The intrinsic mechanism of the generation of subpicosecond electrical pulses by nonuniform illumination of GaAs transmission‐line gaps is investigated using a self‐consistent Monte Carlo approach. It is attributed to the photocarrier‐induced field redistribution, which results in a displacement current pulse that is independent of the carrier lifetime. Partial‐gap illumination and high dark resistivity are the prerequisites for this pulse generation technique. The pulse dependence on light intensity, excitation wavelength, bias voltage, and the asymmetric response to bias polarity are discussed and clarified. It is predicted that this mechanism should also be observable in other semiconductors such as silicon. ©1995 American Institute of Physics.