Science > Physics > Semiconductors > Introduction
Semiconductors are the substances whose conductivity lies between the conductors and insulators e.g. Germanium, Silicon, etc. These elements are members of the fourth group of the periodic table with valency 4. These elements have a partially filled conduction band and partially filled valence band. There are no free electrons for conduction in semiconductors at low temperature (absolute zero). Thus germanium crystal acts as an insulator at absolute zero. As the temperature increases, the width of the energy gap reduces and some electrons jump to the conduction band. Thus the conductivity of a semiconductor increases with the increase in the temperature.
Types of Semiconductors:
Depending upon the working semiconductors are classified into two types.
a) Intrinsic semiconductors and b) Extrinsic semiconductors
Intrinsic Semiconductors:
A semiconductor which is in extremely pure form is called an intrinsic semiconductor. e.g. Germanium, Silicon.
The crystal structure of these elements consists of regular repetition in three dimensions of a unit cell having the form of a tetrahedron, with one atom at each vertex. Consider a semiconductor like germanium having valency four. Germanium atom has four electrons in its outermost shell. Germanium has a crystalline structure in which each atom of germanium shares its valence electrons with four neighboring atoms forming four covalent bonds. The covalent bonds are strong bonds. Thus there is no free electron for conduction in germanium at low temperature (absolute zero). Thus germanium crystal acts as an insulator at absolute zero. A two-dimensional representation is as shown below.
At room temperature, the thermal energy of some electrons increases and they are set free. Thus the crystal shows a small conductivity.
Extrinsic Semiconductors:
The crystal of intrinsic semiconductors shows a small conductivity. The conductivity of semiconductors can be increased by adding a small quantity of some impurity in the pure crystal of the semiconductor. This process is called doping. The ratio of impurity is very low i.e. 1 atom of impurity for every 106 to 1010 atoms of semiconductors. These atoms of impurities are
so less that they do not affect the crystal structure of the semiconductor.
Generally, trivalent or tetravalent elements are added as impurities to semiconductor crystal. Depending upon the impurity the semiconductors are classified into two types a) p-type semiconductor and b) n-type semiconductor
Classification of Extrinsic Semiconductors:
Depending upon the impurity the semiconductors are classified into two types a) p-type semiconductor and b) n-type semiconductor
P-Type Semiconductors:
At absolute zero the conductivity of germanium crystal is zero. At room temperature, germanium shows a small conductivity. To increase the conductivity of germanium crystal small quantity of some impurity is added to it. This process is called doping.
Let us consider that the germanium is doped with an element from the third group say boron (trivalent impurity). Boron has three valency
electrons. Therefore, boron can form only three covalent bonds with neighboring germanium atoms. One of the covalent bonds around each boron atom has an electron missing. The absence of an electron is called a hole. This impurity is called acceptor impurity.
Under the action of an electric field, an electron from a neighboring completely filled covalent bond jumps into this hole creating a hole in the bond from which electron has moved. The process is repeated continuously. Thus the hole appears to move through the crystal from positive end to a negative end. Thus the conductivity of doped germanium increases.
The absence of an electron in the hole means the presence of a positive charge. Hence the doped material is called p-type semiconductor.
Characteristics of p-Type Semiconductors:
- In p-type semiconductors, doping is done with trivalent impurity i.e. impurity from the third group of the periodic table.
- The impurity in the p-type semiconductor is called the acceptor impurity.
- Each atom of impurity creates a hole in the crystal.
- The electrical conductivity is due to the hole.
- When a potential difference is applied across the p-type of semiconductor, the holes appear to move from positive end to a negative end.
- In p-type semiconductors holes are the major charge carriers.
- Example: Germanium crystal doped with Boron.
n-Type Semiconductors:
At absolute zero the conductivity of germanium crystal is zero. At room temperature, germanium shows a small conductivity. To increase the conductivity of germanium crystal small quantity of some impurity is added to it. This process is called doping.
Let us suppose the germanium is doped with an element from the fifth group say phosphorous (pentavalent impurity). Phosphorus has five Valency electrons. Therefore, phosphorous can form four covalent bonds leaving one free electron unbonded. Due to pentavalent doping the number of free electrons increases. This impurity is called the donor impurity.
Under the action of an electric field, free-electron around phosphorous moves through the crystal from the negative end to a positive end. Thus the conductivity of doped germanium increases.
The presence of an electron means the presence of a negative charge. Hence the doped material is called an n-type semiconductor.
Characteristics of n-Type Semiconductors:
- In n-type semiconductors, doping is done with pentavalent impurity i.e. impurity from the fifth group of the periodic table.
- The impurity in the n-type semiconductor is called the donor impurity.
- Each atom of impurity leaves one free electron in the crystal.
- The electrical conductivity is due to electron set free by the electron.
- When a potential difference is applied across n-type semiconductors, the electrons move from a negative end to a positive end.
- In n-type semiconductors, electrons are the major charge carriers.
- Example: Germanium crystal doped with Phosphorous.