Abstract: A heterojunction photodiode with improved wavelength-selectivity and risetime. The problem of short-wavelength diffusion-tail response is avoided by interposing between the window and active layers a barrier layer of higher bandgap than that of the window layer, which prevents high-energy photocarriers generated in the window layer from diffusing to the PN junction. In one embodiment, n-type substrate, active, barrier, and window layers are initially grown, and the window layer is coated with an opaque oxide. A window is opened in the oxide layer, and a p-type dopant is diffused heavily through the opening, through the window layer, and partly into the barrier layer. A PN junction is thus formed in the barrier layer, its depletion region extending through the remaining n-type region of the barrier layer and into the active layer, where photocarriers are generated by photons passing through the window-opening.