Patent Publication Number: US-2021190722-A1

Title: Chemical sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a chemical sensor. 
     BACKGROUND ART 
     Ion-sensitive transistors have been known as chemical sensors that are configured to detect ions in a solution (see, for example, PTL 1). Such transistors can be used to detect pH of the solution. However, such pH sensors have a problem of low detection sensitivity. 
     Also, accumulative chemical/physical phenomenon detection devices have been known (see, for example, PTL 2). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2008-215974 
     PTL 2: Japanese Unexamined Patent Application Publication No. 2002-098667 
     SUMMARY OF INVENTION 
     Technical Problem 
     The traditional chemical sensor has a transistor and others formed on a silicon substrate. The silicon substrate, however, has a problem of a high production cost of the chemical sensor and a problem of difficulty in attaching the chemical sensor to a curved surface or the skin. 
     The present invention was devised in view of such circumstances, and provides a chemical sensor that can be manufactured at low cost and has high detection sensitivity. 
     Solution to Problem 
     The present invention provides a chemical sensor characterized by comprising: a substrate; a semiconductor thin film provided on the substrate and having a first contact region and a second contact region; an injection electrode in contact with the first contact region; a first MIS structure including a part of the semiconductor thin film and a first gate electrode; a second MIS structure including a part of the semiconductor thin film and a second gate electrode; a transfer electrode in contact with the second contact region; and a capacitor electrically connected to the transfer electrode, wherein the semiconductor thin film has a sensing region provided so that an electric potential thereof changes in direct or indirect response to an object to be measured; the injection electrode is configured to inject an electrical charge into the first contact region; the first MIS structure is configured to control a flow of the electrical charge to the sensing region, the electrical charge being injected into the first contact region by the injection electrode; the second MIS structure is configured to control the flow of the electrical charge from the sensing region to the second contact region; and the transfer electrode is configured to allow the electrical charge in the sensing region to flow to the capacitor through the second contact region. 
     Advantageous Effects of Invention 
     The chemical sensor of the present invention has the substrate and the semiconductor thin film disposed on the substrate. This makes it possible to use a flexible substrate as the substrate and to attach the flexible substrate to a curved surface or the skin. Also, using the semiconductor thin film, a production cost of the chemical sensor can be reduced. 
     The semiconductor thin film has the sensing region disposed so that the electric potential thereof changes in direct or indirect response to the object to be measured. The chemical sensor of the present invention also has the first MIS structure and the second MIS structure, the first MIS structure being configured to control the flow of the electrical charge to the sensing region, which was injected into the semiconductor thin film by the injection electrode, and the second MIS structure being configured to control the flow of the electrical charge in the sensing region to the transfer electrode. The first MIS structure configured to control the flow of the electrical charge into the sensing region allows for the injection of the electrical charge from the injection electrode into the sensing region in an amount corresponding to the electric potential of the sensing region that has responded to the object to be measured, and also allows for the storage of the injected electrical charge in the sensing region. The amount of the electrical charge stored in the sensing region is comparable to an amount of an object to be detected. The second MIS structure configured to control the flow of the electrical charge from the sensing region to the transfer electrode allows for the transfer of the electrical charge stored in the sensing region to the capacitor through the transfer electrode. By reading the electrical charge (quantity of electricity) stored in the capacitor as a signal voltage, the object to be detected (for example, pH of the solution) can be detected. Furthermore, by repeating the storage of the electrical charge in the sensing region and the transfer of the electrical charge to the capacitor multiple times, the electrical charges stored in the sensing region can be accumulated in the capacitor, thus increasing the quantity of electrons in the capacitor. This makes it possible to read out the amplified signal voltage, thus increasing detection sensitivity of the chemical sensor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagrammatic top view of a chemical sensor in accordance with an Embodiment of the present invention. 
         FIG. 2( a )  is a diagrammatic cross-section view of the chemical sensor taken along the dashed line A-A of  FIG. 1 ; and  FIG. 2( b )  is a diagrammatic cross-section view of the chemical sensor taken along the dashed line B-B of  FIG. 1 . 
         FIG. 3  is a diagrammatic view of an electric circuit of a chemical sensor in accordance with an Embodiment of the present invention. 
         FIG. 4  is a diagrammatic cross-section view of a chemical sensor in accordance with an Embodiment of the present invention. 
         FIG. 5  is a diagrammatic cross-section view of a chemical sensor in accordance with an Embodiment of the present invention. 
         FIG. 6  is a diagrammatic cross-section view of a chemical sensor in accordance with an Embodiment of the present invention. 
         FIG. 7  is an explanatory drawing of a charge transfer in a chemical sensor in accordance with an Embodiment of the present invention. 
         FIG. 8  shows a photograph and an explanatory drawing of a pH sensor prepared. 
         FIGS. 9( a ) to 9( c )  each show a voltage sequence of one measurement cycle. 
         FIG. 10  shows a graph of results of pH detection experiments. 
