# Vertical GaN MOSFET Power Devices
## 4 Techniques to Boost the Breakdown Voltage
### Field Plate Termination  
Another widely adopted technique used to enhance the \(V_{BD}\) of power devices is the incorporation of field plate (FP) technology. GaN-based HEMTs have particularly benefitted from this approach, as it helps increase the \(V_{BD}\) by mitigating electric field concentration at the edges of the gate or drain electrodes [92; 93; 94]. During the off state of the power device, the electric field distribution is not uniform across the device. Instead, there are areas of high electric field concentration, specifically located at the edges of the gate or drain electrodes. This heightened electric field can lead to increased leakage current in these regions, attributed to the tunneling effect, thereby causing premature breakdown of the device. Moreover, the presence of this high electric field can impact the charging and discharging processes of the device, potentially resulting in switching delays.  
In the vertical GaN MOSFET, there is a highly doped n-type GaN region for the ohmic contact and a lightly doped n-type GaN drift layer for voltage blocking. Due to these structural distinctions, the distribution of electric fields within the device differs from that in lateral GaN devices. Electric field crowding primarily occurs at the interface between the gate metal and the semiconductor. This leads to the formation of a depletion region around the edge of the gate, where the electric field resulting from the space charge accumulation intensifies, significantly enhancing the electric field strength in that region [95]. Figure 8  
Figure 7: **(Top)** PFOM vs. doping concentration, and (**bottom**) breakdown vs. doping concentration. Reprinted with permission from Ref. [85]. 2023, [PERSON].  
depicts the schematic structure of a vertical GaN OG-FET with a field plate [73]. Through the use of the first field plate (FP1), the high electric field at the gate metal edge is reduced from 3.7 MV/cm to 2.6 MV/cm [96]. The field enhancement factor, \(\eta\), which characterizes the degree of field crowding, is used to investigate the effect of field crowding quantitatively and is given by  
\[\eta=\frac{E_{M}}{E_{0}}, \tag{6}\]  
where \(E_{M}\) represents the maximum gate-edge electric field, and \(E_{0}\) denotes the electric field at the opposite end of the gate [95]. In the case of the device without FP, the field enhancement factor exhibits a linear dependence on the depth of the depletion region. As the depletion region becomes deeper, there is a greater concentration of the electric field stemming from the nearby space charge near the gate edge, resulting in an augmented field enhancement factor, \(\eta\)[97]. An empirical formula of \(\eta\) and the depletion region depth (\(W_{DD}\)) is given by [95]  
\[\eta=1+\text{a}W_{DD}, \tag{7}\]  
where the parameter \"\(a\)\" is approximately 1.5 \(\upmu\)m\({}^{-1}\) and exhibits some degree of dependency on various structural parameters. The relationship of \(\eta\) on \(W_{DD}\) is different for different types of breakdown. For parallel-plane breakdown, \(\eta\) is the same as the formula given in Equation (7). For punch-through condition, \(\eta\) is given by  
\[\eta=1+\text{a}T_{GaN} \tag{8}\]  
where \(T_{GaN}\) is GaN drift layer thickness. With FP, the depletion region extends beyond the gate edge, partially overlapping with the FP electrode. Consequently, the concentration of the electric field at the gate edge disperses towards the edge of the FP electrode. This redistribution results in a reduction of the maximum electric field at the contact edge, thereby decreasing \(\eta\). While the incorporation of the FP improves the device's \(V_{BD}\), it also introduces another high-field region situated within the semiconductor and the dielectric material beneath the FP edge region, and the breakdown in this region becomes another issue to be concerned. The effect of FP also depends on the type of dielectric materials as well as the height of the FP from the semiconductor.