Objects can be roughly divided into conductors, semiconductors and insulators according to their conductivity. Let’s talk about semiconductors in detail below.
Pure semiconductor materials are called intrinsic semiconductors. Intrinsic semiconductor materials are doped with group V impurity elements (phosphorus, arsenic, etc.), and the impurities provide electrons, making the concentration of electrons greater than the concentration of holes, forming an N-type semiconductor (Figure 1) material, and the impurities are called donors; At this time, the electron concentration is greater than the hole concentration, which is the majority carrier, while the hole concentration is lower, which is the minority carrier. Similarly, doping group III impurity elements (boron, etc.) into semiconductor materials to make the hole concentration greater than the electron concentration, and crystalline silicon becomes a P-type semiconductor (Figure 2). Take silicon as an example. Doping a little bit of boron, aluminum, gallium and other impurities into high-purity silicon is a P-type semiconductor; doping a little bit of phosphorus, arsenic, antimony and other impurities is an N-type semiconductor. In N-type semiconductors, unbalanced electrons are called unbalanced majority carriers, and unbalanced holes are called unbalanced minority carriers. The opposite is true for P-type semiconductors. In semiconductor devices, unbalanced minority carriers often play an important role.
Whether it is an N-type semiconductor material or a P-type semiconductor material, when they exist independently, they are all electrically neutral, and the charge amount of ionized impurities is equal to the total charge number of carriers. When two semiconductor materials are connected together, for N-type semiconductor materials, electrons are majority carriers and have a high concentration; while in P-type semiconductors, electrons are minority carriers and have a low concentration. Due to the existence of the concentration gradient, the diffusion of electrons is bound to occur, that is, electrons diffuse from the high-concentration N-type semiconductor material to the low-concentration P-type semiconductor material to form a PN junction at the interface of the N-type semiconductor and the P-type semiconductor. Near the PN junction interface, the electron concentration in the N-type semiconductor gradually decreases, and the electrons diffused into the P-type semiconductor and the majority carrier holes in it recombine and disappear. Therefore, near the interface of the N-type semiconductor, due to the decrease of the majority carrier electron concentration, the positive charge number of the ionized impurities is higher than the remaining electron concentration, and a positive charge region appears. Similarly, in P-type semiconductors, since holes diffuse from P-type semiconductors to N-type semiconductors, near the interface, the number of negative charges of ionized impurities is higher than the concentration of remaining holes, and negative charge regions appear. This positive and negative charge area is called the space charge area of the PN junction, which forms an electric field from the N-type semiconductor to the P-type semiconductor, which is called the built-in electric field, or the barrier electric field. Because the resistance here is particularly high, it is also called a barrier layer. This electric field has a counteracting effect on the diffusion of multiple sons in the two regions, and has a helpful effect on the drift of minority sons, until the diffusion flow is equal to the drift flow, and a stable built-in electric field is established on both sides of the interface. The so-called diffusion means that under the influence of an external electric field, a free electron that moves randomly has an accelerated motion in the direction opposite to the electric field, and its speed increases continuously with time. In addition to drifting motion, carriers in semiconductors can also flow due to diffusion. When any particles like gas molecules are concentrated too much, if they are not restricted, they will disperse by themselves. The basic reason for this phenomenon is the irregular thermal motion of these particles. As the diffusion progresses, the space charge region widens and the internal electric field increases. Since the internal electric field acts to hinder the diffusion of multiple carriers and promote the drift of minority carriers, a stable PN junction will be formed when the diffusion motion and drift motion reach a dynamic balance. . The PN junction is very thin, with few electrons and holes in the junction, but there are positively charged ions on the side close to the N-type, and negatively charged ions on the side close to the P-type. Due to the lack of carriers in the space charge region, the PN junction is also called the depletion region.