A circuit for use in an image sensor as well as an image sensing system
using the circuit are set forth. The circuit comprises a memory device
having a non-volatile memory cell, a control gate, a drain and a source.
The circuit also employs a photosensitive semiconductor device that is
positioned for exposure to electromagnetic radiation from an image. A
pixel control circuit is connected to these components to direct the
memory device and the photosensitive semiconductor device to a plurality
of controlled modes. The controlled modes may include an erase mode and
an exposure mode. In the erase mode, at least a portion of an electric
charge is removed from the non-volatile memory cell to place the memory
device in an initialized state. In the exposure mode, the non-volatile
memory cell is charged at least partially in response to a voltage at a
terminal of the photosensitive semiconductor device. The voltage at the
terminal of the photosensitive semiconductor device corresponds to
exposure of the photosensitive semiconductor device to the
electromagnetic radiation from the image. The pixel control circuit may
also direct the memory device and the photosensitive semiconductor device
to further modes including a read mode and a data retention mode. In the
read mode, current flow between the source and drain of the memory device
is detected as an indicator of the charge on the non-volatile memory
cell. In the data retention mode, the charge on the non-volatile memory
cell of the memory device that was acquired during the exposure mode is
maintained notwithstanding further exposure of the photosensitive
semiconductor device to the electromagnetic radiation from the image. The
circuit, and one or more peripheral support circuits, may be implemented
in a monolithic substrate using, for example, conventional CMOS
manufacturing processes.