Question

what are ohmic and non ohmic contacts

Answer 1

An ohmic contact is a non-rectifying junction—a region in a semiconductor device that has been prepared so that the current–voltage (I-V) curve of the region is linear and symmetric.

Similarly a non-ohmic contact is a rectifying junction—a region in a semiconductor device that has been prepared so that the current–voltage (I-V) curve of the region is non-linear and anti-symmetric.

ref: http://en.wikipedia.org/wiki/Ohmic_contact