Hello, This is about a direct band gap semiconductor: Q1) What induces recombination of an electron in the conduction-band with a hole in the valence-band? Is it the electrostatic attraction between a negatively charged electron and a positively charged hole which induces recombination or something else? Q2) Suppose we are sending a stream of photon, one photon at a time, towards a semiconductor (direct band-gap). Each photon carries an energy slightly larger than its predecessor photon. In this way ultimately we arrive at a photon which has energy exactly equal to the band-gap energy. The time gap between two consecutive photons is large enough that any excited electron in the semiconductor falls back to the valence band. The question is, when a photon with energy less than the band-gap is incident on a semiconductor, then: (a) Is it that an electron from valence band absorbs it and lands-up in the middle of energy band-gap, but, since there is no energy-state available for it to occupy, it immediately releases the absorbed energy and falls back to the valence band; and therefore we say that there is no absorption for photons with energy lower than band-gap, or (b) Is it that an electron never absorbs energy from a photon with energy lower than band-gap; it absorbs energy from a photon only when the photon energy is equal to or greater than the band-gap energy? If this option is true than how does an electron come to know that which photon has energy equal to or greater than band-gap ? it indicates that the electron must be interacting with each of the photons, but does not absorb their energy until the photon energy is equal to or greater than band-gap energy. I have attached a schematic; my question is that whether the type of transition shown in the schematic allowed ? Please help me clarify these doubts. Thank you.