Comparison of JFET/MOS/HEMT Based Low Noise Charge Sensitive Preamplifiers for HPGe Detectors in Cryogenic Temperature

Abstract

The paper presents development of three different types of low noise charge sensitive preamplifiers (CSA) for HPGe detectors for basic physics experiments such as CDEX (China dark matter search experiment) and spectroscopy applications, based on JFET, MOS and HEMT transistors respectively. For the JFET and HEMT based CSAs, only the input transistor of the corresponding type was used and their secondary amplifiers were designed with commercial opamps and worked at room temperature. For the MOS based CSA, a monolithic ASIC of CMOS CSA was developed using a 350 nm CMOS process. The noise performances of these CSAs were optimized for a point-contact HPGe detector with ∼2 pF input capacitance and were tested at liquid nitrogen temperature. JFET suffered from the 'freeze-out' effect and the noise performance of the JFET CSA degraded from 64 electrons to 134 electrons when the temperature went from 120 K to 77 K. For comparison, the noises of the MOS and HEMT based CSAs kept getting better down to 77 K. The serial noises of the MOS and HEMT CSAs were similar and the HEMT CSA showed significant parallel noise contribution. The CMOS CSA has also been tested with a 0.5 kg point-contact HPGe detector. The energy spectrum was measured and the electronics noise was monitored to be 186 eV, corresponding to 26 electrons ENC (equivalent noise charge) in rms.