Diodes:

One of the critical components for semiconductor devices is the p-n junction. While p-type and n-type semiconductor materials are placed in contact with each other, the junction acts very differently than either type of semiconductor material alone. Specifically, current shall flow readily in one direction (forward biased) but not in the other direction (reverse biased), creating the basic diode. This non-reversing behaviour arises through the nature of the charge transport process in the two kinds of semiconductor materials of the pn-junction.