It is the junction when the a semiconductor doped with group 13elements is brought in contact with a semiconductor doped with group 15elements.and as the name suggests it is a junction between p type and n type semiconductor.next is the depletion region which is formed when holes and electrons diffuse from p type and n type respectively.and this is due to the tendency of recombination of holes and electrons .so they cross the pn junction (which is formed when the two p type and n type material were brought in contact with each other .At this stage the width of depletion region is zero which is referred to as metallurgical junction).after a certain amount of time the majority carriers can no longer diffuse as the uncovering of immobile ions takes place(immobile ions means the elements that were used for doping are now uncovered ) and only the minority carriers diffuse ( uncovering immobile ions leads to creation of positive ions in the n side and uncovering of negative ions in the p side as a result electric field is formed which always supports the movement of minority carriers and opposes movement of majority carriers).the above scenario is for zero applied voltage