Description
Following is a sample of the information contained on this CD: FIELD OF THE INVENTION The present invention pertains to semiconductor based image sensors and more particularly to Active Pixel image sensors (APS) or Passive Pixel image sensors (PPS) having increased dynamic range. BACKGROUND OF THE INVENTION APS are solid state imagers wherein each pixel contains both a photo-sensing means and at least one other active component, creating a charge that is converted to a signal (either a voltage or current signal). The signal represents the amount of light incident upon a pixel photosite. The dynamic range (DR) of an image sensing device is defined as the ratio of the effective maximum detectable signal level, typically referred to as the saturation signal, (V.sub.sat), with respect to the rms. noise level of the sensor, (.sigma..sub.noise). This is shown in Equation 1. Image sensor devices such as charge coupled devices (CCD) that integrate charge created by incident photons have dynamic range limited by the amount of charge that can be collected and held in a given photosite, (V.sub.sat). For example, for any given CCD, the amount of charge that can be collected and detected in a pixel is proportional to the pixel area. Thus for a commercial device used in a megapixel digital still camera (DSC), the number of electrons representing Vsat is on the order of 13,000 to 20,000 electrons. If the incident light is very bright and creates more electrons that can be held in the pixel or photodetector, these excess electrons are extracted by the anti-blooming means in the pixel and do not contribute to an increased saturation signal. Hence, the maximum detectable signal level is limited to the amount of charge that can be held in the photodetector or pixel. The DR is also limited by the sensor noise level, .sigma..sub.noise. Due to the limitations on Vsat, much work has been done in CCD's to decrease .sigma..sub.noise to very low levels. Typically,