         FIG. 11  shows a graph of results of pH monitoring experiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A chemical sensor according to the present invention comprises: a substrate; a semiconductor thin film provided on the substrate and having a first contact region and a second contact region; an injection electrode in contact with the first contact region; a first MIS structure including a part of the semiconductor thin film and a first gate electrode; a second MIS structure including a part of the semiconductor thin film and a second gate electrode; a transfer electrode in contact with the second contact region; and a capacitor electrically connected to the transfer electrode, wherein the semiconductor thin film has a sensing region provided so that an electric potential thereof changes in direct or indirect response to an object to be measured; the injection electrode is configured to inject an electrical charge into the first contact region; the first MIS structure is configured to control a flow of the electrical charge to the sensing region, the electrical charge being injected into the first contact region by the injection electrode; the second MIS structure is configured to control the flow of the electrical charge from the sensing region to the second contact region; and the transfer electrode is configured to allow the electrical charge in the sensing region to flow to the capacitor through the second contact region. 
     The MIS structure is formed of a three-layer structure including a metal layer, an insulator layer, and a semiconductor layer. 
     It is desirable that the substrate should be a flexible substrate. This makes it possible to attach the chemical sensor of the present invention to a curved surface or the skin. 
     It is desirable that the semiconductor thin film should have a thickness that is the same as or thicker than a thickness of a monoatomic layer but is 200 nm or less. This makes it possible to reduce a cost of production of the chemical sensor. Also, this allows the chemical sensor of the present invention to adhere to the curved surface or the skin. 
     It is desirable that the chemical sensor of the present invention should comprise an extension gate electrode and a third MIS structure. It is desirable that the extension gate electrode should comprise a gate portion and a sensing section that electrically interacts directly or indirectly with the object to be measured. It is desirable that the third MIS structure should include the sensing region of the semiconductor thin film and the gate portion. Such a structure as above is capable of arranging the sensing section, which interacts electrically with the object to be measured, separately from a charge transfer section comprising the MIS structures, thereby preventing the object to be measured adversely affecting the charge transfer section. 
     It is desirable that the chemical sensor of the present invention should be provided with a reference electrode. It is desirable that the reference electrode should be placed around the sensing section of the extension gate electrode. This placement of the reference electrode enables the reference electrode to come in contact with an aqueous solution as the object to be measured, thereby giving a reference point to the electric potential of the aqueous solution. 
     It is desirable that the chemical sensor of the present invention should comprise a fourth MIS structure. It is desirable that the fourth MIS structure should include the first contact region of the semiconductor thin film and a third gate electrode. Applying a gate voltage to this third gate electrode can change an electric potential in the first contact region, leading to a decrease in a Schottky barrier formed between the injection electrode and the first contact region. 
     It is desirable that the chemical sensor of the present invention should comprise a fifth MIS structure. It is desirable that the fifth MIS structure should include the second contact region of the semiconductor thin film and a fourth gate electrode. It is desirable that the transfer electrode should be in contact with the second contact region of the semiconductor thin film. Applying a gate voltage to this fourth gate electrode can change an electric potential in the second contact region, thereby enabling a Schottky barrier formed between the transfer electrode and the second contact region to decrease. 
     Hereinafter, an Embodiment of the present invention will be described with reference to the accompanying drawings. Structures shown in the drawings or described below should be recognized as exemplifications in all respects, and the scope of the present invention is not limited to the drawings and the following descriptions. 
       FIG. 1  is a diagrammatic top view of a chemical sensor in accordance with the present Embodiment.  FIG. 2( a )  is a diagrammatic cross-section view of the chemical sensor taken along the dashed line A-A of  FIG. 1 ; and  FIG. 2( b )  is a diagrammatic cross-section view of the chemical sensor taken along the dashed line B-B of  FIG. 1 .  FIG. 3  is a diagrammatic view of an electric circuit of a chemical sensor in accordance with the present Embodiment.  FIG. 4  to  FIG. 6  show diagrammatic cross-section views of chemical sensors in accordance with the present Embodiments, respectively. 
     A chemical sensor  50  in accordance with the present Embodiment is characterized by comprising: a substrate  1 ; a semiconductor thin film  2  provided on the substrate  1  and having a first contact region  13  and a second contact region  17 ; an injection electrode  3  in contact with the first contact region  13 ; an MIS structure  20   b  including a part of the semiconductor thin film  2  and a first gate electrode  4 ; an MIS structure  20   d  including a part of the semiconductor thin film  2  and a second gate electrode  5 ; a transfer electrode  6  in contact with the second contact region  17 ; and a capacitor  7  electrically connected to the transfer electrode  6 , wherein the semiconductor thin film  2  has a sensing region  15  provided so that an electric potential thereof changes in direct or indirect response to an object  32  to be measured; the injection electrode  3  is configured to inject an electrical charge into the first contact region  13 ; the MIS structure  20   b  is configured to control a flow of the electrical charge to the sensing region  15 , the electrical charge being injected into the first contact region  13  by the injection electrode  3 ; the MIS structure  20   d  is configured to control the flow of the electrical charge from the sensing region  15  to the second contact region  17 ; and the transfer electrode  6  is configured to allow the electrical charge in the sensing region  15  to flow to the capacitor  7  through the second contact region  17 . 
     The chemical sensor  50  in accordance with the present Embodiment can comprise at least one of a source follower circuit  37 , a reference electrode  30 , a reset gate electrode  28 , and a reset electrode  29 . 
