A highly sensitive and wide spectra-range mesa type photodetector having the impurity diffusion along the mesa-sidewall is provided with.A mesa-type hetero-bipolar phototransistor or photodiode having a photo-absorption layer 10 (41) formed by a first semiconductor layer of a first conductivity type, an anode layer 40 (or base layer 4) formed by a second semiconductor layer of a second conductivity type which has an opposite polarity with the first conductivity type, a wide band gap emitter 3 or window layer 42 formed by the third semiconductor layer on the anode layer, and the wide band gap buffer layer 11 of the first conductivity type which has a relatively wide band gap semiconductor as compared with the second semiconductor layer on the substrate 12, which also serves as the cathode layer. And the first semiconductor layer 10, the second semiconductor layer 4 and the wide band gap emitter 3 or window layer 42 is selectively etched to form the mesa structure 7. The diffusion region 6 shaped like a ring in plan view is also provided from the exposed sidewall of this mesa structure toward the center of the device by a predetermined horizontal width, converted into the second conductivity type, which is the same conductivity type as the second semiconductor layer. Further, the diffused region 33 is formed also in a flat surface of the second semiconductor layer 4 parallel to a substrate principal surface toward center of the device from a sidewall with a predetermined width in the horizontal direction, and also the diffused region 32 of the second conductive type is formed in the buffer layer 11 located under the photo-absorption layer 41 by the Zn diffusion in the surface part parallel to the substrate principal surface.