Patent GB-1407847-A (GB)
Title: Semkconductor data storage circuits
IPC: G11C17/08
Filed: 1972-12-06
Abstract: 1407847 Integrated circuits INTERNATIONAL BUSINESS MACHINES CORP 6 Dec 1972 [30 Dec 1971 30 June 1972] 56183/72 Heading H1R [Also in Division H3] In an integrated semiconductor symmetrically structured bi-stable data storage circuit operable both as a read/write cell and as a read-only cell including a pair of cross-coupled transistors, T1, T2, a control element S operably associated. with one of the pair of cross-coupled transistors. and connected to enable an input signal VH to be applied to set the circuit in a predetermined read-only state and a further element S<SP>1</SP> associated with the other of the pair of crosscoupled transistors,. the further element S<SP>1</SP> is substantially identical to the control element S to maintain the symmetry of the circuit but it is so connected to the remainder of the circuit that it is not affected by an input signal applied to the circuit. As shown in Fig. 2A information is read into the cell by applying potentials to the word line WL and to one of the bit lines B0, B1 so that one of the read-write transistors T3, T4 conducts to set the state of the bi-stable FET's T1, T2. Reading out is obtained by applying potentials to the word line WL and the bit lines B0 and B1. To enable the cell to operate as a read-only cell the control element S, in the form of a FET is controlled by a direct voltage or voltage pulse applied to a line VH. A further FET D may be provided to speed up the writing in of information in the read-only operation. The further FET D is controlled by the line VH so that point 1 is charged more quickly. A transistor D<SP>1</SP> which is not operated is then included for symmetry. An integrated construction is disclosed (Fig. 1B, not shown) and Fig. 2B with T1-T6 as thin oxide FET's and the transistors S, S<SP>1</SP>, D and D<SP>1</SP> as thick oxide FET's. N+ diffusion zones which are shaded from left to right in Fig. 2B are introduced into the surface of a P-type substrate. The surface of the substrate and these N+ diffusion zones are covered with a thick insulating oxide layer (not shown) on to which metal surfaces are applied shown in a heavy framing. Below the metal surfaces in the region of each of the gates of the FET's the thick oxide layer is reduced to a thin gate oxide which is represented by the dash-dotted rectangles with corresponding transistor references. In addition the thick oxide layer is provided with contact holes shaded from left to right via which the metal surfaces contact the diffusion zones below. Parasitic thick oxide field effect transistors C1, C2, C3 formed at the crossing of existing diffusion zones cannot cause an asymmetry of the storage cell. In a further embodiment, Fig. 3, which is a modification of the cell described in Specification 1,253,763, information is read in and out of the storage cell T1, T2 by write/read transistors T3, T4. For non-destructive read out in a matrix of cells the potential is raised at the word line W so that all the other write/read transistors T3, T4 in the non- addressed storage cells are rendered non- conductive. The write/read transistors of the non-addressed cells need not be completely non- conductive and the higher read current caused by the addressed storage cell is then determined by a differential amplifier. In the read only operation the state of the bi-stable transistors T1, T2 is set by the additional transistor S. A corresponding transistor S<SP>1</SP> supplies the symmetry for the write/read operation. In write/read operation of the cell the potentials at the word and bit line are selected so that the base emitter diode of the transistors S or S<SP>1</SP> does not carry a high current. The two read/ write transistors T3, T4 together with the load transistors 10, 20 are integrated such as into a common isolation pocket (P+, Figs. 4B and 4C, not shown) with the transistors T3 and T4 formed as vertical transistors and the load transistors 10 and 20 formed as lateral transistors. The two cross-coupled transistors T1, T2 can be vertical transistors in two separate isolation pockets or the two transistors can be arranged in one isolation pocket (Figs. 5B and 5C, not shown) by operating them inversely such that the common emitter zones N1 are formed by an epitaxial layer also serving as the word line W connected to the storage cell. The bulk resistance of the epitaxial layer N1 can be reduced by a highly doped sub-collector zone N+. The storage cells of Fig. 3 may be formed into a storage matrix in an integrated structure (Fig. 6, not shown). The storage cells (25) are disposed at the intersections of the word and bit lines. The word lines WI, WII extend horizontally in the sub-collector zone N+ or in epitaxial layer N1 of the isolation pocket housing the bi-stable transistors T1 and T2. The transistors S, S<SP>1</SP> may be lateral transistors.