A magnesium delta-doped AlGaN last barrier (MDDLB) was introduced in the structure of deep ultraviolet light emitting diodes (DUV LEDs) to improve their light output power. The MDDLB effectively improved hole injection efficiency and increased the hole concentration at the last AlGaN well of DUV LEDs. It also raised the potential barrier for electron transport from multiple quantum wells to the p-side. Therefore, it reduced overflow of electrons into the p-side of DUV LEDs. These phenomena improved light emitting efficiency of DUV LEDs with the MDDLB. In addition, the current crowding effect was suppressed by the MDDLB in DUV LEDs. Therefore, the 350 mA-light output power of DUV LEDs with the MDDLB was approximately 30% larger than that of DUV LEDs without the MDDLB. Furthermore, the largest light output power of DUV LEDs with the MDDLB was 55 mW, which was approximately 46% larger than that of DUV LEDs without the MDDLB. The suppressed current crowding effect by the MDDLB also reduced efficiency droops of DUV LEDs with the MDDLB. Therefore, efficiency droops of DUV LEDs decreased from 64% to 55% when the MDDLB was introduced.