Influence of the surface and the episubstrate interface on the drain current drift of GaAs MESFET's

Long-term drift of GaAs MESFET's under gate bias and under substrate bias was extensively investigated to discover whether the dominant cause of the phenomenon is due to the surface or the epi-substrate interface. It was found that the activation energy of the ordinary drain current drift under gate bias scatters greatly from 0.16 eV to 0.8 eV, depending on the amount of the drift, whereas the activation energy of the drift due to substrate bias is always constant at 0.82 eV. Ordinary drift under gate bias and its activation energy are greatly influenced by surface conditions, but drift due to substrate bias is not so influenced. Changes of the high frequency parameters are also different for each case. These results indicate that the drift in GaAs MESFET's performance, which has often been observed in the past, is dominated by surface conditions, probably mobile charges on the surface.