All of the energy levels in the bottom two bands are completely filled with electrons. What transitions are possible for an electron in this solid? Because the only unfilled energy levels are in the conduction band, only the transitions represented by (a) and (b) are possible. However, transition (a) occurs much more easily than transition (b) does because transition (a) requires less energy. That is why the bands below the valence band do not appreciably affect the electrical properties of a material; the energy required for an electron in one of these low-lying bands to be excited into a higher unfilled energy level is usually too large. So, without sufficient energy, the electron will remain on a lower level and will not affect the electrical properties as much as valence electrons do. Although energy bands exist in all solids, the term is usually used to refer to crystalline solids. Crystalline solids are materials that are made of atoms arranged in regular, repeating patterns. Conduction band (a) Unfilled energy levels Valence band Filled energy levels (b) Figure 24-4 Only transitions into unfilled energy levels, such as those represented by (a) and (b), are possible for electrons in a solid. However, transition (b) requires such a large amount of energy that it does not easily occur. Modern Electronics 869

7 (a) Conductor Valence band Conduction band CONDUCTION AND ELECTRON TRANSITIONS So far, we have considered the band structure of a solid and the way an electron can make transitions between the energy levels within or between these bands. These transitions do not represent a physical motion of the electrons. Rather, they represent changes in the energy of electrons. The physical motion of electrons in a solid the conduction of electricity depends on the arrangement of the electrons in the bands of the solid, because a moving electron moves to an unfilled final energy level. Bound electrons need energy to escape the electrostatic forces that bind them to an individual atom. Electrons can be excited into higher energy levels by either of two important mechanisms: the application of an electric field or thermal excitation. When an electric field is applied to a solid, the electric field does work on individual electrons, giving them enough energy to move to higher energy levels within the atom. The absorption of thermal energy can also excite electrons in a solid. The atoms in a solid experience random vibrations (thermal energy) that can occasionally transfer enough energy to an electron to excite it to a higher energy level. At room temperature, a few electrons near the unfilled energy levels are excited to unoccupied levels. The thermal energy available for this is very small, so only the uppermost electrons can move across the band gap. (b) (c) E g 5 ev Insulator Valence band Conduction band E g 1 ev Valence band Semiconductor Band gap Band gap Figure 24-5 A conductor (a) has a partially filled valence band. Insulators (b) and semiconductors (c) have empty conduction bands and filled valence bands, but the band gap in a semiconductor is smaller than it is in an insulator. A conductor has a partially filled valence band If the valence band overlaps the conduction band, the solid is a conductor. For example, Figure 24-5(a) shows a valence band that is also the conduction band. The band is actually partly filled. The uppermost filled energy level lies in the middle of the band. If energy is added, electrons can be excited from filled energy levels to one of many higher unfilled energy levels. A solid conductor has many electrons that require only a small amount of energy either from the applied field or by thermal excitation to be moved into nearby unfilled energy levels. These electrons are then able to move freely in the conductor with only a small applied field. An important feature of a conductor is the partially filled conduction band. The rest of a conductor s band structure, including the size of the band gaps, is of little importance. An insulator has a full valence band and a large energy gap An insulator has an empty conduction band and a full valence band, as shown in Figure 24-5(b). It also has a very large band gap (about 5 10 ev) between the conduction band and the valence band. In fact, the band gap is so large that it is difficult for an electron to gain enough energy to be excited into the conduction band. 870 Chapter 24

8 For example, at room temperature, thermal excitations typically give electrons about ev, which is much smaller than the band gap. At such a temperature, very few electrons are thermally excited into the conduction band. Thus, although an insulator has many vacant states in its conduction band, there are so few electrons occupying these states that the overall electrical conductivity is very small, resulting in the high resistance typical of substances that are insulators. A semiconductor has a full valence band and a small energy gap Intermediate between a conductor and an insulator is a semiconductor. The valence band of a semiconductor, like that of an insulator, is full, as shown in Figure 24-5(c). However, the band gap in a semiconductor is much smaller than in an insulator (about 1 ev). In fact, the band gap in many semiconductors is small enough for electrons to be thermally excited into the conduction band rather easily. Thus, the conductivity of many semiconductors is strongly dependent on temperature. For example, at temperatures near 0 K, most electrons are in the valence band and there is little thermal energy available for excitation. This makes semiconductors poor conductors at very low temperatures. At higher temperatures, however, a larger number of electrons can be thermally excited from the valence band to the conduction band, which has many empty energy levels. Because thermal excitation across the narrow gap is more probable at higher temperatures, the conductivity of semiconductors improves rapidly with temperature. The application of an electric field to a semiconductor also does work on electrons, increasing their energy so that they can conduct and create a current. Section Review 1. The resistance of conductors increases with increasing temperature, as described in Chapter 19. On the other hand, the resistance of a semiconductor decreases with increasing temperature. How can you explain this property of semiconductors? 2. Which of the following is likely to be a valence electron? a. the innermost electron of a uranium atom b. an electron in a calcium atom that is in the outermost energy level c. an electron in a bromine atom that is in the second-most outer energy level of the atom 3. Which band contains energy levels for electrons that are already free to move about within a semiconductor? Which band contains energy levels for electrons that cannot move within a semiconductor? Modern Electronics 871

