Firstly we should know that there are two types of semiconductors ౼ Intrinsic and Extrinsic.

Intrinsic Semiconductor means a pure semiconductor i.e. a semiconductor without doping. It is also called undoped semiconductor. Examples include pure silicon and pure germanium.

2. Extrinsic Semiconductor: Intrinsic semiconductors generally do not have that good conducting properties at room temperature to be useful in electronic devices. It has been observed that if we add a small amount of suitable impurity to an intrinsic semiconductor, its conductivity at room temperature increases considerably. The process of adding impurities to an intrinsic semiconductor is called doping .



When we dope an intrinsic semiconductor with suitable impurity, then it is known as extrinsic Semiconductor. While preparing an extrinsic semiconductor, we should closely take care of the amount and type of the impurities being added. Generally, per 10⁸ atoms of the semiconductor, one atom of impurity is added.





The purpose of doping, as stated earlier, is to increase the conductivity of the material. Also conductivity of a material at a given temperature depends on the type and amount (or density) of the mobile charge carriers present inside it. So to increase the conductivity of a semiconductor, we should somehow try to increase the number of charge carriers in it. We know that the charge carriers in a semiconductor material are of two types ౼ conduction electrons and holes. The free electrons carry negative charge while the holes (which are basically nothing but deficiency of electrons) can equivalently be considered as positively charged particles.



In a semiconductor, it is the thermal agitation which generates the electron-hole pair. This means that if there is no thermal energy at all i.e. at absolute zero temperature, there will not be any electron-hole pair generation or there will not be any free charge carrier in the semiconductor. That's why we say that a pure semiconductor will behave like an insulatior at absolute zero. Greater the temperature, larger will be the number of electron-hole pairs or charge carriers and therfore the greater conductivity. In other words, conductivity of semiconductors increases with increasing temperature or resistivity of semiconductors decrease with increasing temperature. This is just opposite to the case with metals.







In an intrinsic semiconductor, number of free electrons is equal to the number of holes at any temperature as any free electron is always generated by leaving a hole behind and also if recombination of an electron and a hole takes place, both disappear simultaneously, thus maintaining the number of free electrons and holes equal. To increase the conductivity of a pure semiconductor we should either try to dope an impurity which can increase the number of free electrons (negative charge carriers) or to dope an impurity which can increase the number of holes (positive charge carriers) in it. If we add a pentavalent impurity (having five valance electrons) like arsenic or antimony, a large number of free electrons are produced in the semiconductor material as each pentavalent impurity atom supplies one extra free electron (see the fig. below).



Similarly if we add a trivalent impurity (like gallium, indium etc.) a large number of holes are produced in the semiconductor as each trivalent impurity atom supplies one additional hole (see the figure given below).



Depending upon the type of the impurities added, we classify the extrinsic semiconductor in two categories ̶̶ p type and n type. Here p stands for positive and n stands for negative. In n type semiconductor, number of negative charge carriers (i.e. free electrons) are greater whereas in p type, positive charge carriers i.e. the holes are in majority.

Thus when we add a small amount of pentavalent impurity to an intrinsic semiconductor, it is called as n type semiconductor. Similarly when we add a small quantity of trivalent impurity to a pure semiconductor, it is called p type semiconductor.



Difference between p type and n type Semiconductors The major differences between p type and n type semiconductor is summarized below in the form of a table.





Basis of Comparison N-type Semiconductor

P-type Semiconductor Group of Doping Element Fifth group element is the dopant in n-type semiconductor.

Third group element is the dopant in p-type semiconductor. Nature of Impurity Added Pentavalent Impurity is added.

Trivalent Impurity is added. Effect of Doping Each impurity atom provides one extra free electron (donar impurity)

Each impurity atom creates one extra vacancy (accepter impurity) Majority Carriers Free Electrons are the majority carriers.

Holes are the majority carriers. Minority Carriers Holes are the minority carriers.

Free electrons are the minority carriers. Electron and Hole Density Electron density is much greater than the hole density i.e. nₑ>>nₕ. Hole density is much greater than the electron density i.e. nₕ>>nₑ. Energy Level The impurity energy level is close to the conduction band and away from the valence band.

The impurity energy level is close to the valence band and away from the conduction band. Fermi Level Fermi level lies between the impurity level and the conduction band.

Fermi level lies between the impurity level and the valence band.