We knew there that the most important characteristic of a PN-junction or diode is its ability to conduct current in one direction only and in the other direction it offers very high resistance. Now we should know about its functional theory. That means why it represents such properties, this is our interest here.
Above figure shows a PN-junction just immediately after it is formed. Its left half is P-type and right half is N-type semiconductor. It is known by us that each type has:
- majority carriers (hole for P-type & electron for N-type). These are mobile and created using doping. The pure or intrinsic semiconductor has little current conduction capability due to less mobile charge. But its electrical property can be changed by adding a small amount of suitable impurity. The process of adding impurities to a semiconductor is known as doping.
- minority carriers (hole for N-type & electron for P-type). These are mobile and generated thermally.
- immobile ions
The sample as a whole is electrically neutral and so are the P region and N region considered separately. Therefore in each region the charge of majority carriers is equal to the total charges on its minority carriers and immobile ions. Without an external voltage when the PN-junction is formed, the following process are initiated. Before to discus the process, we need to clear some concept. The hole is a empty place that created after leaving by an electron. So we can think that only electron is mobile. Hole may be mobile due to mobility of electron. The process will explain in a post later. Spontaneous (natural) motion of electron from higher density to lower density is known as diffusion. Motion of electron due to influence of an electric field is known as drift. In the process, majority carriers diffuse from one region to other. Then they combine with each other. Each recombination eliminates a hole and a electron and emit energy as heat or light. The diffusion of majority carriers takes place because they move haphazardly due to thermal energy and also because there is a difference in their concentrations in the two regions.
In this process, the negative acceptor ions in the P region and positive donor ions in the N region in the immediate neighborhood of the junction( connection area among the semiconductors) are left uncompensated. This situation is shown in below.
Now when additional carriers trying to diffuse are repelled by the uncompensated ions. As a result, the diffusion of carriers across the junction occurs for a very short time and total recombination can't occur. The region containing the uncompensated acceptor and donor ions is called depletion region. Since this region has immobile ions which are electrically charged, it is also referred to as the space-charge region. The electric field between the acceptor and the donor ions is called a barrier (because this restricts more charge diffusion). The physical distance from one side of the barrier to the other is referred to as the width of the barrier. The difference of potential from one side of the barrier to the other side is referred to as the height of the barrier. Although barrier discourages the diffusion of majority carriers, it helps the minority carriers to drift across the junction. The minority carriers are constantly generated due to thermal energy. The drift of minority carriers across the junction is counterbalanced by the diffusion of the same number of majority carriers across the junction. These few majority carriers have sufficiently high kinetic energy to overcome the barrier and cross the junction. Thus we conclude that a barrier voltage is developed across the PN-junction even if no external battery is connected.