9 24-2 Semiconductor applications 24-2 SECTION OBJECTIVES Compare the roles of electrons and positively charged holes in conducting current. Describe the process of doping to create n- and p-type semiconductors. Analyze p-n junctions and their role in semiconductor devices. Explain the role of a diode as a rectifier. Explain how a transistor can be used as an amplifier. hole an energy level that is not occupied by an electron in a semiconductor SEMICONDUCTOR DOPING Charge carriers in a semiconductor can be negative or positive. To see why this is so, consider the valence and conduction bands of a semiconductor, as shown in Figure Imagine that a few electrons are excited from the valence band to the conduction band by an electric field. The electrons in the conduction band are free to move through the material. Normally, electrons in the valence band are unable to move because all nearby energy levels are occupied. But when an electron moves from the valence band into the conduction band, it leaves a vacancy, or hole, in an otherwise filled valence band. Because this hole creates an empty energy level in the valence band, another valence electron from this or a nearby atom is free to move into the hole. Whenever an electron does so, a new hole is created at its former location. So, the net effect can be viewed as a hole migrating through the material in a direction opposite the motion of the electrons in the conduction band. In a material containing only one element or compound, there are an equal number of conduction electrons and holes. Such combinations of charges are called electron-hole pairs, and a semiconductor that contains such pairs is called an intrinsic semiconductor. Figure 24-6 is a schematic of an intrinsic semiconductor. In the presence of an electric field, the holes move in the direction of the field and the conduction electrons move opposite the field. Keep in mind that the motion of holes is always in the direction opposite the motion of the electrons. Figure 24-6 An electric field can excite valence electrons into the conduction band, where they are free to move through the material. Holes in the valence band can then move in the opposite direction. Energy Conduction electrons Applied E field Conduction band Narrow forbidden gap Valence band Electrons Holes Doping adds impurities that enhance conduction In the last section, it was explained that the concentration (the number per unit volume) of charge carriers in a semiconductor depends on temperature. Another way to change the concentration of charge carriers is to add impurities, atoms that are different from those of an intrinsic semiconductor. 872 Chapter 24

10 Adding impurities is called doping. Only a few added impurity atoms (about one part in a million) can have a large effect on a semiconductor s resistance. The semiconductor s conductivity increases as the doping level increases. When impurities dominate conduction, the material is called an extrinsic semiconductor. There are two methods for doping a semiconductor: either add impurities that have extra valence electrons or add impurities that have fewer valence electrons compared with the atoms in the intrinsic semiconductor. Semiconductors used in commercial devices are usually doped silicon or germanium. These elements have four valence electrons. Semiconductors are doped by replacing an atom of silicon or germanium with one containing either three valence electrons or five valence electrons. Note that a doped semiconductor is electrically neutral because it is made of neutral atoms. The semiconductor atoms and the impurity atoms remain uncharged as before. The balance of positive and negative charges has not changed, but the number of charges that are free and able to move has. These charges are therefore able to participate in electrical conduction. doping the addition of impurity atoms to a semiconductor NSTA TOPIC: Semiconductors GO TO: scilinks CODE: HF2242 An n-type semiconductor has electrons as majority carriers In Figure 24-7(a), an atom containing five valence electrons, such as arsenic, is added to a semiconductor. Four of the valence electrons participate in the bonding between neighboring atoms. One electron is left over. Such an impurity in effect donates an extra electron to the solid, so it is referred to as a donor atom. The impurity alters the band structure of the solid. Therefore, the extra electron occupies an energy level, or donor level, that lies just below the conduction band, as shown in Figure 24-7(b). The energy spacings between donor levels and the bottom of the conduction band are very small (typically about 0.05 ev). So, only a small amount of thermal energy is needed to cause an electron in these donor levels to move into the conduction band. Semiconductors doped with donor atoms are called n-type semiconductors. The reason is that most of the charge carriers are electrons with negative charges. Positive holes that form in the donor level do not move very easily. The average thermal energy of an electron at room temperature (22 C) is about ev. Semiconductor atoms Conduction band Impurity atom with five valence electrons E g Donor levels E d Extra electron from impurity atom Figure 24-7 An n-type semiconductor is doped with impurity atoms that have extra valence electrons. (a) (b) Valence band E g = 1 ev E d 0.05 ev Modern Electronics 873

11 Semiconductor atoms Conduction band Impurity atom with three valence electrons Hole, or electron deficiency in a bond Figure 24-8 A p-type semiconductor is doped with impurity atoms having fewer valence electrons. (a) Acceptor levels Valence band A p-type semiconductor has holes as majority carriers If a semiconductor is doped with atoms containing three valence electrons, such as indium and aluminum, all three of these electrons form bonds with the neighboring atoms in the p-type semiconductor. This leaves an electron deficiency, or hole, in the fourth bond, as shown in Figure 24-8(a). The energy levels, or acceptor levels, of such impurities lie just above the valence band, as shown in Figure 24-8(b). Electrons from the valence band have enough thermal energy at room temperature to fill these impurity levels, leaving a hole in the valence band. These holes can be filled by other nearby electrons, which leave new holes. Thus, positive charges (holes) can move throughout the material even if no electrons are in the conduction band. Because such an impurity accepts an electron from the valence band, these impurities are referred to as acceptors. A semiconductor containing acceptors is known as a p-type semiconductor because the majority of the charge carriers are positively charged holes. E g (b) E g = 1 ev E a 0.05 ev E a diode an electronic device that allows electric current to pass more easily in one direction than in the other Figure 24-9 The circuit diagram symbol for a diode indicates that it allows electric current in one direction only. DIODES A diode is a device that has an almost infinite resistance in one direction and nearly zero resistance in the other direction. So, a diode is able to pass current in only one direction, similar to the way a one-way valve passes water into a pipeline. A diode used in this manner is called a rectifier. Figure 24-9 shows the electrical circuit symbol for a diode. The arrow that is a part of the circuit symbol for the diode indicates the direction of the current in the diode under most situations. The p-n junction is the contact between a p-type semiconductor and an n-type semiconductor Consider what happens when a p-type semiconductor is joined to an n-type semiconductor to form a p-n junction. A device having one p-n junction can be used as a diode in electrical circuits. It will allow current to pass through the circuit in only one direction. This is because holes and electrons on either 874 Chapter 24

