A potential barrier of \( 0.4 \, \text{V} \) exists across a \( p-\!n \) junction. An electron enters the junction from the \( n \) - side with a speed of \( 6.0 \times 10^{5} \, \text{m s}^{-1} \). The speed with which electron enters the \( p \) side will be \( \frac{x}{3} \times 10^{5} \, \text{m s}^{-1} \) the value of \( x \) is ______.
(Given mass of electron \( = 9 \times 10^{-31} \, \text{kg} \), charge on electron \( = 1.6 \times 10^{-19} \, \text{C} \).)
