Rectifying vs ohmic behavior?
A Ohmic contact is a junction between two (semi)conductors that obeys Ohm's law, i.e. its current is proportional to the applied voltage.

A rectifying contact instead creates a rectifier, i.e. a two terminal component where current is not proportional to applied voltage. In a rectifier current can be any value if applied voltage is above zero, and current is zero if applied voltage is less than zero, as in this image:


(thanks to wikipedia)