A nitride semiconductor structure and a semiconductor light emitting device are revealed. The semiconductor light emitting device includes a substrate disposed with a first type doped semiconductor layer and a second type doped semiconductor layer. A light emitting layer is disposed between the first type doped semiconductor layer and the second type doped semiconductor layer. The second type doped semiconductor layer is doped with a second type dopant at a concentration larger than 5×1019 cm−3 while a thickness of the second type doped semiconductor layer is smaller than 30 nm. Thereby the semiconductor light emitting device provides a better light emitting efficiency.