embodiments of the present invention will be described below with reference to the attached drawings . fig1 shows the cross section of a semiconductor device for various elements forming the sensor circuit of an integrated ion sensor according to the present invention . the sensor circuit is used to process a detected signal . in fig1 designates a silicon substrate on which a plurality of various semiconductor devices are formed ; 2 designates a cmos device composed of an nmos fet ( 2a ) and pmos ( 2b ); and 3 designates an isfet device formed by providing an ion selective membrane with the gate electrode of a mosfet through a conductive member , as described later . the above pmos fet ( 2b ) is separated from other semiconductor devices by an nwell 4 formed to the silicon substrate 1 . the source and drain of the pmos fet ( 2b ) are formed of a p + diffused layer 5 , respectively and sio 2 is used for the gate insulation film 6 thereof . on the other hand , the nmos fet ( 2a ) and isfet device 3 are separated from other semiconductor devices by a pwell 7 formed to the silicon substrate 1 , and an n + diffused layer 8 is used for the source and drain thereof . the isfet device 3 essentially has the same arrangement as that of the mosfet devices . further , an aluminium electrode 10a is formed on a gate electrode 9 composed of polysilicon and connected thereto , and a platinum layer 11 connected to the aluminium electrode 10a through a titanium layer 10b is disposed above the aluminium electrode 10a . the titanium layer 10b serves as an adhesive layer . the upper surface of the ion sensor device is almost entirely covered with a polyimide film 12 and thus the upper surface of the device is entirely protected by the polyimide film 12 . further , a portion of the polyimide film 12 forms at least one groove of , for example , a circular shape and an ion selective membrane 13 is disposed therein . the above platinum layer 11 is extended to the circular opening where the ion selective membrane 13 is disposed in the state that the opening is surrounded with the polyimide film 12 . the extreme end of the platinum layer 11 has substantially the same configuration as that of the ion selective membrane 13 and is covered with the ion selective membrane 13 . with this arrangement , the platinum layer 11 is substantially electrically connected to the gate electrode 9 in the state that the extreme end thereof is covered with the ion selective membrane 13 . the platinum layer 11 corresponds to the above conductive member disposed between the gate electrode 9 and the ion selective membrane 13 . fig2 shows the sensor circuit as a whole formed in the sensor device having the above arrangement . as apparent from the electric circuit diagram , the sensor circuit 17 of the ion sensor is arranged as a voltage follower circuit using the cmos device 2 and isfet device 3 . the voltage follower circuit has the two semiconductor devices of the nmos ( 2a ) and isfet device 3 at the input stage thereof , wherein the isfet device 3 is formed as a non - inverted input terminal and the nmos ( 2a ) is formed as an inverted input terminal . in the voltage follower circuit shown in fig2 a dc power supply such as a battery or the like is connected between terminals 14 and 15 and a measured output voltage is derived from an output terminal 16 . the terminal 14 is a power supply terminal and the terminal 15 is a ground terminal . the sensor circuit having the above arrangement has a function to process a detected signal output from the ion selective membrane 13 . fig3 is a perspective view showing the outside appearance of the above sensor device of the ion sensor . the voltage follower circuit shown in fig2 is formed in the interior of the device as an arrangement including the cmos device 2 and isfet device 3 and denoted at 17 , and the upper surface of the voltage follower circuit is covered with the polyimide film 12 to protect the sensor circuit 17 from a solution to be measured . as described above , the groove composed of a circular opening is defined to the predetermined position of the polyimide film 12 and the ion selective membrane 13 is disposed therein . as described with reference to fig1 the isfet device is provided with the platinum layer 11 connected to the gate electrode of the mosfet on which the isfet device is based and the platinum layer 11 is extended to the position on this side in fig3 where the ion selective membrane 13 is disposed . the configuration of the extreme end of the platinum layer 11 is substantially the same as that of the ion selective membrane 13 , which covers the extreme end of the platinum layer 11 . with this arrangement , the gate electrode of the isfet device 3 is electrically connected to the ion selective membrane 13 through the platinum layer 11 . also , the above power supply terminal 14 , ground terminal 15 , and output terminal 16 are disposed in a line on the silicon substrate 1 at the end thereof opposite to the end where the ion selective membrane 13 is disposed . the integrated ion sensor having the above arrangement is provided with the platinum layer 11 having the function to protect the sensor circuit 17 , which is interposed between the gate electrode 9 of the isfet device and the ion selective membrane 13 , and the platinum layer 11 covered by the ion selective membrane 13 is electrically connected to the gate electrode 9 . the platinum layer 11 prevents the possibility that alkali ions and water molecules enter the interior of the sensor circuit through the ion selective membrane 13 when the device is dipped in a solution to be measured for a long time , so that the platinum layer 11 can protect a gate insulation film and the like . accordingly , the platinum layer 11 can prevent the deterioration of the gate electrode , gate insulation film and the like below the ion selective membrane 13 . further , since the sensor circuit is arranged as the voltage follower circuit and the platinum layer 11 covering the polymer - supported ion selective membrane 13 is electrically connected to the gate electrode of the isfet device 3 serving as the non - inverted input terminal of the voltage follower circuit , the change of a potential in the ion selective membrane 13 corresponding to a target ion concentration in a solution to be measured can be directly output as it is . further , since a sensor output is made to have a low impedance because the sensor circuit is arranged as the voltage follower circuit , noise can be lowered and thus an external measurement circuit having a simple arrangement can be used . note that a non - inverted amplifying circuit may be used in place of the above voltage follower circuit , wherein a detected signal output from the ion selective membrane 13 is amplified by the non - inverted amplifying circuit and then subjected to a signal processing . in the above embodiment , another metal film or metal oxide film may be used as the conductive member in place of the platinum layer 11 as long as they have characteristics similar to those of the platinum layer 11 . the other metal film includes , for example , platinum group metal , gold , silver , and silver alloy and the metal oxide film includes palladium oxide ( pdo ), platinum oxide ( pto 2 ) and iridium oxide ( iro 2 ). in the above embodiment , to enable the ion sensor to be used to the measurement requiring high accuracy in such a case as clinical examinations and the like , for example , a polymer - supported ion selective membrane having an ion selective material dispersed in a polymer together with a plasticizing agent must be used as the ion selective membrane in place of an inorganic material . next , how the integrated ion sensor having the above arrangement is used will be described with reference to fig4 . in fig4 designates a vessel and 21 designates a solution to be measured which is accommodated in the vessel 20 . the ion concentration of the measuring solution 21 is measured by using the ion sensor according to the present invention . the sensor device 22 incorporating the sensor circuit 17 is placed in the measuring solution 21 in a substantially upright state so that the portion of the ion selective membrane 13 is dipped in the measuring solution 21 and the reference electrode 23 spaced apart from the sensor device 22 by a necessary distance is also placed in the solution . in operation , the sensor device 22 having the arrangement shown in fig3 is actually encapsulated in a package 24 so that the entire portion thereof is covered therewith except only the ion selective membrane 13 which is exposed . three lead wires 14a , 15a and 16a are derived from the upper surface of the package of the sensor device 22 : the lead wire 14a derived from the power supply terminal 14 of the above voltage follower circuit is connected to the positive terminal of a battery 25 ; the lead wire 15a derived from the ground terminal 15 of the voltage follower circuit is connected to the negative terminal of the battery 25 ; and the lead wire 16a derived from the output terminal 16 of the voltage follower circuit is connected to one input terminal 26a of a voltmeter 26 . the voltmeter 26 is supplied with an output signal from the sensor device 22 and displays the output signal as a voltage data . the lead wire 23a from the power supply terminal of the reference electrode 23 is connected to the negative terminal of the battery 25 and the negative terminal of the battery 25 is connected to the other input terminal 26b of the voltmeter 26 . the ion sensor cannot carry out a measuring operation only by the sensor device 22 including