Chapter 1 INTRODUCTION TO SEMICONDUCTORS MATERIAL
Objectives Discuss basic structures of atoms Discuss properties of insulators, conductors, and semiconductors Discuss covalent bonding Describe the conductions in semiconductor Discuss Ntype and Ptype semiconductor Discuss the diode Discuss the bias of a diode 2
LECTURE S CONTENT 1.1 Atomic structure 1.2 Semiconductor, conductors and insulators 1.3 Covalent bonding 1.4 Conduction in semiconductors 1.5 Ntype and Ptype semiconductors 1.6 Diode 1.7 Biasing the diode 1.8 Voltagecurrent characteristic of a diode 1.9 Diode models 1.10 Testing a diode 3
1.1 Atomic Structure
1.1 Atomic Structure Basic structure Atomic number Valence electron ATOM Electron shells Ionization Free electron 5
The Atom Atom is the smallest particle of an element that retains the characteristics of that element. An atom consists of the protons and neutrons that make up the nucleus (core) at the center and electrons that orbit about the nucleus. The nucleus carries almost the total mass of the atom. Neutrons are neutral and carry no charge. Protons carry positive charges. The electrons carry negative charges. The number of protons = the number of electrons in an atom, which makes it electrically neutral or balanced. Fig. 4: Bohr model of an atom 6
1.1 Atomic Structure (cont.) Protons (positive charge) ATOM Electrons (negative charge) Neutrons (uncharged) Nucleus (core of atom) 7
1.1 Atomic Structure (cont.) Atomic Number Element in periodic table are arranged according to atomic number Atomic number = number of protons in nucleus which is the same as the number of electron in an electrically balanced atom Electron Shells and Orbits Electrons near the nucleus have less energy than those in more distant orbits. Each distance (orbits) from the nucleus corresponding to a certain energy level. In an atom, the orbits are group into energy bands shells Diff. in energy level within a shell << diff. in energy between shells. 8
K L M N Shells or orbital paths 29 p + 29 n Valence Electron Valence shell is the outermost shell in an atom that determines the conductivity of an atom. The electrons in valence shell are called valence electrons. Valence shell Valence electron Fig.5: Bohr model of copper atom (Cu) 9
The Number of Electrons in Each Shell The maximum number of electrons (N e ) in each shell is calculated using formula below: N e = 2n n = number of shell Example for the copper atom (Cu) shell : 1 st shell (K): 2n 2 = 2(1) 2 = 2 electrons 2 nd shell (L): 2n 2 = 2(2) 2 = 8 electrons 3 rd shell (M): 2n 2 = 2(3) 2 = 18 electrons 4 th shell (N): 1 electrons 2 Total: 29 electrons n = the shell number 10
Ionization When atom absorb energy (e.g heat source) the energies of the electron are raised Valence electron obtain more energy and more loosely bound to the atom compared to the inner electron If a valence electron acquires sufficient energy escape from the outer shell and the process of losing valence electron called ionization. The escape electron is called free electron. 11
1.2 Semiconductors, Conductors and Insulators
1.2 Semiconductors, Conductors, and Insulators In terms of electrical properties Materials Insulators Semiconductors Conductors All materials are made up of atoms that contribute to its ability to conduct electrical current 13
1.2 Semiconductors, Conductors, and Insulators Atom can be represented by the valence shell and a core A core consists of all the inner shell and the nucleus Example of carbon atom: valence shell = 4 e inner shell = 2 e Nucleus: = 6 protons = 6 neutrons +6 for the nucleus and 2 for the two innershell electrons (net charge +4) Fig. 6: Diagram of a carbon atom 14
12 Semiconductors, Conductors, and Insulators (cont.) Conductors material that easily conducts electrical current. The best conductors are singleelement material (e.g copper, silver, gold, aluminum) Only one valence electron very loosely bound to the atom free electron Insulators material does not conduct electrical current (e.g rubber, plastic) valence electron are tightly bound to the atom very few free electron Semiconductors material between conductors and insulators in its ability to conduct electric current in its pure (intrinsic) state is neither a good conductor nor a good insulator most common semiconductor silicon(si), germanium(ge), and carbon(c) which contains four valence electrons. 15