     A charge transfer section  35  includes the semiconductor thin film  2 , the injection electrode  3 , the first gate electrode  4 , the second gate electrode  5 , the transfer electrode  6 , a gate portion  9  of an extension gate electrode  8 , a third gate electrode  22 , a fourth gate electrode  24 , the reset gate electrode  28 , and the reset electrode  29 . 
     Hereinafter, the chemical sensor  50  in accordance with the present Embodiment will be described. 
     The chemical sensor  50  is a sensor configured to detect chemical substances. Specific examples of the sensor include an ion sensor (including a pH sensor), which is configured to detect ions in a solution, and a wearable sensor, which is configured to detect chemical substances contained in sweat secreted from sweat glands (including pH of the sweat) or chemical substances released from the skin. The chemical sensor  50  may be a flexible sensor. 
     The substrate  1  is made of a plate or a sheet that functions as a basic component of the chemical sensor  50 . The substrate  1  may be a flexible substrate. This makes the chemical sensor  50  be the flexible sensor, and makes it possible to stick the chemical sensor  50  on a curved surface, the skin, and so forth. 
     The substrate  1  may be formed of a laminate structure. For example, the substrate  1  of the chemical sensor  50  shown in  FIGS. 2 and 4 to 6  comprises a substrate  1   a  and a substrate  1   b . The substrate  1   a  is made of, for example, a polyimide sheet; and the substrate  1   b  is made of a PET (polyethylene terephthalate) sheet. Due to its heat resistance, the polyimide sheet can be used as the basic component of the chemical sensor  50  at a time of preparing the chemical sensor  50 . The PET sheet functions as a support sheet of the polyimide sheet. 
     The semiconductor thin film  2  is a thin film of a semiconductor to be disposed on the substrate  1 . There may be an insulator layer  26  and the gate electrodes placed between the substrate  1  and the semiconductor thin film  2 . The semiconductor thin film  2  has a thickness that is, for example, the same as or thicker than a thickness of a monoatomic layer but is 200 nm or less. The semiconductor thin film  2  may be an n-type semiconductor or may be a p-type semiconductor. The semiconductor thin film  2  is made of, for example, an oxide semiconductor thin film, a silicon thin film, a carbon nanotube thin film, or an organic semiconductor thin film. As examples of a material for the oxide semiconductor thin film, there may be mentioned IGZO(In—Ga—Zn—O), ITZO(In—Sn—Zn—O), and IGO(In—Ga—O). 
     The semiconductor thin film  2  may have the first contact region  13 , a first control region  14 , the sensing region  15 , a second control region  16 , and the second contact region  17 . 
     The injection electrode  3  is disposed in such a way as to be in contact with the first contact region  13  of the semiconductor thin film  2 . The injection electrode  3  is disposed in such a way as to inject an electrical charge (electrons) into the first contact region  13  of the semiconductor thin film  2 . The injection electrode  3  may be disposed under the semiconductor thin film  2  or on the semiconductor thin film  2 . To the injection electrode  3 , a voltage V input  is applied in such a way as to inject the electrical charge into the first contact region  13 . 
     The injection electrode  3  may be made of a metal monolayer film or a metal laminated film. The injection electrode  3  may have, for example, an Au layer (which is in contact with the semiconductor thin film). 
     The third gate electrode  22  may be disposed in such a way that the first contact region  13  of the semiconductor thin film  2  is placed between the injection electrode  3  and the third gate electrode  22 . The third gate electrode  22  together with the first contact region  13  of the semiconductor thin film  2  and a gate insulating film may be disposed so as to configure an MIS structure  20   a  (which is a three-layer structure including a metal layer, an insulator layer, and a semiconductor layer). When a gate voltage is applied to the third gate electrode  22  having the structure described above, an electric potential in the first contact region  13  is changed. For example, even when a Schottky barrier is formed at an interface between the injection electrode  3  and the first contact region  13 , a height of the Schottky barrier can be lowered by applying the gate voltage to the third gate electrode  22 , with the result that the electrical charge is more easily injected into the first contact region  13  from the injection electrode  3 . The third gate electrode  22  may be omitted. 
     In  FIG. 2( b )  and  FIG. 4  to  FIG. 6 , for example, an insulator layer  26   b  becomes a gate insulating film of the MIS structure  20   a . In  FIG. 2  and  FIG. 4  to  FIG. 6 , the third gate electrode  22  is positioned on the lower side of the semiconductor thin film  2 , and the injection electrode  3  is positioned on the upper side of the semiconductor thin film  2 ; however, the third gate electrode  22  may be positioned on the upper side of the semiconductor thin film  2 , and the injection electrode  3  may be positioned on the lower side of the semiconductor thin film  2 . 
     The gate electrode (the first gate electrode  4 , the second gate electrode  5 , the third gate electrode  22 , the fourth gate electrode  24 , the reset gate electrode  28 , or the gate portion  9  of the extension gate electrode  8 ) may be made of a metal monolayer film or a metal laminated film made from a plurality of metals. The gate electrode may be, for example, an Al electrode. 
