Abstract: The DC characteristics of Si/SiGe/Si HBTs at room temperature and low temperatures are modelled and simulated analytically. The models account for the following: heavy doping effects, heterojunction barrier effects, the temperature dependence of physical parameters, the base current arising from back injection hole current, the recombination current in both quasi-neutral base, and depletion region of BE junction. The Gummel plot and dependence of current gain of collector current of Si/SiGe/Si HBTs at 300K and 77K are given. Furthermore, the effect of minority carrier lifetime in the base on the current gain is studied. Simulation results show that Si/SiGe/Si HBTs have excellent DC performance both at room temperature and low temperatures in low and middle current regions, and the current gain are seriously degraded because of the presence of heterojunction barrier effects under high collector current.