Patent Publication Number: US-2010126885-A1

Title: Sensor device and method of measuring a solution

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to sensor devices and methods of measuring solutions. Particularly, the present invention relates to a sensor device using a transistor having a porous insulating layer. 
     2. Description of the Related Art 
     Various sensors have been researched and developed, their applications ranging from industrial to medical fields. Nowadays sensors are commonly used in general households, forming an indispensable part of modern society. Sensors may be classified by what they sense, how they convert signals, and what material they are made of. In terms of how they convert signals, sensors may be roughly categorized into physical sensors, chemical sensors, and biosensors. 
     Biosensors are a measuring device simulating or directly taking advantage of the excellent molecular recognition capability of living bodies. Biosensors are gaining increasing attention because of their wide potential applications. A typical biosensor consists of a molecular recognition material (receptor) for receiving a certain substance and a transducer for converting a response signal into a detectable signal, such as an electric signal. The molecular recognition material may be immobilized on a membrane. Upon recognition of a target substance, the molecular detection material produces an enzyme response, breathing of a microorganism, or an immune reaction, for example. Such a response is detected as a change in current or thermal quantity, and that change is converted into an electric signal by the transducer and displayed. 
     The molecular recognition material may include enzymes, microorganisms, immune substances such as antibodies, genes such as DNA, and cells. The transducer may include an enzyme electrode, a hydrogen peroxide electrode, an ion electrode, a field-effect transistor, an optical fiber, a photo counter, a crystal oscillator, a surface acoustic wave device, or a thermistor, depending on the object of measurement. 
       FIG. 1  depicts a schematic cross section and an electric circuit diagram of an element of an insulated gate field effect transistor (IGFET)  300  as an example of a conventional bio-transistor. The IGFET  300  includes a gate insulating film  310  on a surface of which a molecular recognition material  320  is immobilized. The molecular recognition material  320  is immersed in a solution  340  together with a reference electrode  330 . 
     The IGFET  300  controls the potential of the solution  340  using the reference electrode  330 , and measures, via the reference electrode  330 , the density of charge due to molecular recognition by the molecular recognition material  320  on the surface of the gate insulating film  310 . Because the electron density in a channel in the silicon (Si) surface varies in response to the charge density on the surface of the gate insulating film  310 , a signal due to the molecular recognition material  320  alone can be detected in principle by measuring a drain current (see “OYO BUTURI”, Vol. 74, No. 12, p. 1555-1562 (2005), for example). 
     It has also been proposed in recent years to apply organic material in the field of electronics to satisfy the need for more light-weight, portable, and flexible devices. Various vertical transistors employing organic material have been proposed. For example, a light-emitting element according to the related art includes a light-emitting layer made of an organic material and a transistor made of an organic material, thus forming both the light-emitting layer and its control element from organic material (see “Thin Solid Films”, Vol. 331 (1998), pp. 51-54, for example). 
     An example of a vertical transistor using an organic semiconductor has also been reported (see Kudo et al., “T.IEE Japan”, Vol. 118-A, No. 10, (1998) pp. 1166-1171, for example) that includes CuPc (copper phthalocyanine) sandwiched by a source electrode and a drain electrode, wherein a slit-like aluminum thin film is embedded in a CuPc layer for a gate electrode. 
     There has also been a report of the performance of a light-emitting element having an organic transistor, specifically a vertical organic light-emitting transistor in which α-NPD(bis-1-N naphthyl N phenylbenzidine) is used as a hole transport material, Alq 3  (8-hydroxyquinolate aluminum complex compound) is used as a light-emitting material, and a gate electrode is disposed in an α-NPD layer (see Ikegami et al., “Technical Report of IEICE”, OME2000-20, pp. 47-51, for example). 
     Hereafter, the concept of a three-phase zone in an anode enzyme electrode reaction system according to an energy conversion technology is described with reference to  FIG. 2 . The three-phase zone is formed by an enzyme catalyst, an ion conductor, and fuel. As illustrated in  FIG. 2 , the resistance involved in proton conduction and the resistance involved in electron conduction are due to polarization in the three-phase zone. This concept provides a guidance concerning the designing of a bio-transistor. Specifically, it can be seen that, because proton production takes place in the three-phase zone at the anode pole, it is important to decrease the resistance involved in electron conduction for energy conversion. 
