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I. the Bipolar Junction Transistor A. Physical Structure

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I. the Bipolar Junction Transistor A. Physical Structure I. The Bipolar Junction Transistor A. Physical Structure: • oxide-isolated, low-voltage, high-frequency design • typical of the bipolar transistor found in a BiCMOS process, n+ polysilicon contact metal contact to base to n+ emitter region metal contact to collector p-type base + + A p n n A' n+ buried layer n+ buried layer p-type substrate field oxide n+ - p - n sandwich (intrinsic npn transistor) (a) (emitter) (base) edge of n+ buried layer field oxide A A' + p p n+ + n emitter area, AE (intrinsic npn transistor) (collector) (b) EECS 6.012 Spring 1998 Lecture 16 B. Circuit Symbol and Terminal Characteristics • Two devices that have complementary characterisitics npn transistor and the pnp transistor • The direction of the diode arrow indicates device type npn pnp C E IC IE + + + VEB B V B − V + CE EC − IB VBE − IB − − −I −I E E C C (a) (b) Normal operation: Normal operation: I positive V positive C CE –IC positive VEC positive I positive V = 0.7 B BE –IB positive VEB = 0.7 -IE positive IE positive EECS 6.012 Spring 1998 Lecture 16 C. npn BJT Collector Characteristics • Similar test circuit as for n-channel MOSFET ... except IB is controlled instead of VBE = IC IC(IB, VCE) + V − CE IB (a) I C = µ (µA) IB 2.5 A 300 = µ IB 2 A 250 200 I = 1.5 µA (saturation) B 150 = µ (forward active) IB 1 A 100 = IB 500 nA 50 I = 0 (cutoff) −3 −2 −1 B 1 2 3 4 5 6 VCE (V) = µ −4 IB 1 A (reverse active) −8 = µ IB 2 A (b) EECS 6.012 Spring 1998 Lecture 16 D. Regions of Operation • Constant-current region is called forward active ... corresponds to MOSFET saturation region (!!!) β IC = F IB • Bipolar saturation region (modeled as a constant voltage) corresponds to MOSFET triode region ≈ VCE VCE() sat = 0.1V • Cutoff ... corresponds to MOSFET cutoff region • Reverse active ... terminal voltages for npn sandwich are flipped so that VCE is negative and VBC = 0.7 V. Only occasionally useful. • Boundary between saturation and forward-active regions: > > VCE VCE() sat and IB 0 EECS 6.012 Spring 1998 Lecture 16 II. Bipolar Transistor Physics A. Forward Active Region of Operation n+ polysilicon base-emitter depletion region n+ emitter p-type base 0 n-type collector x n+ buried layer base-collector outline of “core” depletion region n+pn sandwich B. Game Plan • Understand thermal equilibrium potential and carrier concentrations. ≈ • Apply the Law of the Junction with VBE 0.7 V and VBC < 0 (typical forward-active bias point) to find the minority carrier concentrations at the depletion region edges. • Assume that the emitter and the base regions are “short” (no recombination) and find the diffusion currents. EECS 6.012 Spring 1998 Lecture 16 C. Thermal Equilibrium • Typically emitter is doped two orders of magnitude (at least) more heavily than the base; the collector is an order of magnitude more lightly doped than the base. • Minority carrier concentrations: pnE(x) npB(x) pnC(x) (log) (log) (log) emitter base collector n+ poly- silicon p = 104 cm−3 contact nCo = 3 −3 npBo 10 cm = −3 pnEo 10 cm − − − + x WEo xBEo xBEo 0 WBo WBo xBCo (c) • Electrostatic potential: φ (x) (mV) n+ emitter: o φ = 550 mV 500 o n collector: φ = 360 mV 250 o − − − + x WEo xBEo xBEo 0 WBo WBo xBCo −250 p base: − φ = − 500 o 420 mV EECS 6.012 Spring 1998 Lecture 16 D. Carrier Concentrations under Forward Active Bias • Boundary conditions at the edges of the depletion region are: Emitter-Base: exp[VBE / Vth] >> 0......Base-Collector: exp[VBC / Vth] = 0 • Ohmic contacts return carrier concentration to equilibrium φ(x)(V) 2.5 2 1.5 = VCE 2 V 1 thermal equilibrium 0.5 0 − − − + WE xBE xBE = WB WB xBC x VBE 0.7 V −0.5 (a) pnE(x) npB(x) pnC(x) npB(0) emitter base collector n+ poly- silicon − contact pnE( xBE) pnCo npBo npB(WB) p (W + x ) pnEo nC B BC − − − + x WE xBE xBE 0 WB WB xBC (b) EECS 6.012 Spring 1998 Lecture 16 E. The Flux Picture - Forward Active Bias • Rather than current densities, we use the concept of flux [# per cm2 per second] • The width of the electron flux “stream” is greater than the hole flux stream. n+ polysilicon hole diffusion flux n+ emitter majority electrons majority hole flux from base contact electron p-type base diffusion n-type collector n+ buried layer majority electron flux to coll. contact (minimum resistance path is through the n+ buried layer) (b) • The electrons are supplied by the emitter contact and diffuse across the base • Electric field in the collector depletion region sweeps electrons into the collector • n+ buried layer provides a low resistance path to the collector contact • The holes are supplied by the base contact and diffuse across the emitter • The reverse injected holes are recombined at the polysilicon ohmic contact EECS 6.012 Spring 1998 Lecture 16 III. Forward-Active Terminal Currents p (x) pnE(x)pnE(x) npBn(pBx)(x) pnCnC(x) polysiliconpolysilicon (emitter)emitter (base)base (collector)collector contactcontact - W−W - x − x - x −x 0 W W + x x E E BE BE BE BE0 WB B WB +B xBC BC x A. Collector current: • Electron diffusion current density X emitter area V ⁄ V V ⁄ V BC th BE th n e – e n ()W –n()0 pBo diff pB B pB J ==qD ----------------------------------------------- qD ------------------------------------------------------------------------------------ nB nW n W B B V⁄ V diff qDnnpBoAE BE th I==–JA ------------------------------- e CnB E WB EECS 6.012 Spring 1998 Lecture 16 B. Base current • Reverse-injected hole diffusion current density X emitter area qD p V⁄ V diff p nEo BE th J = –------------------------- (e– 1 ) pE W E V⁄ V diff qDppnEoAE BE th I ==–J A ------------------------------- e– 1 B pE E WE C. Emitter current • Sum of IB and IC according to KCL (negative ... reference is positive-in) qD p A qD n A V⁄ V diff diff p nEo E n pBo E BE th I==J+ J A– ---------------------------------- +--------------------------------- e EnB pE EW W E B EECS 6.012 Spring 1998 Lecture 16 IV. Forward-Active Current Gains α A. Alpha-F - F • The ratio of collector current to the magnitude of the emitter current qD n A ------------------------------n pBo E- I W --------C ==-----------------------------------------------------------------------------------B α F IE qDppnEoAE qDnnpBoAE ------------------------------- +------------------------------- WE WB 1 α= ---------------------------------------------- FDNW paB B 1+----------------------------- D N W n dE E α α • F --> 1 ... typically, F = 0.99. β B. Beta Forward Current Gains - F • The ratio of collector current to base current α I 1 – F I ===– I – I ------C- – I I ---------------- B E C α C C α F F α F I==---------------- IβI. CαBFB 1–F β • A typical value is F = 100 EECS 6.012 Spring 1998 Lecture 16.
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