     The insulator layer  26  (insulator layer  26   a ,  26   b ,  26   c , or  26   d ) is a layer made of an insulator. A material for the insulator layer  26  may be an inorganic insulator or an organic insulator. As examples of the material for the insulator layer  26   a ,  26   b , or  26   d  to function as a gate insulating film, there may be mentioned SiO 2 , Al 2 O 3 , and Si 3 N 4 . The insulator layer  26   a ,  26   b , or  26   d  may be made of a monolayer film or a laminated film comprising a plurality of insulators. The material for the insulator layer  26   c  that covers upper parts of the MIS structures may be an organic insulator such as a polyimide. 
     As shown in  FIG. 2( b ) , the insulator layers  26  to function as the gate insulating films of the plurality of MIS structures (each of which having a three-layer structure including a metal layer, an insulator layer, and a semiconductor layer) may have a two-layer structure including the insulator layer  26   b , which comes in contact with the semiconductor thin film  2 , and the insulator layer  26   a , which comes in contact with the insulator layer  26   b . Some of the plurality of gate electrodes may be placed between the insulator layer  26   a  and the insulator layer  26   b , and the other gate electrodes may be placed on a side of insulator layer  26   a  opposite to the insulator layer  26   b . One of the two adjacent gate electrodes may be placed between the insulator layer  26   a  and the insulator layer  26   b , and the other one may be placed on a side of insulator layer  26   a  opposite to the insulator layer  26   b . This makes it possible to place the insulator layer  26   a  between the two adjacent gate electrodes, thereby inhibiting leak current from flowing between the gate electrodes. The two adjacent gate electrodes may be arranged in such a way that one end of the gate electrode may overlap with one end of the other gate electrode. This makes it possible to prevent any interspace (gap) between the regions (the first contact region  13 , the first control region  14 , the sensing region  15 , the second control region  16 , or the second contact region  17 ) of the semiconductor thin film  2 . 
     In  FIG. 2( b ) , the two-layer structure including the gate insulating films and every gate electrode are arranged below the semiconductor thin film  2 ; however, the two-layer structure including the gate insulating films and every gate electrode may be arranged above the semiconductor thin film  2 . 
     The first gate electrode  4  is arranged in such a way that the first gate electrode  4 , the first control region  14  of the semiconductor thin film  2 , and the gate insulating film(s) configure the MIS structure  20   b . For example,  FIG. 2( b )  shows that the insulator layer  26   a  and the insulator layer  26   b  become the gate insulating films;  FIG. 4  shows that the insulator layer  26   b  becomes the gate insulating film; and  FIG. 5  and  FIG. 6  show that the insulator layer  26   d  becomes the gate insulating film. By applying a gate voltage to the first gate electrode  4 , an electric potential in the first control region  14  of the semiconductor thin film  2  included in the MIS structure  20   b  can be changed. 
     The MIS structure  20   b  is configured to control a flow of an electrical charge to the sensing region  15 , the electrical charge being injected into the first contact region  13  by the injection electrode  3 . By controlling a gate voltage V ICG  to be applied to the first gate electrode  4 , it is possible to limit or promote a flow of the electrical charge from the first contact region  13  to the sensing region  15 . 
     The MIS structure  20   b  is configured so that the first control region  14  is placed between the first contact region  13  and the sensing region  15 . This makes it possible to control the flow of the electrical charge into the sensing region  15 . 
     The first gate electrode  4  may be located below the semiconductor thin film  2 , as shown in  FIG. 2( b )  and  FIG. 4 , or may be located above the semiconductor thin film  2 , as shown in  FIGS. 5 and 6 . 
     The sensing region  15  of the semiconductor thin film  2  is provided so that an electric potential thereof changes in direct or indirect response to the object  32  to be measured. In  FIG. 2  to  FIG. 5 , the sensing region  15  is configured to respond indirectly to the object  32  to be measured with use of the extension gate electrode  8 . In  FIG. 6 , the sensing region  15  of the semiconductor thin film  2  is configured to respond directly to the object  32  to be measured. 
     The extension gate electrode  8  has the gate portion  9 , a sensing section  10 , and a wiring portion  11  connecting the gate portion  9  to the sensing section  10 . The extension gate electrode  8  may be, for example, an Al electrode. 
     The gate portion  9  is arranged in such a way that the gate portion  9 , the sensing region  15  of the semiconductor thin film  2 , and the gate insulating film configure an MIS structure  20   c . In  FIG. 2( b )  and  FIG. 4 , for example, the insulator layer  26   b  becomes the gate insulating film. In  FIG. 5 , the insulator layer  26   d  becomes a gate insulating film. The MIS structure  20   c  is configured so that the sensing region  15  of the semiconductor thin film  2  is placed between the first control region  14  and the second control region  16 . The gate portion  9  may be disposed below the semiconductor thin film  2 , as shown in  FIG. 2( b )  and  FIG. 4 , or may be disposed above the semiconductor thin film  2 , as shown in  FIG. 5 . 
     The sensing section  10  is configured to electrically interact directly or indirectly with the object  32  to be measured. Examples of the object  32  to be measured include an aqueous solution, sweat, and chemical substances released from the skin (for example, acetone). 
