Types Of Semiconductor: Intrinsic And Extrinsic Semiconductor - Practice Questions & MCQ

Intrinsic semiconductor-

It is a pure semiconductor. Silicon and germanium are the most common examples of intrinsic semiconductors. Both these semiconductors are most frequently used in the manufacturing of transistors, diodes and other electronic components.  

Both Si and Ge have four valence electrons.  In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbour atoms, and also to take a share of one electron from each such neighbour as shown in the below figure. This shared pair of the electron is called a Covalent bond or a Valence bond.

The above figure shows the structure with all bonds intact (i.e no bonds are broken). This is possible only at low temperatures.

As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break–away from the conduction band becoming the free electron and creating a vacancy in the bond. This vacancy with an effective positive electronic charge is called a hole.

In intrinsic semiconductors, the number of free electrons ( n_e ) is equal to the number of holes ( n_h )

i.e   where   is called intrinsic carrier concentration.

Semiconductors possess the unique property in which, apart from electrons, the holes also move.

The free-electron moves completely independently as a conduction electron and gives rise to an electron current, I_e under an applied electric field. while Under an electric field, these holes move towards the negative potential generating hole current (I_h).

Hence, the total current (I) is given as  I = I_e + I_h

And apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers.

An intrinsic semiconductor will behave like an insulator at T = 0 K. As shown in the below figure., at T = 0 K, the electrons stay in the valence band and there is no movement to the conduction band.

When the temperature increases, at T > 0K, some electrons get excited. These electrons jump from the valence to the conduction band as shown in the below figure.

The conductivity of an intrinsic semiconductor at room temperature is very low. As such, no important electronic devices can be developed using these semiconductors. Hence there is a necessity of improving their conductivity. This can be done by making use of impurities. because when a small amount of a suitable impurity is added to the pure semiconductor, the conductivity of the semiconductor is increased manifold

Extrinsic semiconductors-

An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications. The deliberate addition of a desirable impurity is called doping and the impurity atoms are called dopants. Another term for Extrinsic semiconductors is ‘Doped Semiconductor’.

The size of the dopant and Semiconductor atoms should be the same, for making sure that the amount of impurity added should not change the lattice structure of the Semiconductor.  

Following types of dopants used in doping the tetravalent (valency 4) Si or Ge:

(i) Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.

This will give  n-type semiconductor

(ii) Trivalent (valency 3); like Indium (In), Boron (B), Aluminium (Al), etc.

This will give  p-type semiconductor

n-type semiconductor

 When a pentavalent impurity is added to an intrinsic or pure semiconductor (silicon or germanium), then it is said to be an n-type semiconductor. Pentavalent impurities such as phosphorus, arsenic, antimony, etc are called donor impurity.

The four valence electrons of each phosphorus atom form 4 covalent bonds with the 4 neighboring silicon atoms.

  The free-electron (fifth valence electron) of the phosphorus atom does not involve in the formation of covalent bonds.

This shows that each phosphorus atom donates one free electron. Therefore, all the pentavalent impurities are called donors.

So, there is a donor energy level between the valence band and conduction band. Just below the conduction band.

 The number of free electrons depends on the amount of impurity (phosphorus) added to the silicon.

Charge on n-type semiconductor-

Even though n-type semiconductor has a large number of free electrons, but the total electric charge of n-type semiconductor is neutral.

Conduction in n-type semiconductor-

When voltage is applied to n-type semiconductors as shown in the below figure; then the free electrons move towards the positive terminal of the applied voltage.  Similarly, holes move towards the negative terminal of the applied voltage.

In an n-type semiconductor, conduction is mainly because of the motion of free electrons.

because In an n-type semiconductor, the population of free electrons is more whereas the population of holes is less (i.e n_e >>n_h)  

 In an n-type semiconductor, free electrons are called majority carriers and holes are called minority carriers.

p-type semiconductor

When the trivalent impurity is added to an intrinsic semiconductor (Si and Ge), then it is said to be a p-type semiconductor. Trivalent impurities such as Boron (B), Gallium (G), Indium(In), Aluminium(Al), etc are called acceptor impurity.

The three valence electrons of each boron atom form 3 covalent bonds with the 3 neighboring silicon atoms.

For the fourth covalent bond, only silicon atom contributes one valence electron. Thus, the fourth covalent bond is incomplete with the shortage of one electron. and This missing electron is called a hole.

This shows each boron atom accepts one electron to fill the hole. Therefore, all the trivalent impurities are called acceptors.

So there is an acceptor energy level just above the valence band.

A small addition of impurity (boron) provides millions of holes.

Charge on the p-type semiconductor-

Even though p-type semiconductor has a large number of holes, but the total electric charge of p-type semiconductors is neutral.

Conduction in p-type semiconductor-

When voltage is applied to p-type semiconductor as shown in the below figure; then the free electrons move towards the positive terminal of the applied voltage.  Similarly, holes move towards the negative terminal of the applied voltage.

In a p-type semiconductor, conduction is mainly because of the motion of holes in the valence band.

because In a p-type semiconductor, the population of free electrons is less whereas the population of holes is more (i.e  n_h >>n_e)  

 In a p-type semiconductor, holes are called majority carriers and free electrons are called minority carriers.

• Number of electrons or holes-

The electron and hole concentration in a semiconductor in thermal equilibrium are related as:

n_e × n_h = n_i²

 On the increasing temperature,  the number of currents carriers increases.

And the relation is given as $n_e=n_h=A T^{\frac{3}{2}} e^{-\frac{E g}{2 K T}}$

where 

E_g = Energy gap

K = Boltzmann Constant

T = Temperature in kelvin