LED

Module 6 : LIGHT EMITTING DIODE (LED)

Lecture : LED - I

	For $x<0.45$ , the alloy remains a direct bandgap material with radiative transitions in red and infrared (630 nm to 870 nm). For $x>0.45$ , the alloy becomes an indirect bandgap material with emission in red, orange and yellow (560 to 700 nm) with pure GaP emitting in red at 700 nm. Another direct bandgap alloy is In $_{0.49}$ Al $_x$ Ga $_{0.51-x}$ P which emits at 590 to 630 nm corresponding to green and red.
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	Indirect band gap material are generally inefficient as light emitting devices. It is, however, possible to circumvent the three body process by a two stage two body processes. If one adds nitrogen impurities to GaAS $_{1-x}$ , nitrogen being isoelectronic with phosphorus, occupy the sites of P. Nitrogen sites act as trapping levels for electrons in the conduction band. Electrons in the conduction band first make a transition to trapping centre. This result in a transfer of momentum but the energy remains the same. A final vertical transition to the valence band is now made with emission of radiation. The emission is in green, yellow and orange.
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	GaP with nitrogen traps also emit in green. Traps are undesirable as they contribute to non-radiative recombination and also reduce carrier life time before recombination. To cover the entire visible spectrum, the need for LEDs which emit blue light has been felt. SiC which has an indirect band gap in the range 2.2 to 3 eV and it emits in blue. However, it makes an extremely inefficient device. Compond II-Vi semiconductors like ZnSe also emit in blue - green. GaN with a band gap of 3.4 eV is a direct bandgap material. An alloy of GaN, viz., InGaN has a smaller direct bandgap and it emits in blue (430 - 460 nm).