Bipolar Junction Transistors 1
Introduction physical structure of the bipolar transistor and how it works How the voltage between two terminals of the transistor controls the current that flows through the third terminal The equations that describe these current-voltage characteristics How to analyze and design circuits that contain bipolar transistors, resistors, and dc sources 2
Introduction Three-terminal device Multitude of applications Signal amplification/digital logic/memory circuit/switch Voltage between two terminals to control the current flowing in third terminal Bipolar junction transistor (BJT) Metal-oxide-semiconductor field-effect transistor (MOSFET) BJT was invented in 1948 at Bell Telephone Laboratories Ushered in a new era of solid-state circuits It was replaced by MOSFET as predominant transistor used in modern electronics. BiCMOS 3
4.1 Device Structure and Physical Operation Simplified structure of BJT Consists of three semiconductor regions: Emitter region (n-type) Base region (p-type) Collector region (n-type) Type described above is referred to as npn Dual of npn is pnp transistor Three terminals: emitter(e), base(b), collector(c) 4
4.1.1 Simplified Structure/Operation Modes Transistor consists of two pn-junctions: Emitter-base junction (EBJ) Collector-base junction (CBJ) Operating mode depends on bias condition Active mode used for amplification Cutoff and saturation modes used for switching application (logic circuits) Bipolar(electron and hole) participate in conduction 5
4.1.2 npn-transistor in the Active Mode Active mode is most important Two external voltage sources are required for biasing to achieve it Refer to Figure 4.3 Figure 4.3: Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.) 6
Current Flow Forward bias on emitter-base junction will cause current to flow This current has two components: Electrons injected from emitter into base Holes injected from base into emitter It will be shown that first (of the two above) is desirable This is achieved with heavy doping of emitter, light doping of base 7
Current Flow Emitter current (i E ) : current which flows across EBJ Flows out of emitter lead Dominate by electron components i E e v BE V T Minority carriers in p-type region These electrons will be injected from emitter into base Small proportion of recombination process Reach most of diffusing electrons to the boundary of the collector-base depletion region Because base is thin, concentration of excess minority carriers within it will exhibit constant gradient 8
Collector Current Reach most of diffusing electrons to the boundary of the collectorbase depletion region Opposite direction to that of the flow of electrons i C = I S e v BE V T I S : constant of proportionality (saturation current) Inversely proportional to W and directly proportional to area of EBJ Typically between 10-12 and 10-18 A Also referred to as scale current i C is independent of the value of v CB As long as collector is positive, with respect to base 9
Base Current Base current consists of i B = i B1 + i B2 i B1 : due to holes injected from base region into emitter i B2 : due to holes that have to be supplied by external circuit to replace those recombined v Each current will be proportional to e BE V T 10
Base Current Common-emitter current gain (β) Is influenced by two factors: Width of base region (W) Relative doping of base / emitter regions (N A /N D ) High Value of β (50~200, >1000) Thin base (small W in nano-meters) Lightly doped base / heavily doped emitter (small N A /N D ) transistor parameter (eq4.2) (eq6.5) i B S vbe / VT (eq6.6) ib e i I C (eq4.3) 11
Emitter Current All current which enters transistor must leave i E = i C + i B Equations (4.7) through (4.13) expand upon this idea α : common-base current gain (less than but very close to unity) this expression is generated through combination of (6.5) (4.5) and (6.7) (4.7) 12 C 1 1 vbe / VT E C Se (eq4.8/4.9) (eq6.8/6.9) i i I (eq4.10) (eq6.10) i i E this parameter is reffered to as common-base current gain (eq4.11) (eq6.11),(eq4.13) (eq6.13) 1 1 (eq4.12) (eq6.1 i E IS e v BE / V T i C
The Emitter Current All current which enters transistor must leave i E = i C + i B Equations (4.7) through (4.10) expand upon this idea α : common-base current gain (less than but very close to unity) Small change in α correspond to very large changes in β this expression is generated through combination of (6.5) (4.2) and (6.7) (4.4) 13 C 1 1 vbe / VT E C Se (eq6.8/6.9) (eq4.5) i i I (eq4.7) (eq6.10) i i E this parameter is reffered to as common-base current gain (eq4.8) (eq6.11),(eq4.10) (eq6.13) 1 1 (eq4.9) (eq6.12) i E IS e v BE / V T i C
n p n ( x) concentration of minority carriers a position x (where 0 represents EBJ boundary) n thermal-equilibrium value of minority carrier (electron) concentration in base regionn vbe voltage applied across base-emitter junctionnp 0 V thermal voltage (constant) n T vbe (eq6.1) 0 n e n p / V T Figure 4.4 Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 14
Minority-Carrier Distribution Concentration of minority carrier n p at boundary EBJ is defined by (4.11) Concentration of minority carriers n p at boundary of CBJ is zero Positive v CB causes these electrons to be swept across junction n p n v BE ( x) concentration of minority carriers a position x (where 0 represents EBJ boundary) np 0 thermal-equilibrium value of minority carrier (electron) concentration in base regionnp 0 voltage applied across base-emitter junctionn V thermal voltage (constant) n T (eq6.1) 0 (eq4.11) n p n e v BE / V T 15
Minority-Carrier Distribution Tapered minority-carrier concentration profile exists It causes electrons injected into base to diffuse through base toward collector As such, electron diffusion current (I n ) exists A E cross-sectiona area of the base-emitter junction q magnitude of the electron charge D n electron diffusivity in base W width of base (eq4.12) q6.2) I (eq6.2) I n E n n A qd dn p x dx dn 0 p AEqDn W this simplification may be made if gradient assumed to be straight line 16
Current Flow Some diffusing electrons will combine with holes (majority carriers in base) Since base is very thin and lightly doped, recombination is minimal Recombination does, however, cause gradient to take slightly curved shape The straight line is assumed 17
n p n ( x) concentration of minority carriers a position x (where 0 represents EBJ boundary) n thermal-equilibrium value of minority carrier (electron) concentration in base regionn vbe voltage applied across base-emitter junctionnp 0 V thermal voltage (constant) n T vbe (eq4.11) (eq6.1) 0 n e n p / V T Recombination causes actual gradient to be curved, not straight. Figure 4.4 Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 18
Collector Current It is observed that most diffusing electrons will reach boundary of collector-base depletion region Because collector is more positive than base, these electrons are swept into collector Collector current (i C ) is approximately equal to I n i C = I n (eq6.3) (eq4.13) (eq6.4) I i C S BE / V 2 E n i ni intrinsic carrier density NA doping concentration of base I S v AE qdnn saturation current: IS W e T A qd n W N A 19
Recapitulation / Equivalent-Circuit Models Present first-order BJT model Assumes npn transistor in active mode Basic relationship is collector current (i C ) is related exponentially to forward-bias voltage (v BE ) It remains independent of v CB as long as this junction remains reverse biased v CB > 0 i B is much smaller than i C Nonlinear voltage-controlled current source 20
Common base model Common emitter model Figure 4.5: Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode. 21