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 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 0.1 nA/cm/sup 2/ have been achieved (for 0.3 mm thick devices, at 23/spl deg/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, and the response of the photodiodes to X-rays and gamma-rays when coupled to CsI(Tl), have been measured. These prototypes are being evaluated with respect to their applications in gamma cameras for medical imaging.