Ideas to implementation

Transcription

1 Physics HSC Course Stage 6 Ideas to implementation Part 4: The age of silicon

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3 Contents Introduction... 2 Using the photoelectric effect... 5 The solar cell...5 The breathalyser...9 Electrons and the atom The atom and light...13 Society and scientists The Bohr atom...17 de Broglie: The electron is a wave!...19 Band theory The resistivity continuum...24 Semiconductors and conductivity Hole current...27 Transistors Valves vs transistors...32 What was wrong with silicon?...35 Thermionic vs solid state...37 Part 4: The age of silicon 1

4 Suggested answers Exercises Part From ideas to implementation

5 Introduction Once the electron was discovered as an entity the harnessing of its potential as a mechanism to accomplish tasks in valves, then semiconductor materials was rapid. The behaviour of the electron surrounding the atom was of great interest to theoretical scientists. The giants of the new quantum mechanics were now coming to the fore of physics. This part begins by exploring the contributions of Bohr and de Broglie to our modern day understanding of the electron and investigates the applications of electron behaviour in the semiconductors that have driven the electronic revolution of our lives. At the end of Part 4, you will have had opportunities to learn to: describe the de Broglie model of electrons in orbits around atoms identify that some electrons in solids are shared between atoms and move freely describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity explain why silicon became the preferred raw material for transistors describe how doping a semiconductor can change its electrical properties identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes. Part 4: The age of silicon 3

6 At the end of this part you will have had opportunities to: identify data sources gather, process and present information to summarise the use of the photoelectric effect in: breathalysers solar cells photocells identify data sources, gather and process information to discuss Einstein and Planck s debate about whether science research is removed from social and political forces discuss differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices perform an investigation to model the difference between conductors, insulators and semiconductors in terms of band structures perform an investigation to demonstrate a model for explaining the behaviour of semiconductors, including the creation of a hole or positive charge on the atom that has lost the electron and the movement of electrons and holes in opposite directions when an electric field is applied across the semiconductor gather, process and present secondary information to discuss how shortcomings in available technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor identify data sources, gather, process, analyse information and use available evidence to assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors. Extracts from Physics Stage 6 Syllabus Board of Studies NSW, originally issued The most up-to-date version can be found on the Board's website at 4 From ideas to implementation

7 Using the photoelectric effect A great deal of modern technology is based on the photoelectric effect. These technologies include solar cells, and the photocell you learned about previously. Among the applications for technology that have used the photoelectric effect to operate are devices such as breathalysers used to determine alcohol levels in people. The solar cell Solar cells are sometimes called photovoltaic cells. They convert light energy from photons directly into electrical energy. There are a number of different types of photovoltaic cells available at the present time. Most rely on similar principles to generate electricity. Photovoltaic cells are made predominantly from semiconductor materials such as silicon or germanium. You may recall from the module Electrical energy in the home that one of the properties of semiconductor materials is that they have four valence electrons. This means the outer shell of electrons of a silicon atom contains four unpaired electrons. For an atom of any element higher than helium to have maximum chemical stability, it needs to have eight electrons in its outer (valence) shell. When silicon atoms combine to form a solid crystal, each atom share some of its electrons with four adjacent atoms (and each of those shares one of its electrons with the first atom). This sharing of electrons is called covalent bonding. The effect of this sharing of electrons is that each silicon atom has the stable eight electrons in its valence shell, even though only four of them technically belong to that atom. For the four bonds to be equally far apart in three dimensions, the basic silicon crystal structure must be a tetrahedron. Its shape is a pyramid consisting of four triangular faces, with a silicon atom at each corner and one in the centre. Part 4: The age of silicon 5

8 The covalent bonds in the silicon tetrahedron ensure there are no free electrons at normal temperatures as is the case in conductors. However, if some of the electrons in a silicon crystal gain energy by absorbing photons of electromagnetic energy through the photoelectric effect, they can leave their valence shells. This has two significant effects: A free electron is produced. Free electrons are what an electric current is comprised. A hole is left in the valence shell where that electron is produced. That hole is a missing electron and is in effect acting as a positive charge. When this occurs in a photovoltaic cell, free electrons move through the semiconductor crystal lattice made up of silicon atoms seeking a hole made by another free photoelectron to fall into. The result of this is that if the pure silicon crystal is subject to an energy gain (such as when it is heated) it begins to conduct electricity. If it has a potential difference applied across it electrons flow in one direction. A flow of electrons in one direction is an electric current. To improve conductivity in semiconductors, a process called doping occurs. Doping of silicon involves the substitution of another element for silicon into the crystal lattice. This usually means the doping atom replaces something like one in every silicon atoms. Silicon is a special material because it can readily be doped with other types of atoms when in crystalline form. This doping occurs simply by adding in atom that has either five electrons or three electrons in its valence shell to replace a silicon that has four valence electrons. For example, if the silicon crystal lattice is doped with something like phosphorus or arsenic (with five valence electrons) the fifth electron is not involved in bonding. It is converted into a free electron easily with little energy gain. Such doped semiconductors are called n-type semiconductors because they have an excess of negative charges. Si Si + free electron not required to pair up with a silicon electron Si A doped silicon tetrahedron. Because it is doped with a pentavalent (five valence electron) phosphorus atom there is one free electron loosely bound left over in the structure. This forms a n-type semiconductor. Si phosphorus ion 6 From ideas to implementation

