A potential barrier of 0.5V exists across a p-n junction. An electron with speed $5\times {10}^{6}m/s$  approaches the p-n junction from the n-side, with what speed will it enter the p-side?

$As the electron crosses the potential barrier, its speed decreases.$

$$\begin{gathered} U\sin g Conservation of Mechanical Energy, \\ \Rightarrow \frac{1}{2}m{V}_i^2=eV+\frac{1}{2}m{V}_f^2 \end{gathered}$$

$\Rightarrow \frac{1}{2}(9.1\times {10}^{-31}){(5\times {10}^6)}^2 =(0.5\times 1.6\times {10}^{-19})+\frac{1}{2}(9.1\times {10}^{-31})V_f^2$

$\Rightarrow V_f=2.7\times {10}^{5}m/s$