RF measurements and characterization of heterostructure field-effect transistors at low temperatures

The RF performance of both conventional AlGaAs-GaAs and superlattice AlAs-GaAs heterostructure field-effect transistors (HFETs) has been investigated at 120 K, and the results are compared with room-temperature values. Both the system used for low-temperature RF measurements up to 12 GHz and the procedure used to extract the equivalent circuit from measured S-parameters of the packaged FET are described. The high-frequency performance of the HFETs is strongly improved at low temperatures but is sensitive to light due to the device structure. The problems of low-temperature measurement and the results of RF investigation are discussed. Although the gate lengths of the HFETs investigated are greater than 1 μm, the method and the results of the analysis can be transferred to submicron devices without any restrictions. Therefore, submicron superlattice HFETs may exhibit high power gain at 300 K as well as at lower temperatures both in the dark and under illumination