Intrinsic and Extrinsic Semiconductors 

1.0Introduction to Semiconductors

Semiconductors are materials with electrical conductivity between that of conductors and insulators. They form the foundation of modern electronic devices. The two primary types of semiconductors are:

• Intrinsic Semiconductors: Pure, undoped materials.
• Extrinsic Semiconductors: Doped materials with controlled impurity levels.

Understanding the differences between these types is crucial for grasping the operation of various semiconductor devices.

2.0Intrinsic Semiconductors or Pure Semiconductor

Semiconductors in their purest form. Free from any type of impurity All the covalent bonds are complete, No free electrons. Valence bands are completely filled. Conduction bands are complete empty. It behaves as perfect insulator.

Some of the covalent bonds are broken due to thermal energy.

On receiving an additional energy, one of the electrons from a covalent bond breaks and is free to move in the crystal lattice.
While coming out of the covalent bond, it leaves behind a vacancy named ‘hole’. This process is called Electron-hole pair generation.

In intrinsic semiconductor, the number of thermally generated electrons always equals the number of holes.

nₑ = free electron concentration, nₕ = hole concentration
nₑ = nₕ

Total Current = Iₕ + Iₑ
Valence Band Current Iₕ, Conduction Band Current Iₑ

Conductivity (σ)

σ = e (nₑ μₑ + nₕ μₕ) = σₑ + σₕ

Where, σₑ = μₑ nₑ e & σₕ = μₕ nₕ e

3.0Properties of Semiconductor

Negative temperature coefficient (α), with increase in temperature resistance decreases.
Crystalline structure with covalent bonding [Face centred cubic (FCC)].
Conduction properties may change by adding small impurities.
Position in periodic table → IV group (Generally)
Forbidden energy gap (0.1 eV to 3 eV)
Charge carriers: electron and hole.

There are many semiconductors but few of them have practical application in electronics like

4.0Concept of Electron Current and Hole Current

In conductors current is caused by only motion of electrons but in semiconductors current is caused by both electrons in conduction band and holes in valence band.

Current that is caused by electron motion is called electron current and current that is caused by hole motion is called hole current. Electron is a negative charge carrier whereas hole is a positive charge carrier.

At absolute zero temperature intrinsic semiconductor behaves as insulator. However, at room temperature the electrons present in the outermost orbit absorb thermal energy. When the outermost orbit electrons get enough energy then they will break bonding with the nucleus of atom and jumps in to conduction band. The electrons present in conduction band are not attached to the nucleus of an atom so they are free to move.

When the valence electron moves from valence band to the conduction band a vacancy is created in the valence band where electron left. Such vacancy is called hole.

5.0Conduction in Intrinsic Semiconductor

The process of conduction in intrinsic semiconductor is shown in below fig. In the below fig, an intrinsic semiconductor is connected to a battery.

Here, positive terminal of battery is connected to one side and negative terminal of the battery is connected to other side. As we know like charges repel each other and opposite charges attract each other. In the similar way negative charge carriers (electrons) are attracted towards the positive terminal of battery and positive charge carriers (holes) attracted towards the negative terminal of battery.

Electrons will experience a attractive force from the positive terminal, so they move towards the positive terminal of the battery by carrying the electric current. Similarly, holes will experience a attractive force from the negative terminal, so they moves towards the negative terminal of the battery by carrying the electric current. Thus, in a semiconductor electric current is carried by both electrons and holes.

In intrinsic semiconductor the number of free electrons in conduction band is equal to the number of holes in valence band. The current caused by electrons and holes is equal in magnitude.

The total current in intrinsic semiconductor is the sum of hole and electron current.

$I=I_{hole}+I_{electron}$

Extrinsic Semiconductors

N-type and P-type Semiconductor

The conductivity of an intrinsic semiconductor depends on its temperature, but at room temperature its conductivity 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. When a small amount, say, a few parts per million (ppm), of a suitable impurity is added to the pure semiconductor, the conductivity of the semiconductor is increased manifold. Such materials are known as extrinsic semiconductors or impurity semiconductors.

Extrinsic semiconductors are semiconductors that are doped with specific impurities. The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors.

The deliberate addition of a desirable impurity is called doping and the impurity atoms are called dopants.

Intrinsic Semiconductor + Dopants = Extrinsic Semiconductor

6.0Doping

Doping is the mixing of impure atoms in a pure semiconductor material. Here the impure atoms refer to the atoms that are different from the pure semiconductor. The most commonly used impure atoms are Boron (B), Aluminum (Al), Arsenic (As), Phosphorus (P) etc.

