Semiconductors refer to materials whose conductivity at room temperature is between conductors and insulators. For example, conductors such as gold, copper, iron, and aluminum are materials that can flow electricity, while insulators such as rubber, plastic, wood, and diamonds are resistant to electricity. Semiconductors are somewhere in between, meaning that sometimes semiconductors conduct electricity and sometimes they don't. Due to the unique conductive properties of semiconductors, their conductivity will increase significantly when the ambient temperature rises or there is light. If an appropriate amount of trace impurities are added to a pure semiconductor, its conductivity can be increased to hundreds of thousands of times. Using this characteristic, semiconductor devices for various purposes (such as diodes, transistors, field effect transistors and thyristors, etc.) have been made.
When a voltage is applied across a semiconductor, conduction occurs. In semiconductors, there are two types of conductive particles that form currents, and they move in opposite directions: one is free electrons, which move directionally in the semiconductor, forming electronic current; the other is valence electrons bound by the nucleus, which fill the holes to form hole current. There are two types of particles that carry electric current in semiconductors: free electrons and holes, which are called carriers. This is the main characteristic of the conductive mode of semiconductors, and it is also the essential difference in the conductive mechanisms between semiconductors and metals.
The essence of hole conduction is that the valence electrons (bound electrons in covalent bonds) in adjacent atoms fill the holes one by one, thereby forming an electric current. Since electrons have a negative charge and electrons and holes move in opposite directions, we say holes have a positive charge.