The theoretical avalanche breakdown in InP MISFETs is described using a two‐dimensional numerical model based on the calculation of Poisson's equation and the current equation. The study investigates various structures with a full analysis of the potential shape and the electric field distribution which gives complete explanations of the breakdown behaviour of the device. The characteristics of the transistor are determined as a function of the geometry of the structure, and the effects of the main parameters are given: The gate‐drain distance, the insulator zone width, the channel thickness, the doping concentration of the active layer and the shape of the recess.