An Evaluation of SiGe HBT Operation at Cryogenic Temperatures

The paper deals with silicon-germanium heterojunction transistor's operation under cryogenic conditions. Evaluating methodology was demonstrated with simple tool set. Families of output and input I-V characteristics in common emitter mode were measured at room and liquid nitrogen temperatures. Some cryogenic effects were observed and discussed, namely: 1) increasing of turn-on voltage due to freezing of major carriers in emitter with cooling, 2) enhancement of current gain due to bandgap difference of Si emitter and SiGe base even at LNT, 3) reducing of breakdown voltage due to more pronounced avalanche multiplication, 4) early voltage decreasing because of neutral base recombination, 5) improvement of power dissipation and reducing of self-heating effect. Despite of these effects, SiGe HBTs save their robust operation at cryogenic temperatures that confirms the capability of used IC technology process for LNT applications.