9 If the silicon crystal lattice is doped with an atom that has only three electrons in its valence shell such as boron or aluminium then the semiconductor material effectively has a hole built in to the portion of the crystal lattice where that doping atom is included. Such doped semiconductors are called p-type semiconductors because they have a deficiency of negative charges. Si Si B Si hole occurs where the missing electron should be Si trivalent impurity atom such as boron or aluminium A doped silicon tetrahedron. The structure is doped with a trivalent (three valence electron) boron atom. This means one of the silicon atoms in the structure doesn t have a paired electron. The missing electron from the pair acts as a hole. This is a p-type semiconductor. Since conduction occurs in semiconductors as free electrons fall into holes you might question why doping helps to make a better solar cell than pure silicon. Before reading on try to work out how a combination of an n-type semiconductor layer and a p-type semiconductor can make a more efficient solar cell. Write down you ideas in the space below. After having given the problem some thought, read on and see how it really works. The photovoltaic cell works because a layer of n-type silicon is overlain on a layer of p-type silicon so that there is contact between the two different types of semiconductor material. The free electrons from the n-type layer are attracted to the positive holes in the p-type layer. Part 4: The age of silicon 7

10 protective glass cover metal contact grid antireflective coating to maximise light absorption n-type silicon p-type silicon back contact A photovoltaic cell showing layers. This results in the following things. the electrons from the n-type layer fill the holes but then a band of electrically neutral semiconductor forms at the contact. A barrier that resists further movement of electrons directly across the contact between the n-type and p-type layers forms. This produces a situation where the negatively charged n-type conductor is separated by the neutral barrier zone from the positively charged p-type conductor. The result is equivalent to an electric field separating the n-type and p-type layers. The p-type layer is effectively negatively charged even though its holes make it relatively positively charged compared to the n-type layer. The n-type layer has lost electrons trying to fill the holes in the p-type layer, so has a deficiency of electrons. When light strikes the cell, some of the electrons in the n-type layer that is facing the light are freed up by the photoelectric effect. They flow out of the n-type layer to a collecting conductor that is also connected to the p-type layer. At the same time, these photoelectrons produce holes in the n-type conductor. These photoelectron induced holes are filled by electrons from the p-type layer that rush in to fill them. In a photovoltaic cell the photoelectrons from the n-type layer travel along the conductor to the p-type layer. That is, they return to the layer from where they came. If the conductor path between the n-type layer and p-type layer includes an external circuit then the flow of electrons that results can be made to do work because it is an electric current. The amount of kinetic energy gained by the free electrons in a solar cell due to the photoelectric effect, is sufficient to allow the electrons to travel a relatively short distance to a point where they will meet a conductor material such as a thin strap of metal. That thin strap of metal, usually printed onto the solar cell, will carry the free moving electrons. If an electric field exists in the solar cell it will force the electrons to flow in one direction. That direction is usually designed to be from the top of the solar cell, where it is exposed to light, to the bottom of the solar cell. 8 From ideas to implementation

11 The resulting flow of electrons picked up in the metallic strap is the electric current produced by the cell. In general, a solar cell will not be exposed to maximum light when placed in sunlight for the entire day. In fact, the amount of sunlight impacting a fixed solar cell is variable depending upon the orientation of the cell. As can be expected with the photoelectric effect, more intense sunlight causes an increase in output current. The voltage output of the solar cell, however, remains close to constant. It does vary slightly but this is due to the changing frequency mix of light hitting the cell. This means that different stopping potentials come into play within the cell at different frequencies of incident light. You will recall from the The world communicates that the Sun appears red at sunset and sunrise because of the increased scattering out of blue light by the molecules of gas in the atmosphere when the light has to travel a longer path to get to you. That change in colour represents a different mix of frequencies in the light. Hence, it is easy to see that during the course of a day many changes in incident frequencies of light may impact the photovoltaic cell. In general, solar cells respond to a broad range of frequencies of light. They tend to be most efficient at converting the energy from the blue to blue green portion of the visible spectrum but will respond to other frequencies of light. Specialist solar cells have been created to respond best to particular portions of the electromagnetic spectrum. These include cells that respond best to the infrared range. These cells often find uses in remote control mechanisms for appliances. Do Exercise 4.1 now. The breathalyser Modern breathalyser technology is largely proprietry. That means that it is kept as a 'trade secret' but in past breathalyser technology the photoelectric effect was used extensively. The figure on the next page outlines how the past breathalyser operated. 1 A sample of breath is taken into the analyser and eventually pumped into a test ampoule for comparison against a similar control ampoule of air. Both ampoules contain a sample of a coloured chemical Part 4: The age of silicon 9

12 called potassium dichromate in solution. Potassium dichromate changes colour from yellow to green when alcohol is present. 2 Before the test the transmission of light through the ampoules is balanced by moving a light back and forth towards a pair of photovoltaic cells located on either side of the figure above. These photovoltaic cells are sensitive to blue light. The test involves bubbling of a sample of breath through the potassium dichromate solution. temp C take analyse empty full off vents galvanometer standard ampoule breathalyser % blood alcohol test ampoule blue filter photovoltaic cell light light carriage adjustment blue filter photovoltaic cell A breathalyser. 3 A blue filter covers each photovoltaic cell. This ensures that the change in intensity of only one wavelength of light is analysed The analysis is therefore consistant. 4 When the electrical current coming from each cell is equalised, samples are compared after the gas from the breath has been bubbled through the potassium dichromate solution. 10 From ideas to implementation

