ECE, ME – I SEM SEMICONDUCTOR PHYSICS AND TRANSISTORS WHAT IS A SEMICONDUCTOR? •Many materials, such as most metals, allow electrical current to flow through them •These are known as conductors •Materials that do not allow electrical current to flow through them are called insulators •Pure silicon, the base material of most transistors, is considered a semiconductor because its conductivity can be modulated by the introduction of impurities SEMICONDUCTORS, INSULATORS AND METALS The electrical properties of metals and insulators are well known to all of us. Everyday experience has already taught us a lot about the electrical properties of metals and insulators. But the same cannot be said about “semiconductors”. What happens when we connect a battery to a piece of a silicon; would it conduct well ? or would it act like an insulator ? THE NAME “SEMICONDUCTOR” IMPLIES THAT IT CONDUCTS SOMEWHERE BETWEEN THE TWO CASES (CONDUCTORS OR INSULATORS) Conductivity : σmetals ~1010 /Ω-cm The conductivity (σ) of a S/C semiconductor (S/C) σinsulators ~ 10 -22 /Ω-cm lies between these two extreme cases. CONDUCTORS AND SEMICONDUCTORS I I 0 VB VB Conductor I ntrinsic Silicon Holes (Ben Franklin) Insulator Electrons Bar of metal Bar of silicon I I VB VB p-Type Silicon n-Type Silicon Hole Conduction Electron Conduction Semiconductors SEMICONDUCTORS A material whose properties are such that it is not quite a conductor, not quite an insulator Some common semiconductors elemental Si - Silicon (most common) Ge - Germanium compound GaAs - Gallium arsenide GaP - Gallium phosphide AlAs - Aluminum arsenide AlP - Aluminum phosphide InP - Indium Phosphide CRYSTAL STRUCTURE OF SILICON Covalent Bond: View along a crystallographic Two atoms Share two electrons axis of silicon Silicon atom: Four electrons/atom CRYSTALLINE SOLIDS In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystal Silicon crystal has a diamond lattice CRYSTALLINE NATURE OF SILICON Silicon as utilized in integrated circuits is crystalline in nature As with all crystalline material, silicon consists of a repeating basic unit structure called a unit cell For silicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron WHAT’S SO SPECIAL ABOUT SILICON? Cheap and abundant Amazing mechanical, chemical and electronic properties The material is very well-known to mankind SiO2: sand, glass Si is column IV of the periodic table Similar to the carbon (C) and the germanium (Ge) Has 3s² and 3p² valence electrons NATURE OF INTRINSIC SILICON Silicon that is free of doping impurities is called intrinsic Silicon has a valence of 4 and forms covalent bonds with four other neighboring silicon atoms INTRINSIC SILICON Temperature effects: • At absolute zero all electrons are bonded to neighbors. There is none available for conduction. • Insulator • At higher temperatures electrons gain enough energy to escape the bonds. • Some conduction but still basically an insulator Silicon: 4 electrons/atom PHOSPHORUS (N-TYPE) DOPED SILICON • Replace silicon atom by a phosphorus atom which had five electrons rather than four (silicon). One impurity atom for 106 to 1010 silicon atoms. • Extra electron is free to move in crystal allowing conduction Phosphorus atom has net (+) charge Extra electron (-) which is fixed which is mobile in position Phosphorous atom SIMPLIFIED REPRESENTATION OF N-TYPE SILICON • Charges are balanced: Number of (+) charges equals number of (-)charges Charges due to one impurity atom Shows only impurity atoms • Net positive charge of each • Extra electron (- charge) for each phosphorus atom phosphorus atom • Fixed position in crystal • Mobile: Moves around crystal due to E-field and diffusion p-Type Silicon • Example with Boron doping atoms which has three electrons/atom. Charges due to one impurity atom • Net negative charge of each • Extra hole (+ charge) for each boron boron atom atom • Fixed position in lattice • Mobile: Moves around crystal due to E-field and diffusion MOTION OF CHARGES Mobile charges move due to two effects: • Forces due to electric field: Force = charge x Electric Field • The resulting current is called “drift current” • Diffusion due to gradient in the charge density: • Charges move to be evenly distributed throughout space • Similar to perfume in room or heat in a solid. • The resulting current is called “diffusion current” ELECTRICAL CONDUCTIVITY in order of conductivity: superconductors, conductors, semiconductors, insulators some representative resistivities (): R = L/A, R = resistance, L = length, A = cross section area; resistivity at 20o C resistivity in m resistance(in )(L=1m, diam =1mm) aluminum 2.8x10-8 3.6x10-2 brass 8x10-8 10.1x10-2 copper 1.7x10-8 2.2x10-2 platinum 10x10 12.7x10-2 -8 silver 1.6x10-8 2.1x10-2 carbon 3.5x10-5 44.5 germanium 0.45 