There
lies the main challenge, to read out these charges. Many schemes named
binning, two-phase, three-phase, four-phase etc. corresponding to the
type of clocking used have been used for transferring the charge packets
cell to cell in the bucket brigade style while protecting the integrity
of each packet. Considering a 3-phase scheme, there would be 3 control
wires passing each cell, each wire being connected to one phase of the
clock. These wires control the height of the potential wells which
pushes and pulls the charge packets along the line. For a three-phase
scheme, each pixel contains one storage and two barrier gates. Each
storage gate is connected to the same phase and sequentially, every
barrier gate is connected to their respective phase clocks. A charge
packet moves from one region into the next region when the second gate
goes to a higher potential and the first gate goes low. This sequential
movement of charges can be demonstrated as in the figure below.
Attempts
were made to use CCD devices as memory devices. But soon after, other
techniques took over and after that CCD devices have been used primarily
in scanning, microscopy and photography. One dimensional array has been
utilized in flatbed scanners where one line is scanned at a time and
the charges are read out after that. The array is moved mechanically
over the entire page to create the 2 dimensional scanned images. Two
dimensional arrays have been used to capture the entire scene in one go
and then transfer the charges to the output.
One dimensional linear array CCD