Semiconductor physics | Part-1 ~ MONU TUTORIALS ACADEMY

Introduction

The semiconductor such as germanium and silicon carbon are the material which are neither conductor nor insulator.there conductivity lies in between semi means middle the conductivity of conductor and insulators. Semiconductor have some useful properties and being extensively used in electronic circuit. Semiconductor are used for the preparation of solid study while like diode transistor etc.

Bohr's atomic structure

Niels bohr, a Danish physicist comma idealized a clear picture of atomic structure. He state that

An atom consists of nucleus which contain neutron and proton.
The electron revolve around the nucleus in different orbits.
the nucleus has a positive charge which attract the electron. The electron would fall into the nucleus if they do not have the required centrifugal force for their motion. Therefore, an electron travel in a stable orbit and moves at a right velocity for centrifugal force to balance the nucleus attraction.
The electron in each permitted orbit have a certain fixed amount of energy. The large the orbit the greater is the energy of electron.
An electron is lifted to the higher orbit if some additional energy is given to it. Then the item is said to be in a state of excitation.This is state does not last long since the electron soon falls back to the original orbit. As gift fall back, it radiates the Aqua energy in the form of heat, light.

A simple two dimensional structure of silicon atom is shown in figure.

Energy levels

it has already been discussed that the electron can keep all only in the permitted orbits and not in any intermediate orbit. Thus, all the radii between R1 and R2 or between R2 and r3 er4 hidden each orbit has fixed amount of energy e associated with it and the electron moving in a particular orbit causes the energy of that orbits. The larger the orbit the greater is its energy.an electron in the outer orbit possess more energy more than the electron in the inner orbit.

the level of energy obtained by different orbit is conveniently represented by the energy level diagram shown in figure the first orbit represented the first energy level coma the second orbit represented the second energy level and so on. The large the orbit of an electron coma the greater is energy and higher in the energy level. Energy level is just another way of representing the orbital radius.

Energy bands

As discussed earlier in case of single isolated atom, the electron revolving in any orbit for as a definite energy. However,in a solid common atom is greatly influenced by the closely packed neighbouring atom. because of this the electron in the same orbit have a range of and rises rather than a single energy. This is known as energy band.

the range ofe possessed of the same orbit in a solid is known as energy band.

Classifications of solid and energy bands

on the basis of electrical conductivity of the solid may be classified as insulator conductors and semiconductors. their electrical behaviour can be explain beautifully with the help of energy bands. To ascertain the electrical behaviour, the valence and conduction bands are of particular importance whereas the electron in the lower energy band a tightly bound to the nucleus and play no part in the conduction process.

1. Insulators :-

The substance like wood, mica. which do not allow the passage of current through them are known as insulators full stop the valence band of this substance is full where the conduction band is completely empty.

2. Conductors :-

The substance like copper, aluminium, silver which allowed the passes of current through them are known as conductors. the valence band of this substance overlap the conduction band as shown in figure due to this overlapping a large number of free electron are available for conduction.

3. Semiconductors :-

The substance like carbon, silicon, germanium, selenium, sulphur which have resistivity in between conductors and insulators are known as semiconductors.

Bonds in semiconductors

in every element, the atom are held together by the bonding action of valence electrons. this wanding action is due to the fact that it is the tendency of each attempt to complete its last orbit biotech wearing it electrons in it. In most of the element the last orbit is incomplete. Therefore, to complete it, the atom become active to enter into bargain with other atoms. In this process the atom mein loss gain or share valence electron with other atoms

in semiconductor bones are formed by sharing of valence electron police dog such bonds are called covalent bonds. In this case each atom contributes equal number of valence electrons and the contributed electrons are shared by the atom engage in the formation of the bond.

From the above discussion, it is concluded that,

In semiconductor coma covalent bonds are formed by sharing of valence electron.
in the formation of covalent bonds, each valence electron of a semiconductor atom form direct bond with the valence electron of n adjacent atom.

