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Department of Electrical Engineering, National Taiwan University
SiGe TechnologySiGe Technology
陳博文R91943105
Temperature Temperature EffectsEffects
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Department of Electrical Engineering, National Taiwan University
OutlineOutline• 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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Department of Electrical Engineering, National Taiwan University
The Impact of Temperature on Bipolar The Impact of Temperature on Bipolar TransistorsTransistors
• 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 wit
h cooling, with fT typically degrading more rapidly than fmax with decreasing temperature
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Current-Voltage CharacteristicsCurrent-Voltage Characteristics• For fixed bias current, VBE increase with cooling.
)exp()exp()( 30 KT
EKTE
TTJ RbigoCO
TJ
JqKT
TV
TV CO
CO
biasBEJ
BEC
1| ,
2
20
033
0
311KT
EET
TT
TTJ
JgoRbiCO
CO
qE
VTT
V gobiasBEJ
BEC ,
1| BEVT
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Department of Electrical Engineering, National Taiwan University
Current-Voltage CharacteristicsCurrent-Voltage Characteristics
)KTqV(T)exp(J(T)J BE
COC
BW
nie
PCO
dxxDxn
xPqJ
0 2 )()()(
(T)J(T)Jln
qKTV
CO
CBE
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Department of Electrical Engineering, National Taiwan University
TransconductanceTransconductance
• We can expect an improvement in gm of roughly 3.9* in cooling from room temperature to liquid nitrogen temperature (Fig.9.1)
KTTqI
KTqVI
KTq
VITg CBE
COBE
Cm
)()exp()(
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Current GainCurrent Gain
• Consider ideal Si BJT (constant doping profiles, metal emitter contact)
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Department of Electrical Engineering, National Taiwan University
Current GainCurrent Gain)exp(
)()()()()(
2
KTqV
TNTWTnTqDTJ BE
abb
ibnbC
)exp()()()()(
)(2
KTqV
TNTLTnTqD
TJ BE
depe
iepeB
)exp()()()(
)()()()()(
KTEE
TNTWTDTNLTqD
TITIT
appge
appgb
abbpe
depenb
B
Cideal
)exp()( KTEEE
ideal
appge
appgbRbiT
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ResistancesResistances
• Simulated effects of carrier freeze-out on the doping profile of a bipolar transistor at 77K
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ResistancesResistances
KTE
bi
TWb
bpbbi
Rbi
eTTR
dxTxpTxquTR
)()(
),(),()(1)(
0
• The result for realistic base profiles shows a quasi-exponential 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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CapacitanceCapacitance
21
)()(
TqNq
ATCbi
Sidepl
dc
• 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 ResponseFrequency Response
)()()(2)())()((
21 TCTr
TvTWTCTC
qIKT
f CBCsat
CBebCBEB
CT
• For fixed bias current, both depletion capacitances will decrease only slightly, while τ b 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
)(2)(
)(2)()(
22
, TKTTqW
TDTWT
nb
b
nb
bSib
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Frequency ResponseFrequency Response
)()(8)(
TCTRTff
CBb
TMAX
• The strong base resistance increase at low temperatures.
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SiGe HBT Performance Down to 77KSiGe HBT Performance Down to 77K
)/)(exp(1/)0(exp/)(|
,
),(,~~
KTgradeEKTEKTgradeE
Geg
GegGegV
Si
SiGeBE
•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
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SiGe HBT Performance Down to 77KSiGe HBT Performance Down to 77K
KTgradeEKTgradeE
KTgradeE
VV
Geg
Geg
GegV
SiA
SiGeABE
)(
))(exp(1))(exp(|
,
,
,
,
,
•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
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SiGe HBT Performance Down to 77KSiGe HBT Performance Down to 77K
))(exp())0(exp( ,,~~
,
,
KTgradeE
KTE
VV GegGeg
SiA
SiGeA
•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
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High Temperature OperationHigh 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 profile14% Ge low-noise profile
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High Temperature OperationHigh 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 OperationHigh 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 OperationHigh Temperature Operation
• Gummel characteristics at 25°C and 275°C for a 14% Ge, low-noise optimized SiGe HBT
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High Temperature OperationHigh Temperature Operation
• 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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