A PHYSICAL MODEL FOR THE BASE TRANSIT-TIME OF ADVANCED BIPOLAR-TRANSISTORS

As the feature size of advanced bipolar junction transistors (BJTs) co ntinues to scale down, the effects of nonuniform base doping, high-lev el injection, current-induced base pushout, and velocity overshoot all become prominent. These effects influence strongly the switching spee d of the BJT as well as the gate delay of the BICMOS. We study in deta il the base transit time tau(B), which is often the limiting factor of the BJTs total delay time when the current density is high, and devel op an analytical tau(B) model valid for arbitrary levels of injection and Gaussian base doping profile. For the devices considered, our calc ulations show that the conventional model (considers uniform base dopi ng profile and no base pushout) overestimates tau(B) by a factor of ab out 2.5 at low injection and underestimates tau(B) by a factor of abou t 1.5 at high injection. The present model compares favorably with exp erimental data measured from a 0.12 mu base width BJT.