In this paper, two-dimensional devices simulation program-MEDICI has been used to simulate AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors (PHEMTs). Doping and electron concentrations, current flow and gate characteristic in PHEMTs are studied. The kink effect in PHEMTs is investigated emphatically as a function of temperature and doping concentration of Schottky layer. The results show that the kink effect is related mainly to the trapping/detrapping process of deep levels that lie in the top layers but not related to the impact ionization alone.