FET ELECTRODE

Abstract: This improved field effect transistor (FET) electrode comprises a FET, a thin carbon film (1) covering the surface of an insulating film at the gate of the FET, and an organic thin film (2) further covering the thin carbon film. The thin film (1) is specified to have in parts a polyhedral structure and to have a specific electric resistance after covering of 10 ohm cm or less. The organic thin film is specified to have a red-ox response, for example an electrolytically polymerised 2,6-xylenol. An ion-selection field effect transistor (ISFET) is produced by further covering the organic thin film (2) with ion selective thin films. An enzyme-sensitive FET which can detect the living substances, can be produced by further covering the ion-sensitive thin film with enzyme-active films.

Description: TECHNICAL FIELD This invention relates to a FET electrode and, more particularly, to an ion-sensitive FET electrode. BACKGROUND ART FET electrodes, long known in the art, utilize the principle of a field-effect transistor (FET). With regard to the structure and operation of a FET electrode, an impurity is diffused in a p-type substrate, which comprises a substrate of a metal oxide/semiconductor insulative membrane (p-type SiO₂/Si₃N₄), on the gate portion thereof, thereby forming an n-type source and a drain electrode. When a positive voltage is applied to the electrode at the gate portion, the potential of the p-type semiconductor in the vicinity of a redox membrane drops to induce electrons within the p-type semiconductor. A layer of these electrons forms a channel along which elections flow from the source to the drain to produce a drain current. The amount of this drain current is controlled by the gate voltage. Since the voltage at the gate is proportional to the H⁺ ion activity, the FET electrode can be used as a pH-MOSFET. However, a FET electrode of this type responds to light, besides exhibiting a large amount of drift and poor stability. Another FET electrode described in patent document JP-A-57 63444 uses a metal chromium layer between a gate insulation layer and an organic polymer layer in order to prevent the organic polymer layer from being separated from the gate insulation layer. In yet another example disclosed in patent document JP-A-59 176662, a layer of conductive material such as metal between a gate insulating film and a selective ion sensitive layer is provided in order to produce a small-size semiconductor sensor having uniform characteristics. DISCLOSURE OF THE INVENTION An object of the present invention is to solve the foregoing problems of the prior art and provide a FET electrode exhibiting little drift, high stability and little response to light. As means for solving the foregoing problems, the FET electrode of the present invention comprises a FET, a carbon thin membrane coating a gate insulator of the FET, and an organic thin membrane coating the carbon thin membrane. In the arrangement of the invention, the FET measures the concentration of H⁺ ion based on a potential, which corresponds to the activity of the H⁺ ion, produced on the gate portion by the organic thin membrane. Meanwhile, the carbon thin membrane reduces drift, stabilizes the adhesion between the gate insulator of the FET and the organic thin membrane and shuts out light. Thus, in accordance with the invention, there is provided a FET electrode exhibiting little drift, high stability and little response to light. Moreover, since the membrane covering the gate produces a potential which corresponds to the H⁺ ion, there can be provided a FET electrode which operates on the principle of the field effect, namely a FET electrode having the following structural arrangement, which is characteristic of the field effect: (1) An amplifier having an high input impedance is unnecessary. (2) Since a negative feedback circuit is constructed by utilizing the amplifying action of the device, the output resistance of the electrode can be kept to a low several thousand kilohms and electrical disturbances can be reduced. Preferably, the above carbon thin membrane has a specific resistance of less than 10 ωcm when applied on the gate insulator. Preferably also, the organic membrane is a membrane which manifests an oxidation-reduction response. