Thermodynamic Limits for Some Light-Producing Devices

A thermodynamic upper limit for the energy efficiency η of light energy output to pump energy input is obtained. It is of a general form, and is applied to lasers and to electroluminescent diodes when these are in a steady state. The theory generalizes previous work, notably the results η<1-TTₚ (Mazurenko, lasers) and η<[1-TTL]^-1 (Weinstein, diodes), where Tₚ and TL are effective temperatures associated with the pumping system and the emitted light, respectively. For a GaAs diode, experimental work at the ambient temperature T=300^∘K has been analyzed to yield η=0.1 to 6% for various forward currents. The internal entropy generation rate for the same diode, per electron crossing the device, is estimated at 30 to 50 Boltzmann constants.