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The electrical and optical characteristics of AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) (265-365 nm) at elevated temperatures (25°C-175°C) were investigated, and compared to those of InGaN-based visible LEDs (400-465 nm). Strong carrier localization and localized-state emission were retained in the InGaN LEDs up to 175°C, leading to temperature-independent emission intensity at low-energy tails. The deep-UV LEDs, however, showed dominant band-edge emission, much smaller alloy broadening, and weaker localization effects. The optical power of the UV LEDs decreased much more rapidly with increasing temperature. The characteristic temperature was in the range of 31-73 K, and decreased with increasing Al content in the active region. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the UV LEDs and suggest that increasing carrier-confining potentials will provide a critical means to improve their radiative efficiencies.