Evidence of avalanche breakdown and microplasma noise in GaAs MESFETs

Noise and prebreakdown properties of VPE GaAs MESFETs with and without VPE buffer layer have been investigated. The 1µm gate elements have been produced in the same common manner and with the same geometry starting from the same wafer. With buffer layer no prebreakdown effects are observed. Without buffer layer the instabilities of the DC characteristics, loops and current jumps, as well as the corresponding noise spikes and the noise spectrum can be related to microplasmas activated by avalanche breakdown in the space charge layer of the epi-substrate interface. This effect is located below the drain contact of the MESFETs and is associated by a substrate current flow, no gate current being present. The higher noise factor, also for frequencies higher than X-band, as well as anomalously low values of the breakdown voltage in power devices without buffer layer seems to be affected by that effect.