A new quasi-two-dimensional HEMT model has been developed which solves the physical device equations. It incorporates a full detailed energy transport scheme in the calculation of the carriers density. The model has been improved in taking into account of drift velocities and impact ionization coefficients relevant of each material layer. Two different InP HEMT structures have been simulated, the first one is a single channel InGaAs (SC) and the second one a double InGaAs/InP (DC). This model has allowed to obtain results in close agreement between simulation and experience for short-gate HEMT's.