Integrated HEMT-based charge amplifier-design and experiment

We have designed and tested a fully integrated high electron mobility transistor (HEMT)-based charge amplifier suitable for applications in high-energy physics experiments and compatible to be directly integrated on the detector chip for compact high-performance X- and γ-ray imagers for medical diagnostics. The width of the input HEMT has been optimized within the constraint of fixed low-power dissipation. The direct current and noise characteristics 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 amplifier. The circuit performances have been characterized in terms of output response, linearity, and noise. For a detector capacitance of 5 pF and a feedback capacitance of 1 pF, the measured rise time is 1.89 ns, while the measured ENC is 627 electrons r.m.s. at 20-ns shaping time. For this condition, the dissipated power is 7 mW