1.2 Semiconductors, Conductors, and Insulators (cont.) Energy Bands Conduction band Energy Energy gap E 4 = 1.8eV E 3 = 0.7eV Energ y Conduction band E 2 E 1 Valence band E = energy level Second band (shell 2) Valence band Fig. 16: Energy band diagram for an unexcited (no external energy) atom in a pure (intrinsic) Si crystal. First band (shell 1) Nucleus 16
12 Semiconductors, Conductors, and Insulators (cont.) Energy Bands Fig. 7: Energy diagram for three types of materials Energy gapthe difference between the energy levels of any two orbital shells Bandanother name for an orbital shell (valence shell=valence band) Conduction band the band outside the valence shell where it has free electrons. 17
12 Semiconductors, Conductors, and Insulators (cont.) Energy Bands at room temperature 25 ev (electron volt) the energy absorbed by an electron when it is subjected to a 1V difference of potential 18
12 Semiconductors, Conductors, and Insulators (cont.) Comparison of a Semiconductor Atom to a Conductor Atom Core of Si atom has a net charge of +4 (14 protons 10 electrons) and +1 (29 protons 28 electrons) for Cu atom. A valence electron in Si atom feels an attractive force of +4 compared to Cu atom which feels an attractive force of +1. Force holding valence electrons to the atom in Si > in Cu. The distance from its nucleus of Copper s valence electron (in 4 th shell) > silicon s valence electron (in 3 rd shell). 19
12 Semiconductors, Conductors, and Insulators (cont.) Valence electrons Valence electrons Core (+4) Core (+1) (a) Silicon atom (a) Copper atom Fig.110: Diagrams of the silicon and copper atoms 20
13 Covalent Bonding
13 Covalent Bonding Covalent bonding holding atoms together by sharing valence electrons 13 Covalent Bonding sharing of valence electron produce the covalent bond To form Si crystal 22
13 Covalent Bonding (cont.) Result of the bonding: 1. The atom are held together forming a solid substrate. 2. The atoms are all electrically stable, because their valence shells are complete. 3. The complete valence shells cause the silicon to act as an insulatorintrinsic (pure) silicon. In other word, it is a very poor conductor. 23
13 Covalent Bonding (cont.) Covalent bonding in an intrinsic or pure silicon crystal. An intrinsic crystal has no impurities. Covalent bonds in a 3D silicon crystal 24
14 Conduction in Semiconductor
14 Conduction in Semiconductor Figure 110 Energy band diagram for a pure (intrinsic) silicon crystal with unexcited (no external energy such as heat) atoms. There are no electrons in the conduction band. This condition occurs only at a temperature of absolute 0 Kelvin. 26
Conduction Electrons and Holes When an electron jumps to the conduction band, a vacancy is left in the vallence band, this vacancy is called a hole and the electron is said to be in an excited state. Recombination occurs when a conductionband electron after within a few microseconds of becoming a free, loss its energy and falls back into a hole in the valence band. The energy given up by the electron is in the form of light and/or heat. Fig.111: Creation of electronhole pairs in a Si atom. (a) energy diagram, and (b) bonding diagram 27
Electron Current At the temperature room, at any instant, a number of free electrons that are unattached to any atom drift randomly throughout the material. This condition occurs when no voltage is applied across a piece of intrinsic Si (as illustrated in Fig. 12). When a voltage is applied across the piece of intrinsic Si, as shown in Fig. 13, the thermally generated free electrons in the conduction band, which are free to move, are now easily attracted toward the positive end. The movement of free electrons in a semiconductive material is called electron current. 28
Fig.12: Free electrons are being generated continuously while some recombine with holes 29
Hole Current At the same time, there are also an equal number of holes in the valence band created by electrons that jump into the conduction band (Fig. 13). Electron remaining in the valence band are still attached to the atom not free to move like free electron. However, valence electron can move into nearby hole leaving another hole it comes from Thus, hole has moved from one place to another in the opposite direction. The movement of electrons in a valence band is called hole current. 30
Fig. 13: Free electrons are attracted toward the positive end 31
14 Conduction in Semiconductor (cont.) Electrons and Holes Current movement of holes Figure 14 Hole current in intrinsic silicon. 32
15 Ntype and Ptype Semiconductors
15 Ntype and Ptype Semiconductors Doping The process of creating N and P type materials By adding impurity atoms to intrinsic Si or Ge to improve the conductivity of the semiconductor Two types of doping trivalent (3 valence e) & pentavalent (5 valence e) ptype material a semiconductor that has added trivalent impurities ntype material a semiconductor that has added pentavalent impurities Trivalent Impurities: Aluminum (Al) Gallium (Ga) Boron (B) Indium (In) Pentavalent Impurites: Phosphorus (P) Arsenic (As) Antimony (Sb) Bismuth (Bi) 34
15 Ntype and Ptype Semiconductors (cont.) Ntype semiconductor Pentavalent impurities are added to Si or Ge, the result is an increase of free electrons 1 extra electrons becomes a conduction electrons because it is not attached to any atom No. of conduction electrons can be controlled by the no. of impurity atoms Pentavalent atom gives up (donate) an electron call a donor atom Current carries in ntype are electrons majority carriers Holes minority carriers (holes created in Si when generation of electron holes pair. Fig. (a): Ntype semiconductor Fig. (b) : Energy diagram (ntype) Fig (a) Energy Conduction band Sb impurity atom Electrons (majority carriers) Valence band Holes (minority carriers) Fig (b) 35
15 Ntype and Ptype Semiconductors (cont.) Ptype semiconductor: Trivalent impurities are added to Si or Ge to increase number of holes. Boron, indium and gallium have 3 valence e form covalent bond with 4 adjacent silicon atom. A hole created when each trivalent atom is added. The no. of holes can be controlled by the no. of trivalent impurity atoms The trivalent atom can take an electron acceptor atom Current carries in ptype are holes majority carries Electrons minority carries (created during electronholes pairs generation). Fig. (a): Ptype semiconductor Fig. (b) : Energy diagram (ptype) Fig (a) B impurity atom Fig (b) 36
16 The Diode
16 The Diode Diode is a device that conducts current only in one direction. ntype material & ptype material become extremely useful when joined together to form a pn junction then diode is created Before the pn junction is formed no net charge (neutral) since no of proton and electron is equal in both ntype and ptype. p region: holes (majority carriers), e (minority carriers) n region: e (majority carriers), holes (minority carriers) 38
16 The Diode (cont.) The Depletion Region 39
16 The Diode (cont.) The Depletion Region Summary: When an ntype material is joined with a ptype material: 1. A small amount of diffusion occurs across the junction. 2. When e diffuse into pregion, they give up their energy and fall into the holes near the junction. 3. Since the nregion loses electrons, it creates a layer of +ve charges (pentavalent ions). 4. pregion loses holes since holes combine with electron and will creates layer of ve charges (trivalent ion). These two layers form depletion region. 5 Depletion region establish equilibrium (no further diffusion) when total ve charge in the region repels any further diffusion of electrons into pregion. 40
Ntype +4 +4 +4 +5 +4 Junction +4 +4 Ptype +3 +4 +4 Total (+) = 21 Total () = 20 Net charge = +1 Fig.118: Depletion layer charges Total (+) = 19 Total () = 20 Net charge = 1 41
16 The Diode Barrier Potential In depletion region, many +ve and ve charges on opposite sides of pn junction. The forces between the opposite charges form a field of forces "called an electric field. This electric field is a barrier to the free electrons in the n region, therefore it needs more energy to move an e through the electric field. The potential difference of electric field across the depletion region is the amount of voltage required to move e through the electric field. This potential difference is called barrier potential. [ unit: V ] Depends on: type of semicon. material, amount of doping and temperature. (e.g : 0.7V for Si and 0.3 V for Ge at 25 C). 42
16 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region Overlapping 43
16 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region Energy level for ntype (Valence and Cond. Band) << p type material (difference in atomic characteristic : pentavalent & trivalent) and significant amount of overlapping. Free e in upper part conduction band in nregion can easily diffuse across junction and temporarily become free e in lower part conduction band in pregion. After crossing the junction, the e loose energy quickly & fall into the holes in pregion valence band. 44
16 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region As the diffusion continues, the depletion region begins to form and the energy level of nregion conduction band decreases due to loss of higherenergy e that diffused across junction to p region. Soon, no more electrons left in nregion conduction band with enough energy to cross the junction to pregion conduction band. Figure (b), the junction is at equilibrium state, the depletion region is complete and diffusion has ceased (stop). Create an energy gradient which act as energy hill where electron at n region must climb to get to the pregion The energy gap between valence & cond. band remains the same 45
17 Biasing The Diode
1.7 Biasing The Diode Bias is a potential applied to pn junction to obtain certain operating conditions. This potential is used to control the width of the depletion layer. By controlling the width of the depletion layer, we are able to control the resistance of the pn junction and thus the amount of current that can pass through the device. Two bias condition: forward and reverse bias Depletion Layer Width Junction Resistance Junction Current Min Min Max Max Max Min The relationship between the width of depletion layer & the junction current 47
1.7 Biasing The Diode (cont.) Forward bias Diode connection 1. Forward bias is a potential used to reduce the resistance of pn junction 2. Voltage source or bias connections are + to the p region and to the n region. 3. Bias voltage must be greater than barrier potential (0.3 V for Germanium or 0.7 V for Silicon). 4. The depletion region narrows 5. R limits the current which can prevent damage to the diode 48
1.7 Biasing The Diode (cont.) Forward bias The negative side of the bias voltage push the free electrons in the nregion > pn junction. Flow of free electron is called electron current. Also provide a continuous flow of electron through the external connection into nregion. Bias voltage imparts energy to the free e to move to pregion. Flow of majority carries and the voltage across the depletion region Electrons in pregion loss energycombine with holes in valence band. 49
1.7 Biasing The Diode (cont.) Forward bias Since unlike charges attract, positive side of bias voltage source attracts the e left end of pregion. Holes in pregion act as medium or pathway for these e to move through the pregion. e move from one hole to the next toward the left. Flow of majority carries and the voltage across the depletion region The holes (majority cariers) move to right toward the junction. This effective flow is called hole current. 50
1.7 Biasing The Diode (cont.) The Effect of Forward bias on the Depletion Region As more electrons flow into the depletion region, the no. of +ve ion is reduced. As more holes flow into the depletion region on the other side of pn junction, the no. of ve ions is reduced. Reduction in +ve & ve ions causes the depletion region to narrow. 51
1.7 Biasing The Diode (cont.) The Effect of the Barrier Potential During Forward Bias Electric field between +ve & ve ions in depletion region creates energy hill that prevent free e from diffusing at equilibrium state > barrier potential When apply forward bias free e provided enough energy to climb the hill and cross the depletion region. Electron got the same energy = barrier potential to cross the depletion region. An add. small voltage drop occurs across the p and n regions due to internal resistance of material called dynamic resistance very small and can be neglected 52
1.7 Biasing The Diode (cont.) Reverse bias Diode connection Reverse bias Condition that prevents current through the diode Voltage source or bias connections are to the p material and + to the n material Current flow is negligible in most cases. The depletion region widens than in forward bias. 53
17 Biasing The Diode Shot transition time immediately after reverse bias voltage is applied + side of bias pulls the free electrons in the nregion away from pn junction cause add. +ve ions are created, widening the depletion region. In the pregion, e from side of the voltage source enter as valence electrons and move from hole to hole toward the depletion region, then created add. ve ions. As the depletion region widens, the availability of majority carriers decrease As more of the n and p regions become depleted of majority carriers, the electrical field between the positive and negative ions increases in strength until the potential across the depletion region equals the bias voltage. At this point, the transition current essentially ceases (stop) except for a very 54 small reverse current.
