# Vertical GaN MOSFET Power Devices
## 1 Introduction  
Power semiconductor devices serve as the fundamental building blocks within a power conversion system, exerting significant influence over factors such as system losses and switching speed. The performance of these power devices is primarily characterized by two key parameters: breakdown voltage (\(V_{BD}\)) and on-resistance (\(R_{ON}\)). These characteristics largely depend on the inherent material properties of semiconductors, including carrier mobility and critical electric field. Traditionally, silicon (Si) has been a cornerstone in the realm of power semiconductor devices. However, Si's material limitations have constrained its further advancement. Consequently, there is an increasing demand for wide bandgap (WBG) semiconductors with bandgaps exceeding 3 eV and even ultra-wide bandgap semiconductors with bandgaps exceeding 4.5 eV [1]. Among the suite of wide bandgap materials, gallium nitride (GaN) has emerged as a pivotal material for crafting power devices owing to its exceptional material properties, including a wide bandgap of 3.4 eV, high critical electric field of 3.3 MV/cm, robust resistance to radiation, and impressive saturation velocity [2; 3]. In addition to their high voltage handling capabilities, GaN-based power devices offer low \(R_{ON}\) and minimal conduction losses due to GaN's electron saturation rate, which is 2.8 times that of silicon [4; 5]. Furthermore, GaN devices, especially GaN HEMTs, exhibit high-speed switching performance attributable to their small junction capacitance.  
At present, GaN devices are categorized into three voltage ranges [6]. (a) Low (mid)-voltage devices: These devices have a maximum drain-to-source voltage (\(V_{DS,\,max}\)) of less than 200 V. They find utility in various fields and are used in applications such as DC-DC power converters, motor drives, class D audio amplifiers, solar microinverters, synchronous rectifications, and LiDAR systems, among others. Depending on specific applicationrequirements, these devices can exhibit \(R_{ON}\) values ranging from approximately 2 m\(\Omega\) to 200 m\(\Omega\)[7; 8; 9; 10]. (b) High-voltage devices: The \(V_{DS,\ max}\) for these devices goes up to 650 V. They are used in applications such as industrial converters, telecommunication servers, servo motor control, power adapters, consumer electronics adapters, class D amplifiers, and data centers [11; 12]. (c) Ultra-high-voltage devices: These devices have a \(V_{DS,\ max}\) greater than 1 kV. Currently, there are few commercially available GaN power devices in the kV range. The highest voltage rating among commercially available GaN devices is approximately 900 V [13], which is used in applications like industrial applications, EV chargers, photovoltaic inverters, and more.  
Leading companies such as Transphorm Inc., Infineon Technologies AG, GaN Systems Inc., Panasonic Corporation, Texas Instruments Inc., and others are actively working on introducing GaN devices with voltage ratings exceeding 1 kV. The distribution of WBG power devices based on voltage range is illustrated in Figure 1. Currently, several research articles demonstrate the feasibility of GaN power devices rated at 1 kV and above [14; 15; 16]. GaN power devices in the kV range will compete with other WBG power devices, including those based on SiC and Ga\({}_{2}\)O\({}_{3}\) in various applications, such as automotive, industrial, and photovoltaic sectors. However, though some groups have reported achieving high blocking voltage GaN devices, the specific on-resistance (\(R_{ON\text{-}SP}\)) of the GaN device is still much higher than that of the SiC counterpart [17; 18].