A Monte-Carlo simulation program has been developed to simulate the motion of electrons in a submicron GaAs diode with an AlxGa1−xAs/GaAs heterojunction cathode. The electric field in the diode is self-consistently determined by solving Poisson's equation under reasonable boundary conditions. The distributions of electron energies and electron velocities, and the profiles of the electron density, electric field and average electron velocity, are computed. Based on these data, various properties of the electron transport in a submicron diode with a hot-electron injection mechanism are discussed.