The steep Ge profile in the SiGe base yields the tradeoff between a large accelerating field in the base region, and a low saturation velocity in the base region near the collector. The Ge mole fraction, electric field and doping concentration dependence of minority electron mobility is obtained from the Monte Carlo simulation. An analytical analysis of the base transit time is given and compared with energy balance simulation with consistency. The high Ge content at the base-collector junction produces large strain in the base and requires the low thermal budget process to avoid the dislocation generation, but does not always reduce the base transit time due to the low saturation velocity in the high Ge content region. The minimum Ge content needed at the base-collector junction of graded SiGe HBTs can be predicted by our analysis.
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