Abstract: Apparatus for parallel-in to serial-out conversion comprises a strip of semiconductor material having electrical contacts at either end, and a detector near one end, sensitive to the local density of ambipolar carriers. Minority carriers (or ambipolar carriers) are injected into the strip at a number of parallel inputs and a spatial distribution of ambipolar carriers accumulated along the strip. The ambipolar carriers are swept to the detector when a pulse of current of suitable amplitude and duration is applied to the contacts. The parallel inputs may be in the form of diode diffusions to which electrical signals may be applied. Alternatively the inputs may be lateral extended portions of the strip and in these ambipolar carriers may be generated, for example, photoelectrically.Two-dimensional radiation detector arrays using the invention are also described.