(Solved): a) Draw a schematic of a heterojunction LED and explain its operation. [6 marks] 'b) The bandgap, ...

a) Draw a schematic of a heterojunction LED and explain its operation. [6 marks] 'b) The bandgap, \( E_{g} \), of a ternary \( A l_{x} G a_{1-x} A \) s alloys follows the empirical expression, \( E_{g}(\mathrm{eV})=1.5+1.3 x+0.3 x^{2} \) What is the composition of the alloy if the peak emission of an LED based on it is at a wavelength of \( 810 \mathrm{~nm} \). [8 marks] (c) Calculate the critical angle when the emitted light from an LED is incident at semiconductor-air interface knowing that the refractive index of AIGaAs is 3.66. What will be the critical angle if the LED is covered with epoxy that has refractive index of \( 1.4 \) ? Calculate reflectance at normal incidence for both cases.