Abstract: In a process for fabricating a radiation detector comprising the step of drifting lithium from one side of a silicon wafer, a boron diffusion layer is formed on the other side of the silicon wafer prior to the drifting step. Therefore, in spite of the tendency of the drift layer to have uneven thickness, the drift layer is allowed to be formed uniformly over the entire area. This eliminates the need to lap the other side of the wafer to expose the drift layer over the entire surface. Also, a PN junction diode is formed on the other side of the wafer, and this makes the completed detector resistant to environmental influences, as opposed to conventional radiation detectors of this type which include a surface barrier type diode.