A two-dimensional, pixellated, monolithic semiconductor radiation detector, in which each detector pixel is essentially a perpendicular mode detector. This is achieved by an arrangement of anode spots, one for each pixel located on the flux-exposed front surface of the detector substrate, surrounding by a cathode array preferably in the form of a network of lines, such that the field between the anodes and cathodes on this front surface has a major component in the direction parallel to the surface, and hence perpendicular to the incident photon flux. The conductivity of the substrate is high near this front surface, since this is where the highest level of absorption of photons takes place, and a significant photoconductive current is thus generated between cathodes and anodes. The conductivity is proportional to the incoming photon flux, and decays exponentially with depth into the detector. Since all of the conduction paths are in parallel to each other, the resultant conductance between each anode and its surrounding cathode is the summation of all those conductances.