Introduction

In power supply design, the following divisions of labor have existed until now.
Low voltage/high frequency: Lateral GaN
Medium to high voltage: SiC MOSFET
General-purpose and low-cost: Silicon SJ MOSFET / IGBT
However, the recent emergence of Vertical GaN (GaN-on-GaN JFET), hereafter referred to as vertical GaN, is blurring these boundaries. This article explains its technological advantages.

Superiority in high-frequency switching performance (loss and frequency)

Increased voltage resistance (future upgrade to 1700V/2000V)

Vertical GaN (vertical power devices with a GaN-on-GaN structure) employs a structure where current flows in the direction of the substrate (vertically), so the drain-source voltage (VDS) breakdown voltage is mainly determined by the thickness of the drift layer and the doping design. This is a fundamentally different characteristic from the breakdown voltage design that was important for lateral GaN and conventional silicon devices, which depended on the distance between electrodes.

In actual devices, breakdown voltages of over 1700V have been observed for a 1.2kV class rating, and further scaling to 1700V to 2000V class is structurally possible by increasing the thickness of the drift layer. This high voltage resistance is supported by a vertical structure that makes maximum use of the high critical field strength (approximately 3 MV/cm²) of the GaN material.

Furthermore, because vertical GaN employs a homoepitaxial structure in which GaN is grown on a GaN substrate, it has fewer defects caused by lattice mismatch, suppressing electric field concentration and increased leakage current. In addition, the breakdown voltage exhibits a positive temperature coefficient, so electric field concentration is mitigated even when the temperature rises, resulting in stable breakdown voltage characteristics that are less prone to thermal runaway. This characteristic is an important advantage in automotive and industrial applications that require operation in high-temperature environments.

Thus, Vertical GaN is a power device that combines the flexibility of voltage resistance design due to its vertical structure with the high field resistance of GaN material, achieving both higher voltage resistance and higher efficiency operation than conventional devices.
ON Semiconductor explained that the Idss and Igss values in the ambient temperature range of 25 °C to 200 °C are for reference only, but they are approximately 100​ ​μA.

Gate structure and gate pressure resistance

A major feature of vertical GaN is that, unlike conventional silicon MOSFETs, SiC MOSFETs, and even lateral GaN (HEMTs), it employs a PN junction structure that does not use an oxide film in the gate. Specifically, current control is performed by the junction between the P-GaN gate and the N-GaN channel, thus eliminating the effects of dielectric breakdown and reliability degradation caused by the gate oxide film.

This structure allows vertical GaN to achieve both high gate breakdown voltage and excellent reliability simultaneously. In typical MOSFETs, the electric field strength of the gate oxide film is a limiting factor, and oxide film degradation due to overvoltage and dV/dt stress during high-speed switching becomes a problem. On the other hand, in vertical GaN, the PN junction shares the electric field, resulting in high electric field tolerance and excellent transient durability.

Furthermore, although the gate-source junction exhibits diode-like characteristics, the gate current is typically kept to a few mA during normal operation. By appropriately limiting the gate current, it is possible to operate it with voltage drive (VGS control) similar to that of a silicon MOSFET. This also offers the advantage of easily utilizing existing gate driver design assets.

Avalanche capability

Unlike conventional horizontal GaN, vertical GaN achieves avalanche operation by having a vertical pn junction consisting of p-GaN and n-GaN.

When a high voltage is applied, a strong electric field is formed near the drift layer and gate junction, causing carrier proliferation through impact ionization and generating an avalanche current. This current flows in a distributed manner across the gate region and channels, suppressing localized current concentration and enabling stable energy absorption.

According to ON Semiconductor, the 1200V class devices have been shown to exhibit avalanche voltages of over 1700V and avalanche energies of 7.44 J/cm², which is comparable to the withstand voltages (6 to 15 J/cm²) of state-of-the-art SiC MOSFETs. Furthermore, the breakdown voltage exhibits a positive temperature coefficient, enabling stable avalanche operation that is less susceptible to thermal runaway.

In this way, the vertical design overcomes the weakness of conventional GaN, which was "destructive operation under overvoltage," and possesses high robustness, allowing the device itself to safely absorb overvoltage energy caused by surges and inductive loads.

Short-circuit withstand capability

While power supply systems generally require a short-circuit withstand capability of around 10µs, conventional lateral GaN (HEMT) has a low short-circuit withstand capability of less than 1µs, making its application to high-reliability applications such as automotive inverters a challenge.
Vertical GaN is a device that structurally solves this problem. According to ON Semiconductor, short-circuit withstand power of over 30µs under 400V application conditions and over 10µs even at 800V has been confirmed, demonstrating robustness equivalent to or better than Si and SiC devices.

This excellent short-circuit withstand capability is due to the device's inherent current self-regulating mechanism. During a short circuit, a rapid rise in channel temperature reduces carrier mobility, and an increase in current across the gate-source PN junction lowers the effective gate voltage, automatically suppressing the drain current. As a result, the current decays significantly after its peak, preventing thermal runaway.

Furthermore, in the vertical configuration, carrier generation by the avalanche is distributed across the channel and gate regions, making current concentration less likely and enabling stable operation even under harsh conditions where short circuits and overvoltages are applied simultaneously. This characteristic makes it possible to achieve short-circuit withstand capability in high-voltage regions, which was difficult with conventional GaN devices.

Furthermore, regarding the failure mode after repeated avalanche testing, it was explained that the drain-source connection becomes open ("fail-open"), which significantly contributes to the overall system safety by maintaining voltage resistance. In addition, since almost no characteristic degradation was observed even after 10,000 hours of bias testing, it was found to have excellent characteristics from the perspective of long-term reliability.

Thus, vertical GaN is a device that achieves breakdown withstand capability equal to or better than conventional devices while maintaining high-speed switching performance, and is expected to be applied to high-reliability applications such as electric vehicle inverters and power converters.

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