Theory, design, and performance of low-blooming silicon diode array imaging targets

The increase in size of an image produced by a high intensity source is particularly troublesome in silicon diode array imaging devices. This increase is caused by inversion of Si under the SiO2between the diodes or by the lateral diffusion of the excess minority carriers in the field-free region of the sensor array. The formation of inversion layers can be eliminated by proper choice of the sheet resistance of the resistive sea when the target is operated above flat-band voltage. However, when the diodes in the image are fully discharged in a time interval shorter than the frametime, the excess carriers diffuse to the adjacent diodes and cause an increase in the size of the image. Such "blooming" is related to the effective minority carrier lifetime that is a function of surface recombination velocity at the imaging side of the target, surface recombination velocity at the diode side of the target, and bulk minority carrier lifetime. Two methods for controlling blooming will be discussed. In one case, the effective minority carrier lifetime is reduced by adjusting the n⁺doping profile at the imaging side of the array. The second technique requires diffusion of a separate p-type region, placed between the sensing diodes elements, which provides a high recombination region when the sensing diodes are discharged due to the image overload. Devices have been constructed with good imaging quality, where a 1 percent scan diagonal bright spot bloomed to 4 percent in the first technique, and to 2.5 percent in the second technique with a 10⁵overload above saturation. The data are in good agreement with the theory.