Mumbai University > First Year Engineering > Sem 1 > Applied Physics 1

Marks: 5M

Year: Dec 2015

Principle:

1. LED is a particular diode which generates photons (light) when a stream of electrons passes through it.
2. To build a diode we use a crystal (electric insulator) which is doped by atoms which have one more electron on their valence band (N doping) or missing one electron on their valence band (P doping).

Construction:

1. The methods used to construct LED are to deposit three semiconductor layers on the substrate.
2. The three semiconductor layers deposited on the substrate are n-type semiconductor, p-type semiconductor and active region.
3. Active region is present in between the n-type and p-type semiconductor layers.
4. When LED is forward biased, free electrons from n-type semiconductor and holes from p-type semiconductor are pushed towards the active region.

1. When free electrons from n-side and holes from p-side recombine with the opposite charge carriers (free electrons with holes or holes with free electrons) in active region, an invisible or visible light is emitted.
2. In LED, most of the charge carriers recombine at active region. Therefore, most of the light is emitted by the active region. The active region is also called as depletion region.

Working:

1. A P-N junction can convert absorbed light energy into a proportional electric current. The same process is reversed here (i.e. the P-N junction emits light when electrical energy is applied to it).
2. This phenomenon is generally called electroluminescence, which can be defined as the emission of light from a semi-conductor under the influence of an electric field.
3. The charge carriers recombine in a forward-biased P-N junction as the electrons cross from the N-region and recombine with the holes existing in the P-region. Free electrons are in the conduction band of energy levels, while holes are in the valence energy band.
4. Thus the energy level of the holes will be lesser than the energy levels of the electrons. Some portion of the energy must be dissipated in order to recombine the electrons and the holes. This energy is emitted in the form of heat and light.
5. The electrons dissipate energy in the form of heat for silicon and germanium diodes but in gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP) semiconductors, the electrons dissipate energy by emitting photons.
6. If the semiconductor is translucent, the junction becomes the source of light as it is emitted, thus becoming a light-emitting diode, but when the junction is reverse biased no light will be produced by the LED and, on the contrary, the device may also be damaged.