Question

Can someone please fully explain how LEDs and the temperature effects the following three things:

I. Luminous flux decreases with T.
II. PN junction voltage decrease with T.
III. Efficacy (Lm / W) decrease with T.

Answer 1

(i) Luminous flux decreases with increase in temperature due to increased recombination of electrons and holes that do not contribute to light emission.

(ii) In forward bias as the temperature increases, the intrinsic carrier concentration also increases. This pushes the fermi level closer to the intrinsic fermi level (the middle of the band gap). Since the built-in potential of a diode is determined by the difference in fermi-levels in the p-type and n-type regions, the fermi level in each region moves closer to the middle of the gap, and the built-on potential(PN juction potential) decreases.

(iii) Energy efficiency of light sources is typically measured in lumens per watt (lm/W), meaning the amount of light produced for each watt of electricity consumed. This is known as luminous efficacy. As the temperature rises, the potential decreases, so luminous efficiency also decreases.