12 side of the p-n junction can migrate across the junction. In doing so, they create a potential barrier (an internal electric field) that allows charge to flow one way and not the other. Let us examine this effect in more detail. Remember that an n-type semiconductor has free electrons and a p-type semiconductor has free holes. When an n-type semiconductor and a p-type semiconductor are brought together to form a p-n junction, these free positive and negative charges move because of thermal diffusion. Electrons from the n side nearest the junction, the blue area in Figure 24-10(a), diffuse toward the p side. Positive ions that are fixed in the solid are left behind. Similarly, on the p side nearest the junction, electrons diffuse so that holes move to the n side, which leaves a region of fixed negative ions. When a freemoving electron and a free-moving hole meet, the charges cancel each other, leaving no charge carriers. As a result, the region where the p-type and the n-type semiconductors meet has no mobile charge carriers. For this reason, it is called a depletion region. The depletion region has a fixed size the presence of the ions in it creates an internal electric field that opposes the further diffusion of electrons and holes that would cause the region to grow larger. This internal electric field is directed from right to left in Figure 24-10(a). On either side of the depletion region are the p-type and n-type regions. So, the p-n junction in a diode consists of three distinct regions, as shown in Figure 24-10(a). (a) (b) Electric potential p Depletion region E n V 0 Figure Diffusion of charge carriers across a p-n junction sets up an electric field within the depletion region. x Diodes allow movement of charge in only one direction The different regions of the diode can be understood in terms of the electric potential diagram in Figure 24-10(b). Because of the electric field within the depletion region, one side of the junction is held at a higher electric potential than the other side is. Positive charge carriers cannot move from left to right. Such a move requires extra energy, q V 0 (see Chapter 18), to overcome the internal electric field in the depletion region. For the same reason, negative charge carriers cannot move from right to left. The extra energy needed to move negative charge carriers from right to left can be supplied by an external source of potential difference. If a large enough positive external voltage is applied to the p side of the junction, the electric potential on the left (positive) side is raised relative to the right (negative) side. Because the left side is now at a higher potential than the right side, charges will move, creating a current. When a diode is connected to an external source of potential difference in this manner, it is said to be forward biased. When a positive external voltage is applied to the n side of the junction, however, the potential barrier is increased even more. This increase further limits the current in the junction. Such a diode is said to be reverse biased. Modern Electronics 875

13 Reverse bias Current (ma) Forward bias Potential difference (V) Figure The slope at any point on this curve is equal to the reciprocal of the resistance of the diode at that voltage and current. NSTA TOPIC: Energy levels GO TO: scilinks CODE: HF2241 Figure shows a typical plot of the current in a diode versus the potential difference across the diode. Notice that a diode does not obey Ohm s law (see Chapter 19). Ohm s law predicts that a plot of current versus potential difference will be a straight line with a slope equal to the reciprocal of the resistance. The resistance of a diode, on the other hand, is not constant. When the diode is reverse biased, the curve is nearly horizontal, which implies that the resistance is effectively infinite. (The fact that the current is slightly negative means that, in reality, a small leakage current still flows in the wrong direction in a reverse-biased diode. Because the leakage current is typically very small, we will ignore its presence.) When the diode is forward biased, the resistance varies, approaching zero for larger currents. Note that the maximum potential difference across the diode under forward bias is small (about V). Diodes can be used to convert alternating current to direct current Figure shows the effect of a diode on an applied alternating potential difference. Figure 24-12(a) is the current produced by the generator without the diode. Figure 24-12(b) is the current in the resistor with the diode in place. Only the positive part of the current remains because the diode is forward biased at these times. The diode therefore has very little resistance. When the diode is reverse biased, it has a very large resistance. In this case, the negative part of the original signal is suppressed by the diode, so there is no current in the resistor from this part of the signal. This process of converting an alternating current into a direct current is called rectification. When a single diode is used as a rectifier, the direct current is not constant; instead, it occurs in pulses. This current is referred to as a pulsed direct current. A smoother direct current can be made from the alternating current if more than one diode and some capacitors are included in the rectifier circuit. An additional diode allows more pulses of direct current between the times of the pulses allowed by the first diode, as shown in Figure 24-12(c). The capacitor acts as a filter that smooths the output of the rectifier. Capacitors store charge when a potential difference is applied and release it when there is no potential difference. So, they keep the current in the circuit steadier, as shown in Figure 24-12(d). Figure The diode rectifies an alternating current (a) into a pulsed direct current (b). With the addition of other diodes, more pulses can be added to the signal (c). Capacitors can act as filters that smooth out the variations in current (d). R I R I I R (a) t (b) I R I R (c) t (d) t t 876 Chapter 24

14 TRANSISTORS A transistor is a more complicated electronic device that is used in many applications. Transistors have two p-n junctions instead of one. In this section, we will see how a transistor can be used to amplify a signal. This property of a transistor arises from the nature of the p-n junction. There are many types of transistors, but the type we are concerned with is the junction transistor. The junction transistor shown in Figure 24-13(a) consists of a semiconducting material with a very narrow n region sandwiched between two p regions. This configuration is called a pnp transistor. Another configuration is the npn transistor, which consists of a p region sandwiched between two n regions. Because the operation of the two transistors is essentially the same, we will describe only the pnp transistor. The structure of the pnp transistor, together with its circuit symbol, is shown in Figure The arrow on the emitter lead in Figure 24-13(b) is necessary to distinguish a pnp transistor from an npn transistor. In an npn transistor, the arrow points in the opposite direction. The outer regions are called the emitter and collector, and the narrow central region is called the base. Notice that the junction transistor contains two p-n junctions: the emitter-base junction and the collector-base junction. Usually, the emitter is more heavily doped than the base, so there are more charge carriers in the emitter compared with what would be found in the base. The transistor, unlike most electrical components you have encountered, has three leads instead of two. Setting the reference voltage is important for transistor operation One potential difference ( V ec ) is applied between the emitter and the collector so that the emitter is at a higher electric potential than the collector, as shown at point A in Figure The base is held at an electric potential between the emitter potential and the collector potential by the potential difference at point B ( V eb ). Remember from our discussion of diodes that if the transistor a device, typically containing three terminals, that can amplify a signal Emitter p (a) (b) Emitter Base n Base Collector Collector Figure (a) A pnp junction transistor consists of an n-type semiconductor sandwiched between two p-type semiconductors. (b) The circuit symbol for a transistor has three leads. p A V ec R Output signal E B C I c I b Input signal Figure V eb B Two batteries are necessary to properly bias this transistor. Modern Electronics 877

