Impact of Output Non-Quasi-Static Effect on Consistency Between Two Cutoff Frequency Extraction Methods in Bipolar Transistor Simulation

In device simulation, f_T is typically extracted at a single low frequency, e.g. 1 MHz, using g_m/2piC. This is generally believed to give the same f_T as traditional -20 dB/decade slope extrapolation of |h₂₁| at high frequencies. A large discrepancy between the two f_T extraction methods is observed, and the degree of discrepancy is found to depend on BGN and temperature. We explain the discrepancy as a consequence of output NQS effect and develop a new f_T equation including output NQS effect. The BGN model dependence can then be understood through ac current gain beta_ac. New methods of simulating f_T are developed, which maintain the simplicity of g_m/2piC extraction while giving the same results as -20 dB/decade extrapolation.