A Doped Surface Two-Phase CCD

In a charge-coupled device, as described by Boyle and Smith, 1 charge moves successively from the semiconductor region under a given electrode to the region under the next electrode. For information to be transferred from one end of the resulting shift register to the other, it is necessary that the charge always move in the same direction. Until now, the way this directionality has been typically achieved is by the use of three or more clock lines." -4 In this type of structure, when charge is transferred from one electrode to the next, the electrode behind the one transferring charge is kept at a potential which repels the free charge and thereby prevents backward flow. The electrode to receive charge, meanwhile, is made more attractive to charge than the one giving up its charge. It can be seen that this arrangement requires three electrodes (at least) for each packet of charge and that each one must be driven by a different clock line. The use of three clock lines (as opposed to two) has significant topological disadvantages because with three clock lines there must be crossovers. These have been fabricated by a diffusion into the semiconductor surface. 2 This diffusion must be contacted once for every bit which is undesirable from the standpoint of yield and packing density. 1849