Ultra-low noise hemts for high-impedance and low-frequency preamplifiers: Realization and characterization from 4.2 k to 77 k

Si JFETs have been for decades the lowest noise field-effect transistor (FET) for high-impedance and low-frequency readout electronics. However, they are based on nondegenerate carriers, and their operating temperature is limited to higher than 100 K due to freeze-out of carriers. Being based on degenerate carriers, the HEMT has no low-temperature limit. Here, we report on the experimental results of specially designed HEMTs made at CNRS/LPN. These HEMTs, with different input capacitances from 92 pF to 5.3 pF, have been characterized from 4.2 K to 77 K, and show a power consumption of 100 μW or below. At 4.2 K, the lowest input noise voltage, 6 nV/Hz^1/2 at 1 Hz, has been obtained with the HEMT having the largest input capacitance; the lowest input noise current of about 3 aA/Hz^1/2 at 1 Hz has been observed with the HEMT with the smallest input capacitance; and the white noise voltage in these HEMTs is of about 0.2 nV/Hz^1/2. By increasing the temperature from 4.2 K to 77 K, noise voltage and noise current increase, but their values are limited within a factor of about 3 compared to their lowest values at 4.2 K. Our results show that the HEMT can be a promising transistor to fill the gap for FETs below 100 K for high-impedance and low-frequency readout electronics.