     This suggests that, in the context of electric conduction of a bio-transistor, such as the IGFET  300  depicted in  FIG. 1 , the resistance involved in charge conduction can be reduced by locating the three-phase zone formed by the molecular recognition material  320  on the surface of the gate insulating film  310 , the ion conductor, and one electrode closer to another electrode. It also suggests that a signal can be acquired at high speed by reducing the resistance involved in charge conduction. However, because the IGFET  300  depicted in  FIG. 1  is based on a horizontal field-effect transistor structure, there is a limit as to how closely the three-phase zone and the electrode position can be positioned to each other. 
     In contrast to horizontal type field-effect transistors, such as MOS (Metal Oxide Semiconductor) transistors, in which current flows horizontally with respect to the conducting layer, current flows vertically with respect to the conducting layer in a vertical field-effect transistor. Thus, in a vertical field-effect transistor, the channel length, i.e., the length of a current path of the transistor, can be reduced to approximately the thickness of the conducting layer. In addition, the drain current can be increased, thus enabling the transistor to operate at high speed. The vertical field-effect transistor also has a simple element structure, allowing the transistor to have a reduced element size. 
     Such features of the vertical transistor make a vertical organic transistor far more suitable and advantageous when used as a control element (also referred to as a switching element) for a light-emitting layer, such as an organic EL layer, than a horizontal organic transistor because a display device using an organic EL layer requires high-speed response. Therefore, research and development of a flexible sheet display using organic vertical transistors as control elements are currently being actively conducted. 
     However, in order to apply this vertical transistor technology to bio-transistors and reduce the resistance involved in charge conduction by bringing the position of the three-phase zone formed by the molecular recognition material on the gate insulating film surface, the ion conductor, and one electrode closer to another electrode, the gate electrode and the gate insulating layer of a vertical-structure transistor need to be designed to facilitate a catalyst reaction. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a small and high-performance sensor device employing a vertical bio-transistor that can operate at high speed. 
     According to one aspect of the present invention, a sensor device includes a porous insulating layer formed of a porous insulating material; a first electrode having a first opening portion and formed on a first side of the porous insulating layer; a second electrode having a second opening portion corresponding to the first opening portion and formed on a second side of the porous insulating layer; an insulating layer formed on the second electrode; and a molecular recognition material disposed on an internal wall of an opening in the porous insulating layer. 
     According to another aspect of the present invention, a sensor device includes a low-resistance substrate having a first opening portion; a first electrode formed on a first side of the low-resistance substrate; a porous insulating layer formed on a second side of the low-resistance substrate; a second electrode having a second opening portion corresponding to the first opening portion and formed on the porous insulating layer; an insulating layer formed on the second electrode; and a molecular recognition material disposed on an internal wall of an opening in the porous insulating layer. 
     According to another aspect of the present invention, there is provided a method of measuring characteristics of a solution or a substance contained in the solution, or an amount of the substance using a sensor device having a porous insulating layer formed of a porous insulating material; a first electrode having a first opening portion and formed on a first side of the porous insulating layer; a second electrode having a second opening portion corresponding to the first opening portion and formed on a second side of the porous insulating layer; an insulating layer formed on the second electrode; and a molecular recognition material disposed on an internal wall of an opening in the porous insulating layer. 
     The method includes immersing the first electrode, the second electrode, and the porous insulating layer of the sensor device in the solution; applying a voltage across the first electrode and the second electrode; and detecting an amount of an electric current that flows through the first and the second electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon consideration of the specification and the appendant drawings, in which: 
         FIG. 1  depicts a schematic cross section and an electric circuit diagram of a conventional bio-transistor; 
         FIG. 2  illustrates the concept of a three-phase zone formed by an enzyme catalyst, an ion conductor, and fuel in an anode enzyme electrode reaction system according to an energy conversion technology; 
         FIG. 3  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor according to Example 1 of the present invention; 
         FIG. 4  depicts a porous portion of the vertical bio-transistor of Example 1; 
         FIG. 5  illustrates an operating principle of a biosensor; 
         FIG. 6  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor according to Example 2; 
         FIG. 7  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor according to Example 3 of the present invention; 
         FIGS. 8A through 8I  illustrate a process of manufacturing a vertical bio-transistor; 
         FIGS. 9A through 9E  depicts perspective, transparent views illustrating fundamental steps of the process illustrated in  FIG. 8 ; and 
         FIG. 10  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor according to Example 4 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, a preferred embodiment of the present invention is described with reference to the drawings. 