13 How does a breathalyser work? Any alcohol in the breath sample ampoule will react with the potassium dichromate solution in the ampoule. That will cause a colour change in the solution and will therefore affect the intensity of light hitting the photocell. The yellow light transmitted through the samples before the tests was mostly blocked by the filter. After a positive test the breath sample is more green in colour. That means it will transmit more of the blue wavelengths of light than before the test. After a positive test, the level of photovoltaic output (current) from the sample will not match that of the control sample because a different level of light is now incident upon it. The distance light must be moved towards the control sample ampoule side to equalise the photovoltaic output again is therefore a measure of the level of potassium dichromate that has reacted with alcohol. This measure is in turn a proxy for the breath alcohol level. Prepare a flow chart in the space below that describes the process of breathalysing step by step based on the material and figure above. 1 One of the critical things that must occur in the breathalyser system described above is the maintenance of a uniform sample size analysed each time the unit is used. Why do you think that is critical? Part 4: The age of silicon 11

14 2 What affect do you think having a greater intensity of blue light hitting the photocell would have on the electrical current produced by the cell? Explain your answer. 3 If the potassium dichromate solution was replaced in the sample ampoules with another chemical that changed colour from yellow to orange with the same intensity of light transmitted by the orange solution as with a potassium dichromate solution would the system still work if all else was left the same? Explain your answer. 4 The photovoltaic cell operating in the breathalyser has been specially designed to operate best when light of the blue colour is incident upon it. Red light has little effect on the current produced by the cell. By referring to the photoelectric effect principles explain what this means. Check your answers. 12 From ideas to implementation

15 Models of the atom The atom and light The concept of the quantum was a great success with its ability to explain various aspects of electromagnetic radiation that the wave model could not. Around the turn of the century, many people were investigating the very nature of matter itself. In other words they were investigating the structure of the atom. Among those workers were Ernest Rutherford and Neils Bohr. These two giants of science had performed experiments to establish the nature of the atom. These experiments and the resulting models of the atom established that: an atom had a nucleus containing positively charged protons and electrically neutral neutrons most of the mass of the atom is in the nucleus around the positive nucleus were stable orbitals in which negatively charged electrons ( C) orbit. It was proposed that: each electron in the atom has a specific energy depending on its orbit the larger the radius of the orbit the electron occupied the higher the electrons energy an electron remaining in an orbit had a constant energy level electrons moving to a new orbit either gained energy or lost energy. Einstein had earlier proposed that the energy required to move an electron to gain energy was related to frequency. This is found in his explanation of the photoelectric effect. Experiments found that an electron may absorb energy from an outside source and so move into an orbit of higher energy, say from E 1 to E 2. The atom was then said to be in an excited state. The electron typically only remained at the higher energy level for a short time (ª 10 8 s). After this it fell back to the lower level, E 1. Part 4: The age of silicon 13

16 When doing so, the electron released the excess energy as a single photon with energy given by the relationship: E = hf. This photon's energy is equal to the difference in energy between the two electron hc energy levels, that is: E 2 - E1 = l E 3 E 3 E 2 E 1 absorption e- photon energy is absorbed increasing energy ground state e- emission of energy as a photon E 2 E 1 photon E = hf photon E = hf Electrons can change energy levels by absorbing or emitting a photon. The larger the energy difference between any pair of electron levels, the higher the frequency of the emitted photon. In this way, atoms may emit or absorb photons of different frequencies depending on the size of the energy level change. Note that electrons cannot absorb or emit just any quantity of energy. They can only absorb or emit energy in the amount (or quanta) equal to the difference in energy levels between orbitals. The explanation for how this could come about was a mystery at the time of Planck. He only knew that it was so. That explanation had to await the revolutionary contribution of Louis de Broglie who you will learn about in the next section of this module. 14 From ideas to implementation

17 Society and scientists Max Planck and Albert Einstein shared many similarities. Both were men who devoted their lives to science, both were leaders in their field, both were born in Germany. They differed in their opinions as to the reality of the concept of the quantum and its application to light but eventually they agreed on that. On the surface it should seem that these two men would probably share ideals and maybe even nationalist goals for their native land. The incomplete picture below will highlight that many other aspects of their careers, personalities and political ideals were different. Planck was a patriot for his native Germany. Einstein sought to leave it from an early age and to take up Swiss citizenship. Planck was respected by the state and was a very stoic and proper man; a gentleman who was working for his native Germany. In many respects Planck was for all intents and purposes a patriotic nationalist. Einstein was more liberal in outlook, and was not apparently loyal to any government. These differences were never highlighted more than at the beginning of World War 1. In 1914 Germany had invaded Belgium, the war propaganda machine in Germany was in full operation. A document was produced called later the Manifesto of the ninety-three German intellectuals to the civilized world. This document was designed to tell the German public that the role Germany was taking in the war was justified. Planck was one of the first to sign the document supporting the role of the state in the war. He then supported research to support the war effort. Einstein was also invited to sign the Manifesto of the ninety-three German intellectuals to the civilized world. He refused to accept the role of the state in war and the nationalist view that it was the role of the scientist to support the war effort. This was despite the fact that he was working in Berlin Germany at the time. Einstein refused to sign. His views were those of a pacifist. He could not accept the role of science in killing his fellow human beings. Part 4: The age of silicon 15

