Measured (symbol) (a) cutoff frequency and (b) impulse response of a typical HBT versus that simulated by using the Agilent (dashed curve) and modified (solid curve) HBT model. In (a), = 1, 2, 3, and 4โV top down. In (b), the input pulse width and amplitude are 0.2โnanosecond and 1.45โV, respectively. = 0. Artificial delays between impulses are added for clarity.