Electrically device with a doped amorphous silicon channel

Abstract: Described is an electrically plastic device comprising an amorphous silicon semiconductor layer including movable dopant formed between a pair of electrodes and; at least one gate electrode formed on said amorphous silicon semiconductor layer through an insulation layer or a high resistance layer; whereby the operation of said gate electrode controls the dopant distribution of said amorphous semiconductor layer, thereby varying the electrical conductivity thereof.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device for use in an information processing due to a neural network and more particularly to an electrical device which shows an electrically non-linear characteristic (conductivity or capacitance) upon receiving an input energy (size or time variation). 2. Description of the Prior Art An information processing due to a neural network has been already known to be an excellent method for treating the information processing in a similar way to that of the human brain. However, it has been very difficult or almost impossible for the prior art to obtain an electronic device provided with the ability the same as the synapse. This has been a big problem for the information processing in future. A neuron tip previously proposed is classified into two groups; a semiconductor device and an optical device. The semiconductor device is rather in a high potential for the practical realization of the neuron tip but is still in a difficult position to obtain a plastic function suitable for the neuron tip. The previous development for the neural network has been directed to a modification of bipolar or CMOS silicon semiconductor provided with an analog operation. On the other hand, attention has been increasingly paid to an electric field effect transistor (FET), a transconductance amplifier (OTA), a capacitor array, and a switched resistor array as possible electronic device provided with an ability of the synapse for giving the neural network a plastic function. Among those possible devices, much attention has been directed to the PET such as a floating gate for use in EPROM and MNOS which is characterized by a low electric energy and a high gain. An amorphous silicon has a number of irregular lattice defects and dangling bonds combined with hydrogen atoms in a different way from that of a single crystal silicon. Therefore, a molecular formula of the amorphous silicon is exactly expressed by SiH x . The amorphous silicon has another feature that it can be formed into a porous structure. Accordingly, the dopant in the amorphous silicon can move freely throughout the defective lattice and controls the electric conductivity in a wide range. The reversible movement of the dopant in the amorphous silicon is disclosed at a paper, R. Konenkamp, Solid State Communication, 73 (5), p.323 (1990). In addition, a conduction variable device using a conductive polymer is developed for a purpose to realize an electrical device which is disclosed in JP-A-63-200 396. In "Journal of Applied Physics", 67 (1990) 15 March, No. 6, New York, pages 3132 to 3135, it is described a tungsten-oxide-based, nonvolatile, electrically reprogrammable, variable resistance device as an analog synaptic memory connection for electronic neural networks. A voltage controlled, reversible injection of H + ions in electrochromic thin films of WO 3 is utilized to modulate its resistance. A hygroscopic thin film of Cr 2 O 3 is the source of H + ions. The resistance of this known device can be tailored and stabilized over a wide dynamic range, and the programming speed is modulated by the control voltage. However, the above-mentioned conventional electronic devices are not provided with a sufficiently satisfactory characteristic for the electrical device in view of the fact that the requirements for an electrical device have been increased in view of more efficient information processing as desired in the near future. SUMMARY OF THE INVENTION An object of the present invention is therefore to provide an electrical device which solves the above problem and particularly has a high gain and an excellent performance. In order to achieve the aforementioned object, according to the present invention, there is provided an electrical device as defined in Claim 1 or Claim 2. The device of the present invention has an increased amplification gain, a high impedance and a high operating speed and is easy to be used in neural networks. Further advantageous embodiments are defined in the sub-claims. It is desirable that the amorphous silicon semiconductor layer is superposed on a movable dopant supporting layer. Another preferable modification is that a dopant permeable separation layer is interposed between the amorphous silicon semiconductor layer and the movable dopant supporting layer.. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: Fig. 1 is a cross-sectional view of the electrical device according to the embodiment of the present invention. Fig. 2 is a cross-sectional view for showing the electrical device and its circuit having two terminals according to another embodiment of the present invention. Fig. 3 is a graph showing a relationship between a gate voltage and a drain current of the electrical device according to the present invention. Fig. 4 is an equivalent circuit illustrating the operation of the electrical device according to the present invention. Fig. 5 is an equivalent circuit of the modification of the electrical device according to the present invention. Fig. 6 is a graph showing a relationship between the gate voltage and the drain current of the electrical device according to the present invention. Fig. 7 is a block diagram of a neural network formed by using a plurality of the electrical devices according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 which is a cross-sectional view of the electrical device according to the embodiment of the present invention, an ion conductive substrate 4 of ceramic plate acts as a supporting layer for a movable dopant and has an amorphous silicon semiconductor layer 3, an insulating layer 6 and a gate electrode 5 formed thereon. A pair of electrodes 1 and 2 are formed on the ion conductive substrate 4 so as to come in contact with the insulating layer 6 and