     &lt;Vertical Bio-Transistor Single Element&gt; 
       FIG. 3  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor  10  according to an embodiment of the present invention. The vertical bio-transistor  10  includes a porous portion  60  on one side of which there is formed a source electrode  20  having a first opening portion  63 . On the other side of the porous portion  60 , there is formed a drain electrode  40  having a second opening portion  64 . An insulating layer  50  is formed on the drain electrode  40 , and a gate electrode  30  is disposed in midair away from the source electrode  20  and the drain electrode  40 . 
     The circle of enlarged view within  FIG. 3  depicts a cross section of the porous portion  60  which is formed by a porous alumina (porous Al 2 O 3 )  62 . The porous portion  60  provides a porous insulating layer and has a number of openings  70  that penetrate the porous portion  60  vertically. The openings  70  allow communication between the one side on which the source electrode  20  is formed and the other side on which the drain electrode  40  is formed. On the inner walls of the openings  70 , there is an immobilized enzyme, such as glucose oxidase (GOD)  72  which is a glucose degrading enzyme. The openings  70  having the GOD  72  immobilized thereon provide spatial gaps vertically penetrating the porous portion  60 . 
     The first and second opening portions  63  and  64  provide openings via which the porous alumina  62  of the porous portion  60  can be exposed to the atmosphere. The first and second opening portions  63  and  64  may have a corresponding shape, such as a rectangular, a circular, or an elliptical shape. The source electrode  20  may be made of an electrically conductive material such as aluminum (Al), providing a first electrode. The drain electrode  40  provides a second electrode. The gate electrode  30 , which is disposed in midair above the drain electrode  40 , provides a third electrode (reference electrode (Pt black)). In this structure, when the source electrode  20  emits carriers that are received by the drain electrode  40 , resistance to charge conduction in the source electrode  20  can be reduced so that a current can flow efficiently between the source electrode  20  and the drain electrode  40 . 
       FIG. 4A  illustrates a schematic perspective view of the porous portion  60 .  FIG. 4B  illustrates a cross section taken along line A-A′ of  FIG. 4A . The entire surfaces of the porous portion  60  may be covered with a layer of the same metal material as that of the drain electrode  40 , such as an aluminum layer  66 . Thus, the porous portion  60  has the same potential as that of the drain electrode  40 . Preferably, the aluminum layer  66  also covers a part of the internal walls of the openings  70 , as illustrated in  FIG. 4B . 
     Example 1 
     In Example 1, the vertical bio-transistor  10  depicted in  FIG. 3  is immersed in a solution  80 , such as a solution of blood, to detect a current that flows between the source electrode  20  and the drain electrode  40 . The property or amount of the solution or a material contained in it is determined based on the detected current value. 
     The GOD  72 , which is a glucose oxidoreductase, oxidizes glucose in accordance with the following expression (1), producing hydrogen peroxide (H 2 O 2 ): 
       C 6 H 12 O 6 +O 2 →C 6 H 10 O 6 +H 2 O 2   (1) 
     Hemoglobin in blood in the case of a blood solution is oxidized by the GOD  72  whereby the ion concentration of the solution changes. Thus, by controlling the increase or decrease in potential by controlling a bias voltage VD S  and a gate voltage V G , the amount of the carrier that travels from the source electrode  20  to the drain electrode  40  changes. Thus, by detecting a change in the threshold voltage of the transistor or a change in its current value for the same potential due to the change in charge density as a result of oxidoreduction, the vertical bio-transistor  10  can be used as a biosensor. 
