2.1 The onward march of third information revolution in the first  (Page 13/14)

Detailed physics of operation

The photoactive region of the CCD is, generally, an epitaxial layer of silicon . It has a doping of p+ ( Boron ) and is grown upon a substrate material, often p++. In buried channel devices, the type of design utilized in most modern CCDs, certain areas of the surface of the silicon are ion implanted with phosphorus , giving them an n-doped designation. This region defines the channel in which the photogenerated charge packets will travel. The gate oxide, i.e. the capacitor dielectric , is grown on top of the epitaxial layer and substrate.

Later on in the process polysilicon gates are deposited by chemical vapor deposition , patterned with photolithography , and etched in such a way that the separately phased gates lie perpendicular to the channels. The channels are further defined by utilization of the LOCOS process to produce the channel stop region.

Channel stops are thermally grown oxides that serve to isolate the charge packets in one column from those in another. These channel stops are produced before the polysilicon gates are, as the LOCOS process utilizes a high temperature step that would destroy the gate material. The channels stops are parallel to, and exclusive of, the channel, or "charge carrying", regions.

Channel stops often have a p+ doped region underlying them, providing a further barrier to the electrons in the charge packets (this discussion of the physics of CCD devices assumes an electron transfer device, though hole transfer, is possible).

The clocking of the gates, alternately high and low, will forward and reverse bias to the diode that is provided by the buried channel (n-doped) and the epitaxial layer (p-doped). This will cause the CCD to deplete, near the p-n junction and will collect and move the charge packets beneath the gates—and within the channels—of the device.

CCD manufacturing and operation can be optimized for different uses. The above process describes a frame transfer CCD. While CCDs may be manufactured on a heavily doped p++ wafer it is also possible to manufacture a device inside p-wells that have been placed on an n-wafer. This second method, reportedly, reduces smear, dark current , and infrared and red response. This method of manufacture is used in the construction of interline transfer devices.

Another version of CCD is called a peristaltic CCD. In a peristaltic charge-coupled device, the charge packet transfer operation is analogous to the peristaltic contraction and dilation of the digestive system . The peristaltic CCD has an additional implant that keeps the charge away from the silicon/ silicon dioxide interface and generates a large lateral electric field from one gate to the next. This provides an additional driving force to aid in transfer of the charge packets.

CCDs containing grids of pixels are used in digital cameras , optical scanners , and video cameras as light-sensing devices. They commonly respond to 70 percent of the incident light (meaning a quantum efficiency of about 70 percent) making them far more efficient than photographic film , which captures only about 2 percent of the incident light.