Formation of barrier potential: A $\mathit{p}-\mathit{n}$ junction is formed by bringing $\mathit{p}$-type and $\mathit{n}$-type semiconductors together in a very close proximity.At the instant of $\mathit{p}$ - $\mathit{n}$-junction, movement of free electrons from $\mathit{n}$-side and free holes from $\mathit{p}$-side diffuse across the junction and combine and thus leave -ve ions in $\mathit{p}$-side and +ve ions in $\mathit{n}$-side.These two layers of +ve and -ve ions form the depletion region and the potential difference thus sets up is called potential barrier.