Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 1 INTRODUCTION to SEMICONDUCTORS By: Syahrul Ashikin Azmi School of Electrical System Engineering UNIMAP Objectives Discuss basic structures of atoms Discuss properties of insulators, conductors, and semiconductors Discuss covalent bonding Describe the conductions in semiconductor Discuss N-type and P-type semiconductor Discuss the diode Discuss the bias of a diode LECTURE’S CONTENT 1.1 Atomic structure 1.2 Semiconductor, conductors and insulators 1.3 Covalent bonding 1.4 Conduction in semiconductors 1.5 N-type and P-type semiconductors 1.6 Diode 1.7 Biasing the diode 1.8 Voltage-current characteristic of a diode 1.9 Diode models 1.10 Testing a diode REVIEW REVIEW WHY WE USE ELECTRONICS? • Electronics are easy to move/control - Easy to move/control electrons than real physical stuff • Move information not things - Phone, fax, internet - Takes much less energy and money History Of Semiconductor Devices 1.1 Atomic Structure 1.1 Atomic Structure Atomic number Basic structure Electron shells ATOM Valence electron Free electron Ionization 1.1 Atomic Structure (cont.) smallest particle of an element contain 3 basic particles: ATOM Protons (positive charge) Nucleus (core of atom) Neutrons (uncharged) Electrons (negative charge) 1.1 Atomic Structure (cont.) This model was proposed by Niels Bohr in 1915. -electrons circle the nucleus that consists of protons and neutrons. Figure 1.1 Bohr model of an atom 1.1 Atomic Structure (cont.) Atomic Number - Element in periodic table are arranged according to atomic number - Atomic number = number of protons in nucleus 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. Valence Electrons - Electrons with the highest energy levels exist in the outermost shell and loosely bound to the atom. The outermost shell – valence shell. - Electron in the valence shell called valence electrons. N e 2n 2 1.1 Atomic Structure (cont.) Ionization - When atoms absorb energy (e.g heat source) – losing valence electrons called ionization. Escape electron called free electron. The Number of Electrons in Each Shell - - The maximum number of electrons (Ne) in each shell is calculated using formula below: n = number of shell Example for 2nd shell N e 2n 2 N 2n 2(2) 8 2 e 2 1.2 Semiconductors, conductors and insulators 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 Carbon atom: -valence shell – 4 e -inner shell – 2 e Nucleus: -6 protons -6 neutrons +6 for the nucleus and -2 for the two inner-shell electrons (net charge +4) 1-2 Semiconductors, Conductors, and Insulators (cont.) Conductors • material that easily conducts electrical current. • The best conductors are single-element 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 • 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. 1.2 Semiconductors, Conductors, and Insulators (cont) 1.2 Semiconductors, Conductors, and Insulators (cont.) Energy Bands 1-2 Semiconductors, Conductors, and Insulators (cont.) Energy Bands • Energy gap-the difference between the energy levels of any two orbital shells • Band-another name for an orbital shell (valence shell=valence band) • Conduction band –the band outside the valence shell where it has free electrons. 1-2 Semiconductors, Conductors, and Insulators (cont.) Comparison of a Semiconductor Atom & Conductor Atom A Silicon atom: • 4 valence electrons • A semiconductor • Electron conf.: 2:8:4 14 protons 14 nucleus 10 electrons in inner shell A Copper atom: • Only 1 valence electron • A good conductor • Electron conf.:2:8:18:1 29 protons 29 nucleus 28 electrons in inner shell 1-3 Covalent Bonding 1-3 Covalent Bonding Covalent bonding – holding atoms together by sharing 1-3 Covalent Bonding valence electrons sharing of valence electron produce the covalent bond To form Si crystal 1-3 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 insulator-intrinsic (pure) silicon. In other word, it is a very poor conductor. 1-3 Covalent Bonding (cont.) • Covalent bonding in an intrinsic or pure silicon crystal. An intrinsic crystal has no impurities. Covalent bonds in a 3-D silicon crystal 1-4 Conduction in Semiconductor 1-4 Conduction in Semiconductor Conduction Electrons and Holes Figure 1-10 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. 1-4 Conduction in Semiconductor (cont.) Conduction Electrons and Holes Absorbs enough energy (thermal energy) to jumps a free electron and its matching valence band hole – electron-hole pair Recombination-when a conduction electron loses energy and fall back into hole in valence band Figure 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free (also called conduction electrons). 1-4 Conduction in Semiconductor (cont.) Conduction Electrons and Holes Figure 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes. 1-4 Conduction in Semiconductor (cont.) Electrons and Holes Current Electron current free electrons Apply voltage When a voltage is applied, free electrons are free to move randomly and attracted toward +ve end. The movement of electrons is one type of current in semiconductor and is called electron current. Figure 1-13 Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons. 1-4 Conduction in Semiconductor (cont.) Electrons and Holes Current movement of holes Figure 1-14 Hole current in intrinsic silicon. 