# Vertical GaN MOSFET Power Devices
## 4 Techniques to Boost the Breakdown Voltage
### Drift Layer Thickness and Doping Concentration  
Although the GaN vertical MOSFET is grown homoepitaxially on a bulk GaN substrate, the amount of dislocation density present is still very high. To achieve a high-performance GaN device, an improved epitaxial layer is required. Additionally, the thickness and doping concentration of the buffer or the drift layer also play important roles in enhancing the device's performance [79]. A thicker drift or buffer GaN layer improves the material quality by reducing defect density and resulting in better surface morphology. Moreover, a thicker drift layer can filter dislocations during epitaxial growth and prevent dislocations from propagating towards the upper layers [80; 81; 82]. Another critical factor for enhancing performance is the optimization of doping concentration in the drift region. To obtain the optimized doping concentration for the drift region, it is imperative to understand the dependence of doping concentration on the \(V_{BD}\) and the \(R_{ON\text{-}SP}\) of the device. One of the most preferred benchmarks for power devices is the _PFOM_ (power figure of merit) given by [83; 84; 85]:  
\[PFOM=\frac{{V_{BD}}^{2}}{{R_{ON\text{-}SP}}} \tag{1}\]  
The connection between the doping concentration and the \(R_{ON\text{-}SP}\) of the drift region can be assessed through majority carrier conduction, as expressed by the following equation:  
\[R_{ON\text{-}SP}=\left(\frac{t_{Drift}}{q\mu_{n}(n)}\right), \tag{2}\]where \(t_{Drift}\) represents the thickness of the drift layer, \(n\) signifies the doping concentration, and \(\mu_{n}\) denotes the electron mobility, which depends on the doping concentration. The correlation between electron mobility and doping concentration is elucidated by the Mnatsakanov empirical mobility model [86], given by  
\[\mu_{N}(n)\mu_{min}+\left(\frac{\mu_{max}-\mu_{min}}{1+(n/N_{G})^{\gamma}}\right), \tag{3}\]  
In this equation, \(\mu_{min}\), \(\mu_{max}\), \(N_{G}\), and \(\gamma\) are the parameters used for fitting. However, establishing a direct relationship between the doping concentration of the drift layer and the \(V_{BD}\) is not a straightforward task. It necessitates a thorough examination of the primary causes of the avalanche effect, which can be attributed to either parallel-plane breakdown or punch-through breakdown [87]. Parallel-plane breakdown, also referred to as non-punch-through breakdown, primarily depends on factors such as the doping concentration of the drift region and the impact ionization coefficient of the material rather than the thickness of the drift layer. The distribution of the electric field within the device for both types of breakdowns is illustrated in Figure 6:  
In the case of parallel-plane breakdown, the electric field distribution takes on a well-defined triangular shape across the drift region. Under these conditions, determining the maximum electric field is a straightforward task, and device breakdown occurs when this maximum electric field reaches the critical electric field (\(E_{crit,NPT}\)) characteristic of the device. The typical critical electric field (\(E_{crit}\)) in GaN can be computed using the following equation, which draws upon values reported by [PERSON] et al. [89]:  
\[E_{Crit}=\left(\frac{2.162\times 10^{6}+800\times T}{1-0.25.\text{log}_{10}(n/10 ^{6})}\right), \tag{4}\]  
where \(T\) is the temperature in kelvin. In the scenario of punch-through breakdown, the electric field distribution within the drift region assumes a trapezoidal shape, primarily because the drift region becomes entirely depleted of charge carriers. Within this configuration, the electric field experiences a more gradual change in the drift region, owing to its lower doping concentration, while it exhibits a rapid variation over distance in the n+ doping region due to the higher doping concentration [90].