SSPD_Chapter 5_Section5.5._Photo Sources.
As already discussed in Section 5.2. only compound semiconductors can act as photo-sources as they undergo Radiative Transition and Radiative Recombination.
Average time taken for radiative recombination = τ RR ;
Average time taken for non-radiative recombination = τ NR ;
Number of Radiative Recombinations /second= R r = 1/ τ RR ;
Number of non-Radiative Recombinations /second= R nr = 1/ τ NR ;
Internal Quantum Efficiency = η :
As already discussed in Section 5.2., Compound Semiconductors with direct band-gap have high internal Quantum Efficiency whereas elemental semi-conductors have practically zero internal Quantum Efficiency.
The principle of Photo-Sources (Light Emitting Diode as well as LASER Diode) is stated in three broad terms:
- First the Compound Semiconductor has to be chosen of the correct band-gap so that the radiative transition generate the required wavelength as given by Equation 5.1.
- A Substrate of matching lattice parameter has to be chosen over which the epitaxial layer of the requisite material will be grown.
- Third the Diode made of the required material must be driven hard in forward direction so as to create heavy injection condition. Under heavy injection condition there is copious amount of excess carriers available on both sides of the depletion layer. The life-time of the minority carriers should have short life-time so that they undergo rapid direct-recombination generating large amount of required wavelength photons. This process is shown in Figure 5.5.1.
Section 5.5.1. Design of 0.87μm (IR) Light-Emitting Diode.
From Equation 5.1., for emitting 0.87μm the required Band –Gap is 1.42eV hence GaAs is the material of choice. In this case GaAs epitaxial layer is grown on GaAs Substrate thereby providing automatic lattice matching between the epitaxial layer and the substrate. We propose to fabricate Planar Surface Emitting LED hence we choose PN+ structure with P-layer being thinner than the reciprocal of absorption coefficient. PN+ structure ensures that majority of recombination takes part in P-Bulk and since P Bulk-Width is thinner than penetration depth hence most of the photons of λ=0.87μm manage to cross the P-Bulk and emerge out of the Planar Surface as shown in Figure 5.5.1. This is an optimum structure for Planar Surface Emitting LED. The near proximity of the emitting surface to the recombination region lowers the internal quantum efficiency because the surface states of the emitting surface leads to higher non-radiative recombinations.
This is a Homojunction LED since same material GaAs is used on two sides. This generally has a poor overall Quantum efficiency.
Section 5.5.2. Design of LED of various visible colours (Red, Orange,Yellow, Green and Blue ).
The system topology of GaAsP ternary compound is given. Ga is Group III element and As and P are Group V element. As and P with stoichiometric coefficient (1-y) and y combine with Ga to form a tetravalent structure which is the basic crystalline structure of all semi-conductors. By controlling the stoichiometric coefficient ‘y’ the band-gap of the resulting ternary compound can be tailored by the relation given below: