AnnpnInP/Ga0.47In0.53As/InP heterojunction bipolar transistor with a unity‐gain frequency of 15 GHz was illuminated with ultrafast optical pulses at wavelengths of 620 and 1530 nm. The device responded to the pulses with an emitter current transient having a duration of 12 ps, corresponding to a bandwidth of ∼40 GHz. A slower photocurrent component, with a decay time of ∼100 ps, was a sensitive function of base bias and, because of the photocarrier dynamics and the grounded‐collector circuit configuration, could be nulled out.