18 Einstein even went as far as signing a rival document that supported the rights of all people to a peaceful world called the Manifesto to the European. This anti-nationalist view was to remain with Einstein all his life. Einstein saw clearly the role of science as something for the good of man that was not to be manipulated for the good of the state. An interesting sideline to this story of the differences between these two scientists is that they were in fact friends. After the war Einstein fought hard to have the rights of German scientists returned when some would have taken them away. It was Planck who supported Einstein s appointment to the research institute where he worked in Berlin. Do Exercise 4.2 now. To learn more about the lives of Planck and Einstein and their respective views on whether science research is removed from social and political forces including English translations of the Manifesto of the ninety-three German intellectuals to the civilized world and the Manifesto to the European see pages on the physics website page at: 16 From ideas to implementation

19 Models of the atom The Bohr atom The quantum theory that explained the emission and absorption of electromagnetic radiation by atoms was first proposed by Planck. It was then used by Einstein to describe the photoelectric effect. In 1913 the physics community was puzzled by the emission of radiation given out as specific wavelengths or in spectral lines by elements when heated. The models proposed to explain these strange phenomena described by Balmer, Rydberg and Zeeman was explained by Neils Bohr. Bohr adapted quantum theory to explain the spectral lines produced by hydrogen gas when the hydrogen atom was excited by either heating or the passage of an electric current through the low pressure gas in a discharge tube. Bohr proposed that electrons moving around the nucleus of an atom existed in stable energy levels or orbitals. He also proposed that the electrons revolving around the nucleus could only exist in these specific stable orbitals for any significant time period. The stable orbitals were the key. Stable orbitals existed and the electrons were confined to those orbitals in large. This was like the idea of a 3-D solar system of electron planets orbiting at fixed distances from the central atomic nucleus. To see pages and applets that explain the Bohr model of the atom see sites on the physics websites page at: To move to a vacancy in a lower energy state (orbital) in the Bohr model, the electron has to give out energy in the form of electromagnetic radiation in a single quantum of energy. That energy is precisely equal to the energy difference between its initial and new lower energy state. Part 4: The age of silicon 17

20 To move to a higher energy state or orbital an electron has to gain energy by absorbing a quanta of energy in the form of a photon of electromagnetic energy precisely equal to the energy difference between its new energy level and its original energy level. Atoms of different elements have their stable orbitals at different energy levels. This means that for any element, the spectral lines or specific wavelengths of electromagnetic radiation it emits or absorbs when its electrons drop to lower energy orbitals or rise to higher energy orbitals are characteristic. These are the spectral emission and absorption lines you learned about in The cosmic engine. You may wish to revisit that material to refresh your memory of the learning. The implication of the Bohr model of the electron in stable orbitals around the atom was that electrons couldn t exist in a stable state at continuous energy levels around the nucleus. There were only certain stable orbital levels out from the nucleus where electrons had a high probability of being located. This concept of the model of the atom looks like a small 3-D solar system with the nucleus at the centre and the electrons able to locate themselves like planets at specific distances from the nucleus. This model is shown in the figure below. e- e- e- E E 2 2 E 1 nucleus increasing energy m m The Bohr model of the atom. Note the nucleus that contains the majority of the mass in the atom is only a small volume of the entire atom that includes the electron orbitals. Each orbital out from the nucleus has increasing energy. If an atom falls from an outer orbital say E 3 to E 2 energy is released as an electromagnetic photon of a specific frequency. For an electron to gain energy and move to one of the higher stable orbitals it must absorb a photon of electromagnetic radiation with a specific frequency or energy. The energy of these specific photons is a quantum of energy. 18 From ideas to implementation

21 The Bohr model quandary The problem with the Bohr model that arose was how to explain why the negatively charged electrons had stable orbitals and didn t simply spiral in to the positive nucleus of the atom because of attraction. In essence the problem that existed was to explain why stable orbitals for electrons existed at all. Take a few minutes to read over this passage again before you proceed and try to think up an explanation for the electron s behaviour. If you can think of one write it down in the space below. Then read on to find out how the insights of a member of the French aristocracy, Prince Louis de Broglie, solved the problem. de Broglie: The electron is a wave! In 1923 de Broglie published a paper in a scientific journal that suggested something revolutionary. If electromagnetic radiation could behave as though it had a particle nature (photons), like quanta, then why couldn t particles behave as waves. He applied this revolutionary thought directly to the problem of the electron revolving around the nucleus of the atom. The process de Broglie went through to develop his explanation of the behaviour of electrons was as follows. de Broglie proposed that every particle had a wavelength equal to Planck s constant divided by the momentum of the particle. For an electron this would mean that an electron with a mass m e and velocity v would have a wavelength of l = h. mv e This can be derived from a reversal of Planck s relationship E = hc combined with Einstein s E = mc2 l Part 4: The age of silicon 19

