The potential difference that occurs between two electrically connected metals or between the base regions of two semiconductors. If two metals with work functions ϕ1 and ϕ2 are brought into contact, their Fermi levels will coincide. If ϕ1 > ϕ2 the first metal will acquire a positive surface charge with respect to the other at the area of contact. As a result, a contact potential difference occurs between the two metals or semiconductors.