1.7 Biasing The Diode (cont.) Reverse Current Extremely small current exist after the transition current dies out caused by the minority carries in n & p regions that are produced by thermally generated electron hole pairs. Small number of free minority e in p region are pushed toward the pn junction by the ve bias voltage. e reach wide depletion region, they fall down the energy hill combine with minority holes in n region as valence e and flow towards the +ve bias voltage create small hole current. The cond. band in p region is at higher energy level compare to cond. band in nregion e easily pass through the depletion region because they require no 55 additional energy.
18 VoltageCurrent Characteristic Of A Diode
1.8 VoltageCurrent Characteristic of a Diode VI Characteristic for Forward Bias When a forward bias voltage is applied, there is current called forward current, I F. In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a nonconducting state. Current is very small. Increase forward bias voltage current also increase. FIGURE 126 Forwardbias measurements show general changes in V F and I F as V BIAS is increased. 57
1.8 VoltageCurrent Characteristic of a Diode (cont.) VI Characteristic for Forward Bias With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly. But the voltage across the diode increase only gradually above 0.7 V. This is due to voltage drop across internal dynamic resistance of semicon material. FIGURE 126 Forwardbias measurements show general changes in V F and I F as V BIAS is increased. 58
1.8 VoltageCurrent Characteristic of a Diode (cont.) VI Characteristic for Forward Bias By plotting the result of measurement in Figure 1 26, you get the VI characteristic curve for a forward bias diode dynamic resistance r d decreases as you move up the curve V F Increase to the right I F increase upward After 0.7V, voltage remains at 0.7V but I F increase rapidly. Normal operation for a forwardbiased diode is above the knee of the curve. zero bias V F < 0. 7V V F 0. 7V Below knee, resistance is greatest since current increase ' very little for given voltage, r d = VF / I Resistance become smallest above knee where a large change in current for given change in voltage. 59 F
1.8 VoltageCurrent Characteristic of a Diode (cont.) VI Characteristic for Reverse Bias V R increase to the left along xaxis while I R increase downward along y axis. When V R reaches V BR, I R begin to increase rapidly. Breakdown voltage, V BR. not a normal operation of pn junction devices. the value can be vary for typical Si. Reverse Current Cause overheating and possible damage to diode. 60
1.8 VoltageCurrent Characteristic of a Diode (cont.) The Complete VI Characteristic Curve CombineForward bias & Reverse bias CompleteVI characteristic curve 61
1.8 VoltageCurrent Characteristic of a Diode (cont.) Temperature Effects on the Diode VI Characteristic Forward biased diode : T,I F for a given value of Barrier potential decrease as T increase. V F For reversebiased, T increase, I R increase. Reverse current breakdown small & can be neglected 62
19 Diode Models
19 Diode Models Diode Structure & Symbol anode cathode Direction of current 64
19 Diode Models (cont.) The Ideal Diode Model The Practical Diode Model DIODE MODEL The Complete Diode Model 65
19 Diode Models (cont.) The Ideal Diode Model Ideal model of diodesimple switch: Closed (on) switch > FB Open (off) switch > RB Barrier potential, dynamic resistance and reverse current all neglected. Assume to have zero voltage across diode when FB. Forward current determined by Ohm s law V I F = R BIAS LIMIT I V R R = 0 A = V BIAS V F = 0V 66
Adds the barrier potential to the ideal switch model r ' d is neglected 19 Diode Models (cont.) The Practical Diode Model From figure (c): V V F F = 0.7V ( Si) = 0.3V ( Ge) The forward current [by applying Kirchhoff s voltage law to figure (a)] By Ohm s Law: I V BIAS V F V LIMIT = I F R V = BIAS R V F R R V LIMIT LIMIT LIMIT F =0 I V R R = 0 = V Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V V F Represent by produced across the pn junction A BIAS Open circuit, same as ideal diode model. Barrier potential doesn t affect RB 67
