Unit – I A

Introduction to Semiconductors

1. The word electronics is coined from the words electron mechanics.
2. The subject of electronics deals with the study of devices in which specific current ( I ) versus voltage ( V) relationship is obtained by controlling the production of electrons, their numbers, and their conduction. Such relationships are different from the one obeyed by Ohm’s law in good conductors in which electric current is directly proportional to the electric potential difference.
3. There are many substances found in nature in which the conduction of electricity is different from the one found in metals. Solid state devices are made having appropriate I – V relations by properly adding impurities in such a substance.
4. Solid state devices are small in size and light in weight. They are very efficient and cheap.
5. Semiconductor devices like the P – N junction diode, transistor LED ( Light emitting diode), solar cell and logic circuits are a basis for digital circuits.

Classification of Substances

On the basis of easiness to allow the current to pass through them, substances are classified as follows:
a) ConductorsThey allow electric current to pass through them easily. The elements in the first three groups of the periodic table like alkali metals, noble metals, Aluminium, etc. are good conductors due to the presence of free electrons. The resistivity of the good conductors increases with temperature.
b) Insulators:
They do not allow electric current to pass through them. Non-metals are bad conductors of electricity due to lack of free electrons. The resistivity of insulators decreases on increasing the temperature.
c) Semiconductors:
The elements in the fourth group of the periodic table like Si and Ge have greater resistance than good conductors but less than bad conductors. The resistivity of the semiconductors decreases on increasing the temperature upto a certain limit.

Energy Bands in Solids::

1. The group of discrete but closely spaced energy levels for the orbital electrons in a particular orbit is called energy band.
2. 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.
3. 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.

Valence Band:

a) The energy band formed by energy levels of valence electrons of atoms in solid is called valence band.
b) Very few electrons have energy sufficient to get transferred from valence band to conduction band. Such electrons with high energy are responsible for conduction.

Conduction Band:

a) A band of energy level which is occupied by the conduction electrons of the solids is called conduction band.
b) In conductors, the gap between valence band and conduction band is very small. In good conductors valence band and conduction band overlap.

Explanation of Conductors on the Basis of Band Theory:

1. If the lowest level in the conduction band happens to be lower than the highest level of the valence band, the electrons from the valence band can easily move into the conduction band.
2. Normally the conduction band is empty. But in the case of metallic conductors, it overlaps on the valence band electrons can move freely into it.
3. Thus at room temperature, a large number of free electrons are available for conduction.

Explanation of Insulators on the Basis of Band Theory:

1. If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are available in the conduction band. This makes the material an insulator.
2. 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.
3. In the case of insulators, the forbidden energy gap between the valence band and conduction band is greater than 3 eV.

Explanation of Semiconductors on the Basis of Band Theory:

1. In semiconductors at absolute zero, there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are
available in the conduction band. This makes the material an bad conductor at absolute zero.
2. In the case of semiconductors, the forbidden energy gap between the valence band and conduction band is less than 3 eV. 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 the conduction band as well as due to vacancies in the valence band.
3. Thus above absolute zero, semiconductors behave like conductors.

Semiconductors:

1. Semiconductors are the substances whose conductivity lies that between the conductors and insulators. e.g. Germanium, Silicon etc.
2. These elements are members of the fourth group of the periodic table with valency 4.
3. These elements have partially filled conduction band and partially filled valence band.
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.
5. As the temperature increases, the width of the energy gap reduces and some electrons jump to the conduction band. Thus the conductivity of semiconductors 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:

1. A semiconductor which is in extremely pure form is called intrinsic semiconductor.
e.g. Germanium, Silicon
2. 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.
4. At room temperature, the thermal energy of some electrons increases and they are set free. Thus the crystal shows a small conductivity.

semiconductors

Extrinsic semiconductors:

1. 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.
2. 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.
3. Generally, trivalent or tetravalent elements are added as impurities to semiconductor crystal.
4. Depending upon the impurity the semiconductors are classified into two types
a) p-type semiconductor and
b) n-type semiconductor

p-type Semiconductor:

1. 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.
2. 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.
3. 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 negative end. Thus the conductivity of doped germanium increases.

4. The absence of an electron in the hole means the presence of a positive charge. Hence the doped material is called p-type semiconductor.

n-type Semiconductor:

1. 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.

semiconductor

2. 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.
3. Under the action of an electric field, free electron around phosphorous move through the crystal from negative end to positive end. Thus the conductivity of doped germanium increases.
4. The presence of an electron means the presence of a negative charge. Hence the doped material is called n-type semiconductor.

Characteristics of p-type Semiconductors

1. In p-type semiconductors, doping is done with trivalent impurity i.e. impurity from the third group of the periodic table.
2. The impurity in the p-type semiconductor is called the acceptor impurity.
3. Each atom of impurity creates a hole in the crystal.
4. The electrical conductivity is due to hole.
5. When a potential difference is applied across p-type of semiconductor, the holes appear to move from positive end to negative end.
6. In p – type semiconductors holes are the major charge carriers.
7. Example: Germanium crystal doped with Boron.

Characteristics of n-type Semiconductors:

1. In n-type semiconductors, doping is done with pentavalent impurity i.e. impurity from the fifth group of the periodic table.
2. The impurity in the n-type semiconductor is called the donor impurity.
3. Each atom of impurity leaves one free electron in the crystal.
4. The electrical conductivity is due to electron set free by the electron.
5. When a potential difference is applied across n-type of semiconductor, the electrons move from negative end to positive end.
6. In n -type semiconductors electrons are the major charge carriers.
7. Example: Germanium crystal doped with Phosphorous.

The topic in Solid State (Chemistry) also has a topic of semiconductors which is a sub topic of this.
You can read that topic: Here.

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