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

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 75mV/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

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

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

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

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

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

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

Relevant approximations : not much larger than 1

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)