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Self-heating effects in graded AlGaAs/GaAs heterojunction bipolar transistors are analyzed using an ensemble Monte Carlo description of electron transport from emitter to subcollector including space-charge effects through a self-consistent solution of Poisson’s equation. For a given collector current density, the temperature rise in the intrinsic portion of the device is calculated using a simple analytical treatment. The temperature dependence of the scattering rates and various device parameters is included in the Monte Carlo simulations. The extent of velocity overshoot in the collector and its dependence on the device temperature is analyzed. The collector delay time is most affected by self-heating effects leading to a degradation of the unity current gain cut-off frequency of graded heterojunction bipolar transistors. For the biasing conditions considered here, the unity current gain cut-off frequency is found to be as much as 18% below its value calculated while neglecting self-heating effects. © 1998 American Institute of Physics.