Theoretical analysis of HEMT breakdown dependence on device designparameters

A two-dimensional numerical analysis is presented to investigate the breakdown characteristics of single- and double-channel AlGaAs/GaAs HEMTs. The influence of the doped layer thickness and the thickness of an undoped i-layer under the gate is analyzed. Impact ionization is considered to be the dominant breakdown mechanism. All simulations reveal the existence of a high electric field region near the gate contact. Breakdown occurs in the gate-drain region and the (breakdown) path which maximizes the ionization integral is entirely in the AlGaAs layer. For increased donor layer thickness, single-channel devices biased near pinchoff have gate-drain breakdown voltages varying from 8 to 14 V with corresponding peak electric field values in the range of 8.2×10⁵ to 2.4×10⁶ V/cm. The breakdown voltage increases with increasing gate bias |V _gs| due to a screening effect of transverse from longitudinal electric field. Double-channel HEMTs have slightly higher breakdown than single-channel, especially near pinchoff and for thin donor layers