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 lim it the resolution of X- and gamma-ray spectrometers employing semicond uctor 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 i n evidence. It is shown how the correlation between the gate and drain current noises in FETs plays a significant role in the equivalent noi se charge of a charge preamplifier. Some state of the art devices, bel onging to different technologies, JFET, MOSFET, MESFET and HFET, are e xamined.