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We fabricated AlGaN/GaN heterostructure field effect transistors (HFETs) with p-GaN substrate layers and p-type ohmic contacts (p-sub HFETs) and measured the substrate-bias (VSUB) -dependent threshold voltage VT variation. From this VT-VSUB characteristic, the acceptor concentration in the buffer layer was determined. This method for doping profile measurement has been widely used for Si MOSFETs. By applying this method to AlGaN/GaN HFETs stressed by negative substrate bias or UV light irradiation, buffer layer deep traps were specifically investigated. The deep traps in the buffer layer were determined to be hole traps with a concentration of 0.5-1 × 1016 cm-3. The energy level was estimated to be approximately 0.71-0.95 eV above the valence band.