one of the most important application in semiconductors is tje p-n junction. when p(holes)-type and n(electrons)-type are joined the current will move in only one direction and not revers.
to see a clear picture about the p-n junction see this link
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html
The current will be the flow of one type of carriers, which has the highest concentration by injection, in the same direction as the applied external filed.
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