Semiconductor Electronics: Materials, Devices and Simple Circuits

Intrinsic Semiconductor

An Intrinsic Semiconductor is the purest form of a semiconductor, elemental, without any impurities. Naturally available elements like silicon and germanium are best examples of an Intrinsic Semiconductor. Let’s know them in further more detail.

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The Lattice Structure of Elements

Intrinsic Semiconductor

They are also called diamond-like structures. In such structures, every atom is surrounded by four neighbouring atoms. Now, both Si and Ge have four valence electrons and in the crystalline structure, each atom shares one of its valence electrons with each of its four neighbours.

Also, it takes one electron from each of its neighbours. This shared pair of electron is called a Covalent bond or a Valence bond. This is how the Si or Ge structure looks in two-dimensions with emphasis on the covalent bond:

Intrinsic Semiconductor

Also, the above image shows the structure with all bonds intact. This is possible only at low temperatures. As the temperature increases and more energy becomes available to the valence electrons, they break away leading to an increase in conductivity of the element.

Now, the thermal energy ionizes only a few atoms. This ionization creates a vacancy in the bond. When an electron, having charge –q, gets excited due to the thermal energy, it breaks free from the bond. This leaves a vacancy there with effective charge +q. This vacancy with an effective positive electronic charge is a hole.

The hole also behaves like a free particle but with a positive charge. In intrinsic semiconductors, the number of free electrons is equal to the number of holes and is called the intrinsic carrier concentration.

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Intrinsic Semiconductor – The Movement of Holes

Another interesting property of semiconductors is that like the electrons, the holes move too. Consider the following image:

Intrinsic Semiconductor

In the image above, you can see that an electron, after being excited due to thermal energy breaks itself away from the bond, generating a free electron. (Site1) A vacancy is created at the site from where the electron releases itself. Now, imagine that an electron from Site 2, as shown in the image, jumps to the hole or vacancy created in Site 1. The hole will now have moved from Site1 to Site2 as shown in the image below:

Intrinsic Semiconductor

It is important to observe that the electron freed from Site 1 is not involved in the movement of the hole. It moves independently like a conduction electron contributing to electron current (Ie) under an applied electric field. Also, movement of the hole is actually a movement of bound electrons.

Under an electric field, these holes move towards the negative potential generating hole current (Ih). Hence, the total current (I) is:

I = Ie + Ih

Another important thing to remember is that apart from the process of the generation of free electrons and holes, a process of recombination takes place simultaneously. In this process, the electrons recombine with the holes. In the state of equilibrium, the rate of generation is equal to the rate of recombination.

Intrinsic Semiconductor at T = 0K

At T = 0K, an intrinsic semiconductor will behave like an insulator.

Structurally, there is a small energy gap between the valence and conduction bands in a semiconductor. When the temperature is low, the electrons are not excited enough to jump to a higher energy state. The image below explains how at T = 0K, the electrons stay in the valence band and there is no movement to the conduction band.

Intrinsic Semiconductor

When the temperature increases, at T > 0K, some electrons get excited. These electrons jump from the valence to the conduction band. Here is how it will look:

Intrinsic Semiconductor

Solved Examples for You

Question: How many valence electrons does silicon have?

  1. 1
  2. 2
  3. 3
  4. 4

Solution: Option (D) Silicon has 4 valence electrons.

Question: If we add an impurity to a metal those atoms also deflect electrons. Therefore,
  1. Electrical and thermal conductivities both increase
  2. Electrical and thermal conductivities both decrease
  3. Thermal conductivity increases but electrical conductivity decreases
  4. Electrical conductivity increases but thermal conductivity decreases

Solution: Option (B).

If the number of electrons increases, collisions between them also increase thus increasing the thermal and electrical resistance. So, electrical and thermal conductivities both decrease.

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