     Because the carriers that move from the source electrode  20  to the drain electrode  40  travel through the spatial gaps of the openings  70  in the first opening portion  63  and the second opening portion  64  of the porous portion  60 , the resistance to charge transfer can be reduced. The carriers move through the effective spatial gaps of the openings  70  in the porous portion  60  with a controlled depletion layer, in accordance with a voltage applied between the source electrode  20  and the drain electrode  40 . Thus, by detecting a change in the threshold voltage of the vertical bio-transistor  10  and a change in its drain current, and determining a standard curve for the change in ion concentration, the vertical bio-transistor  10  can be used as a sensor for diagnosing diabetes, for example, based on the detected amount of glucose. 
     The drain electrode  40  may be formed in various shapes. Preferably, the drain electrode  40  includes a voltage-applied portion of electrically conductive material to which a gate voltage is applied, where a current path in the porous portion  60  is formed adjacent the voltage-applied portion of the drain electrode  40 . 
     As described above, the vertical bio-transistor  10  can be used as a glucose sensor for detecting a change in charge amount due to the production of water (H 2 O) and oxygen ion (O − ) accompanying the production of H 2 O 2  as a result of the redox reaction of glucose by the glucose oxidase (GOD). The same above sensor structure may also be used as a complex adsorption sensor utilizing physical adsorption, chemical adsorption, and a combination of physical adsorption and chemical adsorption, as described in detail below. 
     &lt;Principle of Biosensor&gt; 
     With reference to  FIG. 5 , the principle of a biosensor is described. As shown in  FIG. 5 , a biosensor  11  includes a functional membrane  80 . A change due to various reactions in or on the functional membrane  80  is converted by an electric signal convertor  82  into an electric signal that is detected. The reactions in or on the functional membrane  80  may include reactions involving physical adsorption, chemical adsorption, or molecular adsorption; an enzyme response, an antigen-antibody reaction, a reaction involving a microorganism, an electrochemical reaction, and a reaction involving DNA. The reactions may involve any other combinations of the functional membrane  80  and a substance or matter that cause a potential change or exhibit selectivity. 
     Example 2 
       FIG. 6  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor  12  according to Example 2 of the present invention. The vertical bio-transistor  12  is similar to the vertical bio-transistor  10  according to Example 1 with the exception that the porous portion  60  has a portion  90  in the second opening portion  64  that is extended toward the gate electrode  30 . Specifically, in the second opening  64 , the GOD  72  extends toward the gate electrode  30  with respect to the porous alumina  62 . A channel portion as a current path of the transistor adjacent the drain electrode  40  has a length corresponding to the film thickness of the drain electrode  40 . 
     Thus, the carriers that travel through the spatial gaps of the openings  70  formed in the porous portion  60  can travel more efficiently than in Example 1 through the effective spatial gaps with a controlled depletion layer, the spatial gaps being changed by the voltage applied to the drain electrode  40 . Thus, in this structure, the sensitivity of the vertical bio-transistor  12  is enhanced by the decrease in operating resistance, thereby achieving higher operating speed and faster response. The vertical bio-transistor  12  can also function as a highly accurate sensor because of the increase in current density. 
     The porous portion  60  and the insulating layer  50  may include a metal oxide selected from the group (a) consisting of zinc oxide, titanium oxide, tin oxide, indium oxide, aluminum oxide, niobium oxide, tantalum pentoxide, barium titanate, and strontium titanate. Alternatively, the metal oxide may be selected from the group (b) consisting of nickel oxide, cobalt oxide, iron oxide, manganese oxide, chromium oxide, and bismuth oxide. Alternatively, the metal oxide may be formed by doping an impurity into one of the metal oxides selected from the group (a) or (b). 
     The material of the porous portion  60  may include anodized alumina (Al 2 O 3 ) or zinc oxide (ZnO). Anodization performed under certain conditions enables the formation of a number of uniform pores, thereby achieving a high carrier mobility and realizing a highly sensitive vertical bio-transistor. 
