Ohmic Contact

OHMIC CONTACT

Principle

An ohmic contact is a non-rectifying contact which obeys ohm's law i.e., V=IR. The resistance of the ohmic contact should always be low i.e., conductivity should be large.

Explanation

Before contact

Let us consider a metal of fermi energy E_Fm and an n-type semiconductor of fermi energy E_Fs as shown in Fig. 2.32, in which we can see that when they are separated by a distance (d), the fermi energy (E_Fm) of the metal lies above that of the fermi energy of the semiconductor (E_Fs).

After contact

Now, when the metal and n-type semiconductor are made to have contact with each other, then the energy bands of the n-type semiconductor bend downwards near the contact as shown in Fig. 2.33. The magnitude of the band bending and its extension into the semiconductor is very small.

Thus, as a result there is virtually no potential barrier between the metal and the semiconductor and the resistivity becomes very low (or) almost zero. Therefore the electrons can flow freely through the contact and the current increases at the ohmic contact.

Example: Schottky diode is a very best example, which behaves as a ohmic contact if the impurity concentration is very high.