1. Dynamic characteristic
SiGe Chapter5
Dynamic characteristic
5-1 Intuitive picture
5-2 Charge Modulation effects
5-3 Basic RF Performance Factors
5-4 Linear Two-Port Parameters
5-5 Stability,MAG,MSG,and Mason’s U
5-6 Base and Emitter Transit Times
5-7 ECL Gate Delay

2. 5-1 Intuitive
Si BJT:
-Electron must diffusion across the base (constant doping)
base transit time is significant
limit cutoff frequency
-Applied forward bias on EB juntion
back injection of holes from B to E
emitter charge storage delay time is reciprocally related to ßac
SiGe HBT:
-Ge-gradient-induced drift field
decrease base transit time
ex: 0~10% linearly graded Ge profile over 10nm base , yield mV/50nm=15kV/cm electric field electron saturation velocity = 1e7cm/sec
-Ge-induced band offset at EB juntion
exponentially enhance Jc
decrease emitter charge storage delay time
increase ßac

3. 5-3 Basic RF Performance Factors
For high frequencies
Current density versus speed :

4. -low low but Kirk effect roll off
increase collector doping  decrease breakdown voltage

5. Base Resistance
-limits transistor power gain
-noise performance
“circle impedance base resistance” extraction method

6. Power gain and Assume: 1. Large  open circuit 2.
3. I(test) through small
When decrease to unity

7. 5-4 linear two-port parameters
Z-parameter
Y-parameter
H-parameter
S-parameter (voltage waves)
square with power dimension

9. 5-6 Base and Emitter Transit Times
Moll-Ross transit time relation:
BJT:
SiGe:

10. Relevant approximations :
not much larger than 1

11. Implications and optimization issues for
in SiGe HBTs
Implications and optimization issues for
1.the presence of Ge in base of HBT affects its frequency response through the base and emitter transit time.
depends reciprocally on the Ge grading across the base.
3. for enhancement
,triangular Ge profile is better than box Ge profile
,box one is better, for
a compromise trapezoidal profile ,which generally favors both and improvement, is a logical compromise profile design point.
4. versus power dissipation trade-off .
Ex: portable application(power minimization)