Specific HEMTs for deep cryogenic high-impedance ultra low low-frequency noise read-out electronics

For decades, high-impedance and low-frequency readout electronics with the lowest noise level is based on silicon JFETs (Junction Field-Effect Transistors) with an equivalent input noise voltage level of about 1 nV/Hz^1/2 at 1 kHz. But their operating temperature is limited to be higher than 100 K due to their intrinsic structure. It is well known that HEMTs (High Electron Mobility Transistors) are intrinsically available for very low temperature operation, but conventional HEMTs suffer high gate leakage current and large channel low-frequency noise under cryogenic condition. In order to overcome these two major issues, we have extensively investigated the conventional HEMTs at 4.2 K by the bias-cooling method. At a given working point, the dependence of the channel low-frequency noise on the gate leakage current has been found out and this has allowed us to devise a new transistor structure. Specific AlGaAs/GaAs HEMTs have then been fabricated. At 4.2 K, our HEMTs can attain a noise level lower than 0.8 nV/Hz^1/2 at 1k Hz with an intrinsic gain A_int of 26 and an input gate-source capacitance of 46 pF, and their gate leakage current can be limited about 1 pA This result shows that our specific HEMTs may be a suitable transistor for future ultra-low noise deep cryogenic high-impedance and low-frequency readout electronics.