Integrated HEMT-based charge amplifier - Design and experiment

We have designed and tested a fully integrated high electron mobility trans istor (HEMT)-based charge amplifier suitable for applications in high-energ y physics experiments and compatible to be directly integrated on the detec tor chip for compact high-performance X- and gamma -ray imagers for medical diagnostics. The width of the input HEMT has been optimized within the con straint of fixed low-power dissipation. The direct current and noise charac teristics of different sample transistors have been carried out in order to determine the relevant parameters for the proper design and simulation of the whole charge amplifier. A SPICE model was developed ad hoc to simulate the behavior of the HEMT in the biasing conditions of the designed amplifie r. The circuit performances have been characterized in terms of output resp onse, linearity, and noise. For a detector capacitance of 5 pF and a feedba ck capacitance of 1 pF, the measured rise time is 1.89 ns, while the measur ed ENC is 627 electrons r.m.s. at 20-ns shaping time. For this condition, t he dissipated power is 7 mW.