Diodes
Diodes are electric components which allow
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.