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

As the feature size of advanced bipolar junction transistors (BJTs) continues to scale down, the effects of nonuniform base doping, high-level injection, current-induced base pushout, and velocity overshoot all become prominent. These effects influence strongly the switching speed of the BJT as well as the gate delay of the BICMOS. We study in detail 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 develop an analytical tau(B) model valid for arbitrary levels of injection and Gaussian base doping profile. For the devices considered, our calculations show that the conventional model (considers uniform base doping profile and no base pushout) overestimates tau(B) by a factor of about 2.5 at low injection and underestimates tau(B) by a factor of about 1.5 at high injection. The present model compares favorably with experimental data measured from a 0.12 mu base width BJT.