15 The discovery of the transistor by John Bardeen, Walter Brattain, and William Shockley in revolutionized the world of electronics. For this work, these three men shared a Nobel Prize in NSTA TOPIC: Transistors GO TO: scilinks CODE: HF2243 p region is at a higher electric potential than the n region across a p-n junction, the p-n junction is forward biased. If we think of the transistor as two diodes back to back, we see that the emitter-base junction is forward biased and that the base-collector junction is reverse biased. This particular biasing of the two junctions in a transistor is crucial for its proper operation. The junction transistor can be used as an amplifier Now we can see how a transistor can amplify a signal. First consider the emitter-base junction. It is forward biased so that current enters the transistor through the emitter terminal and charge readily flows across the emitter-base junction. Because the emitter is heavily doped relative to the base, nearly all of the current consists of holes moving from the p-type emitter to the base. Few electrons move from the base to the emitter. In addition, most of the holes do not recombine with electrons in the base, because the base is very narrow. Although only a small number of holes recombine in the base, those that do recombine limit the current that can go from the emitter to the collector through the base. This is because positive charge carriers accumulate in the base and prevent holes from flowing in. Once the surviving holes diffuse through the base, they encounter the basecollector junction. This junction is reverse biased, so normally there is no current in this region. The base-collector barrier with its depletion region prevents electrons from migrating to the right and prevents holes from migrating to the left. In this case, however, the holes are on the n side of the junction, where the charge carriers are usually electrons. So the barrier has an opposite effect on holes migrating to the right; the holes are accelerated across the reverse-biased base-collector junction. A small change in the properties of the base can have a great effect on the movement of charge from the emitter to the collector. One way to cause such a change is to connect the base to a second source of potential difference, labeled V eb, as shown at B in Figure The current from this source, although small, draws some of the positive charge that would otherwise accumulate in the base. As a result, more charge can pass through the base from the emitter to the collector. The small potential difference to be amplified is placed in series with this battery. The input signal produces a small variation in the base current, resulting in a large change in collector current and therefore a large change in potential difference across the output resistor. If the transistor is properly biased, the collector (output) current is directly proportional to the base (input) current and the transistor acts as a current amplifier. This condition may be written as follows: I c = bi b The quantity b (beta) is called the current gain. Values of the current gain typically range from 10 to Chapter 24

16 INTEGRATED CIRCUITS The integrated circuit has been called the most remarkable technology ever created by mankind. Integrated circuits form the foundation for computers, watches, cameras, automobiles, aircraft, robots, space vehicles, and all sorts of communication and switching networks. In simplest terms, an integrated circuit is a collection of interconnected transistors, diodes, resistors, and capacitors fabricated onto a single piece of silicon, known as a chip. State-of-the-art chips easily contain several hundred thousand components in a very small area, as shown in Figure Integrated circuits were invented partly in an effort to achieve circuit miniaturization and partly to solve the interconnection problem spawned by the transistor. Before transistors, power and size considerations of individual components set modest limits on the number of components that could be interconnected in a given circuit. With the advent of the tiny, low-power, highly reliable transistor, design limits on the number of components disappeared and were replaced with the problem of wiring together hundreds of thousands of components. Figure shows a schematic diagram of one of the first types of integrated circuits, an operational amplifier. Operational amplifiers, or op amps, are used extensively in electronics and computers. They are the basic element used to perform key mathematical operations, such as addition and multiplication. The numbers involved in the operation are the values of the input and output potential differences. For example, in one configuration, a single input into an op-amp results in a single output. The magnitude of the potential difference of the output will be the magnitude of the potential difference of the input multiplied by the gain for the device. In another configuration, several potential differences are connected as inputs to an op-amp. The result is a single output that has a potential difference with a magnitude that is the sum of the magnitudes of the individual input potential differences. Figure Although it is smaller than a dime, this computer memory chip can store more than 1 million digital bits of information. V Inputs V Output Figure This is a simplified schematic diagram of an operational amplifier, one of the first types of integrated circuits. Notice the many resistors, diodes, transistors, and interconnections that make up the circuit. Modern Electronics 879

17 The integrated circuit was invented independently by Jack Kilby, at Texas Instruments, in late and by Robert Noyce, at Fairchild Camera and Instrument, in early In addition to solving the interconnection problem, integrated circuits have the advantages of miniaturization and fast response. These two features are critical for high-speed computer operation. The fast response results from the miniaturization and close packing of components. The response time of a circuit depends on the time it takes for electrical signals traveling at about 0.3 m/ns to pass from one component to another. This time is reduced by the closer packing of components. These advances, along with new parallel arrangements of processors that allow a computer to break difficult calculations into many pieces, have continued. Computers are getting smaller and faster every day. In 1997, a computer at Sandia National Laboratories became the first to perform more than 1 trillion calculations per second. Section Review 1. Which of the following are equivalent to a conventional current from left to right? a. electrons traveling from left to right b. electrons traveling from right to left c. holes traveling from left to right d. holes traveling from right to left 2. Explain why it is not necessary for an n-type semiconductor to be negatively charged. 3. What type of fixed charge is predominant in the depletion region of a p-type semiconductor at a p-n junction? 4. A diode is connected to a source of alternating current. The current in the source fluctuates between 1.0 ma and 1.0 ma. What will be the range for the rectified current? 5. The current in the base terminal of a transistor is decreased. How will this affect the output current in the collector terminal? 6. Transistors are widely used as amplifiers in radio receivers as well as in microphones. This is because a typical radio signal from an antenna has only a few microamperes of current and a speaker often requires a current of about 0.1 A to work properly. a. If a transistor circuit is used to amplify a 2.5 ma signal to at least 0.1 A, what must be the minimum gain of the transistor circuit? b. Assume the amplifier consists of a number of transistors in series, each with a gain of 10. How many transistors are needed in this circuit? (Hint: Two such transistors in series would provide a total gain of = 100.) 880 Chapter 24

