JPS6021344B2 - Ion potential measurement circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor ion sensor for measuring the amount of ion activity in an electrolytic solution, and particularly to an improved ion activity measurement circuit including an ion-selective membrane.

Conventionally, the amount of ion activity has been measured using a glass electrode, which is directly immersed in the electrolytic solution of the sample to be measured.

However, with glass electrodes, it is difficult to miniaturize the electrode, and there are many problems that need to be solved, such as the response speed to ions and changes in electrical characteristics over time. Recently, metal oxide semiconductor field effect transistors (hereinafter referred to as M
ene1 0phantom de Semiconduc's "Field"
MOS is an acronym for Ef and ctTnensistor.
The gate insulating film of FET (abbreviated as FET) has a multilayer structure.
Semiconductor ion sensors (hereinafter referred to as ion sensors) have been proposed that include an ion-selective membrane that can selectively respond to various ions in an electrolyte.

For example, a drain diffusion layer and a source diffusion layer are formed with an n-type diffusion region on a P-type silicon substrate, which is a suitable semiconductor substrate, and aluminum electrodes are attached to each as a wiring.

On the other hand, in the gate region between the source and drain, silicon dioxide, a thin film anode, an anode grid, etc. are sequentially laminated on the P-type silicon substrate to obtain an ion sensor as an ion selective membrane. Next, an overview of the ion sensor disclosed in Kochikai No. 52-26292 will be explained using FIGS. 1a and 1b to explain the basic configuration and an example of a circuit for measuring the amount of ion activity. Incidentally, Figure c shows an example of a circuit for measuring the amount of ion activity according to the above proposal. In Figures a and b, the ion sensor 1 is constructed by forming a drain diffusion layer D1b and a source diffusion layer S on a suitable semiconductor substrate, for example, a P-type silicon substrate la.
1c is formed with an n-diffusion region, and aluminum electrodes ld as interconnections are provided thereon by vapor deposition. On the other hand, a portion of the gate region le between the source and drain is directly immersed in an electrolytic solution 2 (not shown) and is wiped away by ions in the solution. The gate le has a multilayer structure, and the lower layer in contact with the P-type silicon substrate la is a thermally oxidized silicon layer (Si) layer on top of which is a silicon nitride layer (Si layer). Si3N4)le2
is provided as a layer having good water resistance and impermeable to ions, and further thereon is formed a sensitive layer le3 that is selectively sensitive to target ions and generates an interfacial potential proportional to the amount of ion activity.

Note that the wiring ld from the drain diffusion layer D and the source diffusion layer S
An insulating layer lf is provided to cover the insulating layer lf. In Figure b, a comparison electrode 4 used to keep the potentials of the ion sensor 1 and the electrolyte 2 constant is immersed in a container 3 containing an electrolyte 2 of a sample to be measured. The positive potential of a voltage source VD is connected to the drain electrode D of the ion sensor 1, while the negative potential of a voltage source Vs is connected to the source electrode S via a resistor 5, and is used as a source follower circuit. . With this circuit configuration, the conductivity is changed at the semiconductor surface under the gate insulating film by the interface potential between the gate insulating film and the electrolyte, and the change in the interface potential at this time is taken out as a source potential change and directly read by a voltmeter 6. A measurement circuit using a roller source follower circuit detects changes in interfacial potential as changes in source voltage, so it is not a perfect constant current circuit, and actually measures the voltage between the source electrode and the ground of the comparison electrode 4. It's just that. A constant voltage is applied to the drain electrode ○ and the source electrode S by voltage sources Vo and Vs, and the drain current flowing through the resistor 5 is treated as a constant current. Since this voltage varies, the drain current is therefore approximately constant. Measure the drain-source voltage in this state with a voltmeter.
Even if the amount of ion activity is measured using a method, highly accurate measurement of the amount of ionic activity cannot be expected. C in the figure shows a container 3 containing an electrolytic solution 2.
The ion sensor 1 and the reference electrode 7 are immersed in the ion sensor, and the positive potential of the voltage source VGs is connected to the reference electrode 7.

