Sensor arrays are used, for example, in video cameras, and generally include a two dimensional array of pixels that is fabricated on a substrate. Each pixel includes a sensing element (e.g., a photodiode) that is capable of converting a portion of an optical (or other radiant source) image into an electronic (e.g., voltage) signal, and access circuitry that selectively couples the sensing element to control circuits dispose on a periphery of the pixel array by way of address and signal lines. In CMOS image sensors, which represent one type of sensor array, metal address and signal lines are supported in insulation material that is deposited over the upper surface of a semiconductor substrate, and positioned along the peripheral edges of the pixels to allow light to pass between the metal lines to the sensing elements through the insulation material. As with other sensor arrays, CMOS image sensors typically contain millions of pixels which transform photons coming from a photographed scene into millions of corresponding voltage signals, which are stored on a memory device and then read from the memory device and used to regenerate the optical image on, for example, a liquid crystal display (LCD) device.
Large area sensor arrays are used for medical imaging applications, and have many requirements that are not always applicable to “normal” (e.g., video camera) sensor arrays. First, large area image sensors must have pixel arrays that area much larger than “normal” arrays, both in the sense that the pixels are larger and total array area is very large. Second, large area sensor arrays must be able to operate in both a high resolution, low frame rate operating mode (e.g., to facilitate x-ray imaging) and in a low resolution, high frame rate operating mode (e.g., to facilitate scanning operation). In addition, high end analog performance such as noise and linearity is required. Finally, the cost of the large area sensor arrays must be minimal without performance compromise.
Conventional large area image sensors for medical x-ray applications are currently produced using a-Si:H technology and CMOS technology. Active Pixel TFT arrays that utilize a-Si:H technology typically include a 1T pixel including a photo-diode and a single transfer transistor. The TFT pixels typically transfer their charges to an off-chip charge amplifier (e.g., using one amplifier per column). A problem with this approach is that large arrays are sensitive to signal noise, and it is not possible using current a-Si:H technology to integrate a charge amplifier at the pixel level.
Large area CMOS image sensors overcome the problems associated with sensors that use a-Si:H technology in that the CMOS process allows for the inclusion of amplifier circuits within each pixel. However, some conventional large area CMOS image sensors utilize pixel level amplifiers that are formed in an integrator configuration, i.e., such that there is a current source per integrator. Therefore, a problem with this conventional large area CMOS image sensor approach is that power consumption may be too high for practical large pixel array applications. Other MOS based large area image sensors use charge amplifiers having complex circuitry and control signals that degrade production yields and, as a result, profitability. In addition, this complex circuitry reduces pixel fill-factor, which means less light will be collected and SNR will be degraded.
What is needed is a low cost, large area CMOS image sensor with high end analog performance that overcomes the problems associated with conventional large area image sensors.