Semiconductor || Basic Notes || My Notes

Semi- Conductor – 

A semiconductor is the substance which has resistivity (10^-4 to 0.5Ω) in between Conductor and Insulator.  The semiconductor is nor a conductor and nor an insulator.

For example - germanium, silicon, carbon etc

Germanium (32)  

Germanium is discovered in 1886, and the atomic number of geranium is 32, it means there is 32 protons and 32 electrons in Germanium. Germanium has become the model substance among the semiconductors because it can be purified relatively well and crystallized easily.

Silicon (14) – Silicon is an element most of the common rocks, The silicon compounds are chemically reduced to silicon which is pure to use as a semiconductor. The atomic number of a silicon is 14.

Properties of Semi-Conductor –

·         The resistivity of Semi-Conductor is less than an insulator and more than a conductor.

·         Semiconductor has negative temperature coefficient of resistance. i.e. the resistance  of a semiconductor decrease with the increase in temperature and vice-versa, For example, germanium is actually an insulator at low temperature              

·         When a suitable metallic impurity (e.g. arsenic, gallium etc.) is added to a semiconductor, its current conducting properties changes appreciably.

Bonds in Semi-Conductor-

In semiconductors, bonds are formed by sharing of valence electrons. Such type of bonds is called the co-valent bond. In this formation of a covalent bond, each atom contribute equal number of valence electrons and the contributed electrons are shared by the atoms engaged in the formation of the bond

Energy band of semiconductor-

A semiconductor almost filled valence band and nearly empty conduction band with a very small energy gap separating the two.  (Gap= ±1 eV)

At a room temperature, some of the valence electrons may acquire sufficient energy to enter into the condition band and thus become free electrons. Therefore, at a room temperature, germanium or silicon is neither a good conductor nor an insulator.

Effect of Temperature

The effects of temperature on semiconductor mentioned below-

·         At absolute zero – At absolute zero temperature, all electrons are tightly held by the semiconductor atoms. In this case, the energy gap between valence band and conduction band is very large at absolute zero; this is why semiconductor behaves as an insulator.

·         Above absolute zero-  When the temperature is raised, some of the covalent bonds in the semiconductor break due to the thermal energy supplied

Intrinsic Semiconductor

In a simple way, an extremely pure form of a Semiconductor is known as Intrinsic Semiconductor. In an intrinsic semiconductor, even at room temperature, hole- electron pairs are created. When the electric field is applied across an intrinsic semiconductor, the current conduction takes place by free electron and holes.                                     

  Free electrons are created due to a breakdown of the covalent bonds and at the same time, holes are created by in the covalent bond.

Extrinsic Semiconductor

The Extrinsic Semiconductor is the impure form of Semiconductor. Extrinsic Semiconductor is formed by adding a small amount of impurity to a semiconductor. This process is also called doping. Well Extrinsic Semiconductors are two types:

1- N-Type Semiconductor                                                 2- P-Type Semiconductor             

In our next Posts, we will read about N-Type semiconductor and P-type semiconductor and also about PN junction.

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