Joule heating
If a device has a positive "dR/dT", an increase in temperature (for example, due to thermal fluctuations or the absorption of a photon) will increase the electrical resistance "R". If the device is biased with a constant voltage "V", this increase in resistance will decrease the Joule power "P = V2/R". The decrease in Joule heating will cause the device to return to its equilibrium temperature. This is known as negative electrothermal feedback, as the change in Joule heating opposes the change in temperature. If the device is instead biased with a constant current "I", the Joule power "P = I2R" will increase if the temperature increases. Thus the Joule heating amplifies a change in temperature, an effect known as positive electrothermal feedback. The situation is reversed for the case of a negative "dR/dT".