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Plans - PowerPoint PPT Presentation

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

• 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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• 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 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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Ebers-Moll

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

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

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• 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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J M Early

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• 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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• 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

multiplication

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• 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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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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bdc = DBLENE /DEWNB

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

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

= bdc e(EGeG-ESiG)/2kT

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Gummel Plot

Hermann-Gummel

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• 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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