18 24-3 Superconductors TEMPERATURE AND CONDUCTIVITY Recall from Chapter 19 that the resistance of many solids (other than semiconductors) increases with increasing temperature. This is because at a nonzero temperature, the atoms in a solid are always vibrating, and the higher the temperature, the larger the amplitude of the vibrations. It is more difficult for electrons to move through the solid when the atoms are moving with large amplitudes. This is similar to walking through a crowded room. It is much harder to do so when the people are in motion than when they are standing still. If the resistance depended only on atomic vibrations, we would expect the resistance of the material that is cooled to absolute zero to go gradually to zero as the superconductivity described in Chapter 19 began to occur. Experiments have shown, however, that this does not happen. In fact, the resistances of very cold solids behave in two very different ways either the substance suddenly begins superconducting at temperatures above absolute zero or it never superconducts, no matter how cold it gets. Lattice imperfections cause resistance in some materials The graph in Figure shows the temperature dependence of the resistance of two similar objects, one made of silver and the other made of tin. The temperature dependence of the resistance of the silver object is similar to that of a typical metal. At higher temperatures, the resistance decreases as the metal is cooled. This suggests that the amplitude of the lattice vibrations is decreasing, as expected. But at a temperature of about 10 K, the curve levels off and the resistance is constant. Cooling the metal further does not appreciably lower the resistance, even though the vibrations of the metal s atoms have been lessened. Part of the cause of this nonzero resistance, even at absolute zero, is lattice imperfection. The regular, geometric pattern of the crystal, or lattice, in a solid is often flawed. A lattice imperfection occurs when some of the atoms do not line up perfectly. Imagine you are walking through a crowded room in which the people are standing in perfect rows. It would be easy to walk through the room between two rows. Now imagine that occasionally one person stands in the middle of the aisle instead of in the row, making it harder for you to pass. This is similar to the effect of a lattice imperfection. Even in the absence of thermal vibrations, many materials exhibit a residual resistance due to the imperfect geometric arrangement of their atoms SECTION OBJECTIVES Identify the cause of electrical resistance for some conductors at absolute zero. Explain the BCS theory of superconductivity. Describe some applications of superconductivity. Resistance (Ω) Tin 0 T c 10 Temperature (K) Silver 20 Figure The resistance of silver exhibits the behavior of a normal metal. The resistance of tin goes to zero at temperature T c, the temperature at which tin becomes a superconductor. Modern Electronics 881

19 Superconductivity was discovered in by the Dutch physicist H. Kamerlingh Onnes as he and a student studied the resistivity of mercury at very low temperatures. Onnes received the Nobel Prize in physics in NSTA TOPIC: Superconductors GO TO: scilinks CODE: HF2244 Electron 2 Electron 1 Lattice ion Figure The first electron deforms the lattice, and the deformation affects the second electron. The net result is as if the two electrons were loosely bound together. Such a two-electron bound state is called a Cooper pair. E Figure shows that the resistance of tin goes to zero below a certain temperature that is well above absolute zero. A solid whose resistance is zero below a certain nonzero temperature is called a superconductor, and the temperature at which the resistance goes to zero is the critical temperature of the superconductor, as described in Chapter 19. BCS THEORY Before the discovery of superconductivity, it was thought that all materials should have some nonzero resistance due to lattice vibrations and lattice imperfections, much like the behavior of the silver in Figure The first complete microscopic theory of superconductivity was not developed until 1957, almost half a century after the discovery of superconductivity. This theory is called BCS theory after the three scientists who first developed it: John Bardeen, Leon Cooper, and Robert Schrieffer. The crucial breakthrough of BCS theory is a new understanding of the special way that electrons traveling in pairs move through the lattice of a superconductor. According to BCS theory, electrons do suffer collisions in a superconductor, just as they do in any other material. However, the collisions do not alter the total momentum of the pair of electrons. The net effect is as if the electrons moved unimpeded through the lattice. Imagine an electron moving through a lattice, such as Electron 1 in Figure There is an attractive force between the electron and the nearby positively charged atoms in the lattice. As the electron passes by, the attractive force causes the lattice atoms to be pulled toward the electron. The result is a concentration of positive charge near the electron. If a second electron is nearby, it can be attracted to this excess positive charge in the lattice before the lattice has had a chance to return to its equilibrium position. Through the process of deforming the lattice, the first electron gives up some of its momentum. The deformed region of the lattice attracts the second electron, transferring excess momentum to the second electron. The net effect of this two-step process is a weak, delayed attractive force between the two electrons, resulting from the motion of the lattice as it is deformed by the first electron. The attractive force between these two electrons is an electronlattice-electron interaction, where the lattice serves as the mediator of the attractive force. The two electrons travel through the lattice acting like a single particle. This particle is called a Cooper pair. In BCS theory, Cooper pairs are responsible for superconductivity. The reason superconductivity has been found at only low temperatures so far is that Cooper pairs are weakly bound. Random thermal motions in the lattice tend to destroy the bonds between Cooper pairs. Even at very low temperatures, Cooper pairs are constantly being formed, destroyed, and reformed in a superconducting material, usually with different pairings of electrons. 882 Chapter 24