the amorphous silicon semiconductor layer 3. The insulating layer 6 can be substituted with a high-resistance layer. This structure is suitable for the case in which the movable dopant supporting layer is composed of an ion conductive substrate made of ceramic plate. Referring to Fig. 2 which shows an electrical device and its circuit having two terminals according to another embodiment of the present invention, an ion conductive substrate 7 has a movable dopant supporting layer 4 formed thereon. An amorphous silicon semiconductor layer 3 is formed on the movable dopant supporting layer 4 and has a pair of electrodes 1 and 2 formed at the terminals thereof. A gate electrode 5 is formed between the pair of electrodes 1 and 2 through an insulating layer 6. The resultant device can be operated in a high speed and in a high sensitivity when the electrodes 1 and 2 (source and drain) are formed between the amorphous silicon layer and the movable dopant supporting layer as shown in Fig. 1. A structure having the movable dopant supporting layer 4 integrated with the amorphous silicon semiconductor layer 3 is an essential part of the electrical device according to the present invention. A more useful structure having a threshold value for controlling of the movement of the dopant can be achieved by providing a dopant permeable separation layer between the amorphous silicon semiconductor layer and the movable dopant supporting layer in accordance with the present invention. The dopant permeable separation layer is electrically insulating and precisely controls exactly the movement of the dopant to stabilize the switching characteristics between the pair of electrodes (source and drain). A dopant permeable separation layer can be composed of a porous film having an ion permeable property and suitably composed of a separator material for use in a battery. Referring to Fig. 4 which is an equivalent circuit illustrating the operation of the electrical device according to the present invention, a cation dopant D + is movable and has a polarity reverse to that of the carrier of the amorphous silicon semiconductor to move depending on the voltage applied across the electrodes. The channel impedance markedly varies with the movement of the cation dopant, which is greatly affected by the electric charge and ion radius of the dopant and the structure and the density of the amorphous silicon semiconductor and so on. Fig. 4 shows a case in which the amorphous silicon semiconductor and the cation dopant are used for an easy illustration, but it should be noted that the present invention is not limited to only the case using the cation dopant. An insulating layer at the gate can be substituted with a high-resistance layer 8 as shown in Fig. 5 which is an equivalent circuit of the substituted device. The high-resistance layer 8 preferably has a high resistivity ranging from 10 5 to 10 12 ohm-cm. It is necessary for the as-doped amorphous silicon semiconductor layer 3 between the pair of electrodes to have a markedly higher conductivity than the movable dopant supporting layer 4 as shown clearly in the equivalent circuit shown in Fig.5. The device according to the present invention is characterized in that the amorphous silicon layer shows a large change in the conduction thereof with a magnitude ranging from (×10 4 ) to (×10 8 ) which locates between the source and the drain. In addition, since it is possible to apply an electric field with a high intensity to the gate in the thickness direction, the response speed of the device can be greatly improved. Fig. 3 is a graph showing, as an example, a controlling operation characteristic of the electrical device according to the present invention comprising an n-type amorphous silicon semiconductor and the cation dopant. It is noted that Fig. 3 shows a case in which the movement of the dopant is shorter than the pulse width, so that in response to the negative gate pulse the learning operation is carried out and in response to the positive gate pulse the reset operation is carried out. A neural network can be formed by using a plurality of these elements as shown in Fig. 7. In the neural network, the elements after undergoing the learning operation show a very low impedance. The neural network executes the learning or the reset in a time of msec order but can operate preferably at a high speed of micro sec order after the learning. The operation of this neural network can be expressed by an I D - V G characteristic as shown in Fig. 6, which is corresponding to a characteristic obtained by combining the enhancement form and the depression form of MOSFET. The amorphous silicon semiconductor layer 3 can formed by various methods such as evaporation, glow discharge sputtering, CVD, MBE and the like. The amorphous silicon semiconductor layer is different in the following point from the conductive polymer that the amorphous silicon semiconductor layer does not show any expansion and shrinkage during the doping and de-doping of the dopant. It is possible to change the electronic structure of the amorphous silicon semiconductor layer by chemically modifying the surface of the amorphous silicon semiconductor layer with a chemical or physical absorption or the chemical bonding. The amorphous silicon semiconductor layer can be changed in the resistivity by doping non-movable dopant and can form either an n-type or p-type semiconductor. The non-movable dopant interacts with the movable dopant to change the resistivity of the amorphous silicon semiconductor layer in an adjusting way. In addition, the interaction between the non-movable dopant and the movable dopant can be used for the threshold value control. In view of the impedance, the neural network is preferably formed from the amorphous silicon semiconductor layer having a high electric resistance. It is not suitable for the neural network to consist of the semiconductor having the non-movable dopant in a high concentration doped therein. The movable dopant supporting layer is a layer in which the dopant moves easily, and is preferably low conductivity. In general, the movable dopant supporting layer is consisting of an ion-conductive material such as glass, ceramic or organic material. The glasses or ceramics can be used both as an electric conduction substrate in addition to