# Vertical GaN MOSFET Power Devices
## 3 Vertical GaN Field Effect Transistors
In 2020 [59], researchers introduced a 1.2 kV vertical GaN planar MOSFET with an effective one-resistance of 1.4 m\(\Omega\cdot\)cm\({}^{2}\), employing an ion implantation process. To enhance the device's \(V_{BD}\), they executed a sequential implantation process using Mg and N. Additionally, a short cell pitch design was implemented to improve the device's \(R_{ON}\). In the most recent developments of 2022 [69], a 1.3 kV vertical GaN MOSFET was introduced on a 4-inch freestanding GaN substrate. This device featured a \(V_{TH}\) of approximately 3.15 V and demonstrated a low \(R_{ON\cdot SP}\) of 1.93 m\(\Omega\cdot\)cm\({}^{2}\). These achievements translated into an impressive power figure of merit (PFOM) of 0.88 GW/cm\({}^{2}\). Figure 4 provides a comparison of the PFOM for vertical GaN MOSFETs with breakdown voltages of 1 kV and above, highlighting the progress made in this field over the years. While GaN MOSFETs hold great promise for high-performance power devices, they encounter two significant challenges during fabrication and operation. Firstly, the device exhibits low mobility within the p-GaN inversion layer, necessitating higher gate voltages for device activation. Secondly, achieving reliable ohmic contacts becomes problematic after the plasma etching process in the buried p-type region (used for body contact) as the generation of n-type compensating vacancies increases during the etching process [70]. Moreover, the p-type GaN layer has lower acceptor activation and much lower mobility compared to the n-type. To overcome these challenges, innovative GaN transistor structures  
Figure 3: Vertical GaN power devices include (**a**) GaN power rectifier, (**b**) GaN CAVET, (**c**) GaN planar MOSFET, and (**d**) GaN Trench MOSFET. have been introduced, including fin structures [71; 72] and in situ oxide-GaN interlayer FETs (OG-FETs) [73; 74]. Figure 5 illustrates schematic diagrams of fin structure GaN transistors and OG-FETs. The novel GaN vertical fin power field-effect transistor features a fin-shaped n-GaN channel enclosed by gate oxide and a gate electrode, enabling precise control of current flow. This device exclusively relies on n-type GaN and eliminates the need for epitaxial growth. The device achieves normally-off operation by ensuring the fin width remains below 500 nm. The difference in work function between the gate electrode and GaN allows for complete electron depletion in the channel, resulting in normally-off operation without requiring a p-GaN layer. In [72], a 200 nm fin width normally-off vertical GaN FinFET with a 1.2 kV \(V_{BD}\) and a 0.8 V \(V_{TH}\) was reported. This device exhibited a low \(R_{ON\text{-}SP}\) of approximately 0.2 m\(\Omega\cdot\)cm\({}^{2}\), translating to an impressive PFOM value of 7.2 GW/cm\({}^{2}\). It was noted that a larger device could achieve a higher current of around 10 A at 800 V. However, both large and small devices faced the challenge of catastrophic breakdown, highlighting the impact of the absence of the p-GaN layer on device robustness. Additionally, strict control over fin width to achieve a normally-off device increased fabrication costs [71]. On the other hand, the OG-FET trench MOSFET structure enhances channel mobility without compromising its normally-off characteristics. Generally, in an MOSFET, oxide growth occurs ex situ using atomic layer deposition (ALD) after trench etching. In contrast, OG-FET leverages MOCVD for a regrowth process [75]. The improved channel mobility can be attributed to the in situ oxide and the regrowth of a thin unintentionally doped (UID) channel layer. The UID GaN channel improves the channel mobility by reducing  
Figure 4: The comparison of PFOM for various vertical GaN MOSFET with a breakdown voltage of 1 kV and above [41; 59; 66; 67; 68; 69]. Figure 5: (a) Fin structure vertical GaN MOSFET and (b) in situ oxide–GaN interlayer FET (OG_FET). impurity scattering. In [73], a 1.4 kV GaN OG-FET with a double field-plated structure was reported.