Editing Semiconductor detector (section)

=== Germanium detectors ===
[[File:HPGe detector.jpg|thumb|100px|right|High-purity germanium detector (disconnected from liquid nitrogen dewar)]]

[[Germanium]] detectors are mostly used for [[gamma spectroscopy]] in [[nuclear physics]], as well as [[x-ray spectroscopy]]. While silicon detectors cannot be thicker than a few millimeters, germanium can have a sensitive layer ([[depletion region]]) thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to a few MeV. These detectors are also called high-purity germanium detectors (HPGe) or hyperpure germanium detectors. Before current purification techniques were refined, germanium [[single crystal|crystals]] could not be produced with purity sufficient to enable their use as spectroscopy detectors. Impurities in the crystals trap electrons and holes, ruining the performance of the detectors. Consequently, germanium crystals were doped with [[lithium]] ions (Ge(Li)), in order to produce an [[Intrinsic semiconductor|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 to a "standard" 3″ x 3″ NaI(Tl) scintillation detector. Crystal growth techniques have since 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 (US$113,000).

{{As of|2012}}, HPGe detectors commonly use lithium diffusion to make an [[N-type semiconductor|n<sup>+</sup>]] [[ohmic contact]], and boron implantation to make a [[P-type semiconductor|p<sup>+</sup>]] contact. Coaxial detectors with a central n<sup>+</sup> contact are referred to as n-type detectors, while p-type detectors have a p<sup>+</sup> 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. The central contact in these detectors is opposite to the surface contact, making the dead layer in n-type detectors smaller than the dead layer in p-type detectors. Typical dead layer thicknesses are several hundred micrometers for a 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 [[nitrogen|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, producing too much electrical noise to be useful as a spectrometer. Cooling to liquid nitrogen temperature (77K) reduces thermal excitations of valence electrons 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.

Commercial systems became available that use advanced refrigeration techniques (for example [[pulse tube refrigerator]]) to eliminate the need for liquid nitrogen cooling.

Germanium detectors with multi-strip electrodes, orthogonal on opposing faces, can indicate the 2-D location of the ionization trail within a large single crystal of Ge. Detectors like this have been used in COSI balloon-born astronomy missions (NASA, 2016) and will be used in an orbital observatory (NASA, 2025) [[Compton Spectrometer and Imager]] (COSI).

Because germanium detectors are highly efficient in photon detection,<ref>{{Cite journal |last1=Sangsingkeow |first1=Pat |last2=Berry |first2=Kevin D |last3=Dumas |first3=Edward J |last4=Raudorf |first4=Thomas W |last5=Underwood |first5=Teresa A |date=June 2003 |title=Advances in germanium detector technology |url=https://doi.org/10.1016/S0168-9002(03)01047-7 |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |volume=505 |issue=1–2 |pages=183–186 |doi=10.1016/s0168-9002(03)01047-7 |bibcode=2003NIMPA.505..183S |issn=0168-9002|url-access=subscription }}</ref> they can be used for a variety of additional applications. High-purity germanium detectors are used by Homeland Security to differentiate between naturally occurring radioactive material (NORM) and weaponized or otherwise harmful radioactive material.<ref>{{Cite web |date=2020-12-18 |title=Deciphering Radiation Alarms: Using High Purity Germanium Detectors for Nuclear Security |url=https://www.iaea.org/newscenter/news/deciphering-radiation-alarms-using-high-purity-germanium-detectors-for-nuclear-security |access-date=2024-05-06 |website=www.iaea.org |language=en}}</ref><ref>{{Cite web |title=High-Sensitivity Detectors {{!}} Homeland Security |url=https://www.dhs.gov/publication/high-sensitivity-detectors |access-date=2024-05-06 |website=www.dhs.gov |language=en}}</ref> They are also used in monitering the environment due to the concern of the use of nuclear power.<ref>{{Cite web |title=Document Display {{!}} NEPIS {{!}} US EPA |url=https://nepis.epa.gov/Exe/ZyNET.exe/9101GWDH.txt?ZyActionD=ZyDocument&Client=EPA&Index=1976%20Thru%201980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5CZYFILES%5CINDEX%20DATA%5C76THRU80%5CTXT%5C00000030%5C9101GWDH.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=8 |access-date=2024-05-06 |website=nepis.epa.gov |language=en}}</ref> Finally, high-purity germanium detectors are used for medical imaging and nuclear physics research, making them a rather diverse detector as far as applications go.<ref>{{Cite journal |last1=Cooper |first1=R.J. |last2=Amman |first2=M. |last3=Luke |first3=P.N. |last4=Vetter |first4=K. |date=September 2015 |title=A prototype High Purity Germanium detector for high resolution gamma-ray spectroscopy at high count rates |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |volume=795 |pages=167–173 |doi=10.1016/j.nima.2015.05.053 |bibcode=2015NIMPA.795..167C |issn=0168-9002|doi-access=free }}</ref>