The energy of photons emitted from a semiconductor with energy gap Eg is given by the bandgap energy, i. e.
h v « Eg. (4.16)
In an ideal diode, every electron injected into the active region will generate a photon.
Conservation of energy thus requires that the energy with which an electron is injected is equal
to the photon energy. Thus energy conservation requires
e V = h v. (417)
That is, the voltage applied to the LED multiplied by the elementary charge is equal to the photon energy. There are several effects that can change the diode voltage from the ideal value given by Eq. (4.17). These effects will be discussed below.