Crystalline structure of semiconductors

in semiconductors, the atoms are arranged themselves in an orderly pattern to form a solid. This orderly pattern is known as crystal in this figure shown the crystalline structure of silicon. it is clear that each atom is surrounding by neighbouring atom in repetitive manner. Therefore a pieces of silicon is generally called silicon crystal.

In semiconductor the eight electron do not belong exclusively to the central atom but they are shared by the four surrounding atom. As the adjacent course have a net positive charge coma they attract the share valence electron and create equal and opposite forces. This pulling force in opposite direction act as a glue that hold atom together and form a covalent bond coma consequently gives a crystalline structure of semiconductors.

Effect of temperature of the conductivity of semiconductors

the change in temperature change the electrical conductivity of semiconductor appreciable. Let us see how conductivity change with the change in temperature.

1. At absolute zero :-

at absolute zero temperature all the electron of semiconductor are held tightly by their atoms. The inner orbital electrons are bound to the nucleus, whereas the valence electron are found by the force of covalent bonds.therefore, at this temperature, no free electron is available in the semiconductor hence the semiconductor crystal behave like a perfect.

2. Above absolute zero :-

when the temperature of semiconductor is rise commerce some of its covalent bonds break due to the thermal energy supplied to it.the break king of bonds at those electrons free which were engaged in the formation of these bond. Thus,at a higher temperature a few free electron exist in the semiconductor and they no longer behave as a perfect insulator.

whenever an electron is jump up to the conduction band former a hole is created in the valence bond.

Hole

when energy is supplied to a semiconductor, a valence electron is lifted to a higher energy level coma the departing electron leaves vacancy in the valence bond stop this vacancies called a hole.

Thusevacancy left in the valence bond because of lifting of an electron from valence band and conduction band is known as hole.

Electron hole pair

whenever, some external energy supplied to a semiconductor coma the valence electron are lifted up to the conduction band one after the other living behind the vacancy in the valence band called hole.the number of electrons to be lifted from valence band to the conduction band depends upon the quantity of external energy supplied to the semiconductor. If only one electron is lifted to the conduction band common than one hole is created in the valence band. Thus, is time and electron hole pair is formed.

Hole current

when some external energy is supplied to a pure semiconductor the covalent bonds are broken forming electron-hole pairs. The electron are lifted up to conduction band leaving behind vacancy in the valence bond. under the influence of electric field the free electrons constituting current. at the same time and other current the whole current also flow in the semiconductor as explained below.

in this figure show the phenomenon of hole current in semiconductor suppose the valence electron at J has become free due to thermal energy creating a hole in the covalent bond at J.the whole, having positive charge, is a strong centre of attraction for the electron. since some electric field is applied across the semiconductor, valence electron from nearby covalent bond comes to fill in the hole at J. This creates a hole at k. Another valence electron in turn made its bond to fill the hole at k. Creating a hole at l which is feather filled by the electron from the nearby covalent bond.

this movement of the whole from positive terminal of the supply to the negative terminal throw the semiconductor constitutes hole current.

Intrinsic semiconductor

An extremely pure semiconductor is called intrinsic semiconductor.

on the basis of energy band phenomena and intrinsic semiconductor at absolute zero temperature is shown in figure. Its valence band is completely filled and the conduction band is completely empty.

when some heat energy supplied to eat some of the valence electron are lifted to conduction band leaving behind holes in the valence band as shown in figure. The electron reaching at the conduction band are free to move at random forest of the holes creating in the crystal also move at random in the crystal. This behaviour of semiconductors so that they have negative temperature coefficient of resistance the resistivity decrease of conductivity increases with the rise in temperature.

Conduction through intrinsic semiconductor at room temperature

when an electric potential difference is applied across a pure semiconductor kept at room temperature the current conduction takes place by kinds of charge carries, namely electron and holes. the free electron available in the conduction band start drifting to what the positive terminal whereas the positive charge carriers start during towards the negative terminal as shown in figure therefore the total current inside the semiconductor is the sum of current due to free electrons and holes.

Drift current :- the flow of current in the semiconductor constituted by the drift of free electron available in the conduction band and wholesale level in the valence band which are formed due to external energy supply to them is known as drift current.

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