22 hc 2 = mc l h = mc l but mc is a momentum because c is a velocity. For any other velocity other than c mv = h l rearranging leads to the relationship l = h mv where m is the mass of the object and v is its velocity. The wave representing the electron revolving around the nucleus in an orbital must fit as a whole number of wavelengths to avoid the wave interfering with itself and hence cancelling out. If the radius of the orbit from the nucleus centre is r then 2pr = nl where n is a whole number or integer and represents the energy level of the electron. Combining these two equations from above leads to the relationship mvr= nh e 2p But the angular momentum of an electron is mvr e. For an electron to travel around an atom in a particular orbital, the angular momentum is a constant. Since h and p are constants this implies the only permissible orbital levels are at full energy levels. This means that if an electron is located in the first energy level, its orbits would be one wavelength long, at the second energy level the orbit would be two wavelengths long and so on. The features of the de Broglie model and electron orbitals are shown in the figures below. The first orbital or energy level out from the nucleus. Note that the complete orbital can only be achieved if the length of the orbital is one wavelength long. The figure on the left shows an incomplete orbital. The figure on the right a complete orbital. The second orbital or energy level from the nucleus. Note that the complete orbital can only be achieved if the length of the orbital is two wavelengths long. 20 From ideas to implementation

23 The figure on the left shows an incomplete orbital. The figure on the right a complete stable orbital. The third orbital or energy level out from the nucleus. Note that the complete orbital can only be achieved if the length of the orbital is three wavelengths long. The figure on the left shows an incomplete orbital. The figure on the right a complete stable orbital. The fourth orbital or energy level out from the nucleus. Note that the complete orbital can only be achieved if the length of the orbital is four wavelengths long. The figure on the left shows an incomplete orbital. The figure on the right a complete stable orbital. The fifth orbital or energy level out from the nucleus. Note that the complete orbital can only be achieved if the length of the orbital is five wavelengths long. The figure on the left shows an incomplete orbital. The figure on the right a complete stable orbital. Do Exercise 4.3 now. Part 4: The age of silicon 21

24 Band theory Louis de Broglie s explanation of the stable orbitals in which electrons could exist was important in the development of band theory. This theory explains why certain materials are conductors while others are semiconductors or insulators. The wave model for electrons around the nucleus allowed for, and explained why electrons could be in a number of orbitals close in and further out from the nucleus. The inner orbitals make up the valence band. The outer orbitals are known as the conduction band. For conducting solids such as metals the energy difference for electrons in the valence band and the conduction band is only slight. This means that electrons can easily move from the valence band to the conduction band and from there to adjacent atoms. This movement of the electrons under the action of a potential difference is conduction. For semiconductors, the energy difference between the valence band and the conduction band is greater than for conductors. This means that the electrons must gain additional energy from the semiconductor being heated or from light energy in order for the electron to be able to move from the valence to the conduction band orbitals. After the electron has moved into the conduction band it is free to move between one atom and another hence conduction can occur. For insulators, the energy gap between electrons in the valence band orbitals and the conduction band orbitals is very great. This means that the electrons in the valence band of the insulating material must gain a lot of energy before they can jump this energy gap to get into the conducting band and move from one atom to another and hence conduct electricity. 22 From ideas to implementation

25 conduction band valence band insulator semiconductor conductor A representation of the band theory in insulators, semiconductors and conductors. Note that in a conductor the valence and conduction bands actually overlap. In an insulator the separation of the bands is a large gap. Only electrons in the conduction band can conduct. Modelling conductors, semiconductors and insulators To do this activity you will need the help of others. The more people the better. You will also need to have a tennis ball. Each person in this model represents an atom, the tennis ball represents an electron that is being conducted. Modelling a conductor 1 All stand in a line next to each other with the person on the end of the line holding the tennis ball. 2 Pass the ball to each other with the ball heading in the one direction. The ball should easily be transferred from person to person without the ball being dropped. The conduction band in this case is easily achieved. The electron ball is easily passed from person to person. The distance between the people is the equivalent to the band. Modelling a semiconductor 1 All stand in a line but this time have a distance of separation of around 5 m between each person with the person on the end of the line holding the ball. Everyone must remain standing in the one spot. 2 Throw the ball to the person beside you without allowing the ball to drop. Getting the ball from one end of the line to the other without the ball being dropped is more difficult. The ball must be given a lot more energy than in the case of the conductor model to make the jump from atom to atom. The 5 m separation between the people is equivalent to the gap between the valence and the conduction bands. There is a larger gap between the conduction and the valence bands in semiconductors than in conductors. Part 4: The age of silicon 23

26 Modelling an insulator 1 This time find an open space such as a football field or park. 2 Stand in a line as in the other models but this time stand around 50 m apart. Everyone must remain standing in the one spot. 3 Attempt to throw the ball from one person to another along the line without dropping the ball. It will be very difficult to do so. The large gap between people is equivalent to the gap between the valence and conduction bands. The ball must be given a great deal of energy to be able to move across the band. For an electron to be conducted by an insulator it must be given a great deal of energy. It is not impossible for an insulator to conduct electricity, just not likely! You have just modelled the movement of electrons in insulators, semiconductors and metals. How would you now compare qualitatively the ability of free electrons to drift from atom to atom in each of these substances? Check your answer. The resistivity continuum You may recall from the module Electrical energy in the home that there is a continuum between the resistivity or degree to which different substances resist the flow of an electric current. This resistivity is really a measure of how conductive a material is. The figure below shows the resistivity of a number of insulators, semiconductors and conductors at 25 C. You can get a qualitative measure of how the relative number of free electrons that can drift from atom to atom varies by examining the resistivity in the table and simply estimating the difference between the resistivities. For example, polythene is around times less conductive than silicon and times less conductive than copper. The conductivity of a substance is defined as the reciprocal of the resistivity. 24 From ideas to implementation

