T. Patrick Xiao

Electroluminescent refrigeration by ultra-efficient GaAs light-emitting diodes

Electroluminescence—the conversion of electrons to photons in a light-emitting diode (LED)—can be used as a mechanism for refrigeration, provided that the LED has an exceptionally high quantum efficiency. We investigate the practical limits of present optoelectronic technology for cooling applications by optimizing a GaAs/GaInP double heterostructure LED. We develop a model of the design based on the physics of detailed balance and the methods of statistical ray optics, and predict an external luminescence efficiency of ηext = 97.7% at 263 K. To enhance the cooling coefficient of performance, we pair the refrigerated LED with a photovoltaic cell, which partially recovers the emitted optical energy as electricity. For applications near room temperature and moderate power densities (1.0–10 mW/cm2), we project that an electroluminescent refrigerator can operate with up to 1.7× the coefficient of performance of thermoelectric coolers with ZT = 1, using the material quality in existing GaAs devices. We also predict superior cooling efficiency for cryogenic applications relative to both thermoelectric and laser cooling. Large improvements to these results are possible with optoelectronic devices that asymptotically approach unity luminescence efficiency.

Impact of interface defects on tunneling FET turn-on steepness

The effects of interface defects on the sharpness of the conductance switch can be seen both in the intrinsic band tail steepness (for shallow defects) observed at all temperatures, as well as in a subthreshold regime clearly dominated by thermal activation (for deep defects). To realize a steep turn-on, reduced dimensionality on both sides of the junction is needed [1]. However, increasing the tunneling probability of the device will increase not only the band-to-band current in the on-state but also leakage into the band tail. In order to achieve a turn-on steepness better than 60 mV/dec over at least six decades of current, a very high quality tunneling interface is needed, comparable to the best achieved interface trap densities in semiconductor systems. This suggests future investigation into the engineering of pristine material interfaces, possibly at atomic levels of control, with high yield.

Highly efficient thermophotovoltaics enabled by photon re-use

Thin-film photovoltaic cells with high reflectivity in the below-bandgap spectral region are ideally suited for thermophotovoltaics. This allows the below-bandgap radiation to be reflected back to the emitter, so that their energy can be used to reheat the source, rather than being lost. In this work, we present a substantial improvement in the theoretical thermophotovoltaic conversion efficiency in the presence of photon re-use. We also predict the achievable conversion efficiency for a system that uses In0.53Ga0.47As photovoltaic cells, and present an experimental optical cavity to be used for future efficiency measurements. Owing to recent advances in thin-film photovoltaics, thermophotovoltaic efficiencies above 50% may soon be realizable.