On the other hand, the source electrode S is grounded, and the drain electrode D is connected to the positive potential of the voltage source Vo through a current. In this circuit, when the reference electrode 7 is normally biased, the electrolyte 2
An electric field is generated in the conduction channel by an interfacial potential between the ion sensor 1 and the multilayered gate membrane of the ion sensor 1, and the strength of the electric field, which is determined by the ion concentration in the electrolyte 2, controls the drain current. , this drain current is measured by an ammeter 8. However, the gate
It is necessary to slightly change the voltage value of the voltage source VGs to be applied between the drains by manual operation or the like, and to make some means to keep the drain current constant. Further, unless the drain current is controlled by keeping the drain-source voltage constant or the drain current is not controlled to a constant current, the measurement and circuitry will become complicated. In view of the above points, the present invention provides an ion potential measuring circuit that can easily and accurately measure minute potential changes by operating a semiconductor ion sensor with a constant current and measuring the potential of a reference electrode at this time. The purpose is to provide

Hereinafter, details of the present invention will be explained based on illustrated embodiments.

FIGS. 2a and 2b show the basic principle and circuit configuration of this invention.

This will be explained using a typical MOSFET characteristic equation. Drain-source voltage V versus drain current los.

Assuming the fresh hold voltage VT in the relationship of s,
■ Condition When Vos<Vcs-VT, 10S=8 [(VGS-vT-thing). v.

3. ．． ．． ．． ．． ．． m■ Condition When Vos>VGs-VT,
1DS = Tortoise (V Kei - VT) 2...■W However, 8 = t・Mountain・Cox Here, L: Channel length, W: Channel width,
Cox: Capacitance per unit area of oxide film.

vT=omS-cross(Q38+Q8)+20F,. ．． 、．
．． The relational expression [3' holds true.

Here, Qss is the charge of Si02, Q8 is the exposure charge of the depletion layer, ms is the difference in work function between metal and silicon.

When the characteristic expression of the ion sensor is made to correspond to the characteristic expression of the MOSFET, the following relationship is established. vTX=vR+oX
-Zhi (Q330QB) +20F.・・・． ...{4} Where, V, x: Threshold voltage at ion concentration X, VR: Potential between electrolyte and reference electrode? x: Interface potential.

The fresh hold voltage VTx is calculated as follows: VTx:? x+A...[51
It is expressed as

(2} and {5' equation).

The relationship 咋筈(-A in VGS)-----'6' is established. Figure 2a shows the ion sensor connected to the MOSFET.
Condition (1) The operation is performed in the region of Vos>VGs-VT.

When the ion concentration (PH) of the electrolyte changes from xl to 〆 by △x, the interfacial potential of the electrolyte changes from xl to 0x2 by △0x1
2 Change. Said side type 1. s★-VGs characteristics also △Jx
Shift parallel to . Therefore, it is sufficient to operate the ion sensor with a constant current. Furthermore, if the potential VGs of the comparison current in the electrolytic solution is controlled and the voltage VGs at that time is measured, the change in ion concentration (PH) 2, and the minute potential change Δgx,2 can be found.
FIG. 2b shows an ion potential measuring circuit constructed based on the basic principle of the present invention. The ion sensor 11 is immersed in the electrolytic solution 13 housed in the container 12, and a constant voltage Vo is applied to the input terminal 14 of the drain electrode D from a voltage source (not shown).
supply. Further, the source electrode S is grounded via a reference resistor 15. A potential control circuit 17 that controls the potential at point B of the comparison electrode 16 so that the potential at point A of the source electrode S is constant is connected between the source electrode S and the comparison electrode 16.
A measuring terminal 18 for measuring the ion potential is provided at the connection point between the potential control circuit 17 and the comparison electrode 16, and a measuring means (not shown) such as a voltmeter or a recorder/digital voltmeter is connected to the measuring terminal 18 via an amplifier or the like. etc. can be connected and measured. As is clear from the explanation of the basic principle above, the operation of this circuit is based on a Drytho current of 1.

The characteristics of the ion sensor shift due to c, and at this time l
If l◇xl is equal to the change in △VGsl and the interfacial potential, and the drain current is a constant flow, it is sufficient to compensate with the gate-source voltage Vcs. Therefore, the ion sensor is operated at a constant current and the voltage Vcs By measuring , it is possible to directly observe the change in interfacial potential. FIG. 3 shows another embodiment of the present invention, which is an ion potential measuring circuit in which the circuit constituting the basic principle described above is made more concrete. A constant voltage VD is supplied from a voltage source (not shown) to the input terminal 24 