One either side of the junction, the region ‘D' becomes free from mobile charge carriers. That is on n-type side of D, no free electrons exist and on p-type side of D no holes exist. In the region ‘D' only immobile donor and acceptor ions exist.
Since on either side of junction J, stationary positive and negative charges exist, there is an electric potential difference VD between the edges 1 and 2 of D. This potential difference (VD) across the junction is called ‘Potential barrier' as it prevents further movement of holes and electrons. This combination described in figure becomes a p-n junction diode.