As in other diodes, when this structure is forward biased, holes from the p-region are injected into the n-region, where electrons are the dominant carrier. Similarly, electrons from the n-region are injected into the p-region, where holes are common. When an electron and a hole are present in the same region, they may recombine by spontaneous emission—that is, the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron and hole states involved. These injected electrons and holes represent the injection current of the diode, and spontaneous emission gives the laser diode below lasing threshold similar properties to a LED. Spontaneous emission is necessary to initiate laser oscillation, but it is a source of inefficienc