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Structure of Semiconductor material and Energy Band Theory

We all know that inside an atom the distribution of electrons is in the shell and sub-shell and each shell is at a certain distance from the nucleus and each shell has its own energy level relative to the nucleus. 

The electron circling in the shell nearest (first) to the nucleus is bound to the nucleus with a strong attraction force and the energy of this electron is also much lower than other electrons. The electron which stays away from the nucleus is bound to the nucleus with less attraction force and its energy level is also very high.

Therefore, the energy level of the electron of the outermost orbit (Valence shell) is very high. And it is bound to the nucleus with a weak attraction force. This electron can easily move out of its orbit due to weak force and can participate in chemical reactions.

Atoms in the solid matter are very close to each other, so electrons of the outer orbit are joined by two or more atoms. The electrons of the Valence shell also influence other atoms and their electrons.

The electrons of the outer shell form chemical bonds by partnering with the electrons of their adjacent atoms. The chemical bond formed due to electron sharing is called Covalent Bond. Therefore, the electrons in the atom of a solid substance do not remain in a free state.

Since the electrons of the outer orbit have the highest energy and in solid matter, these electrons merge from one atom to another to form chemical bonds, similarly the energy-related to these electrons also merge with each other to form a band of energy called energy. 

This is called the Energy Band. Similarly, different energy levels of any atom-like the first second, third, etc. merge with the energy level of each other atom to form an energy band. All these energy levels are divided into three parts as follows: -
  • Valence Band 
  • Conduction Band 
  • forbidden Band 
The energy band in which electrons exist at normal temperature is called the valence band.
The energy band of the atom in which free electrons can move is called the conduction band. At normal temperature about there is no free electron present in the Conduction band of the Semiconductor.
Energy Difference between Valence Band and  Conduction band is called forbidden Band. This is denoted by Eg. The value of Eg in different crystals is different. Some solids have Eg = 0. 
According to the Valence Bond theory, The substance in which the conduction band and Valence band overlap each other is a conductor. On the basis of the Conduction Band, Valence Band, and forbidden Gap, Solid materials are divided into Conductor Insulator and Semiconductor.

Conductor, Semiconductor, and Insulator


Atoms of each solid matter have some electrons that are free from the binding of the force of attraction of the nucleus of the atoms and are free to move throughout the material, these electrons are called free electrons


These electrons act as electrical carriers for electrical conduction in metals, hence they are also called conduction electrons. These free electrons can move anywhere in the entire substance, but they cannot go out except the substance. 


The direct relationship of electrical conductivity in solids depends on the energy difference between the conduction band and the valence band. This energy gap is called Forbidden Energy Gap. This is denoted by  Eg. On the basis of this, solid materials are divided into Conductor Insulator and Semiconductor.

Conductor or Metal

Conductor Energy Band theory

The material in which the number of free electrons is very high is called an electrical conductor. Such as silver, gold, copper, etc. are all electrical conductors. The number of free electrons in these materials is very high.


 The resistivity of these materials is very low and the conductivity is very high. Conduction bands and Valence bands are overlapped on top of each other so there is no Forbidden Energy Gap between them. That is, Eg = 0

Insulators


Substances that do not allow to pass an electrical current is called Insulator. The number of free electrons in the insulator's material is very small. Ebonite, glass, wax sulfur, etc. all these substances do not conduct electricity. 


The resistivity of these substances is very high and the conductivity is very low. There is a huge difference between the Valence band and the Conduction band in these materials. Due to which the forbidden energy gap (Eg) is very high. For example, Eg = 6eV for a diamond. 

This means that if 6eV of energy is given in one atom of the diamond, then electrons will come out of the Valence band into the Conduction Band, then the diamond will conduct the current.

Semiconductor


Semiconductors are those materials whose electrical conductivity is higher than insulators but less than conductors. That is, the electrical conductivity of a semiconductor is the intermediate between the insulator and the conductor. 


Semiconductors are electrical insulators at absolute zero temperature, but its conductivity increases continuously with increasing temperature. Silicon, germanium carbon is such a substance. 

The number of free electrons in these materials is less than the conductors and more than the insulators. The electrons of a semiconductor atom are bound to the nucleus with a common attraction force. The magnitude of this force is neither high nor low.

 Semiconductor energy band theory


 In a semiconductor atom, the Energy difference between the Valence Band and the Conduction Band is very small. At absolute temperature, no free electrons are present in its conduction band and electrons are filled in the valence band.
 


As soon as the electron in the Valence shell of an atom receives energy from an external energy source, it jumps into the Conduction Band and acts as an electrical carrier. Hence the conductivity of the semiconductor increases with increasing temperature. 

As soon as the semiconductor receives energy equal to or greater than the Forbidden Energy Gap, the bonds of the atom begin to break and electrons are released into the Conduction Band, thereby increasing the conductivity of the semiconductor. Eg = 1.1 eV for silicon and Eg = 1.1 eV for germanium.
Semiconductor energy band theory

Due to the thermal disturbance at normal temperature, low energy causes some bonds of the semiconductor atom to break down, leaving some electrons free and reaching the conduction band, allowing the semiconductor to act as a conductor even at normal temperature. 


Due to the breakdown of electrons, electrons are freed, which frees the electron space in the atom. This empty space is called Hole.
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