Roll‐off of the current gain cut‐off frequency inNpNdouble‐heterojunction bipolar transistors for large collector currents has been analyzed. The analysis includes such effects as the electron barrier formed at the collector base junction due to electron accumulation. Included in this investigation is also lateral electron diffusion before injection into the collector space‐charge region at the base‐collector heterointerface once the barrier is formed. The available data obtained in SiGe heterojunction bipolar transistors are in good agreement with this model.