How semiconductor holes conduct electricity

The continuous development of current science and technology has led to continuous innovation in the semiconductor industry. Semiconductor materials that we are familiar with are commonly used P and N types. Silicon (SI) is doped with the element boron (B). Because B is trivalent, compared to the 4 valence of SI, it lacks one electron. This is missing. The part doped with B is called a hole, and this type of B-doped material is called P-type material. Correspondingly, the element phosphorus (P) is doped. Because phosphorus has a 5-valence, compared to the 4-valence of Si, there is one more electron. This electron comes out alone and becomes a free electron. This type of P doping It's called N-type material.

N-type materials can easily conduct electricity because they have independent free electrons. This is easy for everyone to understand. However, all books tell us that holes in P-type materials can also conduct electricity. This is difficult for us to understand. One of the explanations is that holes The conductive essence of holes is the beating of electrons, but it seems that holes are conducting electricity, so it is called hole conduction. But this explanation still gives us the answer that electrons conduct electricity, but it is easier to accept than having no answer. So how do holes conduct electricity?

During this time, I have been reading books on semiconductors, including conduction band, valence band, and forbidden band. There are many concepts. I endured my scalp and read them patiently. Finally, I understood the concept of hole conductivity that has been bothering me for a long time. But I dare not say that it is right. Let me share with you what I understand first. First, look at some basic concepts in the picture below:

As shown in the figure above, the electrons in the material are divided into three parts according to the energy level. Because this part is a region, it is represented by a "band", and the concept of frequency bands is the same. The more common ones are divided into three levels: valence band, conduction band and forbidden band. The lowest energy is the valence band. When the electrons in the valence band gain energy, they will cross the forbidden band and jump to the conduction band. Note that the forbidden band does not allow electrons to exist. This is determined by quantum theory, that is, the energy levels of electrons can only be distributed in a limited number of energy levels, rather than continuously.

The electrons in the outermost layer of a metal conductor are very active and are free electrons. They gain energy and jump into the conduction band at room temperature, so they can conduct electricity. Insulators have no free electrons, so there are no electrons in the conduction band, so they cannot conduct electricity. For semiconductors, N-type materials doped with 5-valent phosphorus have one more electron, which becomes a free electron and enters the conduction band, so it can conduct electricity. This is easy to understand. What about hole type? Hole type There are no free electrons in the conduction band, so there is no conductive characteristic in the conduction band. This is something that needs special attention.

Since hole-type materials do not have free electrons in the conduction band to conduct electricity, how do holes conduct electricity? So it is definitely not conduction in the conduction band, but the essence of holes is that electrons conduct electricity, but this electron does not come from the electrons in the conduction band, but It comes from electrons in the valence band, which is the core of the problem.

There are a large number of low-energy holes in P-type materials, and there are more valence band electrons around the holes. If these electrons only need to obtain a small amount of energy, and this energy reaches the top of the valence band and the low end of the forbidden band, it will It may jump into the surrounding holes without jumping into the conduction band (it cannot jump over). When the external electric field is added, the hole starts to move, so it becomes a hole conductor. After understanding this, we will understand that P-type materials conduct electrons in the valence band, which is equivalent to hole conduction, and N-type materials conduct electrons in the conduction band, which is electron conductivity. The above are some working principles of semiconductor hole conduction. Engineers need to have an in-depth understanding to avoid detours.