Necessity of Doping: The Conductivity of semiconductors is very poor at room temperature. To get a significant amount of conductivity we need to increase the temperature of the semiconductor to a high value. But it is practically impossible to use the semiconductor devices at a very high temperatures above 50°C.

Now, doping can increase the conductivity of semiconductors by a significant amount even at room temperature. At this one can use semiconductor devices comfortably at room temperature.

So, we need the doping in semiconductor materials to increase their conductivity without increasing the temperature which enables us to use semiconductor devices at room temperature.

Properties of Dopants

• It occupies only a very few of the original semiconductor atom sites in the crystal.

• It should not distort the original pure semiconductor lattice.
• Sizes of the dopant and the semiconductor atoms should be nearly the same.
• It should give away holes or electron for conduction very easily.

1. N-type semiconductor

When Si or Ge is doped with a pentavalent impurity, N-type semiconductor is formed. The force of attraction between this mobile electron and the positively charged (+5) impurity ion is weakened. So, such electrons from impurity atoms will have energies slightly less than the energies of the electrons in the conduction band.

The energy required to detach the fifth loosely bound electron is only of the order of 0.05 eV for Si and 0.01 eV for Ge. A small amount of energy provided due to thermal agitation is sufficient to detach this electron and it is ready to conduct current.

Free electron concentration is mainly decided by the donor impurity concentration. (nₑ = Nᴅ)
The number of holes decreases because of increase in rate of recombination.
Free electrons are the majority carriers and holes are the minority carriers. (nₑ >> nₕ)

Total current in semiconductor is mainly due to electron current, I = Iₑ + Iₕ ≈ Iₑ

Overall semiconductor is still neutral.

2. P-type semiconductor

When Si or Ge is doped with a trivalent impurity, P-type semiconductor is formed. The vacancy of trivalent impurity may be filled with an electron from neighbouring atom, creating a hole in that position from where the electron jumped.

The acceptor impurity produces an energy level just above the valence band.

The energy difference between the acceptor energy level and the top of the valence band is much smaller (0.01 eV) than the band gap. Electrons from the valence band can easily move into the acceptor level by being thermally agitated.

Hole concentration is mainly decided by the acceptor impurity concentration. (nₕ = Nₐ)
The number of electrons further decreases due to increase in rate of recombination.
Holes are the majority carriers and free electrons are the minority carriers. (nₕ >> nₑ)

Total current in semiconductor is mainly due to hole current, I = Iₑ + Iₕ ≈ Iₕ

Overall semiconductor is still neutral.

S. No.	Intrinsic Semiconductor	N-type Semiconductor	P-type Semiconductor
1.
2.
3.	Current is due to both electrons and holes	Mainly due to electrons	Mainly due to holes
4.	nₑ = nₕ = nᵢ	nₑ >> nₕ (Nᴅ ≈ nₑ)	nₕ >> nₑ (Nₐ ≈ nₕ)
5.	I = Iₑ + Iₕ	I ≈ Iₑ	I ≈ Iₕ
6.	Entirely neutral	Entirely neutral	Entirely neutral
7.	Quantity of electrons and holes are equal	Majority – Electrons Minority – Holes	Majority – Holes Minority – Electrons

7.0Mass Action Law

At room temperature, most of the acceptor atoms get ionised leaving holes in the valence band. Thus at room temperature the density of holes in the valence band is predominantly due to impurity in the extrinsic semiconductor.

“Rate of generation of charge carriers is equal to rate of recombination of charge carriers”

nₑ nₕ = nᵢ²

Note: nᵢ depends only on the nature of semiconductor material and temperature, it does not depend on the doping level.

Intrinsic vs. Extrinsic Semiconductors difference

8.0Applications of Semiconductors

Semiconductors are the building blocks of almost all electronic devices.

• Diodes: Formed by the junction of a P-type and an N-type semiconductor (P-N junction). Diodes allow current to flow in one direction only.
• Transistors: Used for switching and amplifying electronic signals. Bipolar Junction Transistors (BJTs)and Field-Effect Transistors (FETs) are essential components of logic circuits and microprocessors.
• Integrated Circuits (ICs): Also known as microchips, these are miniaturized electronic circuits built on a single piece of semiconductor material. They are the brains of computers, smartphones, and countless other devices.
• Solar Cells: Convert light energy into electrical energy using the photovoltaic effect in semiconductor materials.