This study compares single-heterostructure (SH) and double-heterostructure (DH) p-GaN gate AlGaN/GaN high-electron-mobility transistors (HEMTs) and clarifies how the DH structure suppresses short-channel effects (SCEs) during gate-length scaling. Experimentally, clear SCEs appear in short-channel SH devices, whereas they are strongly suppressed in the DH structure down to a gate length of $0.5~\upmu $ m. The short-channel-safe range, defined here by the absence of a negative threshold-voltage shift, is extended by at least threefold within the examined range. 2-D simulations reveal that the net negative interfacial sheet charge at the i-GaN channel/i-AlGaN back-barrier interface is the key origin of this improvement. This net negative interfacial charge raises the zero-bias channel barrier and increases the hole concentration near the p-GaN/i-AlGaN barrier interface, thereby suppressing both the drain-side lateral depletion of the p-GaN gate and the resulting localized ${E}_{C}$ lowering near the drain-side gate edge. These results show that the DH advantage arises from both zero-bias barrier enhancement and the suppression of drain-induced p-GaN gate depletion, thereby providing a physics-based design guideline for further scaling of p-GaN gate HEMTs.

The history of gallium nitride (GaN) research fights against GaN crystal growth. This chapter describes entire technology related to GaN power devices; GaN epitaxial growth, technical approaches of GaN lateral devices, device semiconductor processing, practical power applications, and device integrations utilizing advantages of GaN lateral devices. The GaN epitaxial layers on Si consist of the following parts: nucleation layer, buffer layer, and active layer. The chapter summarizes GaN epitaxy on Si substrate including impurity doping, choice of substrates, detailed epitaxial structures to initiate epitaxial growth, manage mechanical stress, and generate two-dimensional electron gas for GaN lateral power devices. GaN HFETs enable to make power conversion systems such as inverters and power supplies higher efficient and smaller size taking advantage of the superior device characteristics. GaN-based power devices are very promising devices for power electronics applications due to the superior device performance surpassing conventional Si-based power devices.

A safe connection of on-board automotive electric power grids is done by insulating HV/LV DC/DC converters. Latest electric car architectures are operating with 800 V batteries and 48 V power grids. Hence, compact 800 V / 48 V DC/DC converters are necessary. Within the following paper advantages and challenges of using SiC MOSFETs in such applications are discussed. A developed compact prototype with maximum power density of ca. 8 kW/dm(exp 3) and power weight of ca. 3 kW/kg is presented which uses different suggested solutions to manage the named challenges when using SiC MOSFETs.

Highly efficient three-phase to three-phase matrix converters using Gallium nitride (GaN) bidirectional switches with both high current and high breakdown voltage are demonstrated. The GaN switch with dual gates works as a bidirectional switch by a single device, while a conventional bidirectional switch consists of four devices by two Insulated Gate Bipolar Transistors (IGBTs) and two diodes. In addition, the GaN bidirectional switch is also free from the voltage offsets for the current conduction so that the GaN-based matrix converter enables small size and highly efficient AC/AC conversion. Improvement of the device performance including the introduction of the recessed gate enables the low on-state resistance with stable operation free from current collapse. The maximum drain current reaches 100 A together with the breakdown voltage of 1340 V. The fabricated three-phase to three-phase matrix converter exhibits the maximum conversion efficiency of 98% at 1 kW output power with the expectation that the maximum output power can reach 10 kW or more by the high current device.

In this paper, we present a state-of-the-art integrated GaN power IC capable of operating in a high frequency (MHz) regime. This realizes system size reduction, 60% maximum, of a power IC. The IC consists of two output power transistors (PT) and two gate drivers (GD). The key devices in the IC are normally-off gate injection transistors (GITs) for PT and GD and a normally-on hetero-junction field effect transistor (HFET) for GD. Novel local control of carrier concentration of an identical 2 dimensional electron gas (2DEG) at an AlGaN/GaN interface which made integration of the transistors with such a large threshold voltage difference possible is described. A specially developed post-passivation interconnection process giving low parasitic components is also described. The IC applied to a 12V-1.8V DC-DC converter shows high frequency switching operation well beyond the limit of Si pointing to future improvement in consumer electronics power supply systems.

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.

