LIGHT EMITTING DIODE (LED)

    

     Now a days we often live without electronic light. Nearly this light can be emitted from a P-N junction operated as LIGHT EMITTING DIODE (LED).

                The LED is made up of p-n junction diode.

                                              HOW CAN A P-N JUNCTION GENERATE LIGHT???

     Consider  first a simple semiconductor. When an electron jump into a holes at the top of the valence band from the bottom of the conduction band, an energy E_g (equal to the gap width) is released. In silicon, germanium, & many other  semiconductors, this energy is largely transform into thermal energy  of vibrating lattice, & as a result no light is emitted.

     But in some semiconductor, however, including gallium arsenide, the energy can be emitted as light (photon of energy hf  at wavelength   )

     To emit light from a LED the material must have a suitably large number of electron-hole transition. This is not possible by the pure semiconductor at room temperature( Because light is emit from a diode when an electron is combine with a hole in the depletion zone), So doping will not help. In doped n-type material the number of conduction electron is greatly increased. But there are less number of hole to be combined; in doped p-type there are plenty of holes but not electron to combine with them. So neither a pure semiconductor nor a doped semiconductor is sufficient for making a practical LED.

     So we need that type of semiconductor which have large number of electron in the conduction band & a large number of holes in the valence band. So this property can be fabricated by placing a p-n junction. In such arrangement the current I through the device serve to inject electron into the n-type material & to inject hole into the p-material. If heavy doped & grate current then the depletion becomes narrow as per only few micro meter width. As a result the density of electron in the n-type and correspondingly the density holes in P-type is increased with in the depletion zone. so many number of holes combine with electron in the depletion region as a result light is emitted.

    Hence this is the way in which a LED emits  light.