Sensors are devices consisting of electrodes arranged on one or both sides of a semiconductor wafer (bulk). Specific geometry and doping of the electrodes, with respect to the bulk, allow different ways to collect the electric signal generated by primary and secondary ionization. In all cases sensor biasing is done in such a way to deplete the free charges in the bulk, to be able to detect the small signals due to the incident radiation.
Most sensors developed by the Service are Silicon Drift Detectors (SDDs), in which an electric field forces charge carriers to drift in a direction parallel to the sensor surface. This approach enables signal reading from small-sized electrodes, thereby increasing the signal-to-noise ratio.
The drift can be arranged radially towards a single central electrode, forming single cells or arrays of cells. If the sensor is designed to capture images at the focus of an X-ray telescope, the cell dimensions can be reduced to a few hundred micrometers, forming a multi-pixel array like a CCD but with the advantage of high temporal resolution. An example of sensors of this type, designed within the framework of the REDSOX projects and fabricated on a 6-inch diameter silicon wafer, is shown in the following figure.
