Development of cryogenic Ge JFETs

We report on initial development of new Ge JFETs for deep cryogenic (liquid-helium range) operation. Both p- and n-channel Ge JFETs were fabricated. Due to unsatisfactory epitaxial growth, the p-channel JFETs do not exhibit usable characteristics. The n-channel Ge JFETS have good de characteristics at room temperature, 77 K and 4 K, although at 4 K they exhibit a threshold and "steps" as also observed in earlier Ge JFETs made by Texas Instruments. Noise measurements at 4 K with V-ds = 1.2 V and I-d = 0.3 mA give e(n) of approximate to 0.5-1 mu V/Hz(1/2) at 1 Hz and approximate to 20-30 nV/Hz(1/2) at 10 kHz (input referred). This is considerably higher than that of the best Si JFETs operating at higher temperature or the best GaAs MESFETs operating at 4 K. However, we believe that with refinement of the design and fabrication the noise of the Ge JFETs can be reduced to a range suitable for deep cryogenic applications.