Modern electronic devices are based on semiconductors. As its name implies, a semiconductor is a material that conducts current, but only partly. Most semiconductors are crystals made of certain materials, most commonly silicon. The electrons in an atom are organized in layers, these layers are called shells. Semiconductors typically have four electrons in their valence shell such as in silicon.
The conductivity of a semiconductor is somewhere between that of an insulator, which has almost no conductivity, and a conductor, which has almost full conductivity.
Pure silicon crystals are not that useful electronically. But if you introduce small amounts of other elements into a crystal, the crystal starts to conduct in an interesting way. The process of deliberately introducing other elements into a crystal is called doping. The element introduced by doping is called a dopant. By carefully controlling the doping process and the dopants that are used, silicon crystals can transform into one of two distinct types of conductors:
- N type semiconductor
- P type semiconductor
N-type semiconductor
N-type semiconductor is created when the dopant is an element that has five electrons in its valence layer. Phosphorus is commonly used for this purpose. Because the phosphorus atom has five electrons in its valence shell, but only four of them are bonded to adjacent atoms, the fifth valence electron is left hanging out with nothing to bond to. The extra valence electrons in the phosphorous atoms start to behave like the single valence electrons in a regular conductor such as copper. They are free to move about. Because this type of semiconductor has extra electrons, it's called an N-type semiconductor.
P-type semiconductor
P-type semiconductor is created when the dopant (such as boron) has only three electrons in the valence shell. When a small amount is incorporated into the crystal, the atom is able to bond with four silicon atoms, but since it has only three electrons to offer, a hole is created. The hole behaves like a positive charge.
When voltage is applied to either an N-type or a P-type semiconductor, current flows, for the same reason that it flows in a regular conductor: The negative side of the voltage pushes electrons, and the positive side pulls them. The result is that the random electron and hole movement that's always present in a semiconductor becomes organized in one direction, creating measurable electric current.
The newly doped N-type and P-type semiconductor materials do very little on their own as they are electrically neutral. However, if we join (or fuse) these two semiconductor materials together, to form a PN junction, they behave in a very different way.