5.7x105 silicon 640 6x108 porcelain 1010 - 1012 1016 - 1018 teflon 1014 1020 blood 1.5 1.9x106 fat 24 3x107 SEMICONDUCTOR CRYSTALLINE STRUCTURE Semiconductors have a regular crystalline structure for monocrystal, extends through entire structure for polycrystal, structure is interrupted at irregular boundaries Monocrystal has uniform 3- dimensional structure Atoms occupy fixed positions relative to one another, but are in constant vibration about equilibrium SEMICONDUCTOR CRYSTALLINE STRUCTURE Silicon atoms have 4 electrons in outer shell inner electrons are very closely bound to atom These electrons are shared with neighbor atoms on both sides to “fill” the shell resulting structure is very stable electrons are fairly tightly bound no “loose” electrons at room temperature, if battery applied, very little electric current flows CONDUCTION IN CRYSTAL LATTICES Semiconductors (Si and Ge) have 4 electrons in their outer shell 2 in the s subshell 2 in the p subshell As the distance between atoms decreases the discrete subshells spread out into bands As the distance decreases further, the bands overlap and then separate the subshell model doesn’t hold anymore, and the electrons can be thought of as being part of the crystal, not part of the atom 4 possible electrons in the lower band (valence band) 4 possible electrons in the upper band (conduction band) ENERGY BANDS IN SOLIDS: In solid materials, electron energy levels form bands of allowed energies, separated by forbidden bands valence band = outermost (highest) band filled with electrons (“filled” = all states occupied) conduction band = next highest band to valence band (empty or partly filled) “gap” = energy difference between valence and conduction bands, = width of the forbidden band Note: electrons in a completely filled band cannot move, since all states occupied (Pauli principle); only way to move would be to “jump” into next higher band - needs energy; electrons in partly filled band can move, since there are free states to move to. Classification of solids into three types, according to their band structure: insulators: gap = forbidden region between highest filled band (valence band) and lowest empty or partly filled band (conduction band) is very wide, about 3 to 6 eV; semiconductors: gap is small - about 0.1 to 1 eV; conductors: valence band only partially filled, or (if it is filled), the next allowed empty band overlaps with it .:: THE BAND THEORY OF SOLIDS ::. The electrons surrounding Allowed a nucleus have certain band well-defined energy-levels. Forbidden band Electrons don’t like to have the same energy in the Allowed same potential system. band Forbidden The most we could get band together in the same energy-level was two, Allowed provided thet they had band opposite spins. This is called Pauli Exclusion 1 2 4………………N Principle. Number of atoms The difference in energy between each of these smaller levels is so tiny that it is more reasonable to consider each of these sets of smaller energy-levels as being continuous bands of energy, rather than considering the enormous number of discrete individual levels. Each allowed band is seperated from another one by a forbidden band. Electrons can be found in allowed bands but they can not be found in forbidden bands. .:: SEMICONDUCTOR, INSULATORS, CONDUCTORS ::. Full band Empty band All energy levels are All energy levels are empty occupied by electrons ( no electrons) Both full and empty bands do not partake in electrical conduction. ENERGY BANDS IN SEMICONDUCTORS The space between the bands is the energy gap, or forbidden band ENERGY-BAND DIAGRAM A very important concept in the study of semiconductors is the energy-band diagram It is used to represent the range of energy a valence electron can have For semiconductors the electrons can have any one value of a continuous range of energy levels while they occupy the valence shell of the atom That band of energy levels is called the valence band Within the same valence shell, but at a slightly higher energy level, is yet another band of continuously variable, allowed energy levels This is the conduction band BAND GAP Between the valence and the conduction band is a range of energy levels where there are no allowed states for an electron This is the band gap E G In silicon at room temperature [in electron volts]: EG 1.1eV Electron volt is an atomic measurement unit, 1 eV energy is necessary to decrease of the potential of the electron with 1 V. 1eV 1.602 10 19 joule INSULATORS, SEMICONDUCTORS, AND METALS This separation of the valence and conduction bands determines the electrical properties of the material Insulators have a large energy gap electrons can’t jump from valence to conduction bands no current flows Conductors (metals) have a very small (or nonexistent) energy