When the conventional p-type multi-quantum-barrier electron-blocking layer (p-MQB EBL) and fixed Al-contents in the p-AlGaN hole-source layer (HSL) was replaced by Al-graded ud-AlGaN EBL and Al-graded p-AlGaN HSL, respectively, the efficiency droop has been remarkably suppressed and almost uniform external-quantum efficiency (EQE) under high injection current was confirmed. Also, the operating voltages under 150 mA drive were significantly reduced from 64 V (conventional p-MQB EBL) to 24 V (new polarized ud-AlGaN EBL) in UVB LEDs. These improvements are attributed to the smooth valance band edge without any potential barrier in both Al-graded ud-AlGaN EBL and Al-graded p-AlGaN HSL for efficient injection efficiency.

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