New BiCMOS delay model to include RC-limited BJT saturation effect

This paper presents an analytical model for the delay in a BiCMOS driver. The model covers all three modes of operation of the bipolar transistor (BJT) in a BiCMOS driver—low-level injection, high-level injection and RC−limited saturation. The associated transient collector current IC(t) of BJT, and the analytical derivation of corresponding propagation delay time τd of the BiCMOS driver is based on two important BJT parameters—the Knee current IKF and the parasitic collector resistance RC. Comparison with SPICE simulation results over load capacitance CL ranges of 5–20 pF shows quite good agreement with parameter variations for RC from 300 to 500 Ω, and IKF from 3 to 9 mA respectively. The model accuracy with different initial output voltage Vouti(τ∗) was also studied and is discussed.