**3. Types of semiconductors**

In general, depending on the level of doping, semiconductors can be classified into two main groups such as *intrinsic semiconductors* and *extrinsic semiconductors*. The intrinsic semiconductors are pure semiconductors and no addition is made. In this type of semiconductor, conductivity is provided by the thermal stimulation of electrons. At the same time, the number of excited electrons and positively charged holes is equal. The behavior here appears as a result of the carrier production and recombination steps [5].

On the other hand, extrinsic semiconductors have low conductivity values, and an important process called *doping* is applied to overcome the problems encountered in applications and to increase the conductivity [6]. This process can be explained simply by adding small amounts of impurities in the concentrations of charge-carrying electrons and positively charged holes, thereby increasing the conductivity level. The aim is to change the electronic structure by impurity addition into the structure without changing the crystal structure. For example, arsenic with five valence electrons to an atom and germanium with four valence electrons will cause the arsenic atom to covalently bond with the germanium atom. The extra fifth electron of the arsenic atom will have the electrical conductivity as it will have the freedom to move from one atom to another [4]. Such semiconductors, which the dopant element donates an electron, are called n-type semiconductors. In addition to producing free electrons in n-type doping, an equal number of positive charges are also produced in pairs with free electrons. As a result, the doped semiconductor material remains electrically neutral. However, these positive charges should not be understood as positively charged holes. These charges occur in the absence of free electrons, but do not contribute to a current flow. Another contribution of free electrons to pure semiconductors is that the donor electron is much closer to the conduction band than an electron in the valence band of the original atom.

In another saying, the energy level of the donor electron is at another level that is much narrower than the energy level for valence electrons and facilitates the flow of current in the n-type semiconductor.

In another type of extrinsic semiconductor, if a pure semiconductor with trivalent electrons (e.g., germanium) is replaced by gallium, this type of semiconductor is called p-type semiconductors. Three of the four covalent bonds are occupied in the structure; the bond remains empty and acts as a relatively moving hole in the opposite direction of a moving electron, which is not a real move. The main phenomenon is defined as a relative movement caused by the movement of electrons from one bond to another and leaving a hole after. Similar to the energy band structure of n-type semiconductors, which differs from pure semiconductors, p-type semiconductors also have a higher acceptor energy level than the valence band.