20 Cooper pairs maintain their momentum as they move through the lattice Calculations of the properties of a Cooper pair have shown that this peculiar bound state of two electrons has zero total momentum in the absence of an applied electric field. When an external electric field is applied, the Cooper pairs move through the lattice under the influence of the field. However, the center of mass for every Cooper pair has exactly the same momentum. This is the crucial feature of Cooper pairs that explains superconductivity. If one electron scatters, the other electron in a pair also scatters in a way that keeps the total momentum constant. The net result is that scattering due to lattice imperfections and lattice vibrations has no net effect on Cooper pairs. APPLICATIONS OF SUPERCONDUCTIVITY Notice in Table 19-3, on page 706, that in order to be superconducting, many materials have to be cooled to very close to absolute zero. This makes the use of these superconductors for everyday applications difficult because it is expensive to cool a material to such low temperatures. In fact, early superconductors required coolant baths of either liquefied helium, which is rare and expensive, or liquid hydrogen, which is very explosive. As shown in Table 19-3, a new class of materials 150 has been found that superconducts at much higher 140 temperatures. In particular, these superconductors 130 can enter the superconducting state at temperatures above 77 K, which is the boiling point for liq uid nitrogen. This is very important, because liquid nitrogen is inexpensive, abundant, and inert. The 100 search continues for materials that will superconduct at room temperatures. Figure illustrates Liquid O 2 the dramatic increase in the highest known critical Liquid N 2 70 temperature for a superconductor since the discovery of this phenomenon Superconductors can reduce energy loss and revolutionize electronics The property of zero resistance to direct currents would be very advantageous in low-loss electrical power transmission. A significant fraction of electrical power is converted to internal energy in normal conductors. If power transmission lines could be made to be superconducting, these losses could be eliminated, and there would be substantial savings in energy costs. Critical temperature (K) Liquid H 2 Liquid He HgBa 2 Ca 2 Cu 2 O 8 Ti Ba Ca Cu O Bi Ba Ca Cu O YBa 2 Cu 3 O 7 La Sr Cu O 40 Nb 3 Ge La Ba Cu O 30 NbN 20 Hg Year of discovery Figure The last 20 years have yielded a dramatic rise in the temperature at which a material becomes superconducting. Modern Electronics 883

21 High-temperature superconductors could also have a major impact on the field of electronics. For instance, the junction of two superconductors, like that of two semiconductors, has special properties. Together, the two superconductors can act as an electronic switch. Superconducting film could be used to connect computer chips, enhancing the speed of computer operation. Superconducting rings could be used as storage devices for electrical current or energy. Because there is no resistance in a superconductor, a current introduced in a superconducting ring will continue indefinitely. The electrical energy in the current could be extracted at a later time. Figure Magnetic resonance imaging (MRI) is one medical application of superconducting magnets. Superconductors have special magnetic properties Recall from Chapter 21 that when a conductor carries a current, a magnetic field is produced around the conductor. If a magnet is near the conductor, the magnet will be attracted to or repelled by the conductor. This is the basis of magnetic levitation, a phenomenon in which an object is suspended in the air because it is repelled by a magnet underneath. Such magnets are usually electromagnets. However, electromagnets dissipate a great deal of energy because of their electrical resistance to current. The solution to this problem is to use superconducting wires in the electromagnet. Magnetic levitation has many potential applications in the field of transportation. In fact, a prototype for a train that runs on superconducting magnets has been constructed in Japan. Superconducting magnets are already used in high-energy particle accelerators. Another important application of superconducting magnets is a diagnostic tool called magnetic resonance imaging (MRI). This technique has played a prominent role in diagnostic medicine. It uses relatively safe radiofrequency radiation rather than high-energy X rays to produce images of body sections. Figure shows a patient undergoing an MRI, along with the resulting images of the patient s brain. Section Review 1. A substance has a nonzero resistance even when cooled to absolute zero. What can you predict about the substance s structure? 2. Do electrons in a superconductor collide with atoms in the superconductor? Explain your answer. 3. How is a superconducting ring like a capacitor? How is it different? 4. MRI scans of the brain rely on radio-frequency waves instead of X rays. Calculate the energy of an X-ray photon with a wavelength of m. Then calculate the energy carried by a radio-wave photon with a frequency of 100 MHz. 884 Chapter 24

22 CHAPTER 24 Summary KEY IDEAS Section 24-1 Conduction in the solid state Solids can be classified according to their electronic properties as conductors, insulators, or semiconductors. Electrons in a solid occupy sets of energy levels called bands. Electrons in a semiconductor can be excited from the valence band into a different band, called the conduction band. When this happens, the semiconductor conducts electric charge. Section 24-2 Semiconductor applications Charge can move in a substance as electrons or as positively charged holes left by an electron-poor substance. The n-type semiconductor contains impurities of an atom that has five valence electrons instead of four. As a result, electrons are the majority charge carriers. The p-type semiconductor contains impurities of an atom that has three valence electrons instead of four. As a result, positively charged holes are the majority charge carriers. A diode allows current in only one direction. A transistor consisting of two diodes placed back to back and properly biased with batteries can be used to amplify a weak signal. Section 24-3 Superconductors A superconductor is a solid whose resistance is zero below some temperature called the critical temperature. According to the BCS theory, electrons travel in pairs and the momentum of the pair doesn t change. KEY TERMS band gap (p. 868) diode (p. 874) doping (p. 873) excited state (p. 867) ground state (p. 867) hole (p. 872) transistor (p. 877) valence electrons (p. 867) Diagram symbols electron hole electric field vector electric current arrow diode transistor Modern Electronics 885

23 CHAPTER 24 Review and Assess VALENCE ELECTRONS AND BAND THEORY Review questions 1. Discuss the differences in the band structures of metals, insulators, and semiconductors. How does the band structure model enable you to better understand the electrical properties of these materials? 2. Which of the energy bands in Figure contain unexcited valence electrons? Band A Band D Conceptual questions 5. When a photon is absorbed by a semiconductor, an electron-hole pair is created. Using the energy-band model, explain how this enables the semiconductor to conduct electricity. 6. The energy of visible light ranges from 1.8 to 3.2 ev. Use this fact to explain why silicon, with an energy gap of 1.1 ev, appears opaque, whereas diamond, with an energy gap of 5.5 ev, appears transparent despite the many similarities in their structures and atoms. Band B Band E SEMICONDUCTORS, DIODES, AND TRANSISTORS Band C Semiconductor Band F Conductor Figure Which of the following is likely to be able to move and conduct electricity? a. an electron in the innermost shell of an indium atom b. an electron in the innermost shell of a copper atom c. a valence electron in a copper atom in a wire d. a valence electron in the semiconductor selenium e. an excited electron in the semiconductor selenium f. an electron in the conduction band of the semiconductor germanium 4. Which is likely to have the larger band gap, iodine (an insulator) or silicon (a semiconductor)? Review questions 7. An electric field is oriented as shown in Figure In which direction will electrons in this field move? In which direction will holes move? 8. Which is likely to contain the most holes, a semiconductor doped with atoms with three valence electrons, four valence electrons, or five valence electrons? 9. What are the majority charge carriers in an n-type semiconductor? 10. What are the majority charge carriers in a p-type semiconductor? 11. Explain how diodes differ from resistors. 12. When a single diode is used as a rectifier, does it provide a constant current? E Figure Chapter 24

