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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
  • 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

bjt real characteristics
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

base width modulation early effect
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
Early Effect: Common Base Input Characteristic


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

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

ECE 663

early effect common emitter output characteristic
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

early voltage
Early Voltage

J M Early

ECE 663

avalanche multiplication breakdown
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
Avalanche Breakdown: Common Emitter
  • IC when M1/dc
  • M only needs to be slightly greater than unity
  • VCEO

ECE 663


W/base width mod

Early Effect

W/base width mod & avalanche


ECE 663

graded base
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
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 heterostructure
Si-Ge Heterostructure
  • Most of the bandgap difference shows up in the valence band

ECE 663

si ge hbt s20
Si-Ge HBT’s


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

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

= bdc e(EGeG-ESiG)/2kT

ECE 663

gain plots
Gain Plots

ECE 663

gummel plot
Gummel Plot


ECE 663

bjt small signal response
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