Using Monte Carlo simulations, we study pseudomorphic uniformly doped AlGaAs/InGaAs/GaAs high electron mobility transistors with very short gate lengths (150 and 50 nm). In open‐channel range of operation the saturation of the rain current is ensured by the existence of a pseudodipolar domain between the gate and the drain where the increase in drain potential is dropped. We describe the short channel effects, such as the high drain conductance, that occur in the pinchoff range of operation (especially in the shortest device). In this case the overheated electrons can easily transfer to the upper large band gap layer and cannot form any domain. The potential barrier that controls the current is gradually lowered by the drain potential increase, which is favoring the electron injection from the source side. Short channel effects are also involved in the relatively weak transconductance obtained in the 50 nm gate device, gm=800 mS/mm, to be compared with 900 mS/mm reached in a 150 nm gate device. Finally, we show that the occurring of short channel effects can be predicted from very simple one‐dimensional calculations along two perpendicular directions of the device, by taking into account the lateral diffusion from the source access zone to the active zone.