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 is proportional to the energy transmitted by the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band. Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit. The holes travel into the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be found.

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, and is dependent upon rise time. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.

Silicon detector
Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers 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 detector
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 detector
Germanium detectors are mostly used for spectroscopy in nuclear physics. While silicon detectors cannot be thicker than a few millimeters, 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. These detectors are also called High-Purity Germanium detectors (HPGe) or Hyperpure Germanium detectors. Before current purification techniques were refined, Germanium crystals could not be produced with purity sufficient to enable their use as spectroscopy detectors. Impurities in the crystals trapped electrons and holes, ruining the performance of the detectors. Therefore, Germanium crystals were doped with Lithium ions (Ge(Li)), in order to produce an intrinsic region in which the electrons and holes would be able to reach the contacts and produce a signal.

When Germanium detectors were first developed, only very small crystals were available. Low efficiency was the result, and Germanium detector efficiency is still often quoted in relative terms, as discussed above. Crystal growth techniques have improved, allowing detectors to be manufactured that are as large as or larger than commonly available NaI crystals, although such detectors cost more than $100,000.

Present-day HPGe detectors commonly still use lithium diffusion to make an n+ contact, and boron implantation to make a p+ contact. Coaxial detectors with a central n+ contact are referred to as n-type detectors, while p-type detectors have a p+ central contact. The thickness of these contacts represents a dead layer around the surface of the crystal within which energy depositions do not result in detector signals. Typical dead layer thickness are several hundred micrometers for an Li diffusion layer, and a few tenths of a micrometer for a B implantation layer.

The major drawback of Germanium detectors is that they must be cooled to liquid nitrogen temperatures to produce spectroscopic data. At higher temperatures, the electrons can easily cross the Band gap in the crystal and reach the conduction band, where they are free to respond to the electric field. The system therefore produces too much electrical noise to be useful as a spectrometer. Cooling to liquid nitrogen temperatures, 77.36 K, widens the band gap so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band. Cooling with liquid nitrogen is inconvenient, as the detector requires hours to cool down to operating temperature before it can be used, and cannot be allowed to warm up during use. Ge(Li) crystals could never be allowed to warm up, as the Lithium would drift out of the crystal, ruining the detector. HPGe detectors can be allowed to warm up to room temperature when not in use. Appropriate care must be taken when working with liquid nitrogen; the major hazards are Cold burn and oxygen depletion as the liquid nitrogen boils, producing a significant volume of nitrogen gas.

Commercial systems are now available that use advanced refrigeration techniques to eliminate the need for liquid nitrogen cooling.