24 13. In Figure 24-23, the p-n diode is conducting electricity. a. Which way do electrons move in the circuit? b. Which way do holes move in the circuit? 14. If the circuit in Figure had the battery s polarity in the opposite direction, would the diode still conduct electricity? 15. Explain how transistors differ from diodes. Circuit with a conducting diode Figure In the pnp transistor described on page 877, between which two points is the input signal connected? 17. In the pnp transistor described on page 877, between which two points is the output signal connected? Graphing calculators Refer to Appendix B for instructions on downloading programs for your calculator. The program Chap24 allows you to calculate the number of free electrons per cubic centimeter of a substance. The program Chap24 stored on your graphing calculator makes use of the following equation for free electrons per cubic centimeter in a substance: F = (D*6.02E23)/(N*M) In this equation, D is the density of the substance, N is the number of atoms per free electron for that substance, and M is the atomic mass of the substance. The number is a constant called Avogadro s constant. You may be familiar with Avogadro s constant from your studies of the ideal gas law in chemistry. a. Recall from your studies of conductance in Section 17-3 that the ability of a material to conduct is related to the number of free electrons in that material. If substance A has more free electrons per cubic centimeter than substance B, which substance is a better conductor? The program Chap24 stored on your graphing calculator makes use of the free electron equation. Once the Chap24 program is executed, your calculator will ask for the density of the substance, the atomic mass, and the number of atoms per free electron. The graphing calculator will use these values to find the number of electrons per cubic centimeter for that substance. Execute Chap24 on the p menu and press e to begin the program. Enter the value for the mass density, then the value for the atomic mass, and finally the value for the number of atoms per free electron. Press e after each value. to enter exponents. When you have entered the appropriate values, the calculator will perform the calculation and will display the number of electrons per cubic centimeter. Determine the number of free electrons per cubic centimeter for the following materials: b. copper, which has a density of 8.92 g/cm 3,an average atomic mass of u, and an average of one atom for each free electron c. silicon, which has a density of 2.33 g/cm 3,an average atomic mass of u, and an average of atoms for each free electron d. Which is a better insulator, copper or silicon? Press e to input a new value or ı to end the program. Modern Electronics 887

25 Conceptual questions 18. Atoms with five valence electrons, such as arsenic, are donor atoms in a semiconductor, while those with three valence electrons, such as gallium, are acceptor atoms. Check the periodic table in Appendix E to determine what other elements might make good donors or acceptors. 19. Is the value for the current gain for an amplifying transistor likely to be one, more than one, or less than one? SUPERCONDUCTORS Review questions 20. If a substance is superconducting, is it likely to contain lattice imperfections? 21. What is conserved as Cooper pairs travel through a superconductor? 22. Are electrons in Cooper pairs directly attracted to each other? Conceptual questions 23. Would a superconductor make a good heating element in a toaster oven? Explain why or why not. 24. Would a superconductor make a good material for a high-voltage transmission line? Explain why or why not. MIXED REVIEW 25. The band gap for silicon at 300 K is 1.14 ev. a. Find the lowest frequency photon that will promote an electron in silicon from the valence band to the conduction band. b. What is the wavelength of this photon? 26. A light-emitting diode (LED) made of the semiconductor GaAsP emits red light with a wavelength of 650 nm. Determine the band gap in the semiconductor. 27. Most solar radiation has a wavelength of 10 6 m or less. What band gap should the material in a solar cell have in order to absorb this radiation? Alternative Assessment Performance assessment 1. Silicon, Si, atoms have four valence electrons. Arsenic, As, atoms used as impurities have five valence electrons. Will the resulting n-type semiconductor have a net negative charge? Draw diagrams to support or refute your claim, and present your arguments in a class discussion. 2. Design an experiment to investigate the effects of varying temperature over a range of 100 C on a metallic conductor s resistance. Review Ohm s law before planning your experiment. List the materials you will need and describe the procedure you will follow, including the measurements you will take. If your teacher approves your plan, obtain the necessary equipment and perform the experiment. What resistances do your results predict at absolute zero? Portfolio projects 3. Research information about the physical properties, sources, and other uses of elements that can be used in superconductors. Obtain pictures of samples, if possible. Organize your findings in a brochure, poster, or computer presentation. 4. Interview someone in the electronics industry, or research careers in this industry. Find out about the types of jobs available and the education and training necessary for these jobs. Prepare a file documenting your search process, letters you wrote, notes from interviews, and other sources of information. Compile a summary report, brochure, or presentation based on your research that could be used by clients of a career guidance center. 888 Chapter 24

26 CHAPTER 24 Laboratory Exercise RESISTORS AND DIODES Ohm s law describes the relationship between the current and the potential difference for an ohmic resistor in a circuit. Ohmic resistors are used to regulate the current in a circuit. For a given potential difference, a higher resistance results in a smaller current, while a lower resistance results in a larger current. Diodes are used in circuits because they have an almost infinite resistance in one direction (reverse biased) and nearly zero resistance in the other direction (forward biased). In this experiment, you will compare the performance of an ohmic resistor in a circuit with the performance of a diode in the same circuit. SAFETY Never close a circuit until it has been approved by your teacher. Never rewire or adjust any element of a closed circuit. Never work with electricity near water; be sure the floor and all work surfaces are dry. If the pointer on any kind of meter moves off scale, open the circuit immediately by opening the switch. Do not attempt this exercise with any batteries, electrical devices, or magnets other than those provided by your teacher for this purpose. PREPARATION OBJECTIVES Examine the relationships between current, potential difference, and resistance in an ohmic resistor circuit. Examine the relationships between current, potential difference, and resistance in a semiconductor diode circuit. Recognize the directional character of the resistance in the diode. MATERIALS LIST 2 multimeters or dc ammeter and voltmeter diode momentary contact switch power supply resistance box resistor wire leads 1. Read the entire lab, and plan the steps you will take. 2. Prepare a data table in your lab notebook with seven columns and eleven rows. In the first row, label the columns Level, I R (A), V R (V), I D-F (A), V D-F (V), I D-R (A), and V D-R (V). In the first column, label the 2nd through 11th rows 1 through 10. PROCEDURE Ohmic resistance 3. Construct a circuit that includes a power supply, a contact switch, a resistance box, a current meter, a resistor, and a voltage meter. Do not close the switch until your teacher approves your circuit. The current meter Modern Electronics 889

