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SiGe TechnologySiGe Technology

陳博文R91943105

Temperature Temperature EffectsEffects

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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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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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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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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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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