Prebreakdown phenomena in GaAs epitaxial layers and FET's

Several phenomena in GaAs field-effect transistors (FET's) which are present at higher voltages, such as light emission, noise power eruptions, and excess interface and substrate current flow, depend on the presence of a buffer layer between the active layer and the substrate as well as the epi-layer thickness below the positive ohmic contact. The origin of the light emission and the noise power is impact ionization in the high-field region near the drain contact. The excess current flow after saturation is primarily due to avalanche multiplication and breakdown of the space-charge layer at the interface between active layer and substrate. This space-charge layer is caused by trapped electrons at the interface between epi-layer and substrate. The electric-field strength and the current density in this critical region below the drain contact control the prebreakdown effects and precede the device burnout.