27 good conductors semi-conductors insulators Wm copper graphite nichrome silicon wood glass polythene Use the information in the figure above to describe quantitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators. Compare the conductivities of copper and graphite, copper and silicon, and wool and polythene. Check your answer. Do Exercise 4.4 now. Part 4: The age of silicon 25

28 Semiconductors and conductivity The atoms of semiconducting elements have four unpaired electrons in their outermost energy levels. You may recall from the module Electrical energy in the home that silicon and germanium are two semiconductor elements. There are four electrons in the outer molecular orbital valence band of each atom of the semiconductor, silicon. These four electrons are shared with the four adjacent silicon atoms. Each of these four silicon atoms sheres its four valence electrons with another four silicon atoms and so on to make up a giant crystal. This type of interaction between adjacent atoms is called covalent bonding and is illustrated in the figure below. the central silicon has 8 electrons in its outer shell by sharing one of its electrons with each of four neighbouring silicons Si Si Si Si Si Si Si Si Si Si Si shared valence electrons form covalent bonds A silicon atom showing covalent bonding involving the sharing of electrons between atoms. In a real crystal of silicon all the atoms would be in a sharing situation in a crystal lattice. Covalent bonding like this results in very few electrons being in the conduction band at any instant although just by chance at any one time a few are. This is because the average energy of the solid doesn t preclude an occasional electron gaining sufficient energy to enter the conduction band. 26 From ideas to implementation

29 An additional input of energy (as heat or light) means that many electrons acquire sufficient energy to move into the conduction band. When this occurs, an electron jumps into the conduction band and moves from one atom to another. For silicon this results in only three electrons in the valence band of the atom that lost the electron. The missing electron has left a hole or a degree of relative positiveness for the atom that lost the electron. Any electron from an adjacent atom that enters the conduction band is then free to jump into the hole in the conduction band of the silicon atom but in doing so will itself create a hole in the valence band of the silicon atom from where it came. free electron Si Si heat energy The energy input gained by the electron enables it to move from the valence band to the conduction band. Such an electron can move to another atom thus producing a current flow. Silicon conducts much better when heated. Hole current When an electron in a semiconductor breaks its bond and becomes free it leaves a hole in the crystal structure. At that point there is a positive charge associated with the hole. This is because the electron has a negative charge. Free electrons that lose some of their energy as they pass through the material can recombine with holes. In particular this will occur if there is no potential difference applied across the semiconductor. The application of a voltage to the semiconductor material causes the energetic free electrons to move from the negative potential toward the positive potential. As these free electrons move toward the positive potential, the holes they generate in becoming free appear to migrate toward the negative potential. In the figures following you can see a hole being generated by an electron breaking free and moving towards the positive potential. Part 4: The age of silicon 27

30 hole made as as electron escapes from the lattice + free electron Hole movement commencing. An electron from an adjacent atom may fall into this hole and effectively moves the hole to the left towards the negative potential. + Hole movement continuing. Another electron from a third atom may now break away and fall into that hole. The hole moves towards the negative potential. Thus a hole current is established. A hole current moves towards the negative potential. the hole continues to move toward the negative potential + A further stage in hole movement continued. Free electrons move almost directly through the material. An important point to note here is that hole current is slower than electron current. Holes move by transferring from one atom to another thus making their movement through a material much slower. Excess free electrons in hot semiconductors can simply flow over adjacent atoms that don t contain holes. 28 From ideas to implementation

31 Hole flow may be visualised as follows: Because electron flow is much faster than hole flow, electronic devices that use electron flow are chosen for applications that require high frequency current fluctuations. For general use a pure semiconductor material has too few free electrons and holes to be useful in electronic devices. In order to increase the number of carriers, certain impurities are added to the pure semiconductor in a process called doping. (1) (2) (3) (4) (5) electron movement hole movement Doping a semiconducting material vastly increases the number of free electrons or holes that can be formed. Doping can result in two situations as outlined below: An ion with three valence is substituted for an atom with four valence electrons in the crystal lattice of the semiconductor. This makes an increased number of holes in the crystal lattice. Such a semiconductor is a p-type semiconductor. In a p-type semiconductor the structure may be interpreted as that of negative ions surrounded by mobile positive holes. fixed negative ions mobile holes An ion with five valence electrons is substituted for an atom with four valence electrons in the crystal lattice of the semiconductor. This results in an increased number of free electrons in the crystal lattice. Such a semiconductor is called an n-type semiconductor. The structure of a n-type semiconductor is that of a fixed positive ion surrounded by mobile negative electrons fixed positive ions mobile electrons The end result of either of these activities is similar. The current carrying capacity of the semiconductors is increased Part 4: The age of silicon 29

