The theory of rectification is based on the special properties of the junction that is created between a semiconductor area of n-type and one of p-type. The junction is often referred to as a P-N junction. On one side (N) of this junction there is an excess of electrons because of the addition of n-type dopants to the semiconductor. On the other side (P) where the p-type dopants have been added, there is an excess of holes. This construction, with one n-type and one p-type area in one unit, is generally called a diode. The diode acts as an electrical rectifier, i.e. it only allows current to pass in one direction.