GaAs slow-wave phase shifter characteristics at cryogenic temperatures

It has been previously demonstrated that a single elemental solid-state GaAs epitaxial device, based on slow-wave electromagnetic propagation, can provide substantial variable phase shift in the microwave frequency regime at room temperature. Further, it has been shown that the insertion loss L of the device is reduced by increasing the conductivity in the transmission line metallization. Low-temperature applications make it desirable to determine if such a device can operate at cryogenic temperatures demonstrating useful phase-shifting properties. We have developed an accurate reliable cryogenic experimental test setup and procedure capable of measuring the phase shift θ and insertion loss L of a 1.6-mm-long device embedded in a waveguide system several meters long. Measurements have been made at 4.2, 77, and 309 K over a frequency range from 2 to 18 GHz. The 300-K results agree extremely well with earlier work and substantiate that very accurate measurements in such a setup are possible. The cryogenic results at 77 K produce a differential phase shift of 155° compared to 229° at 300 K. Loss reduction is most dramatic at frequencies below about 14 GHz and becomes progressively less as 18 GHz is approached. For example, at a Schottky bias voltage of 0.5 V, L is reduced by a factor of 2.3, 2.2, and 2.4 at 2, 10, and 12 GHz, respectively. Similar loss reduction factors are found up to 14 GHz for the 0.0-V bias case. Device operation at 4.2 K produces a differential phase shift of 206° and not much change in loss values compared to 77 K.