Abstract:
A synapse configured of an A-MOS transistor has a learning function and can implement high integration similar to that of a DRAM because of its simplified circuit configuration and compact circuit size. With the presently cutting-edge technology (0.15 μm CMOS), approximately 1G synapses can be integrated on one chip. Accordingly, it is possible to implement a neural network with approximately 30,000 neurons all coupled together on one chip. This corresponds to a network scale capable of associatively storing approximately 5,000 patterns.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a neural network associative memory LSI (Large Scale Integrated circuit). More particularly, the present invention relates to a synapse element with a learning function which can be highly integrated, and to a semiconductor integrated circuit device including the synapse element. 
   2. Description of the Background Art 
   Recently, research and development have been vigorously carried out on a brain-type computer based on the manner of processing information by the brain. Intuitive information processing such as pattern recognition, context association, combinatorial optimization or the like, which is easily carried out by the brain-type computer, is a technique indispensable for smooth communication between information processing machines and human beings. Such a technique is expected to make a breakthrough for the machines to be fitted into and utilized in the society without sense of discomfort. For the practical utilization of the brain-type computer, development of the dedicated hardware is indispensable. In particular, development of the neural network associative memory LSI as a main component of the brain-type computer is strongly required. For the practical utilization of the neural network associative memory LSI, the ultimate task is to realize the high integration of the synapse circuits with the learning functions required to store the associative patterns fast and freely. In the neural network forming the associative memory, a great number of synapses, approximately proportional to the square of the number of the neurons, are required. Accordingly, high integration of the synapses is most effective for the high integration of the associative memories. In addition, in order for the associative memory to be able to store the associative patterns fast and freely, the synapse needs to have the learning function. 
   As the prior art related to the associative memory neural network LSI with the learning function, a brief description will be made about the techniques disclosed in Japanese Patent Laying-Open No. 03-80379, and U.S. Pat. Nos. 5,148,514, and 5,293,457 (inventor: Yutaka Arima et al.).  FIGS. 7A and 7B  show exemplary block diagrams of the conventional neural network LSI. In each of  FIGS. 7A and 7B , a line of neuron circuits is disposed in each of 4 sides of a chip, and the synapses are disposed and arranged, in a matrix, in almost all the other central regions of the chip.  FIG. 7A  illustrates two self-connected neural networks, while  FIG. 7B  shows one interconnected neural network. These arrangements and the interconnection interconnecting between a neuron circuit and a synapse circuit as illustrated in  FIGS. 8A and 8B  enable the neural network for the associative memory to be configured effectively. 
     FIG. 9  shows an example of the synapse circuit with the learning function. In a synapse circuit  100  illustrated in  FIG. 9 , a synapse load value (Wij) is represented by the amount of charges accumulated at a capacitor C 1 . The amount of charges accumulated at capacitor C 1  is corrected in accordance with a learning law (ΔWij=±ηSiSj, where η is a learning coefficient, and is updated in accordance with the pulse number applied to ACP+ and ACP−) by a load correction circuit  101  made from a charge pump circuit, and a learning control circuit  102  applying a correction signal thereto. Si and Sj respectively correspond to the output signals of the neurons i and j applying signals to this synapse. In this prior art example, since a symmetrical synapse coupling (Wij=Wji) is expected, two synapse coupling operational circuits  103  are mounted to synapse circuit  100  producing one synapse load value. 
     FIG. 10  shows an example of the neuron circuit. In a neuron circuit  110  illustrated in  FIG. 10 , currents of the output signals from the synapse are added together at a common node  111  (Kirchhoff adder) to convert the signal to a voltage, and the voltage is compared with a threshold (Vref) of the neuron at a comparator  112 . Two selectors  113  and  114  selectively output either the output of comparator  112  or an educator data SR (T) in a register  116 , in accordance with an attribute data SR (P) in a register  115  within the present neuron circuit and a learning control signal IselS. 
