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Transcript
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
Common Base
ECE 663
Common Emitter
ECE 663
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 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
IE  IF 0 (e qV
EB
/ kT
 1)  R IR 0 (e qV
CB
/ kT
 1)
Ebers-Moll
Assuming –VCB > few kT/q and W/LB << 1
IF 0

W  

cosh  
D
D
 LB    qA DB p
 qA E nE 0  B pB0
B0
 LE
LB
W
W  
sinh  

 LB  

IE  IF 0e
qVEB / kT
DB
 qA
pB0e qV
W
EB
/ kT
• Exponential prefactor will increase as VCB increases (W
decreases)
ECE 663
Early Effect: Common Emitter Output Characteristic
IC  dcIB  ICE 0
 dc 
1
DE W NB 1  W 
  
DB LE NE 2  LB 
2
Weff  W  WEB Base  WCB Base  W  WCB Base
WCB
 2K S  0 N A  ND 

Vbi  VCB 

ND N A
 q

WCB Base
1
2
 NC 
 xn  W Base 

 NC  NB 
• 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
IC
J M Early
VCE
VEarly
Converge ~ at single point called "Early Voltage" (after James Early)
Large "Early Voltage" = Absence of "Base Width Modulation"
= Transistor ~ immune to operating voltage changes
BUT requires wide base => lower gain
ECE 663
Avalanche Multiplication Breakdown
•
•
Common Base: Similar to single p-n junction VCB0  VBD(B-C)
Common Emitter: more complicated
1.
2.
3.
4.
5.
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
IC   dcIB  ICB0
Mdc
ICB0

IB 
1  Mdc
1  Mdc
• 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
Ideal
W/base width mod
Early Effect
W/base width mod
& avalanche
multiplication
ECE 663
How can we mitigate these effects?
ECE 663
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
kT 1 dNB ( x )
E
q NB ( x ) dx
ECE 663
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 BiCMOS
ECE 663
Bandgaps and alignments
Si
Si0.8 Ge0.2
Ge
Vacuum level
ECE 663
Si-Ge Heterostructure
Silicon:
Si0.8 Ge0.2 :
Ec
Eg
1.1eV
small = 20% of Eg
Eg
Ev
1eV
large = 80% of Eg
• Most of the bandgap difference shows up in the valence
band
ECE 663
Band Diagram for SiGe HBT
electron
barrier
hole
barrier
Hole barrier is higher by ~ EV ~ 0.1 eV !!
ECE 663
Si-Ge HBT’s
For EV ~ 0.1 eV, new exponential multiplier equals:
 EV
~e
kT
e
0.1
0.0259
1
50
SiGe Heterojunction cuts backward hole injection by ~ 50:
Use higher gain if needed
If more gain not needed, increase BASE DOPING by 50
- Retain gain of previous pure Si transistor
- Reduce "base resistance"
=> more efficient operation
=> FASTER operation (reduced R-C charging time)
ECE 663
Si-Ge HBT’s
dc = DBLENE /DEWNB
dc = DBLE(ni2/NB) /DEW(ni2/NE)
HBTdc = dc(nSii)2/(nGei) 2
= dc e(EGeG-ESiG)/2kT
ECE 663
Gain Plots

VCE or frequency
On frequency versions can plot either:
Power gain
Current gain
=> rolls downward at frequency = " max
f "
=> rolls downward at frequency = " ft "
(Have seen designers ~ come to blows over which more important!)
ECE 663
Gummel Plot
Log( I)
IC
Hermann-Gummel
IB
ratio = 
VBE
ECE 663
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
BJT Small Signal Response
IB  IB (VBE ,VCE )
IB (VBE  v be ,VCE
IB
IB
 v ce )  I B (VBE ,VCE ) 
v be 
v ce
VBE V
VCE V
CE
BE
IC  IC (VBE ,VCE )
IC (VBE  v be ,VCE
IC
IC
 v ce )  IC (VBE ,VCE ) 
v be 
v ce
VBE V
VCE V
CE
BE
i b  g11v be  g12v ce
i c  g 21v be  g 22v ce
ECE 663
BJT Transient Behavior
As with diodes, switching often limited by external circuit:
IC
IB
Rsource
Vsource
N
Rload
P
Vsupply
N
IE
ECE 663
Right Circuit Loop:
IC
Tra
IC
Vsupply
Rload
"Load-line"
VCE
Vsupply
VCE
Transistor is constrained by load-line:
ECE 663
Vsource
time
IC
time
build up Q steady-state
discharge discharge
limited by of remainder
ext. circuit
ECE 663