The PN junction diode
Understanding the operation of the semiconductor diode is the basis for an understanding of all semiconductor devices. The diode is actually manufactured as a single piece of material but it is much easier to explain the operation if we imagine producing two separate pieces of N type and P type material and then "sticking" them together.
Consider a piece of N type material. It contains mobile charge carriers in the form of free electrons. These electrons will be in motion due to thermal energy. (It is important to realise that this motion does not result in an electrical current because the motion is random and there is not net movement of charge from one area of the material to another. This is similar to the way that even in a perfectly still glass of water the individual molecules will be moving randomly on a microscopic scale.) The net result is that the random motion of the electrons results in them being evenly distributed throughout the N type material. In the P type material it is the positively charged holes that are mobile and for identical reasons to those previously described the holes are evenly distributed throughout the P type material.
Now consider what will happen if these two separate pieces of P and N type material are joined together. The random motion of the mobile electrons in the N type material and the holes in the P type material would tend to cause an even distribution of electrons and holes throughout the semiconductor. And in fact this is what begins to happen.
Consider the electrons in the N type material. The electrons start to migrate across the junction of the two materials. When they cross into the P type material they recombine with the holes (ie they fill in the holes in the valence band by filling in the vacant electron positions around the trivalent donor atoms). This means that the number of holes near to the junction becomes depleted. Also as the electrons leave the previously neutral N type material a positive charge builds up at the junction. (This is because the positive charge from the nucleus of atoms near to the junction is now greater than the negative charge of the electrons in that region. This is due to the reduction in the number of electrons due to those which have moved across the junction.)
Similarly as holes migrate from the P to N type material they recombine with electrons (the free electron from the pentavalent atoms completes the fourth covalent bond around the trivalent atom). This leaves a depletion of free electrons near the junction in the N type material. Also a negative charge builds up near the junction in the P type material due to the loss of positively charged holes.
The net result is that the migration of electrons from N to P type material and the migration of holes from P to N has two effects. It results in a depletion of mobile charge carriers near the junction ( a depletion of electrons in the N type material and a depletion of holes in the P type material). This depletion layer is typically about 1 micrometre wide ( 1 millionth of a meter!). Also a voltage is produce across the junction which is called a barrier voltage. The N type material develops a positive charge close to the junction and the P type develops a negative charge. This prevents any further migration of mobile charge carriers.
The effect of the barrier voltage
The positive charge at the N side of the junction repels any positively charged holes that would tend to migrate across the junction from the P type material. It also attracts free electrons and therefore to prevents them moving out of the N type material. Similarly the negative charge in the P type material close to the junction repels electrons which would tend to migrate from the N type material and it attracts the holes and prevents them moving out of the P type material. The migration of mobile charge carriers across the junction would stop when the barrier voltage had built up to a sufficient level to prevent any further migration. For Silicon this is about 0.6 to 0.7 volts for Germanium it is about 0.2 to 0.3 volts.
