The aim of this study is the physical understanding of the avalanche breakdown phenomenon in PHEMTs (AlGaAs/GaInAs/GaAs), in order to optimise the structure and to improve the breakdown voltage. It is therefore necessary to study the influence of the physical parameters on which this phenomenon depends, such as the layer structure, the doping concentration or the gate recess topology. The study is based on a two-dimensional hydrodynamic modelling, that takes electrons and holes into account, and shows that the highest breakdown voltage is obtained for a double step gate recess with two delta-doping layer plans. (C) 2005 Elsevier Ltd. All rights reserved.