gap electrons easily jump to conduction bands due to thermal excitation current flows easily Semiconductors have a moderate energy gap only a few electrons can jump to the conduction band leaving “holes” only a little current can flow INSULATORS, SEMICONDUCTORS, AND METALS (CONTINUED) Conductio n Band Valence Band Conductor Semiconductor Insulator BAND STRUCTURE AND CONDUCTIVITY HOLE - ELECTRON PAIRS Sometimes thermal energy is enough to cause an electron to jump from the valence band to the conduction band produces a hole - electron pair Electrons also “fall” back out of the conduction band into the valence band, combining with a hole pair elimination pair creation hole electron IMPROVING CONDUCTION BY DOPING To make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holes elements with 5 outer electrons contribute an extra electron to the lattice (donor dopant) elements with 3 outer electrons accept an electron from the silicon (acceptor dopant) IMPROVING CONDUCTION BY DOPING (CONT.) Phosphorus and arsenic are donor dopants if phosphorus is introduced into the silicon lattice, there is an extra electron “free” to move around and contribute to electric current very loosely bound to atom and can easily jump to conduction band produces n type silicon sometimes use + symbol to indicate heavier doping, so n+ silicon phosphorus becomes positive ion after giving up electron IMPROVING CONDUCTION BY DOPING (CONT.) Boron has 3 electrons in its outer shell, so it contributes a hole if it displaces a silicon atom boron is an acceptor dopant yields p type silicon boron becomes negative ion after accepting an electron HOLE AND ELECTRON CONCENTRATIONS To produce reasonable levels of conduction doesn’t require much doping silicon has about 5 x 1022 atoms/cm3 typical dopant levels are about 1015 atoms/cm3 In undoped (intrinsic) silicon, the number of holes and number of free electrons is equal, and their product equals a constant actually, ni increases with increasing temperature np = n 2i This equation holds true for doped silicon as well, so increasing the number of free electrons decreases the number of holes INTRINSIC (PURE) SILICON At 0 Kelvin Silicon density is 5*10 ²³ particles/cm ³ Silicon has 4 valence electrons, it covalently bonds with four adjacent atoms in the crystal lattice Higher temperatures create free charge carriers. A “hole” is created in the absence of an electron. At 23C there are 10¹º particles/cm³ of free carriers There are two types of doping DOPING N-type and P-type. The N in N-type stands for negative. A column V ion is inserted. The extra valence electron is free to move about the lattice The P in P-type stands for positive. A column III ion is inserted. Electrons from the surrounding Silicon move to fill the “hole.” IMPURITIES Silicon crystal in pure form is good insulator - all electrons are bonded to silicon atom Replacement of Si atoms can alter electrical properties of semiconductor Group number - indicates number of electrons in valence level (Si - Group IV) IMPURITIES Replace Si atom in crystal with Group V atom substitution of 5 electrons for 4 electrons in outer shell extra electron not needed for crystal bonding structure can move to other areas of semiconductor current flows more easily - resistivity decreases many extra electrons --> “donor” or n-type material Replace Si atom with Group III atom substitution of 3 electrons for 4 electrons extra electron now needed for crystal bonding structure “hole” created (missing electron) hole can move to other areas of semiconductor if electrons continually fill holes again, current flows more easily - resistivity decreases electrons needed --> “acceptor” or p-type material A LITTLE MATH n= number of free electrons p=number of holes ni=number of electrons in intrinsic silicon=10¹º/cm³ pi-number of holes in intrinsic silicon= 10¹º/cm³ Mobile negative charge = -1.6*10-19 Coulombs Mobile positive charge = 1.6*10-19 Coulombs At thermal equilibrium (no applied voltage) n*p=(ni)2 (room temperature approximation) The substrate is called n-type when it has more than 10¹º free electrons (similar for p-type) Intrinsic silicon: DOPED SEMICONDUCTORS: “doped semiconductor”: (also “impure”, “extrinsic”) = semiconductor with small admixture of trivalent or pentavalent atoms; N-TYPE MATERIAL donor (n-type) impurities: dopant with 5 valence electrons (e.g. P, As, Sb) 4 electrons used for covalent bonds with surrounding Si atoms, one electron “left over”; left over electron is only loosely bound only small amount of energy needed to lift it into conduction band (0.05 eV in Si) “n-type semiconductor”, has conduction electrons, no holes (apart from the few intrinsic holes) example: doping fraction of 10 Sb in Si yields about 5x10 -8 16 conduction electrons per