Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

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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) Al2O3 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 Al2O3/GaN/AlGaN/GaN MIS diode and GaN/AlGaN/GaN Schottky diode only exhibit tiny threshold-voltage hysteresis (ΔV 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 ΔV 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 Al2O3/GaN interface account for the V th-instability in Al2O3/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.

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