     For example, in the case where the object  32  to be measured is the aqueous solution, and an object to be detected is pH of this aqueous solution, the sensing section  10  is provided so as to electrically interact with hydrogen ions (H + ) in the aqueous solution. The sensing section  10 , as shown in  FIG. 2( a ) , for example, may be configured to electrically interact with the aqueous solution through the insulator layer  26   b . Also, the sensing section  10  may have the reference electrode  30  placed therearound. The reference electrode  30  is placed so as to come in contact with the object  32  to be measured. In a case where an electric potential of the reference electrode  30  is constant, and a concentration of hydrogen ions in the aqueous solution changes, a difference in electric potentials between the aqueous solution and the sensing section  10  changes in a three-layer structure including the aqueous solution, the insulator layer  26   b , and the sensing section  10 . Thus an electric potential in the sensing section  10  changes according to the concentration of the hydrogen ions in the aqueous solution. 
     The reference electrode  30  may be, for example, an Ag/AgCl reference electrode. 
     Since the sensing section  10  is connected to the gate portion  9  through the wiring portion  11 , the sensing section  10  and the gate portion  9  are substantially the same in electric potential. Since the sensing region  15  of the semiconductor thin film  2 , the gate insulating film, and the gate portion  9  configure the MIS structure  20   c , an electric potential in the sensing region  15  can be changed by the electric potential in the gate portion  9 . Therefore, the electric potential of the sensing region  15  of the semiconductor thin film  2  changes in indirect response to the object  32  to be measured. 
     The chemical sensor shown in  FIG. 6  is configured in such a way that the sensing region  2  of the semiconductor thin film  2  interacts electrically with the hydrogen ions (H + ) in the aqueous solution (the object  32  to be measured) through the insulator film  26   d  without using the extension gate electrode  8 . The reference electrode  30  is disposed so as to come in contact with the object  32  to be measured. In a case where an electric potential in the reference electrode  30  is constant, and a concentration of the hydrogen ions in the aqueous solution changes, a difference in electric potentials between the aqueous solution and the sensing region  15  changes in a three-layer structure including the aqueous solution, the insulator layer  26   d , and the sensing region  15 . Thus an electric potential in the sensing region  15  changes in response to the concentration of the hydrogen ions in the aqueous solution. The electric potential in the sensing region  15  of the semiconductor thin film  2  is then changed in direct response to the object  32  to be measured. 
     The change in this way in the electric potential in the sensing region  15  of the semiconductor thin film  2  in direct or indirect response to the object  32  to be measured enables an amount of the electrical charge corresponding to this electric potential responded to flow from the injection electrode  3  to the sensing region  15  through the first contact region  13  and the first control region  14 . Since this amount of the electrical charge that flowed into the sensing region corresponds to the object  32  to be measured, this electrical charge is transferred to the capacitor  7 , with the result that a quantity of electricity in the capacitor  7  can be detected, and then pH and others of the object  32  can be detected. Furthermore, the amount of the electrical charge in the sensing region may be transferred to the capacitor  7  more than once, and the electrical charges may be accumulated in the capacitor  7 , thereby increasing a quantity of the electricity in the capacitor  7 , and thus increasing detection sensitivity of the chemical sensor  50 . 
     The chemical sensor  50  configured to detect the chemical substances may have a sensitive film on the sensing section  10  of the extension gate electrode  8 , the sensitive film being responsive to the chemical substances. The sensitive film may have, for example, a catalyst and an adsorbent. The sensitive film is configured to supply an electrical charge to the sensing section  10  once the catalyst, the adsorbent, and so forth react to the chemical substances, which are the objects to be detected. This allows an electric potential in the extension gate electrode  8  and an electric potential in the sensing region  15  of the semiconductor thin film  2  to change according to amounts of the chemical substances, i.e., the objects to be detected, thereby enabling the chemical sensor  50  to detect the chemical substances. 
     The second gate electrode  5  is arranged in such a way that the second gate electrode  5 , the second control region  16  of the semiconductor thin film  2 , and the gate insulating film(s) configure the MIS structure  20   d . For example,  FIG. 2( b )  shows that the insulator layer  26   a  and the insulator layer  26   b  become the gate insulating films;  FIG. 4  shows that the insulator layer  26   b  becomes the gate insulating film; and  FIG. 5  and  FIG. 6  show that the insulator layer  26   d  becomes the gate insulating film. By applying a gate voltage to the second gate electrode  5 , an electric potential in the second control region  16  of the semiconductor thin film  2  included in the MIS structure  20   d  can change. 
     The MIS structure  20   d  is configured to control a flow, to the transfer electrode  6 , of an electrical charge in the sensing region  15  of the semiconductor thin film  2 . By controlling a gate voltage V TG  to be applied to the second gate electrode  5 , it is possible to limit or promote a flow of the electrical charge from the sensing region  15  to the transfer electrode  6 . The MIS structure  20   d  is provided so that the second control region  16  is placed between the sensing region  15  and the second contact region  17 . This allows the flow of the electrical charge from the sensing region  15  to the second contact region  17  to be controlled. 
     The second gate electrode  5  may be positioned below the semiconductor thin film  2 , as shown in  FIG. 2( b )  and  FIG. 4 , or may be positioned above the semiconductor thin film  2 , as shown in  FIGS. 5 and 6 . 
