# Vertical GaN MOSFET Power Devices
## 3 Vertical GaN Field Effect Transistors  
Several types of vertical GaN devices have been developed, including p-i-n diodes [57; 58], vertical GaN Schottky barrier diodes (VSBDs), and vertical GaN transistors. Among vertical GaN transistors, two primary types exist: (a) current aperture vertical electron transistor (CAVET) and (b) vertical GaN MOSFET. Figure 3a-d illustrate the schematic structure of vertical GaN devices. Vertical GaN MOSFETs can be further categorized into two subtypes: (a) planar GaN MOSFET and (b) trench GaN MOSFET. Planar GaN vertical MOSFETs are crafted using ion implantation to create regions like p-well and n+ source contact regions, resulting in a relatively cost-effective fabrication process [59]. However, significant challenges include achieving precise control of the acceptor concentration in the p-well to determine the \(V_{TH}\) and addressing the formation of residual point defects during p-well ion implantation and post-implantation annealing. These factors are crucial for ensuring stability in the \(V_{TH}\) and overall device reliability [60]. As a result, from the early stages, the trench MOS structure of the gate is adopted since the p-well can be formed by epitaxial growth to avoid the immature ion-implantation of p-GaN in device fabrication. The concept of the CAVET structure was introduced in 2004 [61]. This device structure bears similarity to a double-diffused metal-oxide-semiconductor (DDMOS) structure and exhibits a negative VTH due to the presence of a channel region beneath the gate, arising from the AlGaN/GaN heterointerface. However, the vertical trench GaN MOSFET is a normally-off device, which is preferred for safe power device operation. Therefore, trench GaN MOSFETs have gained preference over CAVET devices. Moreover, they offer a relatively straightforward manufacturing process, eliminating the need for regrowth of the AlGaN/GaN layer [62; 63]. In 2007, ROHM Co. Ltd. (Kyoto, Japan) achieved a significant milestone by reporting the first-ever GaN trench MOSFET [64]. The following year, in 2008, a normally-off vertical GaN trench MOSFET, utilizing a common MOS structure, was introduced with an impressive \(V_{TH}\) of approximately 3 V [65]. Fast-forwarding to 2014, [PERSON] et al. achieved a breakthrough by presenting a vertical GaN trench MOSFET on a freestanding GaN substrate. This device boasted a formidable \(V_{BD}\) of around 1.6 kV, along with a 7 V \(V_{TH}\). It employed a field plate termination strategy around the mesa isolation, effectively reducing electric field crowding at the p-n junction edge [41]. Then, in 2015, [PERSON] et al. extended their pioneering work to unveil a vertical trench MOSFET featuring a low \(R_{ON-SP}\) of 1.8 m\(\Omega\cdot\)cm\({}^{2}\). This remarkable device had a \(V_{BD}\) of approximately 1.2 kV and operated at over 20 A [66]. This modified device was based on the authors' prior work [41] and featured a hexagonal trench gate layout, effectively reducing the \(R_{ON-SP}\) by increasing the gate width per unit area. In 2016 [67], the same research group made notable advancements. They demonstrated a single-cell vertical GaN MOSFET featuring an impressive \(V_{BD}\) of around 1.6 kV and \(R_{ON\cdot SP}\) of approximately 2.7 m\(\Omega\cdot\)cm\({}^{2}\) for a single cell. These achievements were realized by decreasing the n-GaN drift layer doping concentration of the previous work [66]. Notably, the group also reported the fabrication of a multicell MOSFET with a chip size measuring 1.5 mm \(\times\) 1.5 mm, which exhibited a \(V_{BD}\) of roughly 1.3 kV. This device could operate at a drain current of up to 23.2 A and demonstrated rapid switching characteristics. Moving on to 2016, a normally-off 1.2 kV \"all-gate\" vertical MOS-gate transistor was designed and successfully fabricated [68]. This innovative structure featured two MOS gates on both sides of the source pillar and achieved an \(R_{ON\cdot SP}\) of approximately 2.8 m\(\Omega\cdot\)cm\({}^{2}\).