Operation of a solar cell can be understood from the equivalent circuit at right. Light, of sufficient energy (greater than the bandgap of the material), creates mobile electron–hole pairs in a semiconductor. Charge separation occurs because of a pre-existing electric field associated with the p-n junction in thermal equilibrium. (This electric field is created from a built-in potential, which arises from the contact potential between the two different materials in the junction.) The charge separation between positive holes and negative electrons across a p-n junction (a diode) yields a "forward voltage", the "photo voltage", between the illuminated diode terminals, which drives current through any attached load. Photo "voltage" is sometimes referred to as the photo "emf", distinguishing between the effect and the cause.