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I have been measuring the voltage response to current of an LED. It's a point source LED with an oval emission area of 40x150um (link to datasheet https://www.daido.co.jp/en/products/point_source_led/pdf/med7p4_e.pdf). I have done the same measurements for a range of temperatures from 10°C-70°C. I expected the decrease in voltage with temperature to be linear, but these graphs don't show that. Is there a physical explanation for this or is it more likely to be some kind of experimental error?enter image description here

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Those currents are so high that the ohmic resistance of the semiconductor material becomes important. It is not just the voltage over the $pn$ junction. (And another effect is that at high currents, the current will affect the temperature.)

Theoretical values can only be expected in the region where the $IV$ curve is exponential. Maybe up to 1 mA or something like that in such devices.

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  • $\begingroup$ I realise that the current can affect the temperature but that doesn't seem to be the issue in this case as I've also measured the junction temperature, which seems normal. The datasheet for the LED says that it can be operated between 0 and 25mA, so it shouldn't be outside of its normal operating range. $\endgroup$ – JJH Sep 3 at 12:44
  • $\begingroup$ The $IV$-curve can be exponential over several orders of magnitude (for example from 1 mA down to less than 1 μA) but not in the normal operating range of such devices. Diode thermometers work with much lower currents than LEDs. $\endgroup$ – Pieter Sep 3 at 13:47

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