EECS 6.012 Spring 1998Lecture 16 I. The Bipolar Junction TransistorA. Physical Structure: • oxide-isolated, low-voltage, high-frequency design• typical of the bipolar transistor found in a BiCMOS process,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,n+n+n+n+n+ polysilicon contactto emitter regionmetal contact to basep+p+np-type basen+ buried layerp-type substratemetal contactto collectorfield oxiden+ buried layer- p - n sandwich,, pedge of n+ buried layerfieldoxide(emitter)(collector)(base)AA'AA'(a)(b)n+ emitter area, AE(intrinsic npn transistor)(intrinsic npn transistor),,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, EECS 6.012 Spring 1998Lecture 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 typepnp npn IC positiveIB positive -IE positiveNormal operation:Normal operation:–IC positive–IB positiveIE positiveVCE positiveVBE = 0.7VEB = 0.7VEC positive+−+−−ICIBVBE−IECBE(a)VCE+−+IE−IBVEB−ICEBCVEC(b)EECS 6.012 Spring 1998Lecture 16 C. npn BJT Collector Characteristics • Similar test circuit as for n-channel MOSFET ... except I B is controlled instead of V BEVCEIC = IC(IB, VCE) (a)150100150200250300IC(µA)VCE (V) 6(b)−1−2−3−4−8IB = 1 µA IB = 2 µA (reverseactive)(forward active)(saturation)IB = 0 (cutoff)IB = 500 nA IB = 1 µA IB = 1.5 µA IB = 2 µA IB = 2.5 µA IB+−5432EECS 6.012 Spring 1998Lecture 16 D. Regions of Operation • Constant-current region is called forward active ... corresponds to MOSFET saturation region (!!!) I C = β F I B • Bipolar saturation region (modeled as a constant voltage) corresponds to MOSFET triode region • Cutoff ... corresponds to MOSFET cutoff region• Reverse active ... terminal voltages for npn sandwich are flipped so that V CE is negative and V BC = 0.7 V. Only occasionally useful.• Boundary between saturation and forward-active regions:VCEVCE sat()≈ 0.1V=VCEVCE sat()> and IB0>EECS 6.012 Spring 1998Lecture 16 II. Bipolar Transistor PhysicsA. Forward Active Region of OperationB. Game Plan • Understand thermal equilibrium potential and carrier concentrations.• Apply the Law of the Junction with V BE ≈ 0.7 V and V BC < 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.0xn+ emittern+ polysiliconp-type basen-type collectorn+ buried layerbase-emitterdepletion regionbase-collectordepletion regionoutline of “core”n+pn sandwichEECS 6.012 Spring 1998Lecture 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:• Electrostatic potential:,, (c)xWBoWBo + xBCo 0−WEo − xBEo pnE(x)(log)npB(x)(log)pnC(x)(log)pnCo = 104 cm−3pnEo = 10 cm−3npBo = 103 cm−3emitter base collectorn+ poly-siliconcontact− xBEo WBoWBo + xBCo 0−WEo − xBEo xφo(x) (mV)250500−250−500 n+ emitter:p base:φo = 550 mV n collector:φo = 360 mV φo = −420 mV −xBEoEECS 6.012 Spring 1998Lecture 16 D. Carrier Concentrations under Forward Active Bias • Boundary conditions at the edges of the depletion region are: Emitter-Base: exp[ V BE / V th ] >> 0......Base-Collector: exp[ V BC / V th ] = 0 • Ohmic contacts return carrier concentration to equilibrium ,, (b)x0pnE(x)pnE(−xBE)pnEonpB(x)npB(0)npBopnC(x)baseemittern+ poly-siliconcontactcollectorWBWB + xBC −WE − xBE 0xφ(x)(V)0.51−0.5(a)1.52VCE = 2 VVBE = 0.7 V2.5thermalequilibrium−xBEnpB(WB)pnC(WB+ xBC)pnCo−WE − xBEWB WB + xBC −xBEEECS 6.012 Spring 1998Lecture 16 E. The Flux Picture - Forward Active Bias • Rather than current densities, we use the concept of flux [# per cm 2 per second] • The width of the electron flux “stream” is greater than the hole flux stream.• 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 ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, majority hole flux from base contact n+ buried layern-type collector p-type base n+ emittern+ polysiliconhole diffusion flux(b)majority electron flux to coll.contact (minimum resistance pathis through the n+ buried layer)electrondiffusion majorityelectronsEECS 6.012 Spring 1998Lecture 16 III. Forward-Active Terminal Currents A. Collector current: • Electron diffusion current density X emitter areaxWB0- xBEWB + xBC- WE - xBEpnC(x)npB(x)pnE(x)(emitter)(base) (collector)polysiliconcontact,, xWBWB + xBC0−WE − xBE pnE(x)npB(x)pnC(x)emitterbasepolysiliconcontactcollector−xBE JnBdiffqDnnpBWB()npB0()–WB-----------------------------------------------qDnnpBoeVBCVth⁄eVBEVth⁄–WB------------------------------------------------------------------------------------==ICJ–nBdiffAEqDnnpBoAEWB-------------------------------eVBEVth⁄==EECS 6.012 Spring 1998Lecture 16B. Base current• Reverse-injected hole diffusion current density X emitter areaC. Emitter current • Sum of IB and IC according to KCL (negative ... reference is positive-in)JpEdiffqDppnEoWE-------------------------e(VBEVth⁄1 )––=IBJ–pEdiffAEqDppnEoAEWE-------------------------------eVBEVth⁄1–==IEJnBdiffJpEdiff+AEqDppnEoAEWE----------------------------------qDnnpBoAEWB---------------------------------+ eVBEVth⁄–==EECS 6.012 Spring 1998Lecture 16IV. Forward-Active Current GainsA. Alpha-F - αF• The ratio of collector current to the magnitude of the emitter current• αF --> 1 ... typically, αF = 0.99.B. Beta Forward Current Gains - βF• The ratio of collector current to base current .• A typical value is βF = 100
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