## 4 Results and Discussion
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Trench depth (\(H_{\text{T}}\)) & 7.5 \(\upmu\)m \\ Gate depth(\(H_{\text{G}}\)) & 1.0 \(\upmu\)m \\ n\({}^{+}\) source junction depth & 0.2 \(\upmu\)m \\ p- body junction depth & 0.8 \(\upmu\)m \\ p- body doping concentration & \(1.5\times 10^{18}\) cm\({}^{-3}\) \\ Trench width (\(W_{\text{T}}\)) & 3.6 \(\upmu\)m \\ Mesa Width (\(W_{\text{M}}\)) & 3.2 \(\upmu\)m \\ Width of E\({}_{1}\) (\(T_{1}\)) & 1.6 \(\upmu\)m \\ Thickness of field oxide (\(t_{1}\)) & 1.2 \(\upmu\)m \\ n-drift region doping concentration (\(N_{\text{D}}\)) & \(1\times 10^{15}\) cm\({}^{-3}\) \\ Doping of lower epitaxial layer (\(N_{2}\)) & \(5\times 10^{15}\) cm\({}^{-3}\) \\ Doping of middle epitaxial layer (\(N_{1}\)) & \(1.5\times 10^{16}\) cm\({}^{-3}\) \\ Doping of upper epitaxial layer (\(N_{2}\)) & \(5\times 10^{15}\) cm\({}^{-3}\) \\ Thickness of gate oxide & 50 nm \\ Thickness of n’ drift region (\(L\)) & 16 \(\upmu\)m \\ Thickness of lower epitaxial layer (\(L_{1}\)) & 3.2 \(\upmu\)m \\ Thickness of middle epitaxial layer (\(L_{2}\)) & 4 \(\upmu\)m \\ Thickness of upper epitaxial layer (\(L_{3}\)) & 8.8 \(\upmu\)m \\ Height of source electrode of SGRSO (\(H\)) & 6.3 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural parameters adopted in the simulations. improved by 402% in contrast to the SGRSO UMOSFET as the doping concentration of the drift region is \(5\times 10^{15}\)/cm\({}^{-3}\) and \(1\times 10^{15}\)/cm\({}^{-3}\) for HDSGRSO and SGRSO UMOSFETs. The BV and FOM (BV\({}^{2}\)/\(R_{\rm{SP}}\)) at different widths of the doped layer \(L_{2}\) are shown in Figure 6b. It gets the optimal FOM and BV when the \(L_{2}\) is 4 \(\upmu\)m. The BV and FOM (BV\({}^{2}\)/\(R_{\rm{SP}}\)) at different thickness of the upper n-drift (\(L_{3}\)) is shown in Figure 6c when the \(L_{2}\) is 4 \(\upmu\)m. It achieves the highest FOM and BV when \(L_{3}\) is 3.2 \(\upmu\)m. Figure 6d shows the BV and FOM (BV\({}^{2}\)/\(R_{\rm{SP}}\)) at different doping concentrations of the high doped epitaxial layer when the n-drift region is \(5\times 10^{15}\)/cm\({}^{-3}\). It is obvious the BV of the HDSGRSO UMOSFET is gradually increasing with the increase of the doping concentration of the high doped epitaxial layer. It gets optimal FOM and BV when the doping concentration of the high doped epitaxial layer is \(1.5\times 10^{16}\)/cm\({}^{-3}\), because the modulation effect of the electric field in the n-drift gradually increases and reaches the optimal level with the increasing of the doping concentration of high doped epitaxial layer. However, when the doping concentration of the middle epitaxial layer is higher than \(1.5\times 10^{16}\)/cm\({}^{-3}\), the charge balance in the n-drift will be broken and the BV will drop steeply. Figure 7 shows the BV and FOM (BV\({}^{2}\)/\(R_{\rm{SP}}\)) at different thicknesses of the drift region. It indicates that there exists a saturated BV at a certain thickness. When the L is 14 \(\upmu\)m, the FOM value and BV reaches the optimal value, respectively, and the BV is about 240 V. With the increase of epitaxial thickness, the \(R_{\rm{SP}}\) gradually increases, resulting in the decrease of FOM. The L is 14 \(\upmu\)m for HDSGRSO UMOSFET, whereas the L' is 16 \(\upmu\)m for SGRSO UMOSFET when the BV is 240 V. Because of the enhancing of the electric field by high doped epitaxial layer, the breakdown occurs before the n-drift is depleted completely. Therefore, it can reduce the thickness of the epitaxial layer and \(R_{\rm{SP}}\). As a result, the FOM has been increased by 464% as compared with the SGRSO UMOSFET when the BV is about 240 V, according to the simulation results. Figure 5: Drain I-V characteristics: (**a**) Breakdown Voltage (BV); (**b**) \(R_{\rm{SP}}\) at different gate voltage with \(V_{\rm{DS}}\) = 0.3 V and percentage reduction in \(R_{\rm{SP}}\) as compared with the SGRSO UMOSFET; (**c**) output characteristics and (**d**) transconductance of two structures.