INFLUENCE OF THE UNDOPED SPACER LAYER THICKNESS ON THE DC CHARACTERISTICS OF N-TYPE GAAS ALAS MESFET/

We have investigated the influence of the thickness of an undoped spac er layer under the channel an the current-voltage characteristics of G aAs MESFETs. To that purpose we have grown the active layers of the ME SFETs 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 a re measured for four MESFETs with identical layer structure and doping but different spacer layer thicknesses. Comparison shows that MESFETs with thin spacer layers can he useful for low-noise applications but lose in transconductance and mobility values.