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, 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). Cadmium telluride and cadmium zinc telluride detectors Cadmium telluride (CdTe) and cadmium zinc telluride (CZT) detectors have been developed for use in X-ray spectroscopy and gamma spectroscopy. The high density of these materials means they can effectively attenuate X-rays and gamma-rays with energies of greater than 20 keV that traditional silicon-based sensors are unable to detect. The wide band gap of these materials also means they have high resistivity and are able to operate at, or close to, room temperature (~295K) unlike germanium-based sensors. These detector materials can be used to produce sensors with different electrode structures for imaging and high-resolution spectroscopy. However, CZT detectors are generally unable to match the resolution of germanium detectors, with some of this difference being attributable to poor positive charge-carrier transport to the electrode. Efforts to mitigate this effect have included the development of novel electrodes to negate the need for both polarities of