     The gate portion  9  of the extension gate electrode  8 , the first gate electrode  4 , and the second gate electrode  5  may all be positioned either above or below the semiconductor thin film  2 . This allows an electric potential in the semiconductor thin film  2  at an interface between the semiconductor thin film  2  and the gate insulating film to be changed by the gate portion  9 , the first gate electrode  4 , and the second gate electrode  5 . 
     The transfer electrode  6  is placed so as to be in contact with the second contact region  17  of the semiconductor thin film  2 . The transfer electrode  6  is electrically connected to the capacitor  7 . The transfer electrode  6  is provided so as to allow the electrical charge in the sensing region  15  to flow to the capacitor  7  through the second control region  16  and the second contact region  17 . The transfer electrode  6  may also be electrically connected to the source follower circuit  37 . The transfer electrode  6  may be placed beneath the semiconductor thin film  2  or on the semiconductor thin film  2 . 
     The transfer electrode  6  may be made of a metal monolayer film or a metal laminated film. The transfer electrode  6  may have, for example, an Au layer (which is in contact with the semiconductor thin film  2 ). 
     The fourth gate electrode  24  may be provided so that the second contact region  17  of the semiconductor thin film  2  is placed between the transfer electrode  6  and the fourth gate electrode  24 . The fourth gate electrode  24  together with the second contact region  17  of the semiconductor thin film  2  and the gate insulating film can be provided so as to configure an MIS structure  20   e  (which is a three-layer structure including a metal layer, an insulator layer, and a semiconductor layer). Upon applying a gate voltage to the fourth gate electrode  24  having the structure as above, an electric potential in the second contact region  17  is changed. For example, even when a Schottky barrier is formed at an interface between the transfer electrode  6  and the second contact region  17 , it is possible to lower the Schottky barrier by applying the gate voltage to the fourth gate electrode  24 , with the result that the electrical charge in the second contact region  17  can easily flow into the transfer electrode  6 . The fourth gate electrode  24  may be omitted. 
     In  FIG. 2( b )  and  FIG. 4  to  FIG. 6 , for example, the insulator layer  26   b  becomes the gate insulating film of the MIS structure  20   e . In  FIG. 2  and  FIG. 4  to  FIG. 6 , the fourth gate electrode  24  is positioned below the semiconductor thin film  2 , and the transfer electrode  6  is positioned above the semiconductor thin film  2 ; however, the fourth gate electrode  24  may be positioned above the semiconductor thin film  2 , and the transfer electrode  6  may be positioned below the semiconductor thin film  2 . 
     The capacitor  7  comprises a first conductive layer, a second conductive layer, and an insulator layer (dielectric layer), which is disposed between the first conductive layer and the second conductive layer. One of the first conductive layer and the second conductive layer may be electrically connected to the transfer electrode  6 ; and the other conductive layer may be electrically connected to a ground. This allows an electrical charge in the sensing region  15  to be transferred to the capacitor  7  through the transfer electrode  6 . This also allows the electrical charge in the sensing region  15  to be transferred to the capacitor  7  more than once, and thus enables the electrical charges to be accumulated in the capacitor  7 . The capacitor  7 , for example, may be electrically connected to the transfer electrode  6  through a wiring  38 , as shown in  FIG. 1 . 
     The source follower circuit  37  is a circuit for reading a quantity of electricity of the capacitor  7 . The source follower circuit  37  is configured to output the quantity of the electricity of capacitor  7  as a signal voltage (output voltage V out ). The source follower circuit  37  may be electrically connected to, for example, the capacitor  7  and the transfer electrode  6  through the wiring  38 , as shown in  FIGS. 1 and 3 . Also, the source follower circuit  37  may have, for example, an electric circuit as shown in  FIG. 3 . 
     The reset electrode  29  may be disposed so as to be in contact with the semiconductor thin film  2 . The reset electrode  29  may also be electrically connected to the ground. 
     The reset gate electrode  28  is arranged in such a way that the reset gate electrode  28 , the third control region  18  of the semiconductor thin film  2 , and the gate insulating film(s) configure an MIS structure  20   f . In  FIG. 2( b ) , for example, the insulator layer  26   a  and the insulator layer  26   b  become the gate insulating films; and in  FIGS. 4 to 6 , the insulator layer  26   b  becomes the gate insulating film. Applying a gate voltage to the reset gate electrode  28  can change an electric potential in the third control region  18  of the semiconductor thin film  2  included in the MIS structure  20   f.    
     The MIS structure  20   f  may be configured to control a flow of an electrical charge from the second contact region  17  and the transfer electrode  6  to the reset electrode  29 . This makes it possible to control a gate voltage V RST  to be applied to the reset gate electrode  28 , with the result that it is possible to limit the flow of the electrical charge from the second contact region  17  and the transfer electrode  6  to the reset electrode  29 , and to let the electrical charge of the capacitor  7  and the sensing region  15  flow to the ground through the reset electrode  29 , thereby resetting the electric potential in the capacitor  7  and the sensing region  15 . The MIS structure  20   f  is configured so that the third control region  18  is placed between the transfer electrode  6  and the reset electrode  29 . 
