Strictly, an electrical contact in which the potential difference across it is linearly proportional to the current flowing through it. In practice, any contact that allows a current flow with minimal voltage drop is termed ohmic. Practical ohmic contacts between a metal and a semiconductor can be formed by having a highly doped surface layer on the semiconductor to ensure that the dominant method of transfer of carriers (electrons in an n-type semiconductor) is due to the tunnel effect. See metal-semiconductor contact.