Patent GB-1470191-A (GB)
Title: Charge coupled devices
IPC: H01L29/768
Filed: 1974-03-18
Abstract: 1470191 Charge coupled devices PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 18 March 1974 [19 March 1973] 11974/74 Heading H1K In a CCD in which a layer of one conductivity type in which the packets of charge carries are transferred is isolated in operation from its surroundings and has a doping and thickness such that voltage applied to the transfer electrode system can deplete the layer throughout its thickness without causing breakdown, as described in Specification 1,414,183 the layer comprises one or more surface adjacent regions beneath the electrode system of higher doping than the rest of the layer. Information may be introduced as majority carriers, e.g. via contact 31 in Fig. 3 or by irradiation, and read out at ohmic contact 32. The electrode system may be constituted by a piezo electric layer along which an acoustic wave is propagated to generate transfer voltages but preferably comprises the set of overlapping electrodes shown running normal to the direction of transfer, two phase operation being achieved by applying oppositely phase square wave, multi-level or sawtooth lock pulses via lines 22 and 24 to adjacent electrode pairs the fixed bias, e.g. 5 v. between electrodes of a pair providing the asymmetry in the underlying potential wells required to ensure unidirectional transfer of charge. Injected electrons accumulated beneath the most positive electrode and are concentrated in the N+ layer 17, but away from its upper surface where trapping might occur, and hence are closely coupled to the electrodes. Ion implanted zones of P type may be provided between adjacent electrodes to eliminate undesired potential wells. The odd and even numbered transfer electrodes are of aluminium and polysilicon respectively and are mutually insulated by oxide formed on the polysilicon. Electrodes making Schottky contact with the layer may also be used. The layer is isolated by PN junction 13 and at its sides by further PN junctions, and/or insert oxide. In a variant (Fig. 10, not shown) adjacent electrodes of a pair are at the same potential, the potential well assymmetry required for unidirectional charge transfer being achieved by confining the N + layer to the areas beneath the Al electrodes. In an alternative embodiment (Fig. 6, not shown) which may be formed on an alumina, e.g. sapphire substrate the layer has N+ surface regions and transfer electrodes at opposed faces to provide four phase operation, the charge packets moving across the thickness of the layer at each charge transfer step. Fig. 8 shows a similar arrangement but in which the structure is epitaxially grown on a support of N type silicon, the electrodes on one surface being constituted by P type buried layers 144, 146 biased to ensure layer isolation. In another structure for two phase operation potential well assymmetry is provided by confining the N+ regions to areas beneath the trailing portions of each electrode. Alternative semi-conductor materials are Ge and A m B v  compounds and alternative insulators silicon nitride, alumina and combinations thereof. A thin P-type layer may be provided at the layer surface to prevent the carriers reaching surface traps.