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High current densities in the collector layer reduce the cutoff frequency of heterojunction bipolar transistors. We develop a model based on analytical expressions that describe this reduction. These expressions represent the contributions from each of six regions defined in the output current‐voltage characteristic. The model has parameters determined entirely by device physical makeup. It has no fitting parameters. Its predictions agree well with experimental data taken on two N/p+/n aluminum‐gallium‐arsenide/gallium‐arsenide transistors having abrupt junctions grown by molecular‐beam epitaxy. Because previous models omitted the effects of high current densities, their predictions agree less favorably. The development of the model considers the effects that compound‐semiconductor properties such as velocity overshoot have on the cutoff frequency.