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SiGe Technology
Temperature Effects
陳博文
R91943105
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Outline
•
•
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The Impact of Temperature on Bipolar Transistors
Cryogenic Operation of SiGe HBTs
Optimization of SiGe HBTs for 77K
Helium Temperature Operation
Nonequilibrium Base Transport
High-Temperature Operation
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The Impact of Temperature on Bipolar
Transistors
• A modest increase in the junction turn on
voltage with decreasing temperature
• A strong increasing in the low-injection
transconductance with cooling
• A strong decrease in ß with cooling
• A modest decrease in frequency response
with cooling, with fT typically degrading more
rapidly than fmax with decreasing temperature
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Current-Voltage Characteristics
• For fixed bias current, VBE increase with cooling.
VBE
VBE , bias KT 1 JCO
| JC 

T
T
q JCO T
J CO
Ego
ERbi
 0 (T )T exp(
) exp(
)
KT
KT
3
E  Ego 
1 J CO
1  3 0
2   Rbi

T

3

T


 

0
2
J CO T
0T 3  T
KT
 

T  VBE 
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E go 
VBE
1
| J C  VBE ,bias 

T
T
q 
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Current-Voltage Characteristics
JC(T)  JCO(T)exp(
JCO 
WB

0
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qV BE
)
KT
q
PP ( x )
dx
2
n ie ( x) Dn ( x )
 JC(T) 
KT

ln

q
 JCO(T)
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Transconductance
I C
qIC (T )
q
qVBE
g m (T ) 

I CO exp(
)
VBE KT
KT
KT
• We can expect an improvement in gm of roughly 3.9*
in cooling from room temperature to liquid nitrogen
temperature (Fig.9.1)
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Current Gain
• Consider ideal Si BJT
(constant doping profiles, metal emitter contact)
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Current Gain
qDnb (T )n 2ib (T )
qVBE
J C (T ) 
exp(
)

Wb (T ) N ab (T )
KT
qDpe (T )n 2 ie (T )
qVBE
J B (T ) 
exp(
)

L pe (T ) N de (T )
KT
qDnb (T ) L pe N  de (T )
app
app

E


E
I C (T )
gb
ge
ideal (T ) 

exp(
)

I B (T ) D pe (T )Wb (T ) N ab (T )
KT
 ideal   (T ) exp(
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E Rbi  E gb
 E ge
KT
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Resistances
• Simulated effects of carrier freeze-out on the doping
profile of a bipolar transistor at 77K
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Resistances


Rbi (T )  




qu
(
x
,
T
)
p
(
x
,
T
)
dx

b
0 pb


Wb (T )
Rbi (T )   (T )e
E Rbi
KT
• The result for realistic base profiles shows a quasiexponential increase below about 200K and is very
sensitive function of the peak base doping,
particularly in strong freeze out below 77K
• One can measure a base freeze activation energy
ERbi
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1
Capacitance
 q Si N dc 
Cdepl (T )  A

 qbi (T ) 
1
2
• The parasitic depletion capacitances will generally decrease
(improve) with cooling, due to the increase in junction built-in
voltage, since for a one-side step junction.
• For the CB junction, which is the most important parasitic
capacitance for switching performance due to Miller effect, CCB
typically decreases by 10-20% form 300K to 77K
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Frequency Response
WCB (T )
1
KT

(CEB (T )  CCB (T ))   b   e 
 rC (T )CCB (T )
2fT qIC
2vsat (T )
Wb2 (T )
qWb2 (T )
 b,Si (T ) 

2Dnb (T ) 2KTnb (T )
• For fixed bias current, both depletion capacitances will
decrease only slightly, while τb and τe will both increase
strongly with cooling
• Unb increase only weakly with cooling since the base is
heavily doped and thus cannot offset the factor of KT
• In addition, enhanced carrier trapping on frozen-out
acceptor sites can further degrade the base transit time
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Frequency Response
f MAX
fT (T )

8Rb (T )CCB (T )
• The strong base resistance increase at low
temperatures.
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SiGe HBT Performance Down to 77K
•Measure and calculated
SiGe-to-Si current gain ratio
as a function of reciprocal
temperature for a
comparably constructed i-p-i
SiGe HBT and i-p-i Si BJT
SiGe
| V    Eg ,Ge ( grade) / KT exp( Eg ,Ge (0) / KT )
Si
1  exp(Eg , Ge ( grade) / KT )
BE
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
SiGe HBT Performance Down to 77K
•Measure and calculated
SiGe-to-Si Early voltage ratio as
a function of reciprocal
temperature for a comparably
constructed i-p-i SiGe HBT and
i-p-i Si BJT
VA,SiGe | VBE
VA,Si
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Eg , Ge ( grade) 

1  exp(
)
Eg , Ge ( grade) 
KT
 exp(
)

Eg , Ge ( grade)
KT


KT


Department of Electrical Engineering, National Taiwan University
SiGe HBT Performance Down to 77K
•Measure and calculated
SiGe-to-Si current gain- Early
voltage product ratio as a
function of reciprocal
temperature for a comparably
constructed i-p-i SiGe HBT
and i-p-i Si BJT
VA, SiGe ~ ~
Eg , Ge (0)
Eg , Ge ( grade)
   exp(
) exp(
)
VA, Si
KT
KT
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High Temperature Operation
• Percent change in
peak current gain
between 25°C and
125 °C for various
Ge profile.
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14% Ge low-noise profile
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High Temperature Operation
• The current gain in SiGe HBTs does indeed have an
opposite temperature dependence from that of a Si
BJT, as expected from simple theory.
• These changes in ß between 25°C and 125 °C,
however, are modest at best (<25%), and clearly not
cause for alarm for any realistic circuit.
• The negative temperature coefficient of ß in SiGe
HBTs is tunable, meaning that its temperature
behavior between, say, 25°C and 125 °C can be
trivially adjusted to its desired value by changing the
Ge profile shape near the EB junction.
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High Temperature Operation
• In the case of the 15% Ge triangle profile, with 0%
Ge at the EB junction, ß is in fact femperature
independent from 25°C to 125 °C.
• It is well known that thermal-runaway in high-power
Si BJT is the result of the positive temperature
coefficient of ß.
• The fact that SiGe HBTs naturally have a negative
temperature coefficient for ß suggests that this might
present interesting opportunities for power amplifiers,
since emitter ballasting resistors (which degrade RF
gain) could in principle be eliminated.
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High Temperature Operation
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• Gummel characteristics at 25°C and 275°C for a 14%
Ge, low-noise optimized SiGe HBT
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High Temperature Operation
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• Current gain as a function of collector current at 25°C
and 275°C for a 14% Ge, low-noise optimized SiGe
HBT
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