A closed-form model of the drain-voltage dependence of the OFF-state channel electric field in a HEMT with a field plate

The channel-field distribution under the field plate in a high-electron mobility transistor (HEMT) in the OFF-state is modeled in terms of drain-voltage and physical parameters. Depending upon the drain-voltage and device structure, this distribution can have up to three peaks-one each at the two ends of the field plate and at the drain. It is shown that the complete distribution can be approximated as superposition of triangular distributions, which are analogous to that in the depletion layer of a p-n junction; consequently, the peaks increase as square root of the drain-voltage. The model fits into two-dimensional simulation results and allows estimation of the minimum drain-gate separation, the electric-field reduction, the breakdown-voltage improvement, and critical field for the onset of a parasitic phenomenon in a HEMT with a field plate.