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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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Presentation Transcript
Plans l.jpg
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)

ECE 663


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Common Base

ECE 663



Bjt real characteristics l.jpg
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

ECE 663


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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

ECE 663


Early effect common base input characteristic l.jpg
Early Effect: Common Base Input Characteristic

Ebers-Moll

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

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

ECE 663


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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

ECE 663


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Early Voltage

J M Early

ECE 663


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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…..

ECE 663


Avalanche breakdown common emitter l.jpg
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

ECE 663


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Ideal

W/base width mod

Early Effect

W/base width mod & avalanche

multiplication

ECE 663



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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

ECE 663


Si ge hbt s for bicmos l.jpg
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

ECE 663


Si ge hbt s for bicmos15 l.jpg
Si-Ge HBT’s for BiCMOS

ECE 663



Si ge heterostructure l.jpg
Si-Ge Heterostructure

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

ECE 663


Band diagram for sige hbt l.jpg
Band Diagram for SiGe HBT

ECE 663


Si ge hbt s l.jpg
Si-Ge HBT’s

ECE 663


Si ge hbt s20 l.jpg
Si-Ge HBT’s

bdc = DBLENE /DEWNB

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

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

= bdc e(EGeG-ESiG)/2kT

ECE 663


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Gain Plots

ECE 663


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Gummel Plot

Hermann-Gummel

ECE 663


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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

ECE 663






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