[Physics Class Notes] on Intrinsic Semiconductors Pdf for Exam

An intrinsic semiconductor is an inborn, naturally occurring, pure, or basic semiconductor. The best examples of intrinsic semiconductors are crystals of Pure Silicon and Pure Germanium.

The Atomic Number and Electronic Configuration of Si and Ge Are

Here,  we will study these two intrinsic semiconductors.

What are Intrinsic Semiconductors?

We know that Si and Ge have 4 valence electrons and these two elements possess properties like Carbon because they are tetravalent.

All four electrons of Si and Ge crystals are involved in covalent bonding and no electron sets free; this is the property of catenation that we can see in Carbon also. The diagram below shows the catenation property of Si that we can see in Ge also:

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Let’s look at the following points on the effect of temperature on semiconductor:

Working Mechanism of Intrinsic Semiconductors

As there are zero electrons in the conduction band, so no electricity formation (zero conductivity) means the semiconductor behaves as an insulator at 0 K.

Now, what we do is, we increase the temperature of Silicon or Germanium crystals, the thermal energy offered to these crystals may break the bond and release a few electrons, and some free electrons generate electricity. Also, the release of the number of free electrons depends on the temperature. 

• At room temperature, i.e.,  300 K or 27 ℃, only one covalent bond breaks out of 1029 atoms that means very few electrons. So, we couldn’t obtain good conductivity at room temperature. 

So, what happens next?

As the temperature rises to 300K, the electron from one of the bond, and vacancy generates in that place. So, the electron that is set free is the thermally free electron. 

The place the electron left is the vacancy or the hole, and this electron gains some energy (in electronvolts), crosses the forbidden energy gap and reaches the conduction band; this migration is responsible for electricity generation.

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Here, we focused on one electron, but there are many crystals, and many crystals mean many electrons. 

On applying an electric field from the top to the bottom direction, the electrons start flowing in the opposite direction because the electric field or current and electrons flow in an opposite direction. 

Concept of Hole and Electron

So far, we understood that on applying an electric field to the semiconductor, the electrons start leaving the conduction band and falls. 

Now, electrons start falling and they unite with vacancies or holes, so the process of the meeting of electrons with holes is called recombination. 

Now, as the temperature rises, more electrons migrate to the conduction band and thus supersedes the recombination process. At this moment, the semiconductor conductivity increases with the temperature rise.

Points to Remember: Intrinsic Semiconductors

• A number of thermally generated electrons equals the number of holes generated. (ne = nh).

• The intrinsic concentration is the intrinsic charge carrier density of the semiconductor, and it is symbolized as ni. The ni value for Si and Ge are as follows:

1. Si = ni = 1.5 x 106 per m3

2. Ge = ni = 2.4 x 1019 per m3

                ni   e-Eg/kT

Here, 

E = forbidden energy gap

k = Boltzmann constant 

T = temperature in Kelvin

So, we understood what is an intrinsic semiconductor. Now, we will differentiate it from the extrinsic semiconductor.

Intrinsic and Extrinsic Semiconductor