Diodes are electric components which force current to flow in only one direction. They are formed by connecting p-type and n-type semiconductors as shown to the right. When current flows from the p-type to the n-type material, the positive holes and the negative electrons are forced into close contact at the boundary. At the boundary, the electrons fill the holes across the boundary while the terminals supply new holes and electrons. Thus, in the forward bias case a continual current flows. In the reverse bias case, the charge carriers are pulled apart. There is no longer an easy way for electrons to tunnel through the barrier as there are no longer many empty holes waiting on the opposite side.
The circuit-diagram representation of a diode is with the arrow representing the direction current is allowed to flow.