Transit time components of a SiGe-HBT at low temperature

The transit time components of a Si/SiGe/Si HBT has been investigated in the low temperature regime through simulation. The effect of reduced temperature has been investigated on each transit time component and effectively an increase in f_T more than 1.8 times has been obtained when operating temperature is lowered from 300K to 120K. To validate the simulated results, we compare our simulation results with the reported experimental results. The agreement of the experimental data with our simulated f_T shows that f_T of a SiGe-HBT is primarily determined by the three components of the transit time, the base minority carrier storage time (∼65% of T_∝), the emitter minority carrier storage time (∼7.6% of T_ec), and the base collector space charge layer transit time (∼16.6% of T_ec) over all the temperature range. In addition, we show that f_T for a Si-BJT decreases below 150K and goes down from 23.7 GHz to 21.6 GHz when temperature decreased from 150K to 120K. Similar results have been also been reported for Si by other authors. On the other hand, it is shown that for a SiGe-HBT, the f_T tends to increase even below 150K showing its superiority for cryogenic applications.