One year after the invention of the CCD Robert Krambeck and Carlo Sequin of Bell Laboratories patented a method of fabricating a two-phase CCD (U.S. Patent No. 3,735,156, dated 5/1973) with an N-type region between two electrodes on a P-type substrate.

Although the possibility of building a single gate CCD (or virtual phase) is briefly mentioned in the book, Charge Transport Devices (Sequin and Tompsett, 1976) it was not fabricated until later. Jaroslav Hynechek of Texas Instruments (TI) was the first to describe it in detail (IEEE Trans. on Electron Devices, Vol. ED-28, No. 5, May 1981) and was also granted a patent on the concept. In his invention the surface regions of th N-type semiconductor substrate, positioned between the respective electrodes, have high concentration regions formed by ion implantation of a P-type impurity. This impurity has the same conductive type as that of the substrate.

Later, in 1984, Yuji Kitamura and Nobuhiro Mitani of Sanyo (Japan) proposed a two-phase CCD with a P-type impurity region between two electrodes on a P-type substrate (U.S. Pat. No. 4,649,407, dated 3/1987).

In March of 1985, Vladimir Berezin et al. filed a Russian Patent No. 1,322,932 and named their approach, a 2.5 phase Virtual Phase (VP) CCD. This invention combined the advantages of the 1.5 VP CCD of J. Hynechek of TI with high Quantum Efficiency in the blue and ultraviolet regions of the spectrum (due to the windows in the polysilicon electrodes) and low dark current (due to the pinned accumulated regions of the substrate in the windows). It also exhibited the advantages of the three phase CCD technology......simpler fabrication process, larger well capacity, bi-directional charge transfer, lower clocked phase voltage sweeps plus the inherent recombination anti-blooming characteristics of the pinned pixel regions.

Finally, James Janesick of JPL published an article "Open Pinned-Phase CCD Technology" (Proc. of the SPIE, Vol. 1159, 11/1989). His paper described a CCD imager with an accumulated pinned phase region with the blue/UV blocking polysilicon gate removed. He had independently conceived a CCD imager, named the OPP CCD; that had all the advantages of the Russian 2.5 phase CCD previously described. Janesick has been granted a U.S. patent for the OPP CCD. A few of these CCDs were fabricated and tested by J.Janesick as reported in his SPIE paper.

In Russia there exists manufactured 2.5 phase CCD (alias OPP) imagers for medical (SPIE, Vol. 1447, pp. 34-43, 1991), industrial and scientific applications (SPIE, Vol. 1447, pp. 184-190, 1991) and astronomy (SPIE, Vol. 1447, pp. 64-68, 1991) applications. This imager is presently being successfully offered for sale on the European market.