An electron detector (30) for detection of electrons comprises a semiconductor wafer (11) having a central portion (12) with a thickness of at most 150 μm, preferably at most 100 μm, formed by etching an area of a thicker wafer. On opposite sides of the central portion (12) there are n-type and p-type contacts (16, 31). In operation, a reverse bias is applied across the contacts (16, 31) and electrons incident on the layer (15) of intrinsic semiconductor material between the contacts (16, 31) generate electron-hole pairs which accelerate towards the contacts (16, 31) where they may detected as a signal. Conductive terminals (24, 32) contact the contacts (16, 31) and are connected to a signal processing circuit in IC chips (28, 37) mounted to the semiconductor wafer (11) outside the active area of the detector (30). The contacts (16, 31) are shaped as arrays of strips extending orthogonally on the two sides of the intrinsic layer (15) to provide two-dimensional spatial resolution. In an alternative detector (10), there is a single contact (19) on one side to provide one-dimensional spatial resolution.