# Vertical GaN MOSFET Power Devices
## 4 Techniques to Boost the Breakdown Voltage
### Edge Termination  
An established technique for improving \(V_{BD}\) entails employing an edge termination method to mitigate the concentration of electric field lines near the edge of the p-n junction. This technique is closely linked to the phenomenon of impact ionization, which exerts a significant influence on the \(V_{BD}\) of the device [98]. Edge termination represents a method  
Figure 8: GaN MOSFET with double field plate Reprinted with permission from Ref. [73]. 2017, [PERSON]. ology for modifying the contour of the depletion layer at the junction's edge. This can be achieved through various means, including etching, anode extension, ion implantation, and dielectric passivation. In the realm of GaN devices, various types of edge termination techniques have been reported, such as mesa etches, floating guard rings, plasma treatment, junction termination extension [99], ion implantation [57], and level termination [100]. The application of edge termination techniques does not compromise the rectifying properties of the device. Among these techniques, ion implantation stands out as a widely adopted method in Si and SiC power devices. However, in the context of GaN devices, ion implantation is still relatively less utilized, as indicated in reference [101]. Ion implantation is employed in GaN devices to achieve isolation by introducing material damage through controlled bombardment, resulting in the creation of mid-gap defects [102; 103; 104], hence, creating a highly resistive edge that facilitates the lateral distribution of the electric field. Another technique that utilizes ion implantation involves managing the charge on the edge termination by offsetting the region through the introduction of counter-doping in p-GaN with Si, as detailed in reference [105]. Additionally, N ion implantation can be employed to create a semi-insulating region by controlling the total dose within the edge termination [104]. The ion implantation method is also employed in techniques such as junction termination extension (JTE) and field-limiting rings (FLR). In the case of JTE, an ion implantation process is used to create a thin, lightly doped layer encircling the diffusion junction area. Figure 9 illustrates a device featuring FLR edge termination, where the extension of this thin layer results in the expansion of the depletion layer, akin to the effect of a field plate [106]. This extension helps to reduce the issue of electric field crowding at the junction. Nevertheless, a major challenge faced by GaN devices lies in the difficulty of establishing p-type regions through ion implantation, as highlighted in reference [107]. Mesa termination stands out as the most widely adopted technique in vertical GaN power rectifying devices. This method involves selectively removing a portion of the device using dry etching techniques. Typically, the use of a 90\({}^{\circ}\) angle mesa termination is avoided because it leads to significant electric field crowding, which can reduce the device's VBD. However, a study conducted by [PERSON] et al. [108; 109] reveals that deep mesa termination can enhance the \(V_{BD}\) and reduce reverse leakage. The device with deep mesa (5-10 \(\upmu\)m) exhibits a uniform electric field distribution across the entire device, while shallow mesa (1-2 \(\upmu\)m) tends to have increased leakage and lower \(V_{BD}\). Another commonly employed mesa termination technique for GaN devices is the beveled mesa, where the heavily doped region is removed to a greater extent than the lightly doped side [90]. Figure 10 illustrates a schematic representation of mesa edge termination in GaN power devices [96]. Furthermore, to mitigate electric field crowding at the edge, a combination of field plates with mesa termination is often used. Mesa termination, when coupled with a field plate, can alleviate the electric field at the junction without the need  
Figure 9: GaN MOSFET with field-limiting rings (FLRs). Reprinted with permission from Ref. [106]. 2022, [PERSON] and [PERSON]. for ion implantation [45; 110].