cubic centimeter at room temperature, i.e. gain of 5x106 over intrinsic Si. N-TYPE MATERIAL P-TYPE MATERIAL acceptor (p-type) impurities: dopant with 3 valence electrons (e.g. B, Al, Ga, In) only 3 of the 4 covalent bonds filled vacancy in the fourth covalent bond hole “p-type semiconductor”, has mobile holes, very few mobile electrons (only the intrinsic ones). advantages of doped semiconductors: can”tune” conductivity by choice of doping fraction can choose “majority carrier” (electron or hole) can vary doping fraction and/or majority carrier within piece of semiconductor can make “p-n junctions” (diodes) and “transistors” P-TYPE MATERIAL P-N JUNCTION Also known as a diode One of the basics of semiconductor technology - Created by placing n-type and p-type material in close contact Diffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type P-N JUNCTION Region of charges left behind (dopants fixed in crystal lattice) Group III in p-type (one less proton than Si- negative charge) Group IV in n-type (one more proton than Si - positive charge) Region is totally depleted of mobile charges - “depletion region” Electric field forms due to fixed charges in the depletion region Depletion region has high resistance due to lack of mobile charges p-n JUNCTION: p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ; “diffusion” = movement due to difference in concentration, from higher to lower concentration; in absence of electric field across the junction, holes “diffuse” towards and across boundary into n-type and capture electrons; electrons diffuse across boundary, fall into holes (“recombination of majority carriers”); formation of a “depletion region” (= region without free charge carriers) around the boundary; charged ions are left behind (cannot move): negative ions left on p-side net negative charge on p-side of the junction; positive ions left on n-side net positive charge on n-side of the junction electric field across junction which prevents further diffusion. DIODES (THE PN JUNCTION) DIODES (THE PN JUNCTION) The free electrons in the n region are randomly drifting in all directions. At the instant of the pn junction formation, the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near the junction. The same is true for the p- type material. THE P-N JUNCTION p-n JUNCTION: THE JUNCTION The “potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region THINK OF THE DIODE AS A SWITCH BIASING THE P-N DIODE Forward Bias Reverse Bias Applies - voltage Applies + voltage to to the n region n region and – and + voltage to voltage to p region the p region NO CURRENT CURRENT! DIODES (THE DEPLETION REGION) When the pn junction is formed, the n region loses free electrons as they diffuse across the junction. This creates a layer of positive charges (ions) near the junction. As the electrons move across the junction, the p region loses holes as the electrons and holes combine. This creates a layer of negative charges (ions) near the junction. These two layers of positive and negative charges form the depletion region. PN JUNCTION Formation of depletion region in pn-junction: P-N JUNCTION – REVERSE BIAS positive voltage placed on n-type material electrons in n-type move closer to positive terminal, holes in p-type move closer to negative terminal width of depletion region increases allowed current is essentially zero (small “drift” current) REVERSE BIASED DIODE Depletion region becomes wider, barrier potential higher DIODES (FORWARD BIAS ) Because like charges repel, the negative side of the bias-voltage source "pushes" the free electrons, which are the majority carriers in the n region, toward the pn junction. This flow of free electrons is called electron current. The negative side of the source also provides a continuous flow of electrons through the external connection (conductor) and into the n region as shown. DIODES (FORWARD BIAS ) •To bias a pn junction, apply an external dc voltage across it. Forward bias is the condition that allows current through a pn junction. The picture shows a dc voltage source connected by conductive material (contacts and wire) across a pn junction in the direction to produce forward bias. •This external bias voltage is designated as VBIAS. Notice that the negative side of VBIAS is connected to the n region of the pn junction and the positive side is connected to the p region. This is one requirement for forward bias. A second requirement is that the bias voltage, V BIAS, must be greater than the barrier potential (0.7V in silicon and 0.3 in germanium). P-N JUNCTION – FORWARD BIAS positive voltage placed on p-type material holes in p-type move away from positive terminal, electrons in n- type move further from negative terminal depletion region becomes smaller - resistance of device decreases voltage increased until critical voltage is reached, depletion region disappears, current can flow freely FORWARD BIASED PN-JUNCTION Depletion region and potential barrier reduced P-N JUNCTION - V-I CHARACTERISTICS Voltage-Current relationship for a p-n junction (diode) CURRENT-VOLTAGE CHARACTERISTICS THE IDEAL DIODE Positive voltage yields finite current Negative voltage yields zero current REAL DIODE THE IDEAL DIODE EQUATION qV I I 0 exp 1 , kT where I 0 diode current with reverse bias q 1602 10 19 coulomb , the electronic ch arg e . 