New high sensitivity silicon photodetectors for medical imaging applications

We have developed a new silicon photodiode design that reduces the dark current and can improve the sensitivity of low noise silicon photodetector arrays for medical imaging applications. The reduction in dark current eliminates the need for cooling, which facilitates the mechanical design and allows for the optimum performance of the scintillators that are coupled to the photodiodes. The photodetectors are based on a planar p+ planar entrance window on n-type silicon substrates, with n+ pixels surrounded by a p+ separating grid. Using this approach, dark currents on the order of 0.2 nA/cm² have been achieved (for 0.3 mm thick devices, at 23°C). The quantum efficiency of the p+ entrance window contact is 83% at 560 nm, which makes it an excellent match for CsI(Tl) scintillators. The current/voltage characteristics, response of the photodiodes to x-rays and gamma-rays when coupled to scintillators has been measured. These prototypes will be evaluated with respect to their applications in gamma cameras for medical imaging.