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view illustrating an integrated-type FET electrode of the present embodiment; Fig. 2 is a view for describing an apparatus for measuring the FET electrode of the present embodiment; Fig. 3 is a view illustrating the results obtained by measuring the FET electrode of the present embodiment; and Fig. 4 is a perspective view of an isolated-type FET electrode. BEST MODE FOR CARRYING OUT THE INVENTION First, examples of forming a carbon thin membrane will be illustrated. (Formation Example 1) By using carbon (high-purity graphite carbon Gl6lAS, manufactured by Tokai Carbon K.K.) as a target, a carbon thin membrane was deposited on the surface of sapphire (on silicon) by a sputtering process. The conditions for sputtering were: 100 W, 10.6 Pa (8 x 10⁻² Torr), 20 hrs, a substrate temperature of less than 150 o C and an argon atmosphere. As a result, there was obtained a carbon-coated sapphire substrate having a carbon thin membrane thickness of about 1.0 µm. (Formation Example 2) The sputtering conditions were the same as those in Formation Experiment 1 except for the fact that a methane gas atmosphere was used. As a result, there was obtained a carbon-coated sapphire substrate having a carbon thin membrane thickness of about 1.2 µm. A strong membrane could be produced, and the specific resistance was 1 x 10⁻³ Ωcm. (Formation Example 3) The sputtering conditions were the same as those in Formation Example 1 except for the fact that a hydrogen gas atmosphere was used. As a result, there was obtained a carbon-coated sapphire substrate having a carbon thin membrane thickness of about 0.8 µm. The specific resistance was 1 x 10⁻³ Ωcm. (Comparison Formation Example) As in Formation Example 1, carbon was used as the target to coat the surface of sapphire (on silicon) with a carbon thin membrane by a sputtering process. The conditions for sputtering were: 600 W, 1.3 Pa (1 x 10⁻² Torr), 20 min, a substrate temperature of 300 o C and an argon atmosphere. As a result, there was obtained a carbon-coated sapphire substrate having a carbon thin membrane thickness of about 10⁻⁷ m (1000 Å). An electrode and a FET electrode using the formed carbon-coated sapphire substrate will now be described. (Example 1) The specific resistance of the carbon-coated sapphire substrate (1 cm x 1 cm in size) obtained in Formation Example 1 was about 10⁻³ Ωcm. The periphery of the substrate was insulated with a silicone resin (KE348W, manufactured by Shinetsu Silicone K.K.). A silver coaxial line (0.6 mm⊘ in size) was attached to one side by means of an electrically conductive adhesive to form a lead wire. The resulting electrode (active electrode) had a response area of about 0.5 mm x 0.5 mm at its tip. The surface of the electrode was coated with a polymeric membrane of 2,6 xylenol by an electrolytic polymerization process carried out under the following conditions: Composition of electrolyte solution 0.5 M 2,6 xylenol 0.2 M NaClO₄ acetonitrile solution (solvent) Electrolytic polymerization conditions The electrolyzing potential was swept three times (sweep rate: 50 mV/sec) from 0 to +1.5 V (vs. SSCE), followed by carrying out constant-potential electrolysis for 10 min at a constant potential of +1.5 V. (Experiment 1) Using the redox membrane-coated carbon-sapphire electrode fabricated in Example 1, the relationship between the electromotive force produced across this electrode and a reference electrode (an Ag/AgCl electrode) and a change in pH in a phosphate buffer solution was determined. As a result, a linear relationship was found over a wide pH range of pH 1.0 - 9.0, and the slope of the straight line was 58 - 59 mV/pH (25 o C), thus substantially approximating the Nernst theoretical equation. The speed of response was substantially the same as that of a redox membrane-coated carbon electrode (coated wire-type electrode), namely 5 - 30 sec (pH range of pH 5 - 9). Thus, a semiconductor substrate (silicon or