5 eV k 8.62 10 , Boltzmann' s cons tan t K SEMICONDUCTOR DIODE - OPENED REGION The p-side is the cathode, the n-side is the anode The dropped voltage, VD is measured from the cathode to the anode Opened: VD VF: VD = VF ID = circuit limited, in our model the VD cannot exceed VF SEMICONDUCTOR DIODE - CUT-OFF REGION Cut-off: 0 < VD < VF: ID 0 mA SEMICONDUCTOR DIODE - CLOSED REGION Closed: VF < VD 0: VD is determined by the circuit, ID = 0 mA Typical values of VF: 0.5 ¸ 0.7 V ZENER EFFECT Zener break down: VD <= VZ: VD = VZ, ID is determined by the circuit. In case of standard diode the typical values of the break down voltage VZ of the Zener effect -20 ... -100 V Zener diode Utilization of the Zener effect Typical break down values of VZ : -4.5 ... -15 V THE ZENER DIODE +I The zener diode exhibits a constant voltage F drop when sufficiently reversed-biased. zener point This property allows the use of the zener diode as a simple voltage regulator. -6 -3 +VF 1 2 3 +V Constant breakdown voltage R Here, Vr will be equal to the reverse breakdown voltage of the zener diode and should be constant. What is the purpose Vr of the resistor in this circuit? Its job is to limit the current flowing through the zener diode: D V Vr I R Kit Building Class Page 70 Lesson 3 TRANSISTORS A transistor allows you to control the current, not just block it in one direction. A good analogy for a transistor is a pipe with an adjustable gate. BIPOLAR JUNCTION TRANSISTORS (BJT) collector collector Three Layers in a BJT p n Collector base Base (very thin) n p has fewer doping atoms P+ Emitter i i n+ Two Types of BJT’s PNP (figure on left) emitter emitter operates with outgoing base current NPN (figure on right) operates with incoming base current BJT SCHEMATIC REPRESENTATION collector iB p base n Corresponds to: i P+ emitter collector n Corresponds to: p i n+ emitter THE PHYSICS OF TRANSISTORS A transistor is made from two p-n junctions back to back. An npn transistor has a p-type layer sandwiched between two n-type layers. A pnp transistor is the inverse. An n-type semiconductor is between two layers of p-type. BJT TRANSISTORS: NPN Transistor Sandwiching a P-type layer between two n- type layers. PNP Transistor Sandwiching a N-type layer between two p- type layers. THE BIPOLAR JUNCTION TRANSISTOR The transistor is a versatile device usually configured to perform as a switch or as an amplifier. The bipolar junction transistor (BJT) is the most common type and has three leads: 3 3 Collector Collector 2 2 Base Base 1 Emitter 1 Emitter PNP Transistor NPN Transistor In a transistor, the flow of current from the collector to the emitter is controlled by the amount of current flowing into the base of the transistor. If no current flows into the base, no current will flow from the collector to the emitter (it acts like an open switch). If current flows into the base, then a proportional amount of current flows from the collector to the emitter (somewhat like a closed switch). HOW A “NPN” TRANSISTOR WORKS? The base-emitter diode (forward) acts as a switch. when v1>0.7 it lets the electrons flow toward collector. so we can control our output current (Ic) with the input current (Ib) by C B E using transistors. backward Forward Transistors have three terminals: Collector Base Emitter Active: Always on Ic=BIb Transistors work in 3 regions Saturation :Ic=Isaturation On as a switch Off :Ic=0 Off as a switch TRANSISTOR CURRENTS -The arrow is always drawn on the emitter -The arrow always point toward the n-type -The arrow indicates the direction of the emitter current: pnp:E B IC=the collector current npn: B E IB= the base current IE= the emitter current By imaging the analogy of diode, transistor can be construct like two diodes that connetecd together. It can be conclude that the work of transistor is base on work of diode. TRANSISTOR OPERATION The basic operation will be described using the pnp transistor. The operation of the pnp transistor is exactly the same if the roles played by the electron and hole are interchanged. One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased. Forward-biased junction Reverse-biased junction of a pnp transistor of a pnp transistor Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated. Majority carriers (+) will diffuse across the forward- biased p-n junction into the n-type material. A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes. The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal. Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material. Applying KCL to the transistor : IE = IC + IB The comprises of two components – the majority and minority carriers IC = ICmajority + ICOminority ICO – IC current with emitter terminal open and is called leakage current. COMMON-BASE CONFIGURATION Common-base terminology is derived from the fact that the : - base is common to both input and output of the configuration. - base is usually the terminal closest to or at ground potential. All current directions will refer to conventional (hole) flow and the arrows in all electronic symbols have a direction defined by this convention. Note that the applied biasing (voltage sources) are such as to establish current in the direction indicated for each branch. To describe the behavior of common-base amplifiers requires two set of characteristics: - Input or driving point characteristics. - Output or collector characteristics The output characteristics has 3 basic regions: - Active region –defined by the biasing arrangements - Cutoff region – region where the collector current is 0A - Saturation region- region of the characteristics to the left of VCB = 0V The curves (output characteristics) clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by IC ≈IE Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be VBE = 0.7V In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha IC = IE IC = IE + ICBO It can then be summarize to IC = IE (ignore ICBO due to small value) For ac situations where the point of operation moves on the characteristics curve, an ac alpha defined by IC IE Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector.The value of is typical from 0.9 ~ 0.998. BIASING Proper biasing CB configuration in active region by approximation IC IE (IB 0 uA) TRANSISTOR AS AN AMPLIFIER SIMULATION OF TRANSISTOR AS AN AMPLIFIER COMMON-EMITTER CONFIGURATION It is called common-emitter configuration since : - emitter is common or reference to both input and output terminals. - emitter is usually the terminal closest to or at ground potential. Almost amplifier design is using connection of CE due to the high gain for current and voltage. Two set of characteristics are necessary to describe the behavior for CE ;input (base terminal) and output (collector terminal) parameters. IB is microamperes compared to miliamperes of IC. IB will flow when VBE > 0.7V for silicon and 0.3V for germanium Before this value IB is very small and no IB. Base-emitter junction is forward bias Increasing VCE will reduce IB for different values. Input characteristics for a common-emitter NPN transistor Output characteristics for a common-emitter npn transistor For small VCE (VCE < VCESAT, IC increase linearly with increasing of VCE VCE > VCESAT IC not totally depends on VCE constant IC IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC IB=0 A ICEO occur. Noticing the value when IC=0A. There is still some value of current flows. BETA () OR AMPLIFICATION FACTOR The ratio of dc collector current (IC) to the dc base current (IB) is dc beta (dc ) which is dc current gain where IC and IB are determined at a particular operating point, Q-point (quiescent point). It’s define by the following equation: 30 < dc < 300 2N3904 On data sheet, dc=hFE with h is derived from ac hybrid equivalent cct. FE are derived from forward-current amplification and common-emitter configuration respectively. For ac conditions an ac beta has been defined as the changes of collector current (IC) compared to the changes of base current (IB) where IC and IB are determined at operating point. On data sheet, ac=hfe It can defined by the following equation: EXAMPLE From output characteristics of common emitter configuration, find ac and dc with an Operating point at IB=25 A and VCE =7.5V. Solution: RELATIONSHIP ANALYSIS BETWEEN Α AND Β COMMON – COLLECTOR CONFIGURATION Also called emitter-follower (EF). It is called common-emitter configuration since both the signal source and the load share the collector terminal as a common connection point. The output voltage is obtained at emitter terminal. The input characteristic of common-collector configuration is similar with common-emitter. configuration. Common-collector circuit configuration is provided with the load resistor connected from emitter to ground. It is used primarily for impedance-matching purpose since it has high input impedance and low output impedance. Notation and symbols used with the common-collector configuration: (a) pnp transistor ; (b) npn transistor. Transistor as a Switch • Transistors