In a LED, a device known as a light-emitting diode ( a semiconductor diode ) produces light when receiving a certain level of current. A semiconductor diode is a device which allows current to flow in one direction, called the forward direction. When a potential difference (p.d.) is applied in the forward direction, current flows through the LED and it glows. No current flow is observed when the p.d. is in the reverse direction. LEDs do not have filaments to burn out or break.