DAY 3 OF ELECTRONICS

SEMICONDUCTOR PHYSICS

SEMICONDUCTOR:

It has electrical conductivity that lies between conductor and insulator.

            e.g., Silicon, Germanium

TYPES

INTRINSIC SEMICONDUCTOR – PURE FORM

EXTRINSIC SEMICONDUCTOR – IMPURE FORM

INTRINSIC SEMICONDUCTOR:

It is a crystalline structure and atoms are arranged in uniform periodic pattern.

CONDUCTIVITY OF INTRINSIC SEMICONDUCTOR:

 Conductivity of semiconductor is almost 0 at room temperature because of the covalent bond (covalent bond is already explained in the previous session- bonding of one atom with another). All the free electrons bond with another results in no free electrons.

The conductivity is initiated by giving some external voltage greater than forbidden energy gap.

When the external voltage applied (or at room temperature), a bond will be broken and an electron is freed from atom and it constitute a current.

And the dislodging electron leaves positive charge in that place called as hole.

This forms electron-hole pair.

The hole has equal magnitude as that of electron.

GENERATION AND RECOMBINATION OF ELECTRONS & HOLES:

How electrons constitute electric current??????????????

      

The external energy required for silicon is 1.1eV and for germanium is 0.72eV.

When the external energy applied, the bond broke and electron moves to conduction band.

The dislodging electron produces hole in that place.

Conduction of electron is also done by increase in temperature.

As the temperature is further increased more and more electrons are produced also the electron-hole pair produces.

Hence electrons in the conduction band keep on increasing simultaneously holes also increasing in the valence band.

This gives a way for conduction of intrinsic semiconductor by electrons.

How holes constitute electric current???????????????

The current is produced by means of flow of charge.

Electron current produces by the movement of electrons from valence band to conduction band.

Similarly, hole current produces by the recombination of holes and electrons inside the crystal.

When an electron moves to conduction band hole (I) produced in that place.

As the temperature further increases the electron in the vicinity dislodge and produce hole(II).

Now the hole at the 1st location recombines with the new electron produced at 2nd location.

This process continues and the movement of hole takes place from one place to another.

So this constitutes hole current.

EXTRINSIC SEMICONDUCTOR:

The process of adding impurity to a pure semiconductor is called extrinsic semiconductor.

The process of adding impurities is called doping. The added impurity is called dopant.

Two types:

N-type semiconductor

P-type semiconductor

N-Type semiconductor:

The n-type material is formed by doping pentavalent impurity atoms like P, As, Sb, etc., at a low rate with silicon/germanium.

The process of adding pentavalent impurities to a semiconductor is called as N-Type semiconductor.

When it is added and forms covalent bond one electron doesn’t form any bond (since contains 5 valence electrons).

That electron remains as free electrons and constitute current.

CONDUCTION:

The pentavalent impurities form an additional level called donor level (near edge of conduction band).

At absolute temperature, valence band is filled, conduction band is empty and donor level is filled.

But at room temp, the valence electrons gain some energy and moves to conduction band and constitute current.

At the same time the electrons in the donor level is also moved to conduction band.

So the combination of electrons from valence band and donor levels gives more current.

As the constitution of current is by means of electrons this material is called n-type material.

NOTE: Even though n-type produces the large number of electrons the atom is still electrically neutral.

P-Type semiconductor:

The p-type semiconductor is formed by adding trivalent impurity such as boron, gallium, indium, etc.,

The process of adding trivalent impurities to a semiconductor is known as P-Type semiconductor.

When it is added and forms covalent bond there will be an insufficient electron and holes produced in that place (3 valence electrons).

That hole constitute current called hole current.

CONDUCTION:

The trivalent impurity produces additional level called acceptor level near the valence band.

At zero temperature valence band is filled by holes, acceptor level is empty and conduction band is also empty ( in terms of holes).

But at room temperature, minimum electrons in valence band moves to conduction band also to acceptor level.

So the holes increased in valence band thus constitute hole current.

Thus the increase in holes concentration constitutes more hole current.

For this reason this type is called p-type semiconductor.