     The molecular recognition material may include one or the other of paired substances in any of the following combinations: (1) an enzyme, which is a substance that catalyzes a chemical reaction that occurs in a living body, such as glucose oxidase, diastase, pepsine, trypsin, papain, bromelain, thrombin, lipase, lipoprotein lipase, monooxygenase, peroxidase, ATP synthase, DNA polymerase, RNA polymerase, nuclease, aminoacyl tRNA synthetase, kinase, phosphatase, glycosyltransferase, or DNA methylase, and its substrate; (2) any of the enzymes in (1) and a coenzyme, such as NAD, NADP, FMN, FAD, thiamin diphosphate, pyridoxal-phosphate, coenzyme A, ribonucleic acid, or folic acid; (3) an antigen, such as  Escherichia coli, Bacillus subtilis  including  Bacillus natto , bacterium including cyanobacterium, virus, or a protein that enters a living body, such as a pathogen, and an antibody that exhibits effective reactivity against such an antigen, such as a glycoprotein molecule produced by T cell, B cell, or NK cell, which are examples of lymphocyte; and (4) a hormone, such as inhibin, parathormone, calcitonin, thyroid stimulation hormone, melatonin, insulin, glucagon, and growth hormone, and its receptor. One of the paired substances in any of the above combinations (1) through (4), such as an enzyme, is immobilized on the internal walls of the openings in the porous alumina  62  as the molecular recognition material, to selectively measure the other substance in the pair, such as a coenzyme. 
     Example 3 
       FIG. 7  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor  14  according to Example 3. The vertical bio-transistor  14  includes a silicon (Si) substrate  110  as a low-resistance member which is disposed between the source electrode  20  and the porous portion  60 . The vertical bio-transistor  10  depicted in  FIG. 3  may be disposed on the substrate  110  that is formed in concave shape. 
     &lt;Process of Manufacturing a Vertical Bio-Transistor&gt; 
     With reference to  FIGS. 8 and 9 , a process of manufacturing the vertical bio-transistor  14  depicted in  FIG. 7  is described.  FIGS. 8A through 8I  illustrate the steps of the process of manufacturing the vertical bio-transistor.  FIGS. 9A through 9E  depict perspective transparent views illustrating fundamental steps of the process illustrated in  FIG. 8 . 
     In  FIG. 8A , an oxide layer is formed on a lower surface of a substrate by thermal oxidation. Specifically, a layer of silicon (Si) oxide  210  having a thickness of about 1 μm is formed on a lower surface of a low-resistance silicon (Si) substrate  200  having a thickness of about 200 μm and plane orientation (001) by thermal oxidation. 
     In  FIG. 8B , a resist  212  is applied to a lower surface of the silicon oxide layer  210  by the spin coat method to a film thickness of about 300 nm. 
     In  FIG. 8C , the resist  212  is exposed and developed, thereby forming a pattern of the resist  212 . 
     In  FIG. 8D , the substrate  200  is immersed in a hydrofluoric acid (HF) solution diluted with water, in order to remove the silicon oxide  210  in an area in which the resist pattern is not formed. In this way, a window opening  201  is formed which may be rectangular, circular, or elliptical. 
     In  FIGS. 8E and 9A , the resist  212  is removed with a solvent or by dry ashing or the like. 
     In  FIGS. 8F and 9B , an aluminum film  214  is formed on the silicon substrate  200  at room temperature by the vacuum evaporation method at the vacuum condition of about 1.3 to about 3.9×10 −3  Pa to a film thickness of about 100 nm and preferably about 10 nm. 
     In  FIGS. 8G and 9C , the aluminum film  214  is anodized with a phosphoric acid aqueous solution at about 30° C. and a current density of about 3 to 7 mA/cm 2 , thereby forming a layer of porous alumina  216 . The layer of porous alumina  216  may be controlled to have a pore diameter of about 5 to 450 nm and a pore pitch of about 10 to 500 nm. 
     In  FIGS. 8H and 9D , a central portion  202  of the silicon substrate  200  is removed using a potassium hydroxide (KOH) solution diluted with water, forming a so-called “forward” mesa of about 54.7°. Thus, the first opening portion  63  depicted in  FIG. 7  is formed. 
     In  FIGS. 8I and 9E , the silicon substrate  200  is immersed in a hydrofluoric acid (HF) solution diluted with water to remove the silicon oxide  210 . 
     Thereafter, glucose oxidase (GOD) is immobilized on the internal walls of the porous alumina  216 , and the source electrode  20  having the first opening portion  63  is formed on the silicon substrate  200 . Further, the drain electrode  40  having the second opening portion  64  corresponding to the first opening portion  63  is formed of an aluminum electrode having a thickness of about 100 nm by vacuum deposition. After providing a metal mask, the insulating film  50  is formed by sputtering silicon oxide (SiO 2 ). 