1-5 N-type and P-type Semiconductors 1-5 N-type and P-type Semiconductors Doping 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-) p-type material – a semiconductor that has added trivalent impurities n-type material – a semiconductor that has added pentavalent impurities Trivalent Impurities: Pentavalent Impurites: • Aluminum (Al) • Phosphorus (P) • Gallium (Ga) • Arsenic (As) • Boron (B) • Antimony (Sb) • Indium (In) • Bismuth (Bi) 1-5 N-type and P-type Semiconductors (cont.) N-type 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 an electron -call a donor atom Current carries in n-type are electrons – majority carriers Holes – minority carriers (holes created in Si when generation of electronholes pair. Sb impurity atom Pentavalent impurity atom in a Si crystal 1-5 N-type and P-type Semiconductors (cont.) P-type 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 p-type are holes – majority carries - electrons – minority carries (created during electron-holes pairs generation). B impurity atom Trivalent impurity atom in a Si crystal 1-6 The Diode 1-6 The Diode - Diode is a device that conducts current only in one direction. - n-type material & p-type 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 n-type and p-type. -p region: holes (majority carriers), e- (minority carriers) -n region: e- (majority carriers), holes (minority carriers) 1-6 The Diode (cont.) The Depletion Region 1-6 The Diode (cont.) The Depletion Region Summary: When an n-type material is joined with a p-type material: 1. A small amount of diffusion occurs across the junction. 2. When e- diffuse into p-region, they give up their energy and fall into the holes near the junction. 3. Since the n-region loses electrons, it creates a layer of +ve charges (pentavalent ions). 4. p-region 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 p-region. 1-6 The Diode Barrier Potential • • • • • • 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 nregion, need 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). 1-6 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region Overlapping 1-6 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region • • Energy level for n-type (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 n-region can easily diffuse across junction and temporarily become free e- in lower part conduction band in p-region. After crossing the junction, the e- loose energy quickly & fall into the holes in p-region valence band. 1-6 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 n-region conduction band decreases due to loss of higherenergy e- that diffused across junction to p-region. Soon, no more electrons left in n-region 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 diffusion has ceased (stop). Create an energy gradient which act as energy ‘hill’ where electron at n-region must climb to get to the p-region. The energy gap between valence & cond. band – remains the same 1-7 Biasing The Diode 1.7 Biasing The Diode Bias No electron move through the pn-junction at equilibrium state. Bias is a potential applied (dc voltage) to a pn junction to obtain a desired mode of operation – control the width of the depletion layer. Two bias conditions : forward bias & reverse bias The relationship between the width of depletion layer & the junction current Depletion Layer Width Junction Resistance Junction Current Min Min Max Max Max Min 1.7 Biasing The Diode (cont.) Forward bias 1. Voltage source or bias connections are + to the p region and – to the n region. 2. Bias voltage must be greater than barrier potential (0 .3 V for Germanium or 0.7 V for Silicon). • Diode connection • The depletion region narrows. R – limits the current to prevent damage to the diode 1.7 Biasing The Diode (cont.) Forward bias Flow of majority carries and the voltage across the depletion region • The negative side of the bias voltage push the free electrons in the n-region -> pn junction. Flow of free electron is called electron current. • Also provide a continuous flow of electron through the external connection into n-region. • Bias voltage imparts energy to the free e- to move to p-region. • Electrons in p-region loss energy-combine with holes in valence band. 1.7 Biasing The Diode (cont.) Forward bias Flow of majority carries and the voltage across the depletion region • Since unlike charges attract, positive side of bias voltage source attracts the e- left end of p-region. • Holes in p-region act as medium or pathway for these e- to move through the p-region. • e- move from one hole to the next toward the left. • The holes move to right toward the junction. This effective flow is called hole current. 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. 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 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. 1-7 Biasing The Diode Shot transition time immediately after reverse bias voltage is applied • • • + side of bias pulls the free electrons in the n-region away from pn junction cause add. +ve ions are created, widening the depletion region. In the p-region, e- from – side of the voltage source enter as valence electrons e- 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. 