This paper introduces a 1.2 kV-class low R(ON)Q(G) normally -off Gate Injection Transistor (GIT) on a bulk GaN substrate. A key highlight of this work is the unique asymmetric gate structure, which effectively reduces the Gs, of the GIT and significantly decreases switching losses. This innovative design alleviates the electric field strength at the pGaN gate edge on the drain side, enabling efficient switching. The insulating GaN buffer layer is optimally designed to achieve a vertical off-state voltage of 1.5 kV and a small C-rss. Additionally, the annealed SiN passivation film minimizes lateral leakage current, resulting in a shorter L-GD and reduced R-ON. The fabricated GIT achieves a maximum drain current of 67 A, R-ON of 60 m Omega, breakdown voltage of 1560 V, and a total Q(G) of 6.0 nC. The resultant R(ON)Q(G), as a figure-of-merit for high-speed switching, is a record 360 m Omega nC among the reported values of 1.2 kV-class devices. Notably, in inductive load switching tests at 800 V/13 A, the turn-on and turn-off speeds are 137 V/ns and 100 V/ns, respectively, with a total switching loss of 58 mu J, which is less than a quarter of that for state-of-the-art 1.2 kV-class GaN-on-sapphire devices. This substantial reduction in switching loss greatly enhances performance in high-speed and high-voltage switching applications, making it a very promising candidate.

This paper introduces a 1.2 kV-class low ${R}_{\mathrm{O}\mathrm{N}} Q_{\mathrm{G}}$ normally-off Gate Injection Transistor (GIT) on a bulk GaN substrate. A key highlight of this work is the unique asymmetric gate structure, which effectively reduces the $C_{\text {iss }}$ of the GIT and significantly decreases switching losses. This innovative design alleviates the electric field strength at the p-GaN gate edge on the drain side, enabling efficient switching. The insulating GaN buffer layer is optimally designed to achieve a vertical off-state voltage of 1.5 kV and a small $C_{\text {rss }}$ . Additionally, the annealed SiN passivation film minimizes lateral leakage current, resulting in a shorter $L_{\mathrm{G}\mathrm{D}}$ and reduced $R_{\mathrm{O}\mathrm{N}}$ , The fabricated GIT achieves a maximum drain current of 67 A, $R_{\mathrm{O}\mathrm{N}}$ of 60 mΩ, breakdown voltage of 1560 V, and a total $Q$ G of 6.0 nC. The resultant ${R}_{\mathrm{O}\mathrm{N}} Q_{\mathrm{G}}$ , as a figure-of-merit for high-speed switching, is a record 360 mΩnC among the reported values of 1.2 kV-class devices. Notably, in inductive-load switching tests at 800 V/13 A, the turn-on and turn-off speeds are 137 V/ns and 100 V/ns, respectively, with a total switching loss of $58 \mu \mathrm{J}$ , which is less than a quarter of that for state-of-the-art 1.2 kV-class GaN-on-sapphire devices. This substantial reduction in switching loss greatly enhances performance in high-speed and high-voltage switching applications, making it a very promising candidate.

We present a novel normally-off vertical GaN transistor on a GaN substrate, distinguished by its low $R_{\text{on}}C_{\text{rss}}$ of 171 $\mathrm{m}\Omega \text{pF}$, where the introduction of a p-GaN shield is a key differentiator. This p-GaN shield, strategically placed over V-shaped grooves, significantly reduces the $C_{\text{rss}}$ to 2.92 pF at $V_{\text{ds}}=500\ \mathrm{V}$, which is less than one-eighth of a conventional vertical GaN transistor. The deviceF exceptional performance, with a maximum drain current of 57 A and a breakdown voltage surpassing 900 V, is attributed to the optimized regrown AlGaN/GaN channel layer and the electric field relaxation effect of the p-GaN shield structure. Notably, the device achieves rapid switching at 350 V/10 A with turn-on and turn-off speeds of 40.6 and 45.5 V/ns, respectively, more than two times faster compared to the conventional. The introduction of the p-GaN shield marks a significant advancement, positioning the vertical GaN transistor as a highly promising candidate for high-power and high-speed applications.

