We present the design and noise performance of a fully cryogenic ( T=4 K ) HEMT-based charge amplifier for readout of massive semiconductor dark matter detectors operating at sub-Kelvin temperatures .
The amplifier has been developed to allow direct detection experiments such as CDMS and EDELWEISS to probe WIMP masses below 10 GeV/ c ^ { 2 } while retaining electromagnetic background discrimination .
The amplifier dissipates only 1 mW of power and has a measured noise performance three times better than traditional JFET-based charge amplifiers .
The predicted ionization energy resolution using the measured intrinsic amplifier noises and typical detector characteristics is \sigma _ { E } \approx 100 \text { eV } _ { ee } ( 33 electrons ) .
We have measured a calibrated baseline energy resolution of \sigma _ { E } = 91 \text { eV } _ { ee } when coupled to a live CDMS-II detector .
To our knowledge , this is the best resolution achieved on such massive ( \approx 150 pF capacitance ) radiation detectors .