What is Intrinsic Semiconductor and Extrinsic Semiconductor?

The electrical property of a material that lies between insulator as well as conductor is known as a semiconductor material. The best examples of semiconductors are Si and Ge. Semiconductors are classified into two type’s namely intrinsic semiconductor and extrinsic semiconductor (P-type and N-type). The intrinsic type is pure kind of semiconductor whereas an extensive type includes impurities to make conductive. At room temperature, the conductivity of intrinsic will become zero whereas extrinsic will become little conductive. This article discusses an overview of intrinsic semiconductors and extrinsic semiconductors with doping and energy band diagrams.

What is Intrinsic Semiconductor?

Intrinsic semiconductor definition is, a semiconductor that is extremely pure is an intrinsic type. On the energy band concept, the conductivity of this semiconductor will become zero at room temperature which is shown in the following figure. The intrinsic semiconductor examples are Si & Ge.

In the above energy band diagram, the conduction band is empty whereas the valence band is filled totally. Once the temperature is increased, some heat energy can be supplied to it. So the electrons from the valence band are supplied toward the conduction band by leaving the valence band.

The flow of electrons while reaching from valence to the conduction band will be random. The holes formed within the crystal can also flow anywhere freely. So, the behavior of this semiconductor will show a negative TCR (temperature coefficient of resistance). The TCR means, when the temperature increases, the material’s resistivity will be decreased & the conductivity will be increased.

What is Extrinsic Semiconductor?

To make a semiconductor like conductive, then some impurities are added which is called extrinsic semiconductor. At room temperature, this kind of semiconductor will conduct a small current; however, it is not helpful in making a variety of electronic devices. Therefore, to make the semiconductor conductive, a little quantity of appropriate impurity can be added to the material through the doping process.

Doping

The process of adding impurity to a semiconductor is known as doping. The amount of impurity which is added to the material has to control in the extrinsic semiconductor preparation. In general, one impurity atom can be added to 108 atoms of a semiconductor.

By adding the impurity, the no. of holes or electrons can be increased to make it conductive. For instance, if a Pentavalent impurity includes 5 valence electrons that are added to a pure semiconductor then the no. of electrons will exist. Based on the kind of impurity added, the extrinsic semiconductor can be classified into two types like N-type semiconductor & P-type semiconductor.

Carrier Concentration in Intrinsic Semiconductor

In this type of semiconductor, once the valence electrons damage the covalent bond & move into the conduction band than two kinds of charge carriers will be generated like holes & free electrons.
The no. of electrons for each unit volume within the conduction bands otherwise the no. of holes for each unit volume within the valence band is known as carrier concentration in an intrinsic semiconductor. Similarly, electron carrier concentration can be defined as the no. of electrons for each unit volume within the conduction band whereas the no. of holes for each unit volume within the valence band is known as hole-carrier concentration.

In intrinsic type, the electrons which are generated within the conduction band can be equivalent to the no. of holes which are generated within the valence band. Therefore the concentration of electron carriers is equivalent to the concentration of hole-carrier. So it can be given as

ni = n = p

Where ‘n’ is the concentration of electron carrier, ‘P’ is the concentration of the carrier of the hole & ‘ni’ is the concentration of intrinsic carrier

In the valence band, the concentration of the hole can be written as

P = Nv e -(E_F-E_V)/K_BT

In the conduction band, the concentration of electron can be written as

N = P = Nc e -(E_C-E_F)/K_BT

In the above equation, ‘KB’ is the Boltzmann constant

‘T’ is the total temperature of intrinsic type semiconductor

‘Nc’ is the efficient density of states within the conduction band.

‘Nv’ is the efficient density of states within the valence band.

The Conductivity of Intrinsic Semiconductor

The behavior of this semiconductor is like a perfect insulator at zero degrees temperature. Because at this temperature, the conduction band is empty, the valence band is full and for conduction, there are no charge carriers. However, at room temperature, the thermal energy can be enough to make a huge no. of electron-hole pairs. Whenever an electric field is applied to a semiconductor, and then electrons flow will be there because of the movement of electrons within one direction & holes within reverse direction

For a metal, the current density will be J = nqEµ

The current density within a pure semiconductor because of the flow of holes & electrons can be given as

Jn = nqEµ_n

Jp = pqEµ_p

In the above equations, ‘n’ is the concentration of electrons and ‘q’ is the charge on hole/electron, ‘p’ is the concentration of holes, ‘E’ is the applied electric field, ‘µ’n is the electron mobility and ‘µ’p is the holes mobility.

The density of the whole current is

J = Jn + Jp

= nqEµ_n + pqEµ_p

I= qE (nµ_n + pµ_p)

Where J = σE, then the equation will be

σE = = qE (nµ_n + pµ_p)

σ = q(nµ_n + pµ_p)

Here ‘σ’ is the conductivity of semiconductor

The no. of electrons are equal to the no. of holes in the pure semiconductor so n=p=ni

‘ni’ is the carrier concentration of intrinsic material, so

J = q(niµ_n + niµ_p)

The pure semiconductor conductivity will be

σ = q(niµ_n + niµ_p)

σ = qni(µ_n + µ_p)

So the conductivity of pure semiconductor mainly depends on intrinsic semiconductor & electrons & holes mobility.

FAQs

1). What is an intrinsic and extrinsic semiconductor?

The pure type of semiconductor is the intrinsic type whereas the extrinsic is, the semiconductor in which impurities can be added to make it conductive.

2). What are the examples of intrinsic type?

They are Silicon & germanium

3). What are the types of extrinsic semiconductors?

They are P-type and N-type semiconductors

4).Why extrinsic semiconductors are used in electronics manufacturing?

Because the electrical conductivity of the extrinsic type is high compare with intrinsic. So these are applicable in designing transistors, diodes, etc.

5). What is the conductivity of intrinsic?

In a semiconductor, the impurities & structural defects have an extremely low concentration is known as the conductivity of intrinsic.

Thus, this is all about an overview of the Intrinsic Semiconductor and Extrinsic Semiconductor and energy band diagram with doping. Here is a question for you, what is the temperature of intrinsic?