We fabricate newly-developed GaN transistors on GaN substrates with reduced RonQoss (Ron : on-state resistance, Qoss : output charge), which enable 1MHz switching operation with smaller conduction losses compared with the counterparts fabricated on Si substrates. When the GaN transistor fabricated on the GaN substrate is employed in a boost DC-DC converter, its peak efficiency reaches 98.2% at the output power of 600W, in which the device-related loss is reduced by 45% compared with the case conventional GaN transistors on Si substrates are employed.

We propose a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltage of 2.5 V and stable switching operations. The employed p-type gate does not give any concern of the gate instability. Note that formation of carbon doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. We also propose a novel vertical GaN-based junction barrier Schottky (JBS) diode with trenched p-GaN region on a bulk GaN substrate. A specific differential on-resistance of the GaN JBS diode is 0.9 mΩ·cm2 while keeping high breakdown voltage of 1.6 kV. These results indicate that the demonstrated vertical GaN devices are very promising for future high power switching applications.

GaN-based normally-off Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction are fabricated on bulk GaN substrates. Thickness of insulating GaN buffer layer is increased up to 16 μm for the presented device from 5 μm for conventional GITs on Si. The thick buffer reduces the parasitic output capacitances, which enables fast turn-off switching. The thick buffer and the use of bulk GaN substrate help to improve the crystal quality of AlGaN/GaN so that the sheet resistance is reduced. Improved crystal quality together with reduced trap density successfully suppresses the current collapse up to 1 kV or higher of the applied drain voltage. The resultant R on Q oss (R on : on-state resistance, Q oss : output charge) as a figure-of-merit for high speed turn-off switching is reduced down to 940 mΩnC that is one third from that of GITs on Si. The resultant turn-off dV ds /dt reaches as large as 285 V/ns that is twice higher than reported values by GITs on Si.

A normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm 2 and high off-state breakdown voltage of 1.7 kV is presented. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltages of 2.5 V and stable switching operations. Note that formation of carbon-doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. These results indicate that the demonstrated vertical GaN transistor is very promising for future high power switching applications.

We present a novel GaN-based diode with low reverse leakage current which ensures the high voltage operation up to 600V. The diode consists of multi-junctions of AlGaN/GaN with a p-GaN overlayer where the anode and cathode are formed on the sidewalls of the channels. The tunneling current which is the origin of the leakage current can be reduced by controlling the potential barrier at the anode sidewall by means of the depletion layer from the p-GaN. The fabricated GaN diode with the p-type barrier controlling layer (BCL) exhibits high forward current of 18A at 1.5V with the breakdown voltages over 600V taking advantages of the reduced leakage current. The fabricated GaN-based diode has smaller RonC of 70 pΩF than 95 pΩF of the commercially available SiC Schottky barrier diode (SBD) indicating that the GaN diode is suitable for power switching. The GaN diode exhibits high conversion efficiency of 98.2 % in the voltage boosting converter at the output voltage of 400V by combining it with a GaN Gate Injection Transistor (GIT). The obtained performance by using the GaN diode is superior to that with a SiC SBD.

A breakdown mechanism of polarized semiconductors represented by GaN-based materials is presented, based on the concept of a natural super junction, which is established by the inherent material polarization. In this concept, owing to the precise matching of positive and negative polarizations of both sides of GaN and AlGaN materials, average charge concentration in the material becomes nearly zero under reverse bias condition, which realizes extremely high breakdown voltage. This model is confirmed by device simulation taking all polarization charges of GaN-based materials into account. Furthermore, experimentally fabricated GaN-based Schottky barrier diodes showed a linear increase of breakdown voltage along the anode-cathode spacing, achieving a record breakdown voltage over 9000 V.

We propose a new breakdown mechanism of GaN-based electron devices called ldquonatural super junctionrdquo. The junction model is supported by device simulations and experiments for newly developed multi-channel diodes with a dual-recessed structure. Based on the model, the on-resistance of the diodes can be reduced keeping high breakdown voltages. The fabricated diode achieves extremely high breakdown voltage of 9300 V with low on-state resistance RonA of 176 mOmegacm2, which is the record low value for GaN-based SBDs with the breakdown voltage over 9000 V.

We report ultra high voltage AlGaN/GaN heterojunction transistors (HFETs) on sapphire with thick poly-AlN passivation. Extremely high blocking voltage of 8300 V is achieved while maintaining relative low specific on-state resistance (Ron*A) of 186 mOmegaldrcm 2 . Via-holes through sapphire at the drain electrodes enable very efficient layout of the lateral HFET array as well as better heat dissipation.