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If two solid conductors are brought into contact, an exchange of electrons takes place between them; at first the electron transfer will occur predominantly from the conductor with a lower work function to the conductor with a higher work function. As a result, the conductors acquire electric charges of opposite sign, which causes the appearance of an electric field that impedes the flow of electrons. Ultimately a state of equilibrium is attained; in this state the electron flow in one direction becomes equal to the flow in the opposite direction, and a contact potential is established between the conductors.
The value of the contact potential is equal to the difference in work functions divided by the charge of an electron.
As doping (i.e. amount of impurity concentration) increases -> free electron-hole concentration increases -> thereby increasing the contact potential