Semiconductor detector

A semiconductor detector is a device that uses a semiconductor (usually silicon or germanium) to detect traversing charged particles or the absorption of photons. In the field of particle physics, these detectors are usually known as silicon detectors.

When their sensitive structures are based on a single diode, they are called semiconductor diode detectors. When they contain many diodes with different functions, the more general term semiconductor detector is used.

Semiconductor detectors have found broad application during recent decades, in particular for gamma and X-ray spectrometry and as particle detectors.

Semiconductor radiation detector
In these detectors, radiation is measured by means of the number of charge carriers set free in the detector, which is arranged between two electrodes. Ionizing radiation produces free electrons and holes. The number of electron-hole pairs depends on the energy transmitted by the radiation to the semiconductor. As a result, a certain number of electrons are transferred from the valence band to the conduction band, and an equivalent number of holes are created in the valence band. Under the influence of an electric field, electrons as well as holes travel to the electrodes, where they give rise to a pulse that can be measured in an outer circuit. The holes travel into the opposite direction and can also be measured.

The energy required for production of electron-hole-pairs is very low compared to the energy required for production of paired ions in a gas detector. Consequently, in semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good. Compared with gaseous ionization detectorss, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semicoductor of relatively small dimensions.

Semiconductor particle detectors
Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometres wide) strips of silicon to make them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents which can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can give an accurate picture of what paths particles take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as cloud chambers or wire chambers. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source) as well as suffer degradation over time from radiation.

Diamond detectors have many similarities with silicon detectors, but are expected to offer significant advantages, in particular a high radiation hardness and very low drift currents. At present they are much more expensive and more difficult to manufacture.

Germanium detectors are mostly used for spectroscopy in nuclear physics. While silicon detectors cannot be thicker than a few millimiters, germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV.