Isothermal Instability in Compound Semiconductor Devices

Catastrophic failure events in GaAs MESFETs and MODFETs (HEMTs) are usually preceded by a set of rather complex phenomena: formation of stationary or traveling domain of electric field, avalanche and tunneling breakdown, and forma tion of strong nonequilibrium states and filaments. A typical microwave compound semiconductor FET has submicrometer dimensions of the active region, multifinger array structure, complex profiles of the multiple-layer double recess structure, complex doping profiles, surface depletion of the Schottky gate region, and buffer ayer. Heterojunction devices add variable compound semiconductor material. The specific of the carrier transport results in nonlinear and nonlocal dependence of the mobility and velocity on the electric field. Therefore, an accurate solution for the electric field and current distribution in these devices is more complex than in silicon. In particular, it requires a numerical simulation of a hydrodynamic equation model. Two basic modes of avalanche breakdown in the common source circuit can be determined as a limitation of safe operation area: the drain-source breakdown and the gate breakdown. The first is realized when the high-electric-field region is localized near the drain n+-n junction. In this case, the drain-source voltage increase results in an increase of the avalanche current component of the drain current. The gate breakdown is observed as a current increase in the gate circuit. In most practical cases of the device design, the drain-source breakdown dominates over the gate breakdown in the on-state operation regimes. On the contrary, the gate breakdown dominates over the drain-source breakdown in the pinch-off condition. Similarly to silicon devices, the breakdown in MESFETs, MODFETs, or HEMTs is not of a “pure” avalanche nature. At some critical drain breakdown current, a positive feedback is observed due to injection from the source junction. In this case, current instability results in a sharp redistribution of the breakdown current along the contacts into the current filament state. Depending on the load parameters, the operation regime, and input power, a scenario with overheating inside the current filament and local burnout is realized. Thus, the instability can determine the drain-source voltage limitation and failure mode rather than the breakdown current level.