A semiconductor light receiving element includes: a substrate; a semiconductor stacked portion that is formed on a first region of the substrate; and a first electrode and a second electrode that are electrically connected to the semiconductor stacked portion. Te semiconductor stacked portion includes: a light absorption layer of a first conductivity type including In.sub.xGa.sub.1-xAs; and a second region of a second conductivity type other than the first conductivity type that is located on the opposite side to the substrate with respect to the light absorption layer and bonded to the light absorption layer. The first electrode is connected to a first portion of the first conductivity type located on the substrate side with respect to the light absorption layer in the semiconductor stacked portion.

The present invention relates to a GaN-based radiation detector capable of detecting radiation such as X-rays. The GaN-based radiation detector includes: an n-doped GaN layer having an electron mobility of 700 cm.sup.2/(V.Math.s) or more and a thickness of 300 m or more and doped with an n-type doping concentration of 310.sup.16/cm.sup.3 or less; a p-doped GaN layer formed on one surface of the n-doped GaN layer and having a thickness of 3 m or less and doped with a p-type doping concentration of 510.sup.18/cm.sup.3 or more; a first metal contact formed on the other surface of the n-doped GaN layer; and a second metal contact formed on one surface of the p-doped GaN layer.