A structure for a gallium-nitride (GaN) based ultraviolet photo detector is provided. The structure contains an n-type contact layer, a light absorption layer, a light penetration layer, and a p-type contact layer, sequentially stacked on a substrate from bottom to top in this order. The layers are all made of aluminum-gallium-indium-nitride (AlGaInN) compound semiconductors. By varying the composition of aluminum, gallium, and indium, the layers, on one hand, can achieve the desired band gaps so that the photo detector is highly responsive to ultraviolet lights having specific wavelengths. On the other hand, the layers have compatible lattice constants so that problems associated with excessive stress are avoided and high-quality epitaxial structure is obtained. The structure further contains a positive electrode, a light penetration contact layer, and an anti-reflective coating layer on top of the p-type contact layer, and a negative electrode on the n-type contact layer.