   In accordance with the above-described prior art, synapse circuits with the learning functions can be packaged in a relatively high integration. In fact, it is reported that the integration of 80,000 synapses and 400 neurons on one chip has been successfully achieved by using a 0.8 μm CMOS technology (Y. Arima, et al. “A Refreshable Analog VLSI Neural Network Chip with 400 Neurons and 40 K Synapses,” IEEE, Journal of Solid-State Circuits, vol. 27, No. 12, pp. 1854–1861, December, 1992.). Furthermore, by using this prior art technology together with the currently cutting-edge technology, which is a 0.15 μm-CMOS (Complementary Metal-Oxide Semiconductor), approximately 2 million synapses and approximately 2,000 neurons can be integrated on one chip. In this case, associative storage of approximately 300 patterns is possible. For the practical utilization, however, the storage capacity is not sufficient. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a synapse element having a learning function and a simplified circuit configuration suitable for high integration required for a neuron associative memory, and a semiconductor integrated circuit device including the synapse element. 
   A synapse element in accordance with the present invention implements a function of a synapse composing a neuron of an element of a neural network, and includes an MOS transistor connected between an internal node and a first voltage and capable of modulating a gain coefficient in analog manner in accordance with a potential of a control gate. The MOS transistor receives at its gate an output signal of another neuron, the potential of the control gate corresponds to a synapse load value, and a source-drain current of the MOS transistor represents an output signal of the synapse element. 
   Preferably, the amount of charges accumulated in the control gate is adjusted by injection of either hot carriers or hot holes. 
   A semiconductor integrated circuit device in accordance with the present invention includes a synapse element implementing a function of a synapse composing a neuron of an element of a neural network, and the synapse element includes a first MOS transistor electrically coupled between an internal node and a first voltage and capable of modulating a gain coefficient in analog manner in accordance with a potential of a control gate. The first MOS transistor receives at its gate an output signal of another neuron, and the potential of the control gate corresponds to a synapse load value, and a source-drain current of the first MOS transistor represents an output signal of the synapse element. 
   Preferably, the semiconductor integrated circuit device includes a plurality of synapse elements connected in parallel between a common internal node and a first voltage, and further includes a capacitative element connected between the control gate of each of the plurality of synapse elements and a voltage node common to the plurality of synapse elements. 
   More preferably, the semiconductor integrated circuit device further includes a load value correction portion provided corresponding to the internal node, and the load value correction portion adjusts the amount of charges accumulated in the control gate of the first MOS transistor connected to the corresponding internal node by injection of either hot carriers or hot holes. 
   Even more preferably, the semiconductor integrated circuit device further includes a comparator producing an output signal of the neuron composed of the plurality of synapse elements based on the comparison between a voltage of the internal node and the prescribed threshold voltage, a delay circuit delaying the output of the comparator for a prescribed period of time before output, a second MOS transistor receiving the output of the delay circuit at its gate and electrically coupled between the internal node and a second voltage, and a third MOS transistor electrically coupled between the second voltage and the internal node and having the gate connected to the internal node. 
   Thus, a great advantage of the present invention is that it provides a synapse element with a learning function, capable of realizing high integration of a DRAM (Dynamic Random Access Memory), because of its simplified circuit configuration and compact circuit size. 
   With the presently cutting-edge technology (0.15 μm-CMOS), approximately 1 G (1×10 9 ) synapse elements of the present invention can be integrated on one chip. Accordingly, it is possible to implement a full-connected neural network with approximately 30,000 on one chip of the semiconductor integrated circuit device. This corresponds to a network scale capable of associatively storing approximately 5,000 patterns. Therefore, this can contribute to the realization of the practical utilization of the associative memory LSI being capable of storing thousands of patterns or more, which has been difficult to achieve with the conventional technique. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an example block of a basic circuit configuration diagram of an element utilizing an A-MOS in a synapse circuit of the present invention. 
       FIG. 2  shows an example of a circuit configuration utilizing the AMOS in the synapse circuit of the present invention. 
       FIG. 3  is a diagram representing an example of the behavior of the various types of signals in  FIG. 2 . 
       FIG. 4  is a diagram representing an example of the states of the various types of signals in learning. 
       FIG. 5  is a graph showing an example of hot electron injection characteristic. 
       FIGS. 6A and 6B  are circuit diagrams representing the operations of correcting synapse load values in learning. 
