(Solved): Light extraction efficiency of LEDs Extraction efficiency or ratio depends on the LED structure an ...

Light extraction efficiency of LEDs Extraction efficiency or ratio depends on the LED structure and the means that have been used to manipulate light to escape from the chip. (a) Figure (a) shows a bare LED chip in which the light from the active region is incident on the semiconductor-air interface. The refractive indices of the semiconductor and ambient are $$n_s$$ and $$n_a$$ respectively. Assume a $$\mathrm{GaN}$$ crystal for the semiconductor with a refractive index of 2.5. For normal incidence, what is the transmittance $$T$$ from the semiconductor to the air? What is the critical angle $${?}_c$$ at this interface? The fraction of output light from the active region within the emission cone defined by the critical angle $${?}_c$$ has been evaluated to be $$(1/2)\left[1?\cos {?}_c\right]$$. Taking into account $$T$$ as well, estimate the extraction efficiency (EE) if the ambient is air, $$n_a=1$$. (a) (b) FIGURE (a) Total internal reflection results in poor light extraction (b) Improvement in the light extraction by the use of a domed epoxy. (b) Figure (b) shows an epoxy dome placed on top of the semiconductor. Suppose that the refractive index of the epoxy is 1.8. Calculate the new critical angle at the semiconductor-epoxy interface and estimate the new EE. What is the critical angle for light rays at the epoxy-air interface?