A neural net for data-processing

Abstract: The neural net includes a plurality of synaptic circuits associated with a matrix including: a plurality of line electrodes (R i ), a plurality of column electrodes (C j ), and a plurality of ordered regions (A ij ) in which line electrodes (R i ) and column electrodes (C j ) are interconnected. At least some of the connecting regions (A ij ) include electrochromic material (ECM).

Description: Field of the invention The present invention relates to a neural net for data-processing. State of the prior art Interest in neural nets based on neuron models has increased considerably in recent times. Many factors have contributed to this: neurobiologists have started to improve their understanding of biological processing mechanisms, the availability of low-cost computational power has enabled detailed analysis of circuit models of biological systems and, finally, interest in parallel processing and analog VLSI (very large-scale integration), which in turn are suitable for embodiment in neuron-type circuits, has increased. These developments have been accompanied by the formulation of new neuron models. According to J.J. Hopfield's model (see Proceedings of the National Academy of Science U.S.A., Vol.79, p. 2554, 1982) a neuron can be imitated by an adder element with a non-linear transfer characteristic whose inputs are "weighted" with coefficients characteristic of the net. In a known circuit (see C. Mead, "Analog VLSI and Neural Systems", Addison-Wesley, 1989), the characteristic weighting coefficients of the net can be defined by the transconductance values of MOS transistors and these values can be defined in turn by the resistance values of resistors. A neural net is thus defined once the values of the weighting coefficients assigned to its inputs have been defined. A neural net for a new and different function, however, has to be constructed from scratch with transistors having correspondingly different transconductances. A neural net which can be adapted to carry out different functions is described in U.S. Patent No. 4,760,437. This known neural net includes a matrix comprising a plurality of line electrodes, a plurality of column electrodes, and a plurality of ordered regions in which line electrodes and column electrodes intersect. An element of photoconductive material is associated with each region of intersection and the electrical resistance of the element is determined by the intensity of light caused to fall thereon. The resistance of the element thus determined corresponds to the value of a weighting coefficient associated with the location in the matrix to which the element belongs. A mask, such as a photographic plate, is therefore associated with a neuron matrix of this type, the mask having zones whose transparencies differ in dependence on the weighting coefficients to be assigned to the various locations in the matrix. The matrix is then illuminated through the mask by a uniform-light source and the matrix thus becomes "specialised" to carry out the desired function. With a neural net according to the United States patent cited above, the mask by means of which the electrical resistances of the photosensitive elements of the matrix are defined has to be replaced in order to modify the function carried out by the net. Obviously, this kind of operation cannot be carried out continuously during processing. In other words, the neural net according to U.S. Patent No. 4,760,437 does not have a so-called "learning capability", that is, a capacity for the "on-line" alteration or up-dating of the weightings associated with at least some locations in the matrix. Definition of the invention The object of the present invention is to provide a neural net of the aforesaid type which is formed so as to have a learning capability. According to the invention, this object is achieved, by means of a neural data-processing net as defined in the attached claim 1. Electrochromic materials are materials which can be coloured reversibly as a result of the passage of an electrical load. Electrochromism is a phenomenon by which a reversible colour-change can be induced in some materials as a result of an electrochemical process. More precisely, electrochromism can be defined as a persistent but reversible electrochemically-induced optical change. Electrochromic materials are therefore suitable for forming elements whose transparency can be changed reversibly by the passage of current pulses through the elements in one direction or the other according to whether their transparency is to be increased or reduced. This property of electrochromic materials enables neural nets with learning capabilities to be formed, as will become clearer from the following. Brief description of the drawings Further characteristics and advantages of the invention will become clear from the detailed description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure 1 is a diagram showing the structure of a neural net matrix according to the invention, Figure 2 shows the structure of the circuitry in a region in which line electrodes and column electrodes of the matrix of Figure 1 are interconnected, Figure 3 shows, by way of example, a circuit diagram of a synapse associated with a region in which line electrodes and column electrodes of the matrix are interconnected, Figure 4 is a perspective view of a portion of the matrix, and Figure 5 is a partial sectional view taken on the line V-V of Figure 4. Detailed description of the invention With reference to the drawings, a neural data-processing net according to the invention includes a support substrate 1 of at least partially transparent material, for example glass. A matrix structure, generally indicated 2 in Figure 1, is formed on one face of the substrate. As will become clearer from the following, the neural net also includes active electronic devices which interact with the matrix 2 and conveniently are formed on the other face of the supporting substrate 1. In the embodiment to which Figure 1 relates, the matrix structure 2 comprises a plurality of line electrodes R i (i = 1, 2 ... n) and a plurality of pairs of column electrodes C k j (j = 1, 2 ... m, m being equal to or different from n; k = 1,2). The matrix 2 of Figure 1 also has a plurality of ordered connecting regions A ij , each of which interconnects a pair of column electrodes and a line electrode. Figure 2 is a symbolic circuit diagram of a generic connecting region A ij : two diodes, indicated D₁ and D₂, are arranged in series between the column electrodes C¹ j and C² j . An element, indicated ECM, of electrochromic material, for example, tungsten oxide WO₃, is deposited on a small area of the region A ij , indicated a ij . This element has two electrodes E₁, E₂, the first of which is connected to the anode of D₁ and to the cathode of D₂, whilst the second is connected to the line electrode R i . The arrangement of the diodes D₁ and D₂ is such that an electric current can flow through ECM in one direction or the other. Thus, if the column electrodes C¹ j and C² j are connected to the positive pole of a direct-current voltage supply and the line electrode R i is connected to the negative pole (or to earth), a current flows through C² j , D₂, ECM, and the line electrode R i . If the column electrodes are connected to the negative pole of the supply and the line electrode R i is connected to the positive pole, however, a current will flow through R i , ECM, D₁ and C¹ j , passing through ECM in the opposite direction from that described above. As a result of the known properties of electrochromic materials (in this connection see, for example, the article "Electrochromism and Electrochromic Devices" by B. Scrosate, Chimicaoggi, June 1989, pp. 41-45) a pulse of current through ECM in one direction or the other has the effect of increasing or reducing (persistently but reversibly) the transparency of the electrochromic element. Synaptic circuits are formed on the other face of the support substrate 1 in regions Bij (Figure 3) situated in positions corresponding to the connecting regions A ij of the matrix 2, for example, according to the layout shown in Figure 3. This layout provides for an analog differential amplifier comprising, in known manner, a pair of p-type MOS transistors, indicated M₁ and M₂, and a pair of n-type MOS transistors, indicated M₃ and M₄. The "drains" of M₁ and M₂ are intended to be connected to the positive terminal of a direct-current voltage supply. The "gates" of these transistors are connected to each other and to the "drain" of M₃. The "sources" of M₁ and M₂ are connected to the "drains" of M₃ and M₄ respectively. The "sources" of M₃ and M₄ are connected to the "drain" of a MOS phototransistor, indicated FM, which, conveniently, is formed in a small area b ij facing (through the support substrate 1) the area a ij on which the element ECM of electrochromic material is formed. The arrangement is such that light can fall on FM only through ECM and the support substrate 1. As a result, the intensity of the light falling on FM depends on the transparency of the associated ECM which, moreover, can be modified and, in particular, can be reduced or increased by the passage of current pulses through ECM in one direction or the other, as described above. The weighting coefficient associated with FM can therefore be modified "on line" so that the neural net as a whole has a learning capacity. Figures 4 and 5 are qualitative illustrations of an embodiment of an individual connecting cell or region A ij of the matrix 2. Conveniently, the column electrodes are made of metal, for example, gold, aluminium, tungsten, molybdenum, niobium, nickel (or even other metals) or alloys thereof. A line electrode R i with which the electrode E₂ of the electrochromic element ECM is formed integrally, is deposited on the column electrodes after the interposition of layers F of an insulating material constituted, for example, by SiO₂. The electrode E₁ of the element ECM is formed in two parts: a first part, indicated E′₁, formed by a mixture including a solid-state polymeric electrolyte, such as a polyethylene oxide, and an alkali metal salt, for example, LiClO₄ or LiCF₃SO₃, and a second part, indicated E″₁, constituted by an alkali metal such as lithium or sodium. Still with reference to Figure 4, the diodes D₁ and D₂ are formed by two p-n junctions, for example of silicon. As can be seen from Figure 5, the phototransistor FM of a synaptic circuit is formed on the lower face of the transparent support 1 in correspondence with the associated electrochromic element ECM. The phototransistor is formed, in known manner, by the deposition of a layer of amorphous silicon 100 suitably doped to form the drain, source and gate regions. The gate, drain and source terminals are indicated 101, 102 and 103 in Figure 5. Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the present invention. The foregoing description describes, with reference to the appended drawings, an embodiment in which the matrix 2 includes a plurality of pairs of column electrodes. As has been seen, this embodiment enables the transparency of individual electrochromic elements to be increased and reduced selectively in order to give the neural net a learning capability. For applications in which there is expected to be a need or opportunity for the neural net to have a learning capacity such that, in operation, the transparency of the elements ECM always has to be modified in the same way, that is, always to increase their transparency or always to reduce their transparency, the matrix could be formed with single column electrodes and without the need for one-way conductive elements. Finally, the definition of lines and columns in the foregoing description is of course purely conventional and does not presuppose the parallelism of the line electrodes or of the column electrodes or necessarily imply that the column electrodes are perpendicular to the line electrodes.

