  We present the noise performance of High Electron Mobility Transistors (HEMT)
developed by CNRS-C2N laboratory. Various HEMT's gate geometries with 2 pF to
230 pF input capacitance have been studied at 4 K. A model for both voltage and
current noises has been developed with frequency dependence up to 1 MHz. These
HEMTs exhibit low dissipation, excellent noise performance and can
advantageously replace traditional Si-JFETs for the readout of high impedance
thermal sensor and semiconductor ionization cryogenic detectors. Our model
predicts that cryogenic germanium detectors of 30 g with 10 eV heat and 20 eVee
baseline resolution are feasible if read out by HEMT based amplifiers. Such
resolution allows for high discrimination between nuclear and electron recoils
at low threshold. This capability is of major interest for Coherent Elastic
Neutrino Scattering and low-mass dark matter experiments such as Ricochet and
EDELWEISS.
