AbstractA two‐dimensional numerical model has been developed for the analysis of operation of a high electron mobility transistor (HEMT) based on the AlGaAs/GaAs heterojunction. In this model, the basic equations are formulated under the assumption that the band structure is continuous and Boltzmann statistics are applicable. Further, it is assumed that the velocity‐electric field curve of the two‐dimensional electron gas system is identical to that of the bulk GaAs. The calculations have been performed for the device with a 1 μm gate length and the following results have been made on the operating mechanism of the HEMT. 1 In the short‐channel HEMT, the electron velocity saturates in the channel directly below the drain‐side edge of the gate electrode and as a result the current also saturates. 2 In the HEMT operated in the saturation region, an electron accumulation region is formed in the GaAs layer below the drain‐side edge of the gate electrode. In this region, the current flow significantly extends into the bulk GaAs from the heterojunction interface. 3' A strong electric field concentration occurs in this region and the major portion of the applied drain voltage is sustained by this region. This concentration is limited in an extremely n