Semiconductors are classified as intrinsic (pure), and extrinsic (impure) types. The extrinsic semiconductors are of N-type and P-type.
A pure semiconductor is called an intrinsic semiconductor. Even at room temperature, some of the valence electrons may acquire sufficient energy to enter the conduction band to form free electrons. Under the influence of an electric field, these electrons constitute an electric current. A missing electron in the valence band leaves a vacant space, which is known as a “hole”.
Holes also contribute to electric current. In an intrinsic semiconductor, even at room temperature, electron-hole pairs are created. When an electric field is applied across an intrinsic semiconductor, current conduction takes place due to free electrons and holes. Under the influence of an electric field, the total current through the semiconductor is the sum of currents due to free electrons and holes.
Though the total current inside the semiconductor is due to free electrons and holes, the current in the external wire is only by electrons. Holes being positively charged move towards the negative terminal of the battery. As the holes reach the negative terminal of the battery, electrons enter the semiconductor near the terminal (X) and combine with the holes. At the same time, the loosely held electrons near the positive terminal (Y) are attracted to the positive terminal. This creates new holes near the positive terminal which again drift towards the negative terminal.
Due to the poor conduction at room temperature, the intrinsic semiconductor as such is not useful in electronic devices. Hence, the current conduction capability of the intrinsic semiconductor should be increased. This can be achieved by adding a small amount of impurity to the intrinsic semiconductor so that it becomes an impure or extrinsic semiconductor. This process of adding impurities is known as doping.
The amount of impurity added is extremely small, say 1 to 2 atoms of impurity in 10⁶ intrinsic atoms.
A small amount of pentavalent impurities such as arsenic, antimony, or phosphorus is added to the pure semiconductor (germanium or silicon crystal) to get an N-type semiconductor.
The germanium atom has four valence electrons and antimony has five valence electrons. Each antimony atom forms a covalent bond with the surrounding four germanium atoms. Thus, four valence electrons of the antimony atom form a covalent bond with four valence electrons of an individual germanium atom and the fifth valence electron is left free and is loosely bound to the antimony atom.
This loosely bound electron can be easily excited from the valence band to the conduction band by the application of an electric field or by increasing the thermal energy. Thus, every antimony atom contributes one conduction electron without creating a hole. Such a pentavalent impurity is called a donor impurity because it donates one electron for conduction. On giving an electron for conduction, the donor atom becomes a positively charged ion because it loses one electron. But it cannot take part in conduction because it is firmly fixed in the crystal lattice.
Thus, the addition of a pentavalent impurity (antimony) increases the number of electrons in the conduction band, thereby increasing the conductivity of an N-type semiconductor. As a result of doping, the number of free electrons far exceeds the number of holes in an N-type semiconductor. So electrons are called majority carriers and holes are called minority carriers.
A small amount of trivalent impurities such as aluminum or boron is added to the pure semiconductor to get the P-type semiconductor. The germanium (Ge) atom has four valence electrons and the boron has three valence electrons. Three valence electrons in boron form a covalent bond with four surrounding atoms of Ge leaving one bond incomplete which gives rise to a hole. Thus, the trivalent impurity (boron) when added to the intrinsic semiconductor (germanium) introduces a large number of holes in the valence band. These positively charged holes increase the conductivity of the P-type semiconductor. A trivalent impurity such as boron is called an acceptor impurity because it accepts free electrons in the place of holes. As each boron atom donates a hole for conduction, it becomes a negatively charged ion. As the number of holes is very much greater than the number of free electrons in a P-type material, holes are termed majority carriers and electrons minority carriers.