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 n-region e- easily pass through the depletion region because they require no additional energy. • 1-8 Voltage-Current Characteristic Of A Diode 1.8 Voltage-Current Characteristic of a Diode V-I Characteristic for Forward Bias -When a forward bias voltage is applied, there is current called forward current, IF . -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 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased. 1.8 Voltage-Current Characteristic of a Diode (cont.) V-I 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 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased. 1.8 Voltage-Current Characteristic of a Diode (cont.) V-I Characteristic for Forward Bias dynamic resistance r’d decreases as you move up the curve -Plot the result of measurement in Figure 126, you get the V-I characteristic curve for a forward bias diode - VF Increase to the right - I F increase upward -After 0.7V, voltage remains at 0.7V but IF increase rapidly. -Normal operation for a forward-biased diode is above the knee of the curve. zero bias VF 0.7V VF 0.7V Below knee, resistance is greatest since current increase very little for given voltage, r ' d VF / I F Resistance become smallest above knee where a large change in current for given change in voltage. 1.8 Voltage-Current Characteristic of a Diode (cont.) V-I Characteristic for Reverse Bias - VR increase to the left along x-axis while IR increase downward along yaxis. - When VR reaches VBR , IR begin to increase rapidly. Breakdown voltage, VBR. - not a normal operation of pn junction devices. - the value can be vary for typical Si. - Cause overheating and possible damage to diode. Reverse Current 1.8 Voltage-Current Characteristic of a Diode (cont.) The Complete V-I Characteristic Curve Combine-Forward bias & Reverse bias CompleteV-I characteristic curve 1.8 Voltage-Current Characteristic of a Diode (cont.) Temperature Effects on the Diode V-I Characteristic Forward biased diode : T , I F for a given value of VF Barrier potential decrease as T increase. For reverse-biased, T increase, IR increase. Reverse current breakdown – small & can be neglected 1-9 Diode Models 1-9 Diode Models Diode Structure & Symbol anode cathode Direction of current 1-9 Diode Models (cont.) The Ideal Diode Model The Practical Diode Model DIODE MODEL The Complete Diode Model 1-9 Diode Models (cont.) The Ideal Diode Model Ideal model of diodesimple switch: •Closed (on) switch -> FB •Open (off) switch > RB •Forward current determined by Ohm’s law • Barrier potential, dynamic resistance and reverse current all VBIAS IF neglected. • Assume to have zero voltage across diode when FB. RLIMIT VF 0V IR 0A VR VBIAS 1-9 Diode Models (cont.) The Practical Diode Model •Adds the barrier potential to the ideal switch model ' r • ‘ d is neglected •From figure (c): VF 0.7V ( Si) VF 0.3V (Ge) The forward current [by applying Kirchhoff’s voltage law to figure (a)] VBIAS VF VRLIMIT 0 VRLIMIT I F RLIMIT By Ohm’s Law: •Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V •Represent by VF produced across the pn junction VBIAS VF I R 0 A IF VR VBIAS RLIMIT •Open circuit, same as ideal diode model. •Barrier potential doesn’t affect RB 1-9 Diode Models (cont.) The Complete Diode Model Complete model of diode consists: •Barrier potential ' •Dynamic resistance, r d •Internal reverse resistance, r ' R •The forward voltage consists of barrier potential & voltage drop across r’d : VF 0.7V I F rd' •The forward current: VBIAS 0.7V IF RLIMIT rd' •acts as closed switch in series with barrier potential and small r ' d •acts as open switch in parallel with the large r ' R 1-9 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’ = 10 at the determined value of forward current. 1.0kΩ 1.0kΩ 10V 5V 1-9 Diode Models (cont.) Example 1 a) Ideal Model: VF 0 VBIAS 10V 10mA R 1000 VRLIMIT I F RLIMIT (10 10 3 A)(1103 ) 10V IF b) Practical Model: V 0.7V F IF (c) Complete model: IF (VBIAS VF ) 10V 0.7V 9.3mA RLIMIT 1000 VRLIMIT I F RLIMIT (9.3 10 3 A)(1103 ) 9.3V VBIAS 0.7V 10V 0.7V 9.21mA ' RLIMIT rd 1k 10 VF 0.7V I F rd' 0.7V (9.21mA )(10) 792mV VRLIMIT I F RLIMIT (9.21mA )(1k) 9.21V 1-9 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. 1-10 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 resistance. K A A K 1-10 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. 1-10 Testing A Diodes By Analog Multimeter – OHMs Function Select OHMs range Good diode: Forward-bias: get low resistance reading (10 to 100 ohm) Reverse-bias: get high reading (0 or infinity) Summary Diodes, transistors, and integrated circuits are all made of semiconductor material. P-materials are doped with trivalent impurities N-materials 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. 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. 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.” • • • 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. 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. Energy increases as the distance from the nucleus increases Basic diode structure at the instant of junction formation showing only majority and minority carriers.