On the basis of electrical conductivity, substances can be classified into three types conductors, insulators, and semiconductors. In this article, we shall have a brief idea of semiconductors.

Semiconductors are the substances whose conductivity lies between the conductors and insulators e.g. Germanium, Silicon, etc. They are covalent solids. These elements are members of the fourth group of the periodic table with outer orbit configuration ns² np² and valency 4. 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 energy gap reduces and some electrons jump to the conduction band. Thus the conductivity of semiconductors increases with the increase in the temperature. Such semiconductors are also called as extrinsic or pure semiconductors. The conductivity of semiconductors can be increased by purposely adding pentavalent or trivalent impurity (doping) to their crystal in small traces.

The structure of Pure Silicon (Semiconductor) Crystal:

We can see that each silicon atom share its four valence electrons with neighbouring 4 silicon atoms to form four covalent bonds. Thus at absolute zero, all the electrons are localized and not available for conduction. Thus at absolute zero silicon behaves as an insulator.

Types of Semiconductors:

Depending upon the working semiconductors are classified into two types.

Intrinsic semiconductors:

A semiconductor which is in extremely pure form is called 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. 3. The two-dimensional representation is as shown below.

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.

At room temperature, the thermal energy of some electrons increases and they are set free. These free electrons get delocalized and are available for conduction. When these electrons get delocalized, a hole is created at their position. 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

p-type Semiconductor:

Let us suppose 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 a 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 a positive end to a negative end.
• In p-type semiconductors, holes are the major charge carriers.

n-type Semiconductors:

Let us suppose the germanium is doped with an element from the fifth group say phosphorous (pentavalent impurity). Phosphorus has five valence electrons. Therefore, phosphorus 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 of semiconductor, the electrons move from a negative end to a positive end.
• In n-type semiconductors, electrons are the major charge carriers.

Uses of Semiconductors:

• Various combinations of n-type and p-type semiconductors are used for making electronic components.
• A diode is a combination of n-type and p-type semiconductors and is used as a rectifier.
• Transistors are made by sandwiching a layer of one type of semiconductor between two layers of the other type of semiconductor. NPN and PNP types of transistors are used to detect or amplify radio or audio signals.
• The solar cell is an efficient photodiode used for the conversion of light energy into electrical energy. It consists of both p-type and n-type semiconductors.

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In the last article, we have studied the dielectric properties of solids. In this article, we shall study the electrical properties of solids.

Electrical Conductivity:

The electrical conductivity of solids is due to the motion of electron or positive holes. The conductivity due to the motion of electron or positive holes is called electronic conductivity. The electrical conductivity may be due to the motion of ions. The conductivity due to the motion of ions is called ionic conductivity. Conductivity due to electrons is called n-type conductivity while that due to holes is called p-type conductivity.

In metals, electrical conductivity is due to the motion of electrons and the electrical conductivity increases with the increase in the electrons available for conduction, the electrical conductivity increases. In pure ionic solids, ions are not available for conduction hence in the pure solid state they are insulators. Due to the presence of defects in the crystal electrical conductivity increases.

Solids exhibit a varying range of electrical conductivities, extending of magnitude ranging from 10^–20 to 10⁷ ohm^–1 m^–1. Solids can be classified into three types on the basis of their conductivities. The difference in conductivities of conductors, insulators, and semiconductors can be explained on the basis of band theory.

Electrical Conductivity on the Basis of Energy Bands:

The group of discrete but closely spaced energy levels for the orbital electrons in a particular orbit is called energy band. Inside the crystal, each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands.

The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band. With no external energy, all the valence electrons will reside in the valence band.

On the basis of energy bands and electrical conductivity, solids can be classified as:

Conductors:

The solids with conductivities ranging between 10⁴ to 10⁷ ohm^–1 m^–1 are called conductors. Metals have conductivities in the order of 10⁷ ohm^–1 m^–1 are good conductors. In conductors, the lowest level in the conduction band happens to be lower than the highest level of the valence band and hence the conduction band and the valence band overlap. Hence the electron in the valence band can migrate very easily into the conduction band. Thus at room temperature, a large number of electrons are available for conduction. Examples: Copper, Aluminium, Silver, Gold, All metals

Characteristics of Conductors:

• The substances which conduct electricity through them to a greater extent are called conductors.
• In conductors, the conduction band and valence band overlap with each other or the gap between them is very small.
• There are free electrons in the conduction band.
• Due to increase in temperature conductance decreases.
• There is no effect of the addition of impurities on the conductivity of conductors.
• Their conductivity ranges between 10⁴ to 10⁷ ohm^–1 m^–1.

Conduction in Metallic Solids:

A metal conductor conducts electricity through the movement of free electrons. Metals conduct electricity in solid as well as a molten state. The conduction of electricity is due to the transfer of electrons and not due to the transfer of matter. The conductivity of metals depends upon the number of valence electrons available per atom. It is nearly independent of the presence of impurity and lattice defects. Conductivity decreases with the increase in the temperature. It can be explained as follows

M → Mⁿ⁺_kernel + ne^–  free electrons

The kernels are fixed. Due to the increase in temperature, the amplitude of vibration of kernels increases. Hence the obstruction to the flow of electron increases.

Insulators :

These are the solids with very low conductivities ranging between 10^–20 to 10^–10 ohm^–1 m^–1. The conduction band and valence band are widely spaced. Thus forbidden energy gap between the valence band and conduction band is large (greater than 3 eV). Hence the electron in the valence band cannot migrate into the conduction band. Hence no electrons are available for conduction. But at a higher temperature, some of the electrons from the valence band may gain external energy to cross the gap between the conduction band and the valence band. Then these electrons will move into the conduction band. At the same time, they will create vacant energy levels in the valence band where other valence electrons can move. Thus the process creates the possibility of conduction due to electrons in conduction band as well as due to vacancies in the valence band. Examples: Glass, wood, paper, plastic, mica.

Characteristics of Insulators:

• In Insulators the conduction band and valence band are widely separated.
• There are no free electrons in the conduction band.
• There is an energy gap between the conduction band and the valence band which is more than 3 eV.
• There is no effect of change of temperature on the conductivity of insulators.
• There is no effect of the addition of impurities on the conductivity of insulator.
• They have very low conductivities ranging between 10^–20 to 10^–10 ohm^–1 m^–1.

Semiconductors :

These are the solids with conductivities in the intermediate range from 10^–6 to 10⁴ ohm^–1 m^–1.

The forbidden energy gap between the valence band and conduction band is less than 3 eV. Thus energy gap between the valence band and conduction band is small. At absolute zero, no electrons are available for conduction.

As the temperature increases, many electrons from the valence band may gain external energy to cross the gap between the conduction band and the valence band. Then these electrons will move into the conduction band. At the same time, they will create vacant energy levels in the valence band where other valence electrons can move. Thus the process creates the possibility of conduction due to electrons in conduction band as well as due to vacancies in the valence band. Examples: Silicon, Germanium

Characteristics of Semiconductors:

• In semiconductors, the conduction band and valence band are very close to each other or the forbidden energy gap between them is very small.
• The electrons of the valence bond can easily be excited to the conduction band.
• There is an energy gap between the conduction band and valence band which is less than 3 eV.
• Due to increase in temperature conductance increases.
• There is an effect of the addition of impurities on the conductivity of semiconductors.
• Their conductivity ranges from 10^–6 to 10⁴ ohm^–1 m^–1.

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