US Patent 7259413 High dynamic range image sensor

A pixel cell with controlled leakage is formed by modifying the location and gate profile of a high dynamic range (HDR) transistor. The HDR transistor may have a dual purpose, acting as both a leaking transistor and either a transfer gate or a reset gate. Alternatively, the HDR transistor may be a separate and individual transistor having the gate profile of a transfer gate or a reset gate. The leakage through the HDR transistor may be controlled by modifying the photodiode implants around the transistor, adjusting the channel length of the transistor, or thinning the gate oxide on the transistor. The leakage through the HDR transistor may also be controlled by applying a voltage across the transistor.