Concept of Drift

by / Thursday, 15 June 2017 / Published in Blog

Real Life Application
This concept will be very helpful in choosing the type of material according to the requirement. It also gives a basic idea about semiconductors.

Semiconductors are preferred in case of Electronic devices as it is required to have control over the flow of charge carriers which is unlikely in metals. In semiconductors, there are two types of charge carriers i.e., electrons and holes. Electron is negative charge and hole is positive charge.
In case of semiconductors, there are two types of currents
1. Drift Current
2. Diffusion Current
In this lecture, we learn about how the drift current flows inside semiconductors. To learn about the concept of drift in semiconductors, we first learn drift in metals and then extend the same to semiconductors for the simple reason, that in metals we simply have single type of charge carriers i.e., electrons.
Under equilibrium, electrons move in random fashion with some velocity (drift) but the net current is zero due to random motion. Now, when potential difference is applied across the metal bar, the electric field produces forces the electron in opposite direction to that of applied field. Under lower fields, the drift velocity increases proportional with applied field. The proportional constant (material constant) is known as mobility. Mobility is the ability of charge carrier to move.
If electric field is applied to semiconductors, the force on electrons and holes are in opposite direction. Velocity with which electrons and holes move inside the semiconductors under the applied field is known as electron drift velocity and hole drift velocity respectively. It is observed that mobility of electrons is higher compared to that of holes owing to effective mass of carriers.
Another important parameter is electrical conductivity which is the product of carrier concentration, charge of carrier and electric field intensity. In semiconductors the total conductivity is sum of electron and hole conductivities. Its important to see the difference between the conductivities of intrinsic and extrinsic conductors. It is only to improve the conductivity of material that we convert intrinsic material to extrinsic.
Drift current density is product of electrical conductivity and electric field intensity. As conductivity is higher in case of extrinsic materials, the current density is also high.

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