Magnetoresistance characteristics of gamma-irradiated Al _0.35Ga_0.65N/GaN HFETs

The effect of ⁶⁰Co gamma-irradiation on the device characteristics of Al_0.35Ga_0.65N-GaN heterojunction field effect transistors (HFET) has been investigated using DC and geometrical magnetoresistance measurements. Cumulative gamma-ray doses up to 20 Mrad(Si) are shown to induce drain current degradation, negative threshold voltage shifts and reverse gate leakage current degradation. Analysis of drain magneto-conductance characteristics measured at 80 K indicated an increase in two-dimensional electron gas (2DEG) sheet concentration with accumulated radiation dose. More importantly, the 2DEG mobility-concentration characteristics are noted to remain approximately constant for total gamma-radiation doses up to 20 Mrad(Si), indicating that the areal density of radiation-induced defects at the heterointerface is likely to be negligible. The threshold voltage shifts are therefore attributable to the introduction of relatively shallow radiation-induced defects in the AlGaN barrier region and/or to defects introduced at the gate-barrier interface. Although the drain conductance characteristics manifested similar degradation trends at 80 and 300 K, the 2DEG parameters obtained at 300 K exhibited significant scatter with increasing dose, possibly a manifestation of device instabilities induced by radiation-induced surface defects in the ungated access region near the edge of the gate. Device failure due to severe gate leakage and loss of gate control over the 2DEG charge, occurred after a total dose of 30 Mrad(Si).