sapphire) could be coated with a stable carbon thin membrane. (Examples 2 and 3) Redox membrane-coated, carbon-membrane silicon substrate electrodes were fabricated under the same conditions as set forth in Example 1. (Experiments 2 and 3) A change in pH with respect to the emf developed by the electrodes prepared in Examples 2 and 3 was measured as in Experiment 1. As a result, a linear relationship was found over a wide pH range of pH 1.0 - 9.0, and the slope of the straight line was 58 mV/pH (25 o C), thus substantially approximating the Nernst theoretical equation. The speed of response was a quick 5 - 30 sec. (Example 4) As shown in Fig. 1, a FET electrode 10 was prepared by coating the gate insulator of a MOSFET with a carbon thin membrane, and coating the latter with a polymeric membrane of 2,6 xylenol (set forth in Example 1) as an organic thin membrane by means of an electrolytic polymerization process. Numeral 1a denotes a drain diffusion area, 1b a drain electrode, 2a a source diffusion area, 2b a source electrode, 3 the polymeric membrane of 2,6 xylenol, 4 the carbon thin membrane, 5 a silicon nitride membrane, 6 a silicon oxide membrane, 7 p-type silicon, and 8 sapphire. The emf developed by the FET electrode 10 thus prepared was measured with respect to the pH of a liquid specimen 12 using the measurement apparatus shown in Fig. 2. Numeral 11 denotes a reference electrode, 13 a measurement circuit, and 14 a digital voltmeter. Measurement was performed under the following conditions: I s = 100 µA, V DS = 4 V, temperature = 25 o C. As indicated by the results shown in Fig. 3, the slope of V OUT /pH (= -58.3 mV/pH) is a straight line. This well approximates the Nernst theoretical equation. The speed of response obtained was 5 - 30 sec. The MOSFET characteristics (I D - V GS and I D - V DS ) of the FET sensors are the characteristics peculiar to the respective FET's. Though the present embodiment has been described with regard to the integrated-type FET electrode shown in Fig. 1, the same results can be obtained even with an isolated gate-type FET electrode of the kind shown in Fig. 4. The same results can also be obtained using a p-type silicon substrate or an n-type silicon substrate as the substrate. Preferably, the specific resistance of the carbon-coated sapphire substrate is less than 10 Ωcm, particularly less than 1 Ωcm and, most preferred over all, less than 1 x 10⁻³ Ωcm, as illustrated in the present embodiment. The FET electrode can be used as a biosensor, such as an ion-selective FET sensor, a gas sensor for oxygen or the like and an enzyme sensor, by coating the organic thin membrane (3) of the FET electrode of the present embodiment with an ion-selective membrane (21) (a neutral carrier membrane), an oxygen-active membrane or further an enzyme-fixed membrane, etc.

Claims: A FET electrode comprising a FET, characterized in that it further comprises : a carbon thin membrane (4) coating a gate insulator of said FET, and an organic thin membrane (3) coating said carbon thin membrane. The FET electrode according to claim 1, wherein the carbon thin membrane (4) has specific resistance of less than 10 Ω cm when applied on the gate insulator. The FET electrode according to claim 1, wherein the organic membrane (3) is a membrane which manifests an oxidation-reduction response. The FET electrode according to claim 1, wherein there is further provided an ion-selective membrane coating the organic thin membrane (3), whereby an ion-selective FET sensor is constructed. The FET electrode according to claim 4, wherein there is further provided an enzyme-fixed membrane coating the ion-selective membrane whereby there is constructed an enzyme sensor for measuring the concentration of a biological substrate.

Inventors: OYAMA NOBORU, SHIMOMURA TAKESHI TERUMO KABUS, SUZUKI TAKANAO TERUMO KABUSHIK, YAMAGUCHI SHUICHIRO TERUMO KAB, OYAMA, NOBORU, SHIMOMURA, TAKESHI TERUMO KABUSHIKI KAISHA, SUZUKI, TAKANAO TERUMO KABUSHIKI KAISHA, YAMAGUCHI, SHUICHIRO TERUMO KABUSHIKI KAISHA
Applicants: TERUMO CORP, TERUMO KABUSHIKI KAISHA
Application year: 1987
Main classification: G01N27
IPC codes: G01N27