An Investigation of Deep Levels in GaAs FETs by Selective De-Excitation of the Deep Donor Level EL2

A modified photo-FET technique is presented where the Idss photoresponse before and after EL2 quenching is measured. The technique has been applied both to ion-implanted FETs and to VTPE grown devices with buffer thicknesses ranging from 0.7 ¿m to 3 pm grown on chroinum-doped substrates. The removal of the EL2 contribution to the photoresponse leads to a modification of the charge states of the remaining deep levels whose presence can be detected by examining the difference between the quenched and unquenched Idss spectra. In the case of VPE FETs, a minimum buffer thickness to avoid chromium diffusion problems can be established.