October 23, 2021



Three questions: 1. defector level to valence band 2. Cause of re-excitation and 3. different colours. (Sorry, my question is long because I have written my doubt in detail).

My question is about phosphorescence… In indirect semiconductors, recombination of EHP is delayed due to defector level trapping electrons from the conduction band. Q1. Why can’t electron go down from defector level to valence band to recombine with the hole?

They say that even if we turn off the excitation, we will still have photons emitting for a few minutes in indirect semiconductors, unlike the direct semiconductors where it switches off in 10^-8 seconds.

Q2. My doubt is that when we turn off the excitation, what is it that causes the re-excitation of the electron from defector level to the conduction band… because without this re-excitation, we shouldn’t have recombination taking place.

Q3. Phospher substance like ZnS is used in colour tv screen for different colours. But isn’t energy band gap ( E(g) ) fixed for a particular material? And if so, how can we have different wavelengths(which we must have for different colours) from one material because hv=E(g) for the emitting photon and thus we should get just one v or one wavelength.

Thanks for reading. Please help.

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