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Plans. How do the computed BJT I-Vs compare with expts? Can we understand the discrepancies? What does the gain look like? AC properties (small signal and transient response). Common Base. Common Emitter. BJT – Real Characteristics. What’s wrong with these pictures? Common Base:

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  • How do the computed BJT I-Vs compare with expts?

  • Can we understand the discrepancies?

  • What does the gain look like?

  • AC properties (small signal and transient response)

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Common Base

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Common Emitter

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BJT – Real Characteristics

  • What’s wrong with these pictures?

  • Common Base:

    • Input characteristic shows VCB dependence

    • Output shows breakdown at VCB0

  • Common Emitter

    • Input characteristic pretty good agreement

    • Output characteristic:

      • Upward slope in IC – quasilinear VEC dependence

      • Breakdown at VCE0

      • Upturn prior to breakdown

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Base Width Modulation: “Early” Effect

  • Base width has been assumed to be constant

  • When bias voltages change, depletion widths change and the effective base width will be a function of the bias voltages

  • Most of the effect comes from the C-B junction since the bias on the collector is usually larger than that on the E-B junction

Base width gets smaller as applied voltages get larger

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Early Effect: Common Base Input Characteristic


  • Exponential prefactor will increase as VCB increases (W decreases)

Assuming –VCB > few kT/q and W/LB << 1

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Early Effect: Common Emitter Output Characteristic

  • If NC << NB most of the depletion is in the collector and modulation of base width is minimized – reduced Early Effect

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Early Voltage

J M Early

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Avalanche Multiplication Breakdown

  • Common Base: Similar to single p-n junction VCB0  VBD(B-C)

  • Common Emitter: more complicated

    • holes injected by FB emitter to base

    • holes generate e-p pairs in C-B depletion

    • e- drift back into base

    • e- injected to emitter

    • more holes into base…..

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Avalanche Breakdown: Common Emitter

  • IC when M1/dc

  • M only needs to be slightly greater than unity

  • VCEO<VCB0 – Breakdown voltage is lower for common Emitter mode than common Base mode or p-n breakdown voltage due to amplification effect within the transistor

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W/base width mod

Early Effect

W/base width mod & avalanche


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How can we mitigate these effects?

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Graded Base

  • Implant or diffusion leads to doping profile

  • Doping profile leads to E field

  • If Emitter is on top layer – E field acts to push carriers toward the collector

  • Improved speed if limited by base transport time

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Si-Ge HBT’s for BiCMOS

  • Dilemma for bipolar transistors:

    • For high frequency operation want low base resistance – high base doping

    • For high current gain want to minimize hole injection into emitter (npn) – low base doping

  • Solution HBT – heterojunction bipolar transistors

  • For CMOS integration use Si1-x Gex system

    • Bandgap difference (1.12 eV Si, 1.0 eV, Si0.8Ge0.2)

    • 80% EG in VB

    • 0.1 eV additional barrier for holes to emitter

    • Higher base doping w/same gain

  • Selective growth of pseudomorphic Ge on Si substrate

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Si-Ge HBT’s for BiCMOS

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Bandgaps and alignments

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Si-Ge Heterostructure

  • Most of the bandgap difference shows up in the valence band

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Band Diagram for SiGe HBT

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Si-Ge HBT’s

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Si-Ge HBT’s


bdc = DBLE(ni2/NB)/DEW(ni2/NE)

bHBTdc = bdc(nSii)2/(nGei) 2

= bdc e(EGeG-ESiG)/2kT

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Gain Plots

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Gummel Plot


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BJT Small Signal Response

  • Assume the transistor can follow AC voltages and currents quasistatically (frequency not too high). Also neglect capacitances of pn junctions and other parasitics

Common Emitter equivalent circuit model

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BJT Small Signal Response

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BJT Transient Behavior

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