On-wafer RF measurement setup for the characterization of HEMTs and high T/sub c/ superconductors at very low temperatures down to 20 K

A cryogenic on-wafer measurement setup for the characterization of semiconductor devices and superconducting materials is introduced. It allows an exact small-signal analysis of the investigated devices at temperatures down to 20 K in the frequency range up to 40 GHz. Measurements on a pseudomorphic HEMT (high-electron-mobility transistor) with a gate length of 0.55 mu m and on a superconducting filter have been carried out and show the applicability of the system. Although the HEMT is optimized for low-temperature operation, the device exhibits an improved microwave performance at low temperatures. While the transit frequency of the transistor increases from about 43 GHz at room temperature to 54 GHz at 20 K, the cutoff frequency of the unilateral gain rises from 120 GHz up to 195 GHz. The filter characteristic of the high-T/sub c/ superconductor, represented by the Q-factor and the center frequency, was found to be strongly dependent on temperature. However, near the critical temperature the Q-factor increases from Q approximately=100 at 20 K to Q approximately=3000 at 76 K.<>