A conventional formula for the total emitter-to-collector transit time (tau(cc)) or the short circuit current gain cut-off frequency (f(T)) in heterojunction bipolar transistor (HBT) has been modified to include the effects of the intrinsic emitter dimensions (width, length, area) and the extrinsic parasitic components. The formula shows how the emitter dimensions and the device parasitics affect the total transit time and f(T). The validity of the formula has been verified experimentally for the case of InP/InGaAs HBTs with polyimide planarization process. (C) 2002 Elsevier Science Ltd. All rights reserved.