Drain Feedback - A Novel Feedback Technique for Low-Noise Cryogenic Preamplifiers

This paper describes a novel technique of charge restoration - the drain feedback, based on the characteristics of the gate-to-drain junction of the input FET. The restoration charge required at the input of the preamplifier is generated by impact ionization in high-field regions of the FET. Actually the feedback is obtained by regulating the drain voltage according to the input energy count-rate product and consequently adjusting the electric field for the necessary charge generation. In the first part of the paper the principles of impact ionization in semiconductors at cryogenic temperatures are outlined and applied to junction FET's (JFET's) under saturation conditions. Then, the properties of FET gate leakage current generated by impact ionization are analyzed. Finally, the continuous mode of operation of the new feedback method is presented, and results from its application in x-ray spectroscopy with silicon and germanium detectors are given. Superior noise and count-rate performance coupled with simplicity and reliability are the outstanding features of the drain feedback method. It is the first feedback method in which the cryogenic input stage comprises the detector and the FET without the parasitic increases of stray capacitance or light-induced leakage currents characteristic of the other resistorless configurations.