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Member #789164

Member Since: March 24, 2016

Country: United States

  • Yet another comment is that all junctions that are conducting (including both diodes and transistors) are LEDs. It’s just that “common” junctions (say a normal rectifier diode or a transistor) with a drop of 0.7V gives off photons with around 1770 nm wavelength, solidly in the “thermal infrared” range. (I’ve had way more than my share of X-rays in my life – an X-ray machine essentially uses a vacuum tube as an “LED” operating at several KV to generate the X-rays.)"

    That's not strictly speaking true, since the light from an LED is specifically the release of a photon via a quasi-electron's relaxation from the conduction band to the valence band in a direct-bandgap semiconductor. That fact leads to the semi-monochromatic light coming off an LED under normal operation (and also why you see a spectral shift when you run the LED at lower than its designed forward voltage). While it is true that all hot things radiate, and most room-temp things radiate most strongly in the far infrared, that's blackbody radiation. Blackbody is what e.g. incandescent light-bulbs radiate, and is not monochromatic light. If you built a diode out of an indirect-bandgap material like silicon or germanium, the relaxation happens non-radiatively. While that can be heat (which can in turn be radiated off as blackbody radiation), it doesn't have to be (nor does it have to be transported away as blackbody radiation). It could, for instance, lead to the creation of excitons, or any number of other interesting quasi-particles.

    You can look at e.g. Streetman & Banerjee's "Solid State Electronic Devices" for a breakdown of LEDs in particular.

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