Question

1. Explain why it is difficult to make light emitting devices out of indirect band gap materials.

2. When a direct semiconductor is excited by absorption of photons with energy greater than the band gap, it is generally found that the luminescence spectrum is independent of the exciton frequency. Explain the phenomenon.

3. The radiative life time τR of the laser transition in titanium doped sapphire is 3.9 μs. The lifetime τ of the excited stste is measured to be 3.1 μs at 300K and 2.2 μs at 350K. Explain why τ is different from τR , and suggest a reason why τ decreases with increasing temperature. Calculate the radiative efficiencies at the two temperature.

4. A laser diode emits at 830nm when operating at an injection current of 100mA. (i) Calculate the maximum power that can be emitted by he device. (ii) Calculate the power conversion efficiency , if the actual power output is 50 mW and the operating voltage is 1.9 V.

5. Unpolarized light with an intensity of 100 W/m2 passes through two polarizing filters that are at angle of 40 degree to each other. What is the intensity of the light that emerges from each filter?

6. Unpolarized light with an intensity of 72 W/m2 passes through two polarizing filters. If the light that emerges from the second filter has an intensity of 9 W/m2, what are the angle in degree between the two filters?

7. Unpolarized light with an intensity of Io passes through three polarizing filters. The second filter makes a 45 degree angle relative to the first one. The third filter makes a 90 degree angle relative to the first filter. Determine the intensity of the light emerges from each filter in terms of Io?

Answer 1

In LED an electron and hole pair recombine to eject a photon, so their momenta must match or we can say that they have to be in the same vertical position on the E-k(Energy-momentum) diagram. For indirect bandgap,  It requires interaction with a third pseudo-particle, the phonon for matching the momentum materials. So the probability of ejecting photons is much lower. in indirect bandgap materials.