What are they?

Semiconductors are material that have the ability to conduct and not conduct current, which is a useful property for producing electronic circuits. This property is useful because computers operate in binary at the lowest level; in 0s and 1s. Those 0s and 1s are simulated physically by semiconductors alternating between conducting and not conducting current. Semconductors are typically made from silicon in the form of crystals.

To understand how semiconductors work, it's important to start with the electron layers of atoms. The most important electron layer when it comes to semiconductors is the outermost layer, called the valence shell. This is where the valence electrons are located, which are the electrons that are readily available to interact with electrons from other atoms.

In silicon semiconductors, there are typically four electrons in the valence shell. This number is important when there are multiple silicon atoms grouped closely together because those valence electrons align the atoms together into a crystalline structure. This is how silicon crystals are created. Since each silicon atom typically has four valence electrons, this means each atom is bonded to four other atoms.

It's important to note that silicon crystals by themselves are not very useful in applications like producing semiconductors. In order to change silicon's electrical characteristics to become useful in semiconductors, silicon crystals undergo a process called doping. This process is where a foreign elements are introduced into a crystal in order to change their electrical characteristics.

One example of this when an element with five valence electrons is doped into a silicon crystal, which usually ends up being phosphorus. Through this process, an N-type semiconductor is created. While previously, in a pure silicon crystal, all the valence electrons were paired with each other, there will in fact be one unpaired valence electron per phosphorus atom. Consequently, this means there are extra valence electrons able to freely move about, just as the extra valence electrons in metals like copper. It is well known that metals like copper are excellent conductors of electricity, so through this process of doping, the silicon crystal has similarly become an excellent conductor, which is indeed very useful for applications like semiconductors.

Another example of doping is when an element with three valence electrons is introduced into a silicon crystal. Compared to the perfect pairing of pure silicon in which there are no unpaired electrons, there will be one missing valence electron for every phosporus atom. This is called a hole. These holes act like positive charges in that they attract electrons because they are ready to be filled in to complete bonding. An interesting phenomenon that occurs with these holes is that whenever an electron moves to fill in a hole, the electron leaves a hole where it used to be, thus creating this constant movement of valence electrons within the doped crystal. This is how a P-type semiconductor is created.

Thus, it should be evident that although both N-type and P-type semiconductors are different in what element they introduce to a pure silicon crystal, they both cause the constant movement of valence electrons within the final product. Moreover, when voltage is applied to these semiconductors, it causes current to flow in one organized direction. This is how electric current is created in semiconductors.