32 1 In which direction do holes move in response to an applied voltage? 2 In which direction do free electrons move in response to an applied voltage? Check your answers. Modelling conduction in a semiconductor Conduction in semiconductors involves movement of electrons in one direction when a potential is applied across the semiconductor. The movement of holes or spaces where electrons were in the opposite direction to electron motion. To perform this activity you will require the following: five friends six chairs a video camera (optional). In this activity each person will be acting as an electron. Each chair is an atom. Conduction involves the movement of electrons from atom to atom so in this activity the people will have to move from chair to chair. For this activity the people sitting on the chair must all move chairs in only the one direction. This is equivalent to the application of a potential difference across the semiconductor that would ensure that the electrons in the semiconductor carrying the electric current was only able to travel in the one direction. Procedure 1 Line up the chairs in a row all facing the same direction. 2 All take a chair. Only one person can be sitting in a chair at one time. 3 The person on the end of the row at the right hand end should stand and walk to the other end of the row and prepare to sit down again when the end chair in the row becomes vacant. By standing and leaving their chair they are acting as an electron that has gained sufficient energy to break free of their atom. 4 The person who has moved has left a vacant chair. That vacant chair is the equivalent to a hole. 30 From ideas to implementation

33 Holes are missing electrons and are therefore positive in character even though they do not carry a charge as a particle does. The person next to the hole acting as an electron moves into the hole, or vacant chair. By doing that the electron has moved forward, the hole has moved backward. 5 The person next to the vacant chair should then repeat the procedure in step 4. The result is a movement of electrons forward and at the same time a movement of holes backward. Note that the chair beside a person must be vacant, that is a hole, otherwise the person cannot move into the chair. Electrons cannot move in a semiconductor unless they move into a hole. 6 When the whole sequence is completed through once or twice, video tape the events and review the movement of the hole backwards and the electron forward in the model of a semiconductor carrying a electric hole current you have created. This model of the conduction of an electric current by a semiconductor can also be done very effectively with a Chinese checkers set. You should describe how this could occur in the space below. If you have a Chinese checker set lying around you should have someone read instructions and then try to use them to model conduction through a semiconductor. Discuss with the person using your instructions whether they found the instructions to be clear and easy to understand. Remember, when you do the HSC exam your answers will have to be clear and tell the examiner exactly what you really mean. Part 4: The age of silicon 31

34 Keeping semiconductors cool In most cases the semiconductor materials operate at the prescribed limits when the semiconductor is conducting just the right amount of electrical current. Consequently the semiconductor cannot be allowed to become too hot otherwise it will conduct too much current. To avoid this problem in electronic devices such as computers, silicon based chips are cooled by fans or are attached to heat sinks. Often these heat sinks are made from a conductive metal such as aluminium. The heat is conducted away from the chip to the heat sink that dissipates the heat to the environment. Germanium transistors are more susceptible than silicon to breakdown and conduct large currents when they get hot. Explain this in terms of the band structure theory. Check your answer. 32 From ideas to implementation

35 Transistors Ask your grandparents, or someone over the age of 50 about the radio they remember from their youth. Then ask them about the television from their youth. Ask them if they remember the delay in hearing the radio after they switched it on as the valves in the radio warmed up. They will tell you about the 30 or so second delay between switching the radio on and the sound coming from the radio. A similar story would be told about televisions that used valves. The warm up period was the time it takes to heat the filament in the vacuum tube to get it to begin giving off electrons. Older people may tell you they didn t have television when they were young. Ask them about the television repair man from the early days of TV. They will tell you about valves breaking down and the TV repairman making multiple visits to repair the television. Modern television sets and radios rarely break down. Where a fault does occur in a television it is most likely in the picture tube. The reason for the change in reliability is the switch from a technology based on evacuated glass tubes to solid state devices. Valves vs transistors The first valves were invented in 1904 by John Fleming. The valve he made was a diode. It was essentially a device designed to allow an electric current to flow in only one direction. Because the anode couldn t produce free electrons, no current would flow when the polarity of the voltage across the tube was reversed. The device was really just a vacuum tube with a heated cathode and anode in it. The most common type of valve in use in the earliest electrical devices such as the radio and the television was the triode. It was invented in 1907 and made popular by around 1914 by Lee de Forest. He invented his audion or triode that was specifically used to amplify electric currents passing through a vacuum tube. Part 4: The age of silicon 33

36 anode grid cathode heating filament A schematic figure of a triode. The first solid state style replacement for the triode was the transistor and although theoretically described in 1926 the first transistor was not invented until 1947 by John Bardeen and Walter Brattain at the Bell Laboratories. To see a page that describes the first transistor ever built see a page on the physics websites page at: So what was the problem with the transistor? The theory of how to make one was around for 21 years before the first one was ever built! The answer to that question is an interesting one. It comes down to a basic problem. The only substance that could be made pure enough to act as a semiconductor with the correct properties that would act as a transistor was germanium. Germanium transistors, although an improvement over the triode, did suffer problems. They would stop working if they got too hot. This was not an insignificant problem because, as you learned in the unit Electrical energy in the home, as an electric current passes through any conducting substance showing resistance, heating occurs. The answer to the germanium problem in transistors was to use another semiconductor in the production of the transistors. The obvious solution was to use silicon. Silicon is immediately above germanium in the periodic table, has similar chemical properties and is the second most abundant element in the Earth s crust. It should have been the first choice for a substance to use in transistors! 34 From ideas to implementation

