Charge Storage In The Quasi-Neutral Regions Of Si Homojunction, Algaas/Gaas Heterojunction, And Si/Sige Heterojunction Bipolar-Transistors

Abstract

Charge storage in the quasineutral regions of a bipolar transistor often limits the switching speed of the device, particularly when the transistor is operated in saturation mode (both emitter-base and base-collector junctions are forward biased), a bias condition which occurs frequently in logic circuits. This paper studies and compares the free-carrier charges stored in the quasineutral regions of the conventional Si homojunction bipolar transistor (Si BJT) and the increasingly important AlGaAs/GaAs and Si/SiGe heterojunction bipolar transistors (HBTs). The diffusion capacitances associated with this stored charge, which are used in the Gummel-Poon bipolar transistor model, are also calculated and discussed. It is shown that the AlGaAs/GaAs HBT has the least carrier accumulation in the quasineutral regions among the three devices due primarily to the relatively large energy band gap of GaAs-related materials. Also, the Si/SiGe HBT is expected to operate at a higher switching speed compared to the Si BJT, because the valence band discontinuity at the Si/SiGe base-collector heteroface impedes the minority-carrier injection from the base into the collector