       FIGS. 7A and 7B  are block diagrams showing the configurations of the conventional neural network LSI. 
       FIGS. 8A and 8B  are schematic diagrams representing the relations of the interconnections of a neuron circuit and a synapse circuit respectively. 
       FIG. 9  is a diagram showing a configuration of the conventional synapse circuit. 
       FIG. 10  is a diagram showing a configuration of the conventional neuron circuit. 
       FIG. 11  shows an example of a diagram representing a configuration of an A-MOS device. 
       FIG. 12  shows an example of the configuration parameter of the A-MOS device. 
       FIGS. 13A and 13B  show examples of β modulation of the A-MOS. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1 , a synapse circuit  10  in accordance with the present invention is configured of an A-MOS (Adjustable β-MOS) transistor  11  provided between a common node Nm and a ground voltage GND. As will be described in detail in the following, the present invention is characterized in that the A-MOS device is applied to the synapse circuit composing a neural network. 
   The configuration and characteristic of the A-MOS transistor will simply be described hereinafter. Note that the A-MOS transistor is specifically disclosed in pamphlet of International Publication No. WO02/059979A1. 
   With reference to  FIG. 11 , the A-MOS transistor has source  21 , drain  22  and gate  23  (hereinafter also referred to as a “normal MOS gate”), as in a typical MOS transistor, and in addition thereto the A-MOS transistor further has a control gate  25 . Source  21 , drain  22 , normal MOS gate  23  and control gate  25  are each provided with a contact, as appropriate. In the A-MOS transistor, a gain coefficient β is modulated in analog manner in accordance with a voltage applied to control gate  25 . 
   In the A-MOS transistor, a gain coefficient β modulation characteristic is determined by device geometry parameters shown in  FIG. 12 , more specifically, gate length L and width W of normal MOS gate  23  and an angle θ formed by normal MOS gate  23  and control gate  25 . In other words, by designing these parameters, a ratio between a minimal β and a maximum β can be set relatively freely. 
     FIGS. 13A and 13B  show a principle of β modulation in the A-MOS transistor.  FIG. 13A  shows that a conductance of a channel of control gate  25  is set to be equivalent to that of normal MOS gate  23 . In the figure, the hatched portion indicates an effective gate region. 
   By contrast,  FIG. 13B  shows that the conductance of the channel of control gate  25  is set to be sufficiently larger than that of normal MOS gate  23 , with the hatched portion indicating an effective gate region. 
   Thus in the A-MOS transistor a voltage applied to control gate  25  can be changed to control a direction of an electric field in a channel region to modulate effective gate length L# and width W# in analog manner. Consequently, in accordance with the control gate voltage the gain coefficient β can be modulated in analog manner. 
   In the A-MOS transistor, a source-drain current Isd is expressed by the following equation (1) with a normal gate voltage Vg and a control gate voltage Vcg.
 
Isd∝Vg 2 ×Vcg 1˜2   (1)
 
   Thus, by providing a load with a diode characteristic to the drain side of the A-MOS transistor, an approximation of the product of normal gate voltage Vg and control gate voltage Vcg can be output. In addition, by connecting the drains of a plurality of A-MOS transistors with a common node to add together the source-drain currents of these A-MOS transistors, a product-sum operation can be expressed. 
   Referring back to  FIG. 1 , an output signal Sj from another neuron is connected to normal gate  23  of A-MOS transistor  11 , and control gate  25  is floated. A synapse load value Wij is represented by the accumulated charges of control gate  25 . As a result, source-drain current Isd flowing through A-MOS transistor  11  changes in accordance with the product of output signal Sj of a neuron and synapse load value Wij. 
   As shown in  FIG. 2 , control gate  25  of A-MOS transistor  11  is floated, and is capacitively coupled to a node supplying a learning control voltage Vc with a capacitor  30  posed therebetween. Furthermore, the neural network may have a circuit configuration in which drain nodes  22  of a plurality of A-MOS transistors  11  are connected together through a common node Nm to make common node Nm serve as an axon signal line so that the sum of their currents is regarded as an input signal to the neuron. 