the movable dopant supporting layer. According to the present invention the movable dopant supporting layer comprises at least one element selected from alkali ion conductor based on β-aluminium oxide redox ion conductor containing Ag + or Cu + ion. Another suitable material is a porous ceramic such as zeolite. Further the movable dopant supporting layer can be formed from an anisotropic conduction film which is of a high insulating property in a parallel direction to the film and of a high conduction property in a perpendicular direction to the film. In the device according to the present invention, the amorphous silicon semiconductor layer produces the movement of the dopant due to the operation of the gate electrode, upon having the dopant injected therein. On the other hand, ions of the reverse polarity remained at the movable dopant supporting layer is subjected to the redox (reduction and oxidation) reaction. Therefore, the movable dopant supporting layer must be composed of a material to contain the movable dopant stable and reversible against the redox. It is desirable that the movable dopant supporting layer consists of ion-conductive glasses or ceramics forming a complex compound with the movable dopant. This structure provides the electrical device with the threshold value characteristic and the ensured memory characteristic. An ion inclusion compound can be used for this structure. The electronic redox usually occurs with the movement of the movable dopant at the operation of the gate electrode but can be made reversible and stable with this structure of the device according to the present invention. An electrical device according to the present invention is not limited to the structure shown in Figs. 1 and 2 and can be formed into a structure having a substrate of a silicon single crystal which has an insulating film of SiO 2 formed thereon. An use of a plurality of the electrical devices according to the present invention makes it possible to form an integrated circuit in a similar way to the fabrication process of semiconductor integrated circuits and to accomplish neuron tips. The following description is directed to an embodiment of the present invention. Example Referring to Fig. 2, an ion conductive ceramic layer 4 in a thickness of 1mm having a Na-β-aluminum oxide layer in a thickness of 3µm and an amorphous silicon semiconductor layer 3 in a thickness of 1 µm sequentially are formed on a substrate 7. A pair of electrodes 1 and 2 are formed on the amorphous silicon semiconductor layer 3. Then, a gate electrode 5 is formed on the amorphous silicon semiconductor layer 3 through an insulating layer 6 as shown in Fig. 2 so as to complete an electrical device. The gate and the source of the electrical device are made short circuited so that a negative gate pulse of 10 V is applied for the purpose of the learning and the positive gate pulse of 10 V is applied for the purpose of the reset. A channel impedance shows a great variation of 10 6 ohm between the operations of the negative and positive gate pulses. A network shown in Fig. 7 is formed from a plurality of the electrical devices. Then, by repeating the learning, a low impedance can be obtained with the devices positioned on a line of the shortest distance between the output and input terminals. Accordingly, the network can study a relation between the output signal and the input signal by the variation in the electric resistance.

Claims: An electrical device with reversible doping of an active layer, comprising an active layer (3) formed between a pair of electrodes (1, 2) and including movable dopant to impart electrical conductivity; a movable dopant supporting layer (4) adjacent to said active layer (3), said movable dopant coming from said movable dopant supporting layer (4); and at least one gate electrode (5) formed on said active layer (3) through an insulating layer (6) ; whereby the operation of said gate electrode (5) controls the dopant distribution of said active layer (3), thereby varying the electrical conductivity thereof; characterized in that the device has a MOS-FET structure, wherein said active layer is made of an amorphous silicon semiconductor, and said movable dopant supporting layer (4) comprises at least one material selected from the group consisting of an alkali ion conductor based on β-aluminum oxide, a redox ion conductor including Ag + or Cu + , and a porous ceramic, and wherein said amorphous silicon semiconductor layer is superposed on said movable dopant supporting layer (4). An electrical device having a FET-structure with reversible doping of an active layer, comprising an active layer (3) formed between a pair of electrodes (1, 2) and including movable dopant to impart electrical conductivity; a movable dopant supporting layer (4) adjacent to said active layer (3), said movable dopant coming from said movable dopant supporting layer (4); and at least one gate electrode (5) formed on said active layer (3) through a high resistance layer (8) whereby the operation of said gate electrode (5) controls the dopant distribution of said active layer (3), thereby varying the electrical conductivity thereof; wherein said active layer is made of an amorphous silicon semiconductor, and said movable dopant supporting layer (4) comprises at least one material selected from the group consisting of an alkali ion conductor based on β-aluminum oxide, a redox ion conductor including Ag + or Cu + , and a porous ceramic, and wherein said amorphous silicon semiconductor layer is superposed on said movable dopant supporting layer (4). An electrical device according to claim 1, wherein said pair of electrodes (1, 2) and amorphous silicon semiconductor layer (3) are formed on a substrate (4) composed of an ion-conductive glass or ceramic plate acting as a movable dopant supporting layer. An electrical device according to claim 1, wherein a dopant permeable separation layer being permeable for said movable dopants which come from said movable dopant supporting layer (4) is interposed between said amorphous silicon semiconductor layer (3) and said movable dopant supporting layer (4).

Inventors: KISHIMOTO YOSHIO, SUZUKI MASAAKI, KISHIMOTO, YOSHIO, SUZUKI, MASAAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H01L29
IPC codes: G06N3, G11C11, H01L29, G11C16