Evaluation of Silicon Detectors With Integrated JFET for Biomedical Applications

This paper presents initial results from electrical, spectroscopic and ion beam induced charge (IBIC) characterisation of a novel silicon PIN detector, featuring an on-chip $n$ -channel JFET and matched feedback capacitor integrated on its $p$-side (frontside). This structure reduces electronic noise by minimising stray capacitance and enables highly efficient optical coupling between the detector back-side and scintillator, providing a fill factor of close to 100%. The detector is specifically designed for use in high resolution gamma cameras, where a pixellated scintillator crystal is directly coupled to an array of silicon photodetectors. The on-chip JFET is matched with the photodiode capacitance and forms the input stage of an external charge sensitive preamplifier (CSA). The integrated monolithic feedback capacitor eliminates the need for an external feedback capacitor in the external electronic readout circuit, improving the system performance by eliminating uncontrolled parasitic capacitances. An optimised noise figure of 152 electrons RMS was obtained with a shaping time of 2 $\mu$s and a total detector capacitance of 2pF. The energy resolution obtained at room temperature (21 $^{\circ}$C) at 27 keV (direct interaction of I-125 gamma rays) was 5.09%, measured at full width at half maximum (FWHM). The effectiveness of the guard ring in minimising the detector leakage current and its influence on the total charge collection volume is clearly demonstrated by the IBIC images.