Thermodynamic Limits for Some Light-Producing Devices

A thermodynamic upper limit for the energy efficiency $\eta$ 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 $\eta <\frac{1-T}{T_p}$ (Mazurenko, lasers) and $\eta <{\left[\frac{1-T}{T_L}\right]}^{-1\mathrm{}}$ (Weinstein, diodes), where $T_p$ and $T_L$ are effective temperatures associated with the pumping system and the emitted light, respectively. For a GaAs diode, experimental work at the ambient temperature $T={300}^{\circ }$K has been analyzed to yield $\eta =0.1\mathrm{}$ 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.