Criteria of choice of the front-end transistor for low-noise preamplification of detector signals at sub-microsecond shaping times for X- and gamma-ray spectroscopy

We present an analysis of the electronic noise contributions which limit the resolution of X- and gamma-ray spectrometers employing semiconductor detectors operating at room temperature and at signal processing times in the sub-microsecond range. The figures of merit of the front-end transistors, relevant to attain the highest resolution, are put in evidence. It is shown how the correlation between the gate and drain current noises in FETs plays a significant role in the equivalent noise charge of a charge preamplifier. Some state of the art devices, belonging to different technologies, JFET, MOSFET, MESFET and HFET, are examined.