Optoelectronic diode

An optoelectronic diode is a diode which includes optics along with electronics. It means optoelectronic diode either emit light or they work when light falls on them.
Here we will discuss about light emitting diode as an optoelectronic diode.

Light emitting diode (LED)

In simple p-n junction diodes (Ge, Si), the energy bands are indirect.
When these diodes are forward biased, then the recombination of electron-holes across the junctions starts.
When electron jumps from higher energy level to lower energy levels then the energy emitted by electrons in these diode will be in the form of heat and hence we can not see it.
In some semiconductor (GaAs), the energy bands are direct.
When these semiconductor diodes are forward biased then in the process of recombination of electron-hole pairs, the electron jumps from higher energy level to lower energy levels.
The energy emitted during this process lie in the visible region, and it is clearly seen due to transparent material of the crystal.
Since these diodes emit light energy on forward biased, hence they are known as light emitting diode.

Light emitting diode (a) Arrangement and (b), (c) Circuit symbol

In LED generally we use GaAs or In-P.
In these diodes n-type semiconducting layer grow on a p-type substrate by diffusion.
After it a thin layer of p-type material is grow up on n-type layer.
At the external sides of these layers a thin metallic contacts are attached, which are connected to cathode (K) and anode (A) respectively.
The whole arrangement is placed in a transparent plastic cover or envelope.
The colour of plastic cover depends on the light emitted by diode.
It is used in low forward bias.
The wavelength (colour) emitted by the diode depends on the the % error of As in GaAS-P diode.
Common formula for it is GaAs_1-x P_x.
GaAs gives I.R. light and GaP gives light of blue colour.
If the energy band of above diode is 1.8 eV, we get light of red colour.
If this diode is doped with nitrogen we get light of all the colours ranging from yellow to green in visible region.
Wavelength of emitted light

λ= hc / E_g = (1.26 / E_g) μm

Here h = Planck's constant, c = velocity of light and E_g = energy gap between valence band and conduction band.
If we join seven segment of LED, then we can get any number from 0 to 9. Here a number 4 and number 8 are shown.

Seven segment LED

LED is used in calculators.
It is used in watches.
LED is also used in platform signals.
Light emitting diode is also used as a pilot lamp in electronic device for showing on-off position.
Theses diodes are also used in optical communication systems.

To know more about light emitting diode (LED) please click on the link https://youtu.be/EyTUJV-NXH0 for English and on https://youtu.be/WuOKbuHpq1Q for bilingual (Hindi/English)