Complete model of diode consists: Barrier potential 19 Diode Models (cont.) The Complete Diode Model Dynamic resistance, r ' d Internal reverse resistance, The forward voltage consists of barrier potential & voltage drop across r d : V = + F ' 0.7V IFrd The forward current: I F V = R BIAS LIMIT 0.7V + r ' d r ' R acts as closed switch in series with barrier potential and small r ' d acts as open switch in parallel with the large r ' R 68
19 Diode Models (cont.) Example 1 (1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd = 10Ω at the determined value of forward current. 1.0kΩ 1.0kΩ 10V 5V 69
19 Diode Models (cont.) Example 1 a) Ideal Model: V I V F F R = 0 VBIAS 10V = = = 10mA R 1000Ω 3 3 = I F RLIMIT = (10 10 A)(1 10 Ω) = 10V LIMIT b) Practical Model: (c) Complete model: V F = 0. 7V I V F F R ( V = R BIAS V LIMIT F ) 10V 0.7V = = 9.3mA 1000Ω 3 3 = I F RLIMIT = (9.3 10 A)(1 10 Ω) = 9. 3V LIMIT I V V F F R V = R BIAS LIMIT = 0.7V + 0.7V ' + r I F r d ' d 10V 0.7V = = 9.21mA 1kΩ + 10 = 0.7V + (9.21mA)(10Ω) = = I F RLIMIT = (9.21mA)(1kΩ) = 9. 21V LIMIT 792mV 70
19 Diode Models (cont.) Typical Diodes Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of circuit they will be used in. 71
110 Testing A Diodes By Digital Multimeter Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode. With the diode check function a good diode will show approximately 0.7 V or 0.3 V when forward biased. When checking in reverse bias, reading based on meter s internal voltage source. 2.6V is typical value that indicate diode has extremely high reverse K A A K resistance. 72
110 Testing A Diodes (By Digital Multimeter) When diode is failed open, open reading voltage is 2.6V or OL indication for forward and reverse bias. If diode is shorted, meter reads 0V in both tests. If the diode exhibit a small resistance, the meter reading is less than 2.6V. 73
110 Testing A Diodes By Analog Multimeter OHMs Function Select OHMs range Good diode: Forwardbias: get low resistance reading (10 to 100 ohm) Reversebias: get high reading (0 or infinity) 74
Summary Diodes, transistors, and integrated circuits are all made of semiconductor material. Pmaterials are doped with trivalent impurities Nmaterials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction. A diode is nothing more than a PN junction. At the junction a depletion region is formed. This creates barrier which requires approximately 0.3 V for a Germanium and 0.7 V for Silicon for conduction to take place. 75
Summary A diode conducts when forward biased and does not conduct when reverse biased The voltage at which avalanche current occurs is called reverse breakdown voltage. Reverse breakdown voltage for diode is typically greater than 50V. There are three ways of analyzing a diode. These are ideal, practical, and complete. Typically we use a practical diode model. 76
Your destiny is in your hand There once was a wise man that was known throughout the land for his wisdom. One day a young boy wanted to test him to prove that the wise man a fake. He thought to himself, I will bring one live bird to test the old man. I will ask him whether the bird in my hand is dead or alive. If he says that it is alive, I will squeeze hard to kill the bird to prove that he is wrong. On the other hand if he says that it is dead, I will let the bird fly off, proving that he is wrong. Either way the wise man will be wrong. 77
With that idea in mind, he approached the wise man and asked, Oh wise man, I have a bird in my hand. Can you tell me if the bird is dead or alive?. The wise man paused for a moment and replied, Young man, you indeed have a lot t learn. That which you hold in your hand, it is what you make of it. The life of the bird is in your hand. If you wish it to be dead, then it will die. On the other hand if you desire it to live, it will surely live. The young boy finally realized that the answer given was indeed that of a man of wisdom. 78
Success principles Our dreams are very fragile, just like the little bird. It is our own decision, if we decide to kill it, or allow others to steal it away from us. However, it is also our own choice to nurture it and let it grow to fruition. Success comes to those who allow their dreams to fly high, just like the little bird, which will soar into the sky if the young boy released it from his grasp. 79
Energy increases as the distance from the nucleus increases 80
Basic diode structure at the instant of junction formation showing only majority and minority carriers. 81