Feasibility of a fully integrated HEMT based charge amplifier: design and experiment

We have designed and tested a fully integrated 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 Xand γ-ray imagers for medical diagnostics. The width of the input HEMT has been optimised with the constraint of a fixed low power dissipation. The DC and noise characteristics of different sample transistors have been measured in order to determine the relevant parameters for the proper design and simulation of the whole charge amplifier. A SPICE model was ad-hoc developed to simulate the behaviour of the HEMT in the biasing conditions of the designed amplifier. The circuit performances have been characterised 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