can be used as switches.1 Transistor Switch •Transistors can either conduct or not conduct current.2 •ie, transistors can either be on or off.2 A TRANSISTOR SWITCH In many electronic circuits a small voltage or current is used to switch a much larger voltage or current. Transistors work very well for this application because they behave like switches that can be turned on and off electronically instead of using manual or mechanical action. A TRANSISTOR SWITCH The resistance difference between “on” and “off” for a transistor switch is good enough for many useful circuits such as an indicator light bulb in a mechanical circuit. BJT OPERATION REGIONS Operation IB or VCE BC and BE Mode Region Char. Junctions Cutoff IB = Very Reverse & Open Switch small Reverse Saturation VCE = Small Forward & Closed Switch Forward Active VCE = Reverse & Linear Linear Moderate Forward Amplifier Break-down VCE = Large Beyond Overload Limits LIMITS OF OPERATION Many BJT transistor used as an amplifier. Thus it is important to notice the limits of operations. At least 3 maximum values is mentioned in data sheet. There are: a) Maximum power dissipation at collector: PCmax or PD b) Maximum collector-emitter voltage: VCEmax sometimes named as VBR(CEO) or VCEO. c) Maximum collector current: ICmax There are few rules that need to be followed for BJT transistor used as an amplifier. The rules are: i) transistor need to be operate in active region! ii) IC < ICmax ii) PC < PCmax Note: VCE is at maximum and IC is at minimum (ICmax=ICEO) in the cutoff region. IC is at maximum and VCE is at minimum (VCE max = VCEsat = VCEO) in the saturation region. The transistor operates in the active region between saturation and cutoff. Refer to the fig. Step1: The maximum collector power dissipation, PD=ICmax x VCEmax (1) = 18m x 20 = 360 mW Step 2: At any point on the characteristics the product of and must be equal to 360 mW. Ex. 1. If choose ICmax= 5 mA, subtitute into the (1), we get VCEmaxICmax= 360 mW VCEmax(5 m)=360/5=7.2 V Ex.2. If choose VCEmax=18 V, subtitute into (1), we get VCEmaxICmax= 360 mW (10) ICmax=360m/18=20 mA TRANSISTOR SPECIFICATION SHEET TRANSISTOR TERMINAL IDENTIFICATION TRANSISTOR TESTING 1. Curve Tracer Provides a graph of the characteristic curves. 2. DMM Some DMM’s will measure DC or HFE. 3. Ohmmeter TRANSISTOR AS AN AMPLIFIER TRANSISTOR AMPLIFIER BASICS It is critical to understand the notation used for voltages and currents in the following discussion of transistor amplifiers. This is therefore dealt with explicitly ‘up front’. As with dynamic resistance in diodes we will be dealing with a.c. signals superimposed on d.c. bias levels. TRANSISTOR AMPLIFIER BASICS We will use a capital (upper case) letter for a d.c. quantity (e.g. I, V). We will use a lower case letter for a time varying (a.c.) quantity (e.g. i, v) TRANSISTOR AMPLIFIER BASICS These primary quantities will also need a subscript identifier (e.g. is it the base current or the collector current?). For d.c. levels this subscript will be in upper case. We will use a lower case subscript for the a.c. signal bit (e.g. ib). And an upper case subscript for the total time varying signal (i.e. the a.c. signal bit plus the d.c. bias) (e.g. iB).This will be less common. TRANSISTOR AMPLIFIER BASICS ib 0 + IB = iB TRANSISTOR AMPLIFIER BASICS It is convention to refer all transistor voltages to the ‘common’ terminal. Thus in the CE configuration we would write VCE for a d.c. collector emitter voltage and VBE for a d.c. base emitter voltage. A TRANSISTOR AMPLIFIER One of the most important uses of a transistor is to amplify a signal. In electronics, the word “amplify” means to make the voltage or current of the input signal larger without changing the shape of the signal. Transistor as an amplifier: Transistors are often used as amplifiers to increase input signal in radios, televisions and some other applications .The circuit may be designed to increase the current or voltage level. The power gain is the product of current gain and voltage gain (P=V*I). A TRANSISTOR AMPLIFIER In an amplifier circuit, the transistor is not switched fully “on” like it is in a switching circuit. Instead, the transistor operates partially on and its resistance varies between a few hundred ohms and about 10,000 ohms, depending on the specific transistor. DC BIASING CIRCUITS The ac operation of an +VCC amplifier depends on the initial dc values of IB, IC, and VCE. RC By varying IB around an initial RB dc value, IC and VCE are v out made to vary around their initial dc values. DC biasing is a static operation since it deals v in vce ib with setting a fixed (steady) level of current ic (through the device) with a desired fixed voltage drop across the device.
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