AbstractA two‐dimensional device simulator HESPER, which can numerically analyze the AlGaAs/GaAs heterostructure devices, has been developed. This program was applied to a heterostructure bipolar transistor and its performance was compared with a homostructure device. As a result, it was shown that the transistor characteristics such as current gain and cutoff‐frequency are improved greatly by the introduction of a heterostructure. In addition, from carrier and potential distributions, the advantage of heterostructure devices was clarified. Moreover, HESPER was applied to a two‐dimensional electron gas FET, and the calculated transconductance values have agreed well with the measured results for different AlGaAs thicknesses. Furthermore, it was also suggested that the increasing mobility in the low electric field region and/or the saturation velocity mobility are effective to realize the high‐speed operation. That is, we can expect more excellent performance of the two‐dimensional electron gas devices at 77 K and/or in velocity oversho