Threshold Voltage Instability in Al₂O₃/GaN/AlGaN/GaN Metal–Insulator–Semiconductor High-Electron Mobility Transistors

Abstract

The threshold voltage ( V th ) instability in GaN-based metal–insulator–semiconductor high-electron mobility transistors (MIS-HEMTs) with 15-nm atomic-layer-deposited (ALD) Al 2 O 3 as gate dielectrics is systematically investigated by dc current–voltage ( I – V ), high-frequency capacitance–voltage ( C – V ) (HFCV), and quasi-static C – V (QSCV) characterizations. Both Al 2 O 3 /GaN/AlGaN/GaN MIS diode and GaN/AlGaN/GaN Schottky diode only exhibit tiny threshold-voltage hysteresis (Δ V th ) (<10 mV) in double-mode (up and down sweep) HFCV characteristics as the maximum forward bias ( V Fmax ) during the sweep is set to 0 V, while an apparent Δ V th (as large as 0.9 V) emerges as V Fmax is increased to +5 V for the MIS diode. The stability of V th in the corresponding MIS-HEMTs is thus studied by increasing the maximum V GS ( V GSmax ) in the measurement of transfer characteristics. Significant positive V th shift occurred once the V GSmax exceeds +1 V, while such V th -instability is still absent in Schottky-gate AlGaN/GaN HEMTs. It is suggested that the acceptor-like deep states at Al 2 O 3 /GaN interface account for the V th -instability in Al 2 O 3 /GaN/AlGaN/GaN MIS-HEMTs. As the filling and emission processes of these interface states are slow, they are successfully captured by low-frequency QSCV techniques.