Kinetic studies on the current collapse of a normally-OFF AlGaN/GaN heterostructure gate-injection transistor (GIT) subject to current collapse have been performed above room temperature. The current collapse becomes more severe as the temperature increases, for which we clarified the physical mechanism based on a device simulation study that the hole traps in the epilayer play an important role. As the temperature increases, hole emission from the hole traps is stimulated, which causes sharper potential bending on the drain side in the OFF state, leading to more severe current collapse. The detailed dynamics of holes and the resultant energy profiles in the switching are discussed. (C) 2016 The Japan Society of Applied Physics

Due to the superior characteristics of GaN to Si, they can be utilized to drastically increase the efficiency and minimize the size of power converter system. However, there has been a critical issue of the so-called current collapse where ON-state resistance is increased once GaN transistor is exposed to high voltage. From the temperature dependence of the switching characteristics of an enhancement-mode GaN transistor, we found that deep "hole traps" play an important role in the current collapse. Based on this finding, we proposed a new device structure where hole emission in the OFF state is compensated by the holes injected from a drain-side pGaN. It was found that the proposed device is free from current collapse up to 800V. In this paper, we proposed the mechanism for the suppression of the current collapse in the proposed GaN transistor.

GaN-based natural superjunction diodes and Gate Injection Transistors (GITs) with p-type gate on AlGaN/GaN hetero structure are promising power devices with lower on-state resistance and higher breakdown voltage for power switching applications. In this paper, the current status of the devices for integrated circuits and their application to power switching systems are reviewed, after explaining the basic technologies for decreasing on-resistance, increasing breakdown voltage, and suppressing current collapse. In addition, a solution to increase switching frequency of GITs for smaller system size is described. The effects of integrated circuit of DC/DC converter consisted of gate driver, high-side GIT and low-side GIT with short gate length are also examined.

Current collapse is suppressed up to 800 V of drain voltage in our proposed device, Hybrid-Drain-embedded Gate Injection Transistor (HD-GIT), where an additional p-GaN layer is grown on the AlGaN barrier layer and is connected to the drain electrode. We present, based on a device simulation and electroluminescence study, that the hole injection from the additional drain-side p-GaN at the OFF state compensates the hole emission in the epilayer. As a result, the gate-drain access region is not negatively charged at the OFF state, resulting in the drastic suppression of current collapse in HD-GIT.

Highly efficient conversions from 12 V to 1.2 V are confirmed at 2-5 MHz with high output current by DC-DC converters using Gallium Nitride (GaN) Gate Injection Transistors (GITs). Reduction of the gate lengths of the GITs with optimized structure designs reduces the RonQg down to 19.1 mOmeganC, which is a figure-of-merit for high speed switching. The peak operating efficiency at 2 MHz reaches 90% together with high output current of 50 A by the single-phase operation. The operating frequency can be increased to 5 MHz by the GaN-based DC-DC converter with the peak efficiency as high as 81%, which cannot be achieved by existing Si-based converters.

In this paper, we present a novel compact DC-DC converter IC in which normally-off GaN-GITs (Gate Injection Transistors) and gate drivers are integrated into one chip. The DC-DC converter IC can achieve higher efficiency and smaller chip size by reducing parasitic inductances between switching power devices and gate drivers. The gate driver, having a DCFL (Direct Coupled FET Logic) with a buffer amplifier which is consisted of a GaN-HFET (Hetero-junction FET) and GaN-GITs can operate with higher speed and lower power consumption. The fabricated DC-DC converter IC exhibits a peak efficiency as high as 86.6% at 2MHz for the 12V-1.8V conversion.

GaN-based Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction serve normally-off operations with low on-state resistances owing to the conductivity modulation by injection of holes. Established basic technologies on the GIT have shown promising features for switching applications. Further improvement of the performances would extend the applications and lead to the widespread use. In this paper, recent technologies on the GITs to improve the performances and extract the full potential are described. These include extension of the wafer diameter of Si up to 8 inch, InAlGaN quaternary alloy to reduce the series resistances, shortening the gate length to improve the device performances, integration of the gate driver and flip-chip assembly for faster switching.

Ion implantation technique can be applied for planar isolation of AlGaN/GaN heterojunction field-effect transistors (HFETs), which enables high-density integration of the power switching transistors. So far, the reported isolation using ion implantation for GaN devices has never maintained high isolation voltages after high-temperature processing over 800 °C which is commonly used for the fabrication. In this paper, we present detailed analysis and mechanism of thermally stable isolation of GaN devices by Fe ion implantation keeping high breakdown voltage between the devices after high-temperature annealing. Ion species forming deep levels at atomic sites in GaN are examined by using first-principle calculation prior to the experiments. The calculation indicates that the Fe ions stay at Ga sites with deep levels in GaN. The following experiments using various ion species well agree with the aforementioned predictions, where implanted regions by other ions than Fe exhibit reduction of the resistivity after high-temperature annealing to recover the processing damage by the ion implantation. As a result, it is experimentally found that Fe is the only choice to serve high resistivity after the annealing. The Fe ion implantation enables high breakdown voltage of 900 V after the annealing at 1200 °C. This technique is indispensable to enable monolithic integration of the lateral AlGaN/GaN HFETs for high-voltage power switching systems.