An LED is a semiconductor light source. In the unbiased condition a potential barrier is developed across the p-n junction of the LED (Fig 1). When we connect the LED to an external voltage in the forward biased condition, holes from the p type and electrons from the n type region start to enter the junction to enable the current flow and as a result the height of potential barrier across the p-n junction is reduced. At a particular voltage in this forward biased condition, the height of the potential barrier becomes so low that the current flow increases rapidly and the LED starts glowing as the excited electrons (the electrons which are crossing the junction) comes to the ground state (valence band). This particular voltage is called the knee voltage or the threshold voltage. Once the knee voltage is reached, the current may increase but the voltage does not change.


The light energy emitted during forward biasing is given as

where, c is the velocity of light, h is the Planck’s constant and 𝜆 is the wavelength of the emitted light. If V is the forward voltage applied across the LED when it begins to emit light (the knee voltage), the energy given to electrons crossing the junction is: