Semiconductor, Energy band diagram

What is semiconductor?

Solid- state materials are broadly classified into three, they are

(i)    Conductors,

(ii)   Semiconductors and

(iii)   Insulators.

A semiconductor is a solid chemical element that conducts under some conditions. The conductivity of semiconductor lies between conductor and insulator. Carbon, Silicon and Germanium are the examples of Semiconductor.

Semiconductors in periodic table:

Periodic Table

From the periodic table we know that the semiconductors are in between metals and non metals and they exhibit some of the properties of metals and non metals and thus they are known as metalloid. Semiconductors belong to 3rd ,4th and 5th group in the periodic table.

Example of semiconductor:

Silicon is one of the semiconductor .Chemical symbol of silicon is Si,and its atomic number is 14. The Earth’s crust is comprised of 25.7% silicon in its various forms. Silicon is found in sand, clay, granite, quartz, glass, cement and ceramics. It is the second most abundant element found on earth, after oxygen.  Mostly Silicon is used as Semiconductor in the manufacture of electron devices. Pure silicon does not conduct electricity it’s an insulator. When the temperature is increased or when impurity is added they start conducting.

Arrangement of electrons in the shells:

Number of electrons in each shell

The number of electrons in each shell is arranged according to the above table. So in the first shell of silicon atom there are 2 electrons in the 2nd shell there are 8 electrons and in the valence shell there are 4 electrons. To form a tight crystalline structure the 4 valence electrons forms a covalent bond with other 4 silicon atoms, so that they might stay in a place. At room temperature there is no free electrons so they behave like an insulator. 

Structure of Silicon

Working of semiconductor:

When the temperature is increased it may break the covalent bond and becomes a free electron of negative charge, while leaving a vacancy or a hole of positive charge. In an electric field, a free electron may move from valence band to conduction band. Thus they start to conduct. When impurity from group 3 or group 5 elements are added they have free valence electrons or holes and thus they start to conduct.

Energy band diagram of semiconductor:

Energy band diagram of Semiconductor

 

Energy band:

Band is the collection of energy levels. In material when atoms are closer they are subjected to force from the other atoms. So when they move they produce energy. Electrons in the first orbit forms an energy level and second orbit another energy level. Electrons in the atom have different energy levels and according to their energy levels they are grouped together. There are many energy levels mainly there are three energy bands conduction band, valence band and forbidden gap.

Valence band:

The collection of energy level in the outermost shell of an atom is the Valence band. Electrons can jump into the conduction band when excited. Valence band can be half filled or completely filled and can never be empty.

Conduction band:

When the valence electrons gain sufficient energy they become free and they move around the material. The collection of energy levels of all free electrons is called conduction band.

Forbidden gap:

The energy required by the valence electron to become free is called forbidden energy.

In semiconductors the forbidden gap is small so that when the temperature is increased the electrons in the valence shell breaks the covalent bond and become free leaving a hole of positive charge and thus starts to conduct.