Engineers at Germanium Power Devices are refining a transistor that works at liquid helium temperature. The germanium field-effect transistor was pioneered by Texas Instruments but never put into extensive production. The Massachusetts company, funded by a Small Business Innovation Research grant, is optimizing the device for deep cryogenic operation. Current transistors do not operate well at extremely low temperatures, causing poor device performance. The devices fill a need at NASA for transistors in cryogenically cooled infrared detectors, and the agency plans to use them in its missions. Other applications include x-ray astronomy and gravitational-wave detection.