Page "Thermopower" Paragraph 18

Charge carriers tend to respond to a temperature gradient by moving in the opposite direction, i. e. from the hot end to the cold end.

They tend to respond to an electric field in different ways depending on their charge: positive charges tend to move in the same direction as the field, while negative charges move in the opposite direction of the field.

For equilibrium to be reached, these two tendencies have to cancel out.

Thus, for purely p-type materials which have only positive mobile charges ( holes ), the electric field and temperature gradient should point in the same direction in equilibrium, giving S > 0.

Likewise, for purely n-type materials which have only negative mobile charges ( electrons ), the electric field and temperature gradient should point in opposite directions in equilibrium, giving S < 0.

In practice, real materials often have both positive and negative charge-carriers, and the sign of S usually depends on which of them predominates.