By calculating the current–voltage relation for a quantum-size GaAs/AlGaAs heterojunction field effect transistor, we have shown that the electronic near-ballistic transport from the source to the drain is ideal only at zero source-drain bias in the absence of scattering centers. The electron transmission coefficient of the near-ballistic transport reduces following the increase of the source-drain bias so that the source-drain current is lower than the one at ideal transmission condition. The current is further reduced when impurities are introduced and the current reduction depends on the concentration and spatial configuration of impurities. Together with our early work we have shown that the ionized impurities and heterointerface roughness are two important factors in determining the ballistic transport properties in a heterojunction field effect transistor. An optimal current is reached when the AlGaAs spacer in a usual GaAs/AlGaAs heterojunction transistor is about 100 Å thick. ©1997 American Institute of Physics.