Semiconductor Detectors

In X-ray astronomy, semiconductor ionization detectors are primarily used as non-dispersive spectrometers of high energy resolution. (See also Single Photon Calorimeters for another use of semiconductors).

Semiconductor, or solid state detector, is a term which is usually meant to exclude scintillation counters. The first practical devices were small germanium surface barrier devices made in the 1950s. Since then, improvements in material purity and microelectronics have given rise to an array of detector types based on electron-hole pair creation in cooled silicon or germanium, or in a number of room temperature materials such as mercuric iodide.

Research has been driven by the need for high spectral resolution. Thus, charge coupled devices (CCDs) and silicon drift chambers, both of which are imaging devices with low noise and good energy resolution, have been frequently used as focal plane X-ray detectors. However, silicon avalanche photodiodes have never really found their niche, because the presence of internal gain creates an energy blur due to the statistical fluctuations in (and spatial non-uniformity of) the avalanche process.

All solid state X-ray detectors consist of a volume of semiconducting material, subdivided by impurity doping into regions of differing conductivity, within which a charge collecting electric field can be established by the application of appropriate bias voltages to a set of surface contacts. The usual result of soft X-ray absorption in a semiconductor is the creation of electron-hole pairs (analogous to electron -positive ion pairs in a counting gas). Both the negatively and positively charged charge carriers are free to move, in opposite directions, under the influence of the applied electric field. The number of pairs created is N=E/w, where w is the ionization energy of the material.