Question

Explain why does the peak emission wavelength of LEDs increase (red shift) with temperature? Why does the peak emission intensity decrease with temperature? Use sketches and curves for the answer.

Answer 1

In the question it is asked as to why the peak emission wavelength of LEDs increases with temperature. Also it is asked that why the peak emission intensity decreases with the increase in temperature. The answers for both the problems are discussed below

It is known that the peak wavelength of the emitted radiation from an LED light changes with ambient temperature. LED is a semiconductor device where light is emitted when an electron acquires sufficient energy to cross the forbidden energy gap, Eg. When this electron returns to a lower energy level, a photon is emitted with a wavelength, lambda, which is related to the energy gap of the semiconductor by

Where, h is the Planck constant and

c is the speed of light in a vacuum.

The magnitude of Eg depends on temperature and in semiconductor materials, Eg decreases as temperature increases. Thus the peak emission wavelength of an LED should increase with temperature, as wavelength is inversely proportional to Eg.

Now as the LEDs are usually made up of semiconductor material Silicon, which has high resistivity values than the conductors. Now as the temperature across the LED increases the density of free electrons in the LED junction increases which causes an increase in electric current. Thus the resistance of the LED increases. Now as the current increases in the LED, the velocity of the electrons passing through the semiconductor is increased and the wavelength increases, thus causing the intensity of the LED to decrease.