Extremely low-noise cryogenic amplifiers for radio astronomy: past, present and future

Improvements in the noise temperature of cryogenic field-effect transistors (FET's) and, later, heterostructure field-effect transistors (HFET's) over the last several decades have been quite dramatic. In 1970, a noise temperature of 120 K was reported at 1 GHz and at a physical temperature of 77 K. Currently, noise temperatures of <; 2 K, 10 K and 25 K are reported at 4, 40 and 100 GHz, respectively, for amplifiers at a physical temperature of about 15 K. This article reviews developments in this field and attempts to identify important milestones within the broader context of other technological developments (SiGe HBTs, CMOS). Noise models of unipolar and bipolar transistors are reviewed with an emphasis on their common noise properties, and on the allowable values of transistor noise parameters. The natural limits of low noise performance upon further scaling of gate or emitter size are also discussed.