   A threshold voltage Vref and a voltage produced at a p-MOS transistor  42  connected to common node Nm are applied to a comparator  40 , shown in  FIG. 2 , which determines and outputs the output signal of the neuron. In addition, the drain of p-MOS transistor  44  is connected to common node Nm. Because of the configuration in which an inverted delay signal (with a delay time td produced by delay circuit  50 ) of neuron signal Si is applied to the gate of p-MOS transistor  44 , the neuron signal is turned ON, after a certain period of delay time, p-MOS transistor  44  is turned ON, and the potential of common node Nm increases. Consequently, the output of the neuron is forced to be OFF. 
   After a certain period of delay time subsequent thereto, p-MOS transistor  44  is turned OFF. After a time period corresponding to the drivability of synapse circuit  10  of the present invention (which may be referred to as “AMOS synapse” hereinafter) configured of an A-MOS and connected to common node Nm, an output of a neuron i is again made ON, and the series of operations are repeated. In other words, frequency of pulse generation of an output signal of a neuron is modulated in accordance with the total sum of currents flowing through common node Nm. 
     FIG. 3  shows an example of the behavior of the various types of signals related to the signal output of the neuron. A pulse signal period T of output signal Si of the neuron is given by the following equation (2):
   T= 2 td+tr+tw   (2) 
where tr corresponds to a delay time produced when p-MOS transistor  44  transmits a signal to common node Nm, and tw corresponds to a delay time produced when the entire synapses transmit signals to common node Nm. Thus, as the total signals from the synapses increase (as the currents increase), tw becomes smaller and pulse signal period T also becomes shorter. Accordingly, frequency of generation of the output pulse signal of the neuron increases as the total signals from the synapses increase.
 
     FIG. 4  shows the states of various types of signals during neural network learning period for storing associative patterns, i.e. when a synapse load value is corrected. A correction to a synapse load value corresponds to a correction to the amount of charges accumulated in control gate  25  of each A-MOS synapse  10 . In order to correct the amount of charges accumulated in control gate  25 , learning control voltage Vc and a power-supply voltage for P-MOS transistor  44  and delay circuit  50  are controlled as appropriate so that the amount of charges accumulated in the control gate can be corrected in accordance with the learning law (ΔWij=±ηSiSj). 
   In learning, the power-supply voltage is set at VdH sufficiently higher than Vd. This enables injection of hot carriers near the drain of the A-MOS transistor. At this time, threshold voltage of comparator  40  is also changed to VrefM in accordance with learning control voltage Vc. When Hebb learning law (ΔWij=±ηSiSj) is to be implemented, learning control voltage Vc is driven to “Low” (˜GND). This shifts the potential of control gate  25  to a lower value. As is apparent from the exemplary characteristic of hot carrier injection in  FIG. 5 , this makes injection of holes by avalanche predominant, and the synapse load value increases only when both Si and Sj are ON. 
   Meanwhile, when anti-Hebb learning law (ΔWij=−ηSiSj) is to be implemented, control voltage Vc is driven to “High” (˜VdH) to shift the potential of the control gate to a higher value so that injection of channel hot electrons becomes predominant and the synapse load value can be decreased only when both Si and Sj are ON. 
     FIGS. 6A and 6B  show the states of the potentials at synapse elements when the learning laws are implemented.  FIG. 6A  shows a state in which a synapse load value is corrected to a value with the negative sign (ΔWij=−ηSi×Sj) by injection of hot electrons, while  FIG. 6B  represents a state in which a synapse load value is corrected to a value with the positive sign (ΔWij=+ηSi×Sj) by injection of hot holes. 
   As described above, the present invention is characterized in that the A-MOS transistor is utilized for the synapse circuit composing the neural network. In the circuit configuration in accordance with the present invention, the synapse load value is represented by the potential of the A-MOS control gate. The β variable characteristic of the A-MOS transistor implements the product of the synapse load value (the control gate voltage) and the neuron signal (the input gate voltage). The A-MOS control gate is floated, and the adjustment of the learning control voltage capacitively coupled thereto enables the correction of the synapse load value by injection of hot electrons to attain the learning function. 
   Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.