InP/InGaAs heterojunction bipolar transistors with a 550-Å-thick base and 3000-Å-thick collector layers are fabricated to study the electron velocity overshoot effect in collector depletion layers. The intrinsic delay time evaluated at room temperature measurement remains almost constant at 0.7-0.8 ps in the investigated base/collector bias range of 0 to 1.1 V at a collector current density of 1×105 A/cm2. According to Monte Carlo simulation results, such bias-independent behavior is associated with the sufficiently large overshoot effect caused at high collector biases, where the overshoot velocity is enhanced by the high electric field.
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Kurishima et al. (1992) studied this question.