37 What was wrong with silicon? Unfortunately a problem existed with silicon. Making silicon crystals pure enough to act appropriately as a semiconductor was an extremely difficult task! Gordon Teal solved the problem of purity in He was working for the firm Texas Instruments at the time. From that date on silicon based transistors began to dominate the electronics component world. The way that Teal announced his development of the silicon transistor is a great story. It demonstrates clearly the advantage of training someone to a high level of expertise and then the importance of keeping that key person happy in their job. To see a page that discusses the first use and announcement of the silicon transistor, see pages on the physics website page at: Silicon manufacture The process of refining silicon is as follows. 1 Modern day pure silicon is manufactured by reacting silicon dioxide with carbon in an electric furnace. The silicon gives up the oxygen to the carbon. The oxygen comes off as carbon dioxide gas. 2 The resultant impure silicon is then reacted with chlorine before being reduced to slightly impure silicon. The silicon is now at refining grade. Refining grade means impurities are still at an unacceptable level for transistor or microchip production. Part 4: The age of silicon 35

38 3 The impure silicon is then put into a zone furnace where a band heater heats the molten silicon from the bottom of a large crucible up. This results in the impure silicon rising above the heating band. When the band heater is raised slowly up the crucible from the bottom the material crystallising below the band is pure silicon. Pure silicon appropriate for use in transistors and microchips has impurities of less than one part in one billion. Getting the silicon ready to use in microchips and microprocessors involves the manufacture of silicon wafers. Wafers are the thin discs of silicon, on which the microprocessor or chip is built. The silicon wafer manufacturing process begins by melting ultra-pure silicon in a furnace filled with inert argon gas. A rotating needle with a small crystal of silicon on the tip is lowered into the furnace. Under strict temperature control the small silicon crystal attracts the molten silicon in the furnace. As the silicon crystal grows and the needle rotates, it is slowly drawn back out of the molten silicon effectively growing into one giant silicon crystal called a silicon ingot. The needle and the final ingot looks like a 1.3 m long, 20 cm wide silver cylinder. The ingot is machined to a uniform diameter then sliced and ground to produce highly polished wafers. Imperfect wafers are rejected from the process. The pure silicon crystal must cut into thin wafers before being used to make microprocessor chips. Explain why the use of silicon as a transistor material was a problem in the 1950s? List some of the problems with using silicon. Check your answers. The next issue was the doping of the silicon to improve its conductivity. That was a whole new ball game. It presented huge difficulties and required the development of a whole new set of technologies. To see a page that has a series of slides that explain doping of silicon to produce semiconductor materials see pages on the physics websites page at: 36 From ideas to implementation

39 In the space below list some of the recollections of others you have spoken to about the impact of the replacement of thermionic technology devices with the solid state devices of today. Consider things such as the perceived reliability of the devices in the past compared to today, the physical size of appliances compared to today, the cost of similarly functioning appliances relative to salary in the past and today. Were you surprised by the answers you received on the questions above. The high relative cost and lack of reliability of thermionic devices made the search for better ways to do the job done by these devices a matter of priority for many research organisations, particularly in the United States. Communication devices that used thermionic devices were a big deal. It was the fastest growing segment of a very profitable consumer market. The poor reliability of thermionic devices was seen as a significant problem to be overcome. Thermionic vs solid state Valves like cathode ray tubes are sometimes referred to as thermionic devices or components. The source of the flow of electrons in the thermionic device is a heated filament or cathode. The device itself operates in an evacuated glass tube under the same principles as a cathode ray tube. Valves and solid state devices do the same job. Solid state devices do, however, have a number of advantages in everyday equipment over thermionic devices. Those advantages are listed on the next page. Solid state devices are much more reliable. They have a consistent performance for a longer time. Thermionic devices are large. Transistors can now be made extremely small. Millions of transistors making up complex circuits can be fitted onto microchips. Valves are large and do not allow for the miniaturisation demanded from modern electronic devices. Part 4: The age of silicon 37

40 Solid state devices consume far less energy than thermionic devices. They also produce far less heat energy as a waste product. Thermionic devices require a heater to stimulate the cathode to emit electrons. Thermionic valves are much more expensive to produce than solid state devices that do an identical job. Of course, when solid state devices were first invented this was not so, but it was within a decade of the invention of the first transistor. Thermionic devices are made from glass. This means they are inherently fragile. Thermionic devices have a longer warm up period while the filament is heated to make it give off electrons. Considering all this, it may surprise you to know that there are still some applications where thermionic devices are used (other than in antique electrical equipment). Hi-fi enthusiasts claim that music produced by stereo equipment utilising thermionic devices is of superior tone. In certain specialised applications, higher current flows preclude the use of solid state devices that cannot handle such high currents. It is fair to say that overall the use of solid state devices is the way of the present and the foreseeable future. Without them devices such as computers and mobile phones could not exist. The rapid way in which transistors took over from their thermionic counterpart, the triode valve is illustrated in the timeline sequence below. Year Event 1947 Transistor invented Public told of the invention of the transistor Transistors used in switching equipment in telephone exchanges and military computers Transistors used in hearing aids The first transistor radios transistor units manufactured. The revolution from thermionic to solid state had occurred. 38 From ideas to implementation

41 The last great use for a thermionic device is the picture or display tube in televisions and computer display monitors. Even there the thermionic device is under threat of replacement by technologies such as the plasma and LCD display. Only the low cost base of the proven technology of the picture tube is preventing its replacement. In the future this may change. Do Exercise 4.5 now. Part 4: The age of silicon 39