Abstract: A radiation-sensitive semiconductor device includes a radiation-detecting avalanche diode which has a semiconductor layer structure made up of four layers of the same type conductivity. The fourth semiconductor layer is located above the third layer and has a higher doping concentration than that of the third layer. This fourth layer substantially improves the noise properties of the device, by a factor of about 2. The radiation-sensitive semiconductor device is manufactured by a method in which the first and third layers of the semiconductor layer structure are provided by epitaxial growth, while the second and fourth layers of the structure are provided by ion implantation. The structure and method of the invention are particularly useful in the manufacture of avalanche photodiodes with an improved noise factor.