The semiconductors are just average conductors, a higher resistance than metal conductors, but a lower resistance than insulators. The two most commonly used semiconductor elements are Silicon and Germanium. Their +4 valence electrons mean that they have a very stable covalent bond structure.
Intrinsic semiconductors are pure semiconductors that contain no impurities. When temperature increases, the conduction property of the intrinsic type also increases. This is because, at high temperatures, electrons are excited to higher energy levels and create holes. These holes are positively charged and flow in the direction opposite to that of electrons thus causing electricity. In an intrinsic semiconductor, the number of holes and electrons are equal. Other causal agent of electricity in this type is crystal defects.
Extrinsic semiconductors when impurities are added to intrinsic semiconductors, extrinsic semiconductors are formed, meaning they are not in their natural form. The process of adding impurities to the semiconductor is called doping.
Doping a pure semiconductor with a small amount of material with a valence electron of +5 (which inclues Phosphorus, Arsenic, and Anitmony) creates an n-type semiconductor. It is referred to this because of the excess of free electrons in the material.
Similiarly, doping a pure semiconductor with a small amount of material with valence electron of +3 (Boron, Aluminum, Gallium, and Indium) creates a p-type semiconductor. This results because of a hole that is left by the absence of an electron in the covalent bond structure.
Note that doping a semiconductor does not add or remove any charge. The resulting product is still electrically neutral. Doping simply redistributes valence electrons so more or less free charges are available for conduction.
The PN Junction is formed by joining n-type and p-type semiconductor. The extra electrons in the n-type semiconductor attempt to move over into available holes in the p-type semiconductor. At the same time, some of the holes in the p-type seiconductor end up moving over to the n-type to meet up with electrons.When this happens, we end up with an excess of electrons on the p-type side and extra electrons on the n-type side, creating an electrical imbalance. This electrical imbalance is known as the barrier potential.
When we apply a positive voltage to the p-type semiconductor and a negative voltage to the n-type semiconductor, we are applying a forward bias to the semiconductor. First, the negative voltage at the n-type semiconductor is going to attempt to push electrons towards the junction in the middle. The positive voltage at the p-type semiconductor will push the holes towards the barrier as well. This reduces the barrier potential. If the barrier potential is reduced enough, the charge carriers can move through the barrier and out the other side. This means that current flows.
Applying a reverse voltage to our semiconductor material is known as reverse bias. In this condition, the electrons are pulled away from the barrier on the n-type side and the holes are pulled away from the barrier on the p-type side. This results in a larger barrier, which creates a much greater resistance for charges to flow through. The net result is that no current flows through the barrier.