What condition causes a light emitting diode (LED) to illuminate?

A light emitting diode (LED) illuminates when it is forward biased. This means that the positive voltage is applied to the anode (the longer lead) and the negative voltage to the cathode (the shorter lead). Forward biasing reduces the width of the depletion region at the junction of the diode, allowing current to flow easily through the device. As the electrons gain energy and recombine with holes within the semiconductor material, they release energy in the form of photons, which produces light.

In contrast, when an LED is reverse biased, it does not emit light because the depletion region widens, preventing current flow. The device may even be damaged if the reverse voltage exceeds its maximum rating. Simply receiving a signal pulse does not guarantee illumination unless the pulse is appropriately forward biasing the LED. Using an AC source can be ineffective or damaging to an LED unless properly rectified, as LEDs typically require direct current to function correctly.