Enhancement of thermionic emission and conversion characteristics using polarization- and band-engineered n-type AlGaN/GaN cathodes

Enhanced thermionic emission (TE) and conversion characteristics are observed by controlling spontaneous and piezoelectric polarization and the band diagram of n-type AlGaN/GaN thermionic cathodes. Reduction in the electron affinity and work function by the insertion of an N-polarity thin n-type AlGaN (n-AlGaN) film including tensile biaxial strain is confirmed by an ultraviolet photoemission spectroscopy analysis. The obtained TE current from N-polarity n-AlGaN films grown on an n-type GaN (n-GaN) substrate is 0.29 mA at 500 °C in a Cs gas atmosphere in the vacuum gap between the cathode and a stainless steel anode. This TE current is 5.0 times and 1.6 times higher than that from the surface of Ga-polarity n-GaN substrate and that of the Ga-polarity n-AlGaN film on the substrate, respectively.