     Next, a charge transfer mechanism of the chemical sensor  50  in accordance with the present Embodiment will be described using  FIG. 7 . V input  is an electric potential in the injection electrode  3 ; a region indicated as V ICG  is a band diagram of the first control region  14  controlled by the first gate electrode  4 ; a region indicated as pH is a band diagram of the sensing region  15 ; and a region indicated as V TG  is a band diagram of the second control region  16  controlled by the second gate electrode  5 . 
       FIG. 7( a )  shows a band diagram of an initial state. In the state shown in  FIG. 7( a ) , an aqueous solution as the object  32  to be measured has not yet been dropped into the chemical sensor  50 . 
     When the aqueous solution as the object  32  to be measured is dropped into the chemical sensor  50  (see, for example,  FIG. 2( a )  and  FIG. 6 ), the band diagram changes, as shown in  FIG. 7( b ) . More specifically, an electric potential of the sensing region  15  changes in response to hydrogen ions in the aqueous solution, which is the object  32  to be measured. 
     Next, the voltage V ICG  of the first gate electrode  4  is changed, as shown in  FIG. 7( c ) ; and the electrical charge flows from the injection electrode  3  into the sensing region  15 . At this time, an amount of the electrical charge comparable to an amount of the hydrogen ions in the aqueous solution, which is the object  32  to be measured, flows into the sensing region  15 . 
     Next, as shown in  FIG. 7( d ) , the voltage V ICG  of the first gate electrode  4  is changed so that no electrical charge flows into the sensing region  15 . By doing so, the amount of the electrical charge comparable to the amount of the hydrogen ions is confined in the sensing region  15 . 
     Next, as shown in  FIGS. 7( e ) and 7( f ) , the voltage V TG  of the second gate electrode  5  is changed; and the electrical charge in the sensing region  15  is transferred to the capacitor  7  through the transfer electrode  6 . 
     Then, as shown in  FIG. 7( b ) , the voltage V TG  in the second gate electrode  5  is changed so that the electrical charge in the sensing region  15  does not flow into the transfer electrode  6 . 
     By repeating operations (charge transfer cycle) shown in  FIG. 7( b ) ,  FIG. 7( c ) ,  FIG. 7( d ) ,  FIG. 7( e ) , and  FIG. 7( f )  multiple times in this order, the amount of the electrical charge corresponding to the hydrogen ions in the aqueous solution, which is the object  32  to be measured, can be transferred to the capacitor  7  multiple times, thereby accumulating the electrical charges in the capacitor  7 . 
     The source follower circuit  37  is then used to read out a quantity of electricity in the capacitor  7  as an output voltage V out  (signal voltage). Since this output voltage V out  is a value that corresponds to the amount of the hydrogen ions in the aqueous solution, which is the object  32  to be measured, pH of the solution can be calculated from the output voltage V out  using a calibration curve. Since the charge transfer cycle is repeated multiple times, the amplified signal voltages can be output; and the pH of the aqueous solution, which is the object  32  to be measured, can be detected with high sensitivity. The calibration curve used here is prepared beforehand. 
     Thereafter, the voltage of the reset gate electrode  28  is changed, and the electrical charges stored in the capacitor  7  and the electrical charge in the sensing region  15  are allowed to flow into the reset electrode  29 , restoring the electric potential in the capacitor  7  and the electric potential in the sensing region  15  to their initial state. 
     The charge transfer cycle is repeated multiple times to read the quantity of the electricity stored in the capacitor  7  and to repeat the measurement cycle multiple times by applying the voltage to the reset gate electrode  28 , thereby monitoring changes in the pH of the object  32 . 
     Although the detection of the pH of the solution has been described here, the chemical sensor  50  of the present Embodiment can also be used to detect concentrations of chemical substances contained in sweat secreted by sweat glands and concentrations of chemical substances (such as acetone) released from the skin. 
     Experiment in Preparation of Chemical Sensor 
     A chemical sensor (pH sensor) as shown in  FIGS. 1 to 3  was prepared as follows. 
     (1) A polyamide acid solution was spin-coated on an Si/SiO 2  handle wafer and was calcinated at 350° C., forming a polyimide layer (10 μm thick or less) (substrate  1   a ) on the wafer.
 
(2) By vapor-depositing an Al layer on the polyimide layer and patterning the Al layer using wet etching, a first gate electrode  4 , a second gate electrode  5 , and a reset gate electrode  28  were formed.
 
(3) An Al 2 O 3  layer (50 nm thick) and an SiO x  layer (10 nm thick) (insulator layer  26   a ) were formed on these gate electrodes.
 
(4) By vapor-depositing an Al layer on the insulator layer  26   a  and patterning the Al layer using wet etching, a third gate electrode  22 , an extension gate electrode  8 , and a fourth gate electrode  24  were formed.
 
(5) An Al 2 O 3  layer (50 nm thick) and an SiO x  layer (10 nm thick) (insulator layer  26   b ) were formed on these gate electrodes.
 
(6) An amorphous InGaZnO thin film (30 nm thick) was deposited on the insulator layer  26   b  using a sputtering method; and a semiconductor thin film  2  was formed by patterning.
 
(7) By depositing a Cr/Au layer on the semiconductor thing film  2  and patterning the layer, an injection electrode  3 , a transfer electrode  6 , and a reset electrode  29  were formed.
 
(8) The laminate thereby formed was heat-treated at 200° C. for 90 minutes in a vacuum atmosphere and then was heat-treated at 250° C. for 20 minutes in a forming gas.
 
(9) An Ag/AgCl reference electrode ink was applied around a sensing section  10  of the extension gate electrode  8 , and was heat-treated at 120° C. for 2 minutes in air, forming a reference electrode  30 .
 
(10) By applying polyimide tapes over areas other than the sensing section  10  of the extension gate electrode  8  and the reference electrode  30 , an insulator layer  26   c  was formed.
 
(11) The polyimide layer was removed from the Si/SiO 2  handle wafer; and a PET film (substrate  1   b ) with a temperature sensor was attached to the polyimide layer.
 
     A chemical sensor was prepared according to procedures (1) to (11). In procedures (2) to (8), a source follower circuit  37 , a capacitor  7 , a wiring  38 , and so forth are also formed at the same time. 
     A photograph of the prepared pH sensor and its explanatory drawing are shown in  FIG. 8 . 
     pH Detection Experiment 
     By using the pH sensor prepared, a relationship between an output voltage V out  and a pH of a solution, which is an object to be measured, was analyzed. More specifically, voltage sequences, as shown in  FIG. 9( a )  to  FIG. 9( c ) , were performed as the aqueous solution with pH pre-adjusted was dropped on the sensing section  10  of the extension gate electrode  8 , thereby outputting the output voltage V out . The number of charge transfer cycles is changed according to the measurement. 
       FIG. 9( a )  shows the voltage sequence of a voltage V ICG  to be applied to the first gate electrode  4  associated with one measurement cycle;  FIG. 9( b )  shows the voltage sequence of a voltage V TG  to be applied to the second gate electrode  5  associated with one measurement cycle; and  FIG. 9( c )  shows the voltage sequence of a voltage V RST  to be applied to the reset gate electrode  28  associated with one measurement cycle. 
     When a voltage of 4 V is applied to the first gate electrode  4 , as shown in  FIG. 9( a ) , an electrical charge flows into the sensing region  15 , as shown in  FIG. 7( c ) . When a voltage of 4 V is applied to the second gate electrode  5 , as shown in  FIG. 9( b ) , the electrical charge in the sensing region  15  is transferred to the capacitor  7 , as shown in  FIGS. 7( e ) and 7( f ) . By repeating such a charge transfer cycle including the application of the voltage to the first gate electrode and to the second gate electrode a plurality of times, the electrical charges are accumulated in the capacitor  7 , leading to an increase in quantity of the electricity in the capacitor  7 . 
     After the repetition of the charge transfer cycle is completed, the source follower circuit  37  outputs the quantity of the electricity in the capacitor  7  as the output voltage (signal voltage) V out . 
     After the output voltage V out  is outputted, a voltage of 2 V is applied to the reset gate electrode  28 , as shown in  FIG. 9( c ) ; and the electrical charge of the capacitor  7  and the sensing region  15  flows to ground. 
       FIG. 10  is a graph showing the experimental results and the relationship between the output voltage V out  and the pH of the aqueous solution, which is the object measured. As the object measured, a pH 2.8 aqueous solution, a pH 6.9 aqueous solution, or a pH 11.2 aqueous solution was used. The number of the charge transfer cycles for the measurement was 10, 20, 60, or 100 cycles. 
     As shown in  FIG. 10 , the pH of the aqueous solution, which was the object measured, was found to be proportional to the output voltage V out . It was also found that as the number of the charge transfer cycles was increased, a slope of a regression line increased, leading to an increase in detection sensitivity. 
     pH Monitoring Experiment 
     By repeating the measurement cycle (the number of the charge transfer cycles: 100) shown in  FIG. 9  using the prepared pH sensor, pH of the solution was monitored. 
     More specifically, the object  32  to be measured on the sensing section  10  of the extension gate electrode  8  was changed from a pH 6.9 aqueous solution to a pH 2.8 aqueous solution to a pH 6.9 aqueous solution to a pH 11.2 aqueous solution to monitor pH of the aqueous solutions. 
     The change in the output voltage V out  at the time of changing the pH of the object  32  measured is shown in  FIG. 11 . 
     From the measurement results shown in  FIG. 11 , it was confirmed that the output voltage V out  changed according to the pH of the object  32  measured. 
     It was thus confirmed that the pH of the object  32  measured was monitored using the pH sensor prepared. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1   a ,  1   b : substrate 
           2 : semiconductor thin film 
           3 : injection electrode 
           4 : first gate electrode 
           5 : second gate electrode 
           6 : transfer electrode 
           7 : capacitor 
           8 : extension gate electrode 
           9 : gate portion 
           10 : sensing section 
           11 : wiring portion 
           13 : first contact region 
           14 : first control region 
           15 : sensing region 
           16 : second control region 
           17 : second contact region 
           18 : third control region 
           20   a  to  20   f : MIS structure 
           22 : third gate electrode 
           24 : fourth gate electrode 
           26 ,  26   a  to  26   d : insulator layer 
           28 : reset gate electrode 
           29 : reset electrode 
           30 : reference electrode 
           32 : object to be measured 
           35 : charge transfer section 
           37 : source follower circuit 
           38 : wiring 
           50 : chemical sensor