the sensor circuit 17 for effecting a signal processing and thus carries out the measuring operation in combination with the reference electrode 23 . more specifically , the sensor circuit 17 cannot be set to an active state until a predetermined voltage relationship is established between the sensor circuit 17 reference electrode 23 and the . the ion sensor arranged as described above can be operated as the ion sensor only by connecting the reference electrode 23 to the negative terminal of the battery 25 for supplying an electric power to the sensor device 22 and thus an exclusive power supply for the reference electrode 23 is not needed . further , the reason why the sensor circuit can be operated only by connecting the reference electrode 23 to the negative terminal of the battery 25 and the sensor circuit as described above is that any one or both of the threshold values of the above nmos ( 2a ) and isfet device 3 provided with the input stage of the voltage follower circuit can be controlled in the fabricating process of the sensor device 22 to be set to a proper value , which will be generally described with reference to fig5 . fig5 shows the static characteristics of the output from the integrated ion sensor , wherein 27 shows the change of the output voltage from the sensor with respect to the voltage of the reference electrode when the threshold value of the sensor device 22 is not controlled . in this case , a power supply voltage to the sensor circuit is 5 volt and a buffer solution containing tris - boric acid of ph 7 . 0 is used as a measuring solution . as apparent from the static characteristics 27 , when a zero volt is imposed on the reference electrode , the sensor produces no output . accordingly , the ion sensor could not be used unless a voltage imposed on the reference electrode was set to an intermediate value relative to the value of the power supply voltage to the sensor circuit by the provision of an exclusive power supply to the reference electrode . conversely , when , for example , the threshold value of the isfet device is controlled in the process of fabricating the sensor device 22 , as described above , to thereby arrange the isfet device as a depletion type , a characteristic curve can be shifted to a negative side as shown by 28 in fig5 . with the shifted characteristic curve 28 , even if a zero volt , i . e ., a ground potential is imposed on the reference electrode , the ion sensor can perform the function thereof . more specifically , in the integrated ion sensor according to the present invention , the exclusive power supply is not needed to the reference electrode 23 and the sensor circuit 17 can be operated by setting the reference electrode 23 to the zero volt value by connecting it to the negative terminal of the battery 25 for the sensor device 22 , whereby the arrangement of the ion sensor can be simplified . in particular , the number of the lead wires between the battery 25 as the power supply and the sensor device 22 including the sensor circuit 17 and the reference electrode 23 can be reduced , which is very advantageous when measurement is in vivo . note that the simple change of the threshold value in the control enables the sensor circuit 17 to be operated by connecting the reference electrode 23 to the negative terminal of the battery 25 . as described above , according to the integrated ion sensor of the present invention , since the threshold value of at least the isfet device at the input stage thereof can be controlled when the device is fabricated , the voltage relationship between the reference electrode and the sensor circuit is adjusted so that the power supply for the sensor circuit can be also used by the reference electrode , whereby the number of the power supplies is reduced , the circuit arrangement is simplified , and the number of the lead wires is reduced . further , the provision of the platinum layer and the like between the gate electrode and ion selective membrane can stabilize the measurement effected by the ion sensor for a long time . fig6 and 7 show the result of the measurement carried out by the integrated ion sensor according to the present invention . fig6 is the response characteristics of a k + sensor using a k + ion selective membrane . as the k + ion selective membrane , variomycin was used for ligand , totm [ tri -( 2 - ethlhexyl ) trimellitate ] was used for a plasticizer member , and pvc ( polyvinyl chloride ) was used for a polymer matrix . as apparent from the result of the measurement , a linear response could be obtained in the range of from 10 - 1 to 10 - 5 mol / l and a sensitivity of 59 . 