27 should be connected in series with the power supply and the resistor. The voltage meter should be connected in parallel with the power supply. Make sure the voltage meter and the current meter are connected properly. 4. When your teacher has approved your circuit, make sure the power supply is turned to its lowest setting and the dials on the resistance box are turned all the way to the right. Turn on the power supply, and close the switch to read the voltage meter. Turn the dial on the power supply clockwise until the voltage meter reads 2.5 V. Open the switch, and turn off the power supply. 5. Remove the voltage meter from the power supply, and wire the voltage meter in parallel with the resistor. With the switch open, make sure that the meters are wired correctly. Do not close the switch or turn on the power supply until your teacher approves your circuit. 6. When your teacher has approved your circuit, turn on the power supply, but do not close the switch. Slowly turn the dials on the resistance box to the left, starting with the 100 kω dial. Periodically close the switch just long enough to read the potential difference across the resistor. Turn the dial until the voltage meter reads 0.2 V. Record the potential difference across the resistor, V R, and the current in the resistor, I R, in your data table under Level Repeat the procedure in step 6, turning the dials on the resistance box until the potential difference across the resistor is 0.4 V. Record V R and I R in your data table under Level Repeat the procedure in step 6, increasing the potential difference across the resistor in 0.2 V steps to 2.0 V. Record the current and the potential difference for Levels 3 through 10. Open the switch. Figure Step 3: Make sure all connections are correct. Step 4: Use the voltage meter to measure the potential difference across the power supply. Close the switch only long enough to take your measurements. Step 5: Rewire the circuit to measure the potential difference across the resistor. Step 9: Replace the resistor with the diode. Connect the end of the diode with the colored bands to the black terminal of the power supply. Step 11: Reverse the diode and repeat. Diode 9. Replace the resistor with the diode, as shown in Figure The diode should be connected in the forward-biased position so that the end of the diode with the colored bands is connected to the wire leading to the black terminal of the power supply. Make sure that the meters are connected properly. Do not close the switch until your teacher approves your circuit. 890 Chapter 24

28 10. Repeat the procedure in steps 6 through 8 to find the current and the potential difference at each level. Adjust by steps of 0.2 V from 0.2 V to 2.0 V. Record each potential difference across the forward-biased diode, V D-F, and current in the forward-biased diode, I D-F. 11. Remove the diode from the circuit, and reverse its connections in the circuit so that the end of the diode without the colored bands is connected to the black terminal of the power supply. Do not close the switch until your teacher approves your circuit. 12. Repeat the procedure in steps 6 through 8 above to find the current and the potential difference for the diode at each voltage level. Adjust the potential difference by steps of 0.2 V from 0.2 V to 2.0 V, and record the potential difference across the reverse-biased diode, V D-R, and the current in the reverse-biased diode, I D-R, for each level. 13. Clean up your work area. Put equipment away safely so that it is ready to be used again. ANALYSIS AND INTERPRETATION Calculations and data analysis 1. Organizing data For each level, calculate the ratio between the potential difference across the ohmic resistor and the current in the circuit. 2. Graphing data Using a graphing calculator, computer, or graph paper, plot the following graphs: a. I R versus V R for the ohmic resistor b. I D-F versus V D-F for the forward-biased diode c. I D-R versus V D-R for the reverse-biased diode Conclusions 3. Analyzing results Compare the ratios found in item 1. What is the relationship between the potential difference and the current? Is this true for all trials? Explain. 4. Analyzing graphs Use your graphs to answer the following questions. a. What is the relationship between current and voltage in the resistance circuit? b. What is the relationship between current and voltage in the diode circuit? 5. Comparing results Based on your results, explain the relationship between current, voltage, and resistance for the diode. Explain both the forward-biased and reverse-biased diode. Modern Electronics 891

29 Science Technology Society What Can We Do With Nuclear Waste? For about the past 40 years, people have been arguing about what to do with radioactive waste. Since the waste is harmful to humans as well as to the environment deciding where to put it is a serious problem. Protection for years As radioactive isotopes decay, nuclear waste emits all common forms of radioactivity a-particles, b-particles, g-radiation, and X rays. When this radiation penetrates living cells, it knocks electrons away from atoms, causing them to become electrically charged ions. As a result, vital biological molecules break apart or form abnormal chemical bonds with other molecules. Often, a cell can repair this damage, but if too many molecules are disrupted, the cell will die. This ionizing radiation can also damage a cell s genetic material (DNA and RNA), causing the cell to divide again and again, out of control. This condition is called cancer. Because of these hazards, nuclear waste must be sealed and stored until the radioactive isotopes in the waste decay to the point at which radiation reaches a safe level. Some kinds of radioactive waste will require safe storage for at least years. Low-level waste includes materials from the nuclear medicine departments at hospitals, where radioactive isotopes are used to diagnose and treat diseases. The greatest disposal problem involves highlevel waste, or HLW. Nearly all HLW consists of used fuel rods from reactors at nuclear power plants; about a third of these rods are replaced every year or two because their supply of fissionable uranium-235 becomes depleted, or spent. When nuclear power plants in the United States began operating in 1957, engineers had planned to reprocess spent fuel to reclaim fissionable isotopes of uranium and plutonium to make new fuel rods. But people feared that the plutonium made available by reprocessing might be used to build bombs, so that plan was abandoned. Since that time, HLW has continued to accumulate at power plant sites in temporary storage facilities that are now nearly full. When there is no more storage space, plants will have to cease operation. Consequently, states and utility Questions of disposal 892

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