A Cryogenic Preamplifier using a GaAs Field Effect Transistor Input Stage
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A Cryogenic Preamplifier using a GaAs Field Effect Transistor Input Stage
Diploma Thesis by Björn Starmark
Department of Physics
Göteborg University/Chalmers University of Technology
Göteborg, February 1996
Abstract
We have successfully implemented a preamplifier capable of operating in an
ambient temperature of 4.2 K. The amplifier utilizes a GaAs Metal-on-
Semiconductor Field Effect Transistor (MESFET) input stage which can be
immersed in liquid helium. Two MESFETs have been tested, the first had a gate
area of 0.049 mm2 while the other had an area of 0.012 mm2. With the larger
MESFET, the amplifier had a spot noise at 100 kHz and 4.2 K of 1.2 nV/(Hz)½ at
a gain of 9000 and a pass band from 400 Hz to 700 kHz. The smaller FET was
found to be noisier with anomalous low frequency fluctuations for drain
currents above 1 mA. The smaller MESFET achieved a spot noise voltage at 100
kHz and 4.2 K of 2.8 nV/(Hz)½ at a gain of 5700. It was possible to turn on the
amplifier at 4.2 K indicating a substantial amount of free charges in the material
at this temperature.
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