4 mv / decade was obtained by the sensor . fig7 shows the response characteristics of a na + sensor using a na + ion selective membrane . as the na + ion selective membrane , bis [( 12 - crown - 4 ) methyl ] methyldodecyl - malonate was used for ligand , totm was used for a plasticizer member , and pvc was used for a polymer matrix . as apparent from the result of the measurement , although the linear range of the na + sensor was a little narrower than that of the k + sensor , a linear response was obtained in the range of from 10 - 1 to 10 - 4 mol / l and a sensitivity of 56 . 5 mv / decade was obtained by the sensor . a second embodiment of the integrated ion sensor according to the present invention will be described with reference to fig8 and 9 . the same numerals as used in the components in the above embodiment are used to designate substantially the same components . in fig8 a na ion selective membrane 29 , k ion selective membrane 30 and reference electrode 31 are provided at the extreme end of a sensor device 22 . accordingly , in this embodiment , two kinds of the ion selective membranes 29 , 30 and the reference electrode 31 are formed on the single sensor device 22 . as shown in fig9 the various ion selective membranes 29 , 30 and reference electrode 31 are separately formed on the oxide film 32 formed on a silicon substrate 1 . a polyimide film 33 having holes , with which the various ion selective membranes and the gel for the reference electrode are filled , patterned thereto is used for the separation . each of the ion selective membranes 29 , 30 is composed of a mixed member of ligand and plasticizer member using polyvinyl chloride as a polymer matrix . further , the na ion selective membrane 29 and k ion selective membrane 30 are covered with a platinum layer 11 at their under surface , respectively . since each of the platinum layers 11 is directly adhered to the silicon oxide film 32 with difficulty , titanium layers 10b are provided as an adhesive layer . on the other hand , the reference electrode 31 is composed of a silver electrode 36 having a silver chloride 35 formed thereon , and electrolytic gel 34 mixed with potassium chloride , which covers the silver chloride 35 . since the two ion selective membranes 29 , 30 are disposed on the single device 22 , two exclusive sensor circuits 17 are disposed in the device 22 . in fig8 output terminals 37 , 38 correspond to the sensor circuits 17 for the ion selective membranes 29 and 30 , respectively . further , since the reference electrode 31 is integrally assembled in the device 22 , the reference electrode 31 is directly connected to the ground terminal 15 on the sensor device 22 which is connected to the negative terminal of a battery 25 . note that the ground terminal 15 serves as a ground terminal of the buffer amplifier of each sensor circuit 17 . the change of potentials of the ion selective membranes , which have arisen in the ion selective unit when it comes in contact with a solution to be measured , are transmitted to the isfet devices of the non - inverted input terminal in the sensor circuits 17 corresponding to each ion selective membrane . each of the sensor circuits 17 is energized by an external power supply through the respective lead wires connected to the power supply terminal 14 and ground terminal 15 of the sensor device 22 . an output signal , i . e ., sensor detection signal from each of the sensor circuits 17 is transmitted to an external measurement circuit through lead wires connected to respective output terminals 37 , 38 to thereby calculate the concentrations of na ions and k ions in a solution to be measured . the na ion sensor of the above embodiment could obtain 58 . 2 mv / decade and the k ion sensor thereof could obtain 59 . 3 mv / decade , similarly to the ion sensor of the first embodiment , and thus a sensitivity substantially near to a theoretical value could be obtained . further , a problem of crosstalk did not arise between the sensors . in the above second embodiment , when the number of the isfet devices is generally set to n , the number of lead wires to be derived to the outside is n + 2 . in addition , when a conventional insulating gate type isfet is used , lead wires for the drain and source of each isfet are needed , and thus the sensor device 22 needs 2n lead wires for each isfet and a single lead wire for the reference electrode . accordingly , the integrated sensor device according to the present invention can greatly reduce the number of the lead wires . although , in the above embodiments , the object to be measured is an electrolytic component in a solution , dissolved components such as ammonia gas and the like can be measured by forming hydrophobic porous membranes to hold various inner solution layers on the upper portion of the ion selective unit or a biochemical component such as glucose can be measured by forming an immobilized enzyme membrane in place of the ion selective membrane , in a similar way .