Ultra low noise cryogenic amplifiers for radio astronomy

Cryogenic cooling of receivers to reduce their noise temperature is especially important in radio astronomy, as the antenna noise temperature is determined by the cosmic microwave background radiation (2.725 K) modified by the presence of atmosphere. For frequencies up to 120 GHz direct amplification at cryogenic temperatures is typically employed using InP heterostructure field-effect transistors (HFETs) or, more recently, SiGe heterostructure bipolar transistors (HBTs). This article reviews developments in this field and presents the current state-of-the-art. Examples of noise performance of amplifiers using InP HFETs and SiGe HBTs are compared with the model predications. Some gaps in our current understanding of experimental results are emphasized, and some comments on possible future developments are offered.