FIG. 1 depicts a simplified block diagram of a first CCD image sensor that performs charge multiplication in accordance with the prior art. Pixel array 100 includes vertical charge-coupled device (CCD) shift registers (not shown) that shift charge packets from a row of pixels 102 one row at a time into low voltage horizontal CCD (HCCD) shift register 105. Low voltage HCCD shift register 105 serially shifts the charge packets into a high voltage charge multiplying HCCD shift register 110. Charge multiplication occurs in charge multiplying HCCD shift register 110 through the application of large electric fields to the gate electrodes (not shown) overlying HCCD shift register 110 during charge transfer. The large electric fields produce a signal larger than originally collected in the pixels in pixel array 100. The large electric fields are created by overdriving the gate electrodes over the extended HCCD shift register 110 with sufficiently larger voltages. Typically, charge multiplying HCCD shift register 110 can multiply the number of charge carriers in each charge packet by a factor of two to one thousand. The multiplied charge packet output at the end of charge multiplying HCCD shift register 110 is sensed and converted into a voltage signal by output amplifier 120.
A conventional output amplifier can have a minimum noise level of eight charge carriers, meaning the output amplifier is unable to detect a signal when a charge packet contains less than eight charge carriers. One advantage of a multiplying HCCD shift register 110 is the ability to amplify or multiple charge packets that would not normally be detected by an output amplifier. For example, a charge multiplying HCCD shift register can take an input of just one undetectable charge carrier (e.g., electron) and convert it to a larger detectable group of one thousand charge carriers. The output amplifier is now able to detect the charge packet and convert the charge packet to a voltage signal.
One drawback to a charge multiplying HCCD shift register is its dynamic range. If the charge packet entering the multiplying HCCD shift register has two hundred charge carriers and if the gain is one thousand, the two hundred charge carriers are multiplied to 200,000 charge carriers. Many charge multiplying HCCD shift registers are unable to hold 200,000 or more charge carriers, and the charge carriers bloom (spread out) into the pixels adjacent to the HCCD shift register. When the capacity of the charge multiplying HCCD shift register is 200,000 charge carriers and the gain is one thousand, the maximum signal that can be measured by a charge multiplying HCCD shift register is 200 charge carriers with a noise floor of one charge carrier. That is a dynamic range of 200 to 1. To illustrate how poor that dynamic range is, an output amplifier with a minimum noise level of eight electrons can easily measure charge packets containing 32,000 charge carriers for a dynamic range of 4000 to 1.
To overcome this limitation, prior art CCD image sensors (see FIG. 2) have added a second output amplifier 200 to HCCD shift register 105. If the image is known to contain charge packets too large for the charge multiplying HCCD shift register 110, the charge packets are serially shifted through HCCD shift register 105 to output amplifier 200 instead of towards the charge multiplying HCCD shift register 110. One disadvantage to this implementation is the entire image must be read out of either output amplifier 200 or output amplifier 120. If an image contains both bright and dark regions, the image must be read out of output amplifier 200 so the bright regions do not bloom (flood) the charge multiplying HCCD shift register 110. But when the entire image is read out of output amplifier 200, dark regions in the image are not shifted through the charge multiplying HCCD shift register and do not receive the benefit of charge multiplying HCCD shift register 110.