An ideal MOS capacitor has boron doping-concentration of $10^{15}cm^{-3}$ in the substrate. When a gate voltage is applied, a depletion region of width $0.5$ $\mu m$ is formed with a surface (channel) potential of $0.2$ $V$. Given that $\varepsilon _{0}= 8.854\times 10^{-14}F/cm$ and the relative permittivities of silicon and silicon dioxide are $12$ and $4$, respectively, the peak electric field (in $V/\mu m$) in the oxide region is _______________.