Influence of the undoped spacer layer thickness on the DC characteristics of n-type GaAs/AlAs MESFETs

We have investigated the influence of the thickness of an undoped spacer layer under the channel on the current–voltage characteristics of GaAs MESFETs. To that purpose we have grown the active layers of the MESFETs on top of a 50 nm AlAs layer to separate the device effectively from the undoped GaAs substrate. Substrate leakage currents are of the order of 10−7 A at 30 V source–substrate bias. DC characteristics are measured for four MESFETs with identical layer structure and doping but different spacer layer thicknesses. Comparison shows that MESFETs with thin spacer layers can be useful for low-noise applications but lose in transconductance and mobility values.