Chapter 5: Bipolar Junction Transistor''s Goals is Explore the physical structure of bipolar transistor, Study terminal characteristics of BJT, Explore differences between npn and pnp transistors, Develop the Transport Model for bipolar devices.
Chapter 5 Bipolar Junction Transistors Chapter Goals • • • • • • Explore the physical structure of bipolar transistor Study terminal characteristics of BJT Explore differences between npn and pnp transistors Develop the Transport Model for bipolar devices Define four operation regions of the BJT Explore model simplifications for the forward active region • Understand the origin and modeling of the Early effect • Present a PSPICE model for the bipolar transistor. Discuss bipolar current sources and the current mirror Physical Structure • • • The BJT consists of 3 alternating layers of n and ptype semiconductor called emitter (E), base (B) and collector (C) The majority of current enters collector, crosses the base region and exits through the emitter. A small current also enters the base terminal, crosses the base emitter junction and exits through the emitter Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates the iv characteristics of the BJT Transport Model for the npn Transistor • • • The narrow width of the base region causes a coupling between the two back to back pn junctions The emitter injects electrons into base region; almost all of them travel across narrow base and are removed by collector • • Baseemitter voltage vBE and basecollector voltage vBC determine the currents in the transistor and are said to be positive when they forward bias their respective pn junctions The terminal currents are the collector current(iC ), the base current (iB) and the emitter current (iE) The primary difference between the BJT and the FET is that iB is significant, while iG = 0 npn Transistor: Forward Characteristics Base current is given by i B i 20 I F F F v S exp BE V T F 500 is forward current gain Emitter current is given by Forward transport current is i C i F v I exp BE S V T IS is saturation current 10 18A I S 10 A i E i i C B 0.95 F I v S exp BE V T F F F 1 1.0 In this forward active operation region, VT = kT/q =0.025 V at room temperature i C i B F i C i E F npn Transistor: Reverse Characteristics 20 is reverse current gain R Base currents in forward and reverse modes are different due to asymmetric doping levels in the emitter and collector regions Emitter current is given by Reverse transport current is i R i E v I exp BC S V T i C Base current is given by i B i R R I v S exp BC V T R I R v S exp BC V T R R R 1 0.95 npn Transistor: Complete Transport Model Equations for Any Bias i C v I exp BE S V T v exp BC V T I i E v I exp BE S V T v exp BC V T I i B I S exp vBE V T F I S exp R S exp R S exp F v BC V T v BC V T v BE V T 1 The first term in both the emitter and collector current expressions gives the current transported completely across the base region Symmetry exists between baseemitter and basecollector voltages in establishing the dominant current in the bipolar transistor pnp Transistor: Operation • The voltages vEB and vCB are positive when they forward bias their respective pn junctions • Collector current and base current exit the transistor terminals and emitter current enters the device. pnp Transistor: Forward Characteristics Base current is given by: i B i F I F v S exp EB V T F Emitter current is given by: Forward transport current is: i C i F I v exp EB S V T i E i i C B I S v exp EB V T F 1 pnp Transistor: Reverse Characteristics Base current is given by: i B i I F R v S exp CB V T R Emitter current is given by: Reverse transport current is: i R i E I v exp CB S V T i C I S v exp CB V T R 1 TwoResistor Bias Network for BJT: Example • Problem: Find the Qpoint for the pnp transistor in the 2resistor bias circuit shown below. • Given data: F = 50, VCC = 9 V • Assumptions: Forwardactive region operation with VEB = 0.7 V • Analysis: V 18,000I 1000(I I ) EB B C B V 18,000I 1000(51)I EB B B 9V 0.7V 120 A B 69,000 I 50I 6.01mA C B I V EC 1000(I I ) 2.87V C B Qpoint is : (6.01 mA, 2.87 V) PNP Transistor Switch Circuit Design Emitter Current for PNP Switch Design BJT PSPICE Model • Besides the capacitances which are associated with the physical structure, additional model components are: diode current iS, capacitance CJS, related to the large area pn junction that isolates the collector from the substrate and one transistor from the next • RB is the resistance between external base contact and intrinsic base region • Collector current must pass through RC on its way to the active region of the collectorbase junction • RE models any extrinsic emitter resistance in the device BJT PSPICE Model Typical Values Saturation Current = 3 e17 A Forward current gain = 100 Reverse current gain = 0.5 Forward Early voltage = 75 V Base resistance = 250 Collector Resistance = 50 Emitter Resistance = 1 Forward transit time = 0.15 ns Reverse transit time = 15 ns Minority Carrier Transport in Base Region • With a narrow base region, minority carrier density decreases linearly across the base, and the Saturation Current (NPN) is: I S n qADn bo W B qADn n i N W AB B where NAB = the doping concentration in the base ni2 = the intrinsic carrier concentration (1010/cm3) nbo = ni2 / NAB Dn = the diffusivity = (kT/q) n I S p qAD p bo W B qAD p n i N W DB B • Saturation current for the PNP transistor is: • Due to the higher mobility ( ) of electrons compared to holes, the npn transistor conducts higher current than the pnp for equivalent doping and applied voltages Diffusion Capacitance • • For vBE and hence iC to change, charge stored in the base region must also change Diffusion capacitance in parallel with the forwardbiased baseemitter diode produces a good model for the change in charge with vBE C D • dQ dv BE Q v qAnboWB exp BE V V T T po int I T V F T Since transport current normally represents collector current in the forwardactive region, I C C D V F T Early Effect and Early Voltage • • • • As reversebias across the collectorbase junction increases, the width of the collectorbase depletion layer increases and the effective width of base decreases. This is called “basewidth modulation” In a practical BJT, the output characteristics have a positive slope in the forwardactive region, so that collector current is not independent of vCE “Early” effect: When the output characteristics are extrapolated back to where the iC curves intersect at common point, vCE = VA (Early voltage), which lies between 15 V and 150 V Simplified F.A.R. equations, which include the Early effect, are: F v CE FO V A i B S exp FO v i C v I exp BE CE S V V T A I I F B v BE V T BJT Current Mirror • • • The collector terminal of a BJT in the forwardactive region mimics the behavior of a current source Output current is independent of VCC as long as VCC ≥ 0.8 V. This puts the BJT in the forwardactive region, since VBC ≤ 0.1 V Q1 and Q2 are assumed to be a “matched” pair with identical IS, FO, and VA, I REF V V BB BE R I I I C1 B1 B2 BJT Current Mirror (continued) V I exp BE S V T V CE1 V A I V I exp BE REF V T FO V CE V V V CE BE A I I exp I REF C2 S V V V BE T A V V A FO CE2 I V O A MR is the "Mirror Ratio" V I BE REF V A FO S With an infinite FO and VA (ideal device), the mirror ratio is unity. Finite current gain and Early voltage introduce a mismatch between the output and reference currents of the mirror BJT Current Mirror: Example • • Problem: Find output current for given current mirror Given data: FO = 75, VA = 50 V • Assumptions: Forwardactive operation region, VBE = 0.7 V • Analysis: I REF I O V V BB BE R MR I REF 12V 0.7V 202 A 56k 12 75 (202 A) 223 A 0.7 75 50 VBE = 6.7333e01 IC2 = 5.3317e04 IC21 = 5.3317e04 BJT Current Mirror: Altering the Mirror Ratio I A E I SO A where ISO is the saturation current of a BJT with one unit of emitter area: AE =1(A). The actual dimensions of A are technology dependent The Mirror Ratio of a BJT current mirror can be changed by simply changing the relative sizes of the emitters in the transistors. For the “ideal” case, the Mirror Ratio is determined only by the ratio of the two emitter areas S I O V CE V A n.I REF V BE V A FO n A E2 A E1 BJT Current Mirror: Output Resistance • A current source using BJTs doesn’t have an output current that is completely independent of the terminal voltage across it, due to the finite value of Early voltage. The current source seems to have a resistive component in series with it V v CE2 ce2 V A i i I O C2 REF V BE V A FO Ro • io vo Q pt I V vo CE V A I REF V BE V A FO C2 V V A CE V A I O Ro is defined as the “small signal” output resistance of the current mirror .. .Chapter? ?Goals • • • • • • Explore the physical structure of? ?bipolar? ?transistor Study terminal characteristics of BJT Explore differences between npn and pnp? ?transistors Develop the Transport Model for? ?bipolar? ?devices... vBE V T F I v S exp BC V T R Operation Regions of the? ?Bipolar? ? Transistor Baseemitter? ?junction Forward Bias Reverse Bias Basecollector? ?junction Forward Bias Reverse Bias Forward active region... characteristic of a pn? ?junction? ?diode Setting vBC =0 in the collectorcurrent expression: i C v I exp BE S V T Junction? ?Breakdown Voltages • • • • If reverse voltage across either of the two pn junctions in the transistor