     In this way, the vertical bio-transistor  14  according to Example 3 is manufactured. The silicon substrate  200  depicted in  FIGS. 8 and 9  corresponds to the silicon substrate  110  depicted in  FIG. 7 , and the porous alumina  216  depicted in  FIGS. 8 and 9  corresponds to the porous alumina  62  depicted in  FIG. 7 . 
     Example 4 
       FIG. 10  depicts a schematic cross section and an electric circuit diagram of a single element of a vertical bio-transistor  16  according to Example 4. The bio-transistor  16  may be manufactured by extending the porous alumina  216  in a second opening portion  218  in the steps depicted in  FIGS. 8G through 8H  toward the gate electrode  30 . 
     More specifically, glucose oxidase (GOD)  72  is immobilized on the internal walls of the openings  70  in the porous portion  60  of the vertical bio-transistor  16  in the second opening portion  218  facing the gate electrode  30  to the same height as the surface of the porous alumina  62 . Compared to the vertical bio-transistor  14  depicted in  FIG. 7 , the vertical bio-transistor  16  can exhibit an improved charge generation efficiency with respect to a voltage applied to the drain electrode  40  and an increase in source-drain current. 
     Example 5 
     In Example 5, the porous alumina  62  of the vertical bio-transistor  14  depicted in  FIG. 7  is replaced with a film of zinc oxide. In Example 5, too, similar sensor operation to the foregoing examples can be achieved. 
     Example 6 
     In Example 6, the porous alumina  62  of the vertical bio-transistor  14  depicted in  FIG. 7  is replaced with a chromium oxide film. In this case, too, similar sensor operation to the foregoing examples can be achieved. 
     Example 7 
     In Example 7, the drain electrode  40  of the vertical bio-transistor  14  depicted in  FIG. 7  is formed using Au instead of aluminum. In this case, too, similar sensor operation to the foregoing examples can be achieved. 
     Example 8 
     In Example 8, the drain electrode  40  of the vertical bio-transistor  14  depicted in  FIG. 7  is formed using Pd instead of Al. In this case, too, similar sensor operation to the foregoing examples can be achieved. 
     Example 9 
     In Example 9, the drain electrode  40  of the vertical bio-transistor  14  depicted in  FIG. 7  is formed using an electrically conductive metal oxide, specifically zinc oxide doped with Al, instead of Al. In this case too, similar sensor operation to the foregoing examples can be achieved. 
     Example 10 
     In Example 10, the drain electrode  40  of the vertical bio-transistor  14  depicted in  FIG. 7  is formed using electrically conductive polyaniline, instead of Al. In this case, too, similar sensor operation to that of the foregoing examples can be achieved. 
     When the I-V characteristics of the vertical bio-transistors according to Examples 1 through 10 were measured, substantially the same measurement results were obtained, indicating that the vertical bio-transistors according to Examples 1 through 10 can provide similar effects. 
     Example 11 
     The metal oxide in the aforementioned insulating layer may include a material selected from the group consisting of silicon oxide, tantalum oxide, titanium oxide, aluminum oxide, hafnium oxide, zircon oxide, lanthanum oxide, scandium oxide, praseodymium oxide, bismuth oxide, niobium oxide, tungsten oxide, yttrium oxide, and silicon nitride. 
     Thus, in accordance with an embodiment of the present invention, a high-performance sensor device having a novel structure and exhibiting sensor characteristics unique to a vertical bio-transistor can be provided. Further, an embodiment of the present invention provides a sensor device and a circuit structure using a vertical transistor that exhibit a high carrier mobility and a steep rise signal in output current (source-drain current), thus enabling a high operating speed. Another embodiment of the present invention provides a method of measuring a solution using the sensor device according to an embodiment of the present invention. 
     Although this invention has been described in detail with reference to certain embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 
     The present application is based on the Japanese Priority Applications No. 2008-298827 filed Nov. 21, 2008 and No. 2008-328408 filed Dec. 24, 2008, the entire contents of which are hereby incorporated by reference.