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