KEYWORDS: Field effect transistors, Germanium, Temperature metrology, Cryogenics, Signal to noise ratio, Transistors, Silicon, Sensors, Liquids, Nitrogen
Cooled detectors for IR, visible, UV and X-rays often require a preamplifier that can operate at deep cryogenic temperature, down to the liquid-helium range. Preamplifiers based on available silicon or gallium arsenide transistors have not been entirely satisfactory. With this in mind, we have been developing junction field-effect transistors (JFETs) based on germanium. Our objective has been to achieve stable dc characteristics and very low noise down to as low a cryogenic as possible. We have obtained good dc characteristics down to liquid-helium temperature and low noise down to ≈30 K with Ge JEFTs. Between approximately 30 and 80 K, low-frequency gate-referred noise voltage is ≈30-60 nV/rtHz at 1 Hz, decreasing to <2 nV/rtHz above ≈1 kHz for 40 μm by 1560 μm gate n-channel Ge JFETs with drain current of 330 μA and dissipating approximately 400 μW. For lower and higher drain current the "1/f" noise remains approximately the same, but the white noise increases or decreases as expected. We are continuing development with the goal of extending the same low noise characteristics down to 4K.
Low-frequency, low-noise, low-power cryogenic electronics to read out photodetectors is being investigated for the star-tracking telescope of the Gravity Probe B spacecraft. We report additional results in evaluating low-frequency '1/f' noise of commercial and non-commercial GaAs field-effect transistors (FETs) at room and liquid-helium temperatures. No correlation was found between noise at these two temperatures. For our dc biasing conditions, the lowest- noise non-commercial GaAs FETs give a typical value of Kf (equals Af x gate area) approximately equals 2 X 10-22 V2 (DOT) m2; this corresponds to a noise voltage of approximately equals 80 nV/Hz1/2 at 1 Hz for a gate area of 3 X 104 micrometers 2, only a factor of approximately equals 3 higher than the best Si JFETs of comparable gate area operated at their optimum temperature. RTSs (random telegraph signals) were observed for many GaAs MESFETs at 4 K, for gate areas up to approximately equals 5000 micrometers 2. We also examined low-frequency '1/f' noise in relation to FET materials, processing, and pinch- off voltage but the results were inconclusive.
We describe the status of the development and testing program for the inertial reference system for the Gravity Probe B gyroscopes. The gyroscope housings are attached to a cryogenic telescope with a 14 cm aperture that continuously points at a guide star. The star image is split to provide quadrant pointing information which is used to steer the spacecraft. This data is also combined with the gyro readout data to provide an absolute precession measurement. Motion of the guide star is independently checked by reference to background galaxies. Room temperature testing of a prototype telescope has been completed and preparations are being made for low temperature tests.
Low-frequency, low-noise, low-power cryogenic electronics to read out photodetectors is being investigated for the star-tracking telescope of the Gravity Probe B spacecraft. We report our initial findings from evaluating more than 20 types of GaAs FETs, both commercial and non-commercial, for this application. Most exhibit useable dc characteristics at cryogenic temperatures, although gate leakage and hysteretic effects (presumably due to charge trapping) could be troublesome. Low-frequency noise (based primarily on grounded-gate measurements) at 4 K is '1/f-like' and for the quietest GaAs FETs appears to be at least as low as the lowest noise values reported for Si MOSFETs at 4 K. Further investigation is needed in several areas.
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