Claims: A neural net for data-processing including active electronic devices associated with a matrix structure (2) comprising a plurality of line electrodes (R i ), a plurality of column electrodes (C j ), and a plurality of regions (A ij ) in which line electrodes (R i ) and column electrodes (C j ) are interconnected, said active electronic devices comprising photosensitive elements individually associated with corresponding interconnecting regions of said plurality of interconnecting regions (A ij ), characterized in that at least some of the interconnecting regions (A ij ) of the matrix structure (2) include an element of an electrochromic material (ECM), in that it comprises a support substrate (1) of an at least partially transparent material having said matrix structure (2) formed on one face of the substrate and said photosensitive elements formed on the other face thereof, so that light from an external light source can fall on said photosensitive elements through the electrochromic element (ECM) associated to said photosensitive elements through corresponding interconnecting regions (A ij ) within which said electrochromic elements are included and to the support substrate (1), in that said plurality of column electrodes (C j ) comprises pairs of column electrodes (C¹ j , C² j ) and each of said at least some of the interconnecting regions (A ij ) is disposed between one column electrode of a pair of column electrodes (C¹ j , C² j ) and one of the line electrodes (R i ), and in that each of said at least some of the interconnecting regions (A ij ) of the matrix structure (2) includes a first electrode (E₁) connected between the respective element (ECM) of electrochromic material and each column electrode of a pair of column electrodes (C¹ j , C² j ) by first and second one-way conductive elements (D₁, D₂) respectively, and a second electrode (E₂) connected between said element (ECM) of electrochromic material and a line electrode (R i ); the one-way conductive elements (D₁, D₂) being adapted to enable a current to flow in the electrochromic element (ECM) selectively from the first electrode (E₁) of the electrochromic element to the second (E₂) or from the second electrode (E₂) to the first (E₁). A neural net according to claim 1, characterized in that each electrochromic element (ECM) includes a layer of a metal oxide and in that the first electrode of the electrochromic element (ECM) comprises: a first portion (E₁) arranged in contact with the metaloxide layer and constituted by a complex including a solid-state polymeric electrolyte and an alkali metal salt, and a second portion (E″₁) connected to the column electrodes and including an alkali metal. A neural net according to Claim 2, characterized in that said metal oxide is WO₃. A neural net according to Claim 2 or 3, characterized in that said solid-state polymeric electrolyte is a polyethylene oxide and said alkali metal salt is a lithium salt. A neural net according to Claim 2,3 or 4, characterized in that the second portion (E″₁) of the first electrode (E₁) of the electrochromic element (ECM) is made of lithium. A neural net according to Claims 4 and 5, characterized in that the alkali metal salt is LiClO₄ or LiCF₃SO₃. A neural net according to any of the preceding claims, characterized in that each of the one-way conductive elements (D₁,D₂) includes a p-n semiconductor junction. A neural net according to any of the preceding claims, characterized in that the support substrate (1) is of glass. A neural net according to any of the preceding claims characterized in that the photosensitive elements are phototransistors (FM). A neural net according to Claim 9, characterized in that the phototransistors (FM) are MOSFETs.

Inventors: MINNAJA NICOLA, MINNAJA, NICOLA
Applicants: ENIRICERCHE SPA, ENIRICERCHE S.P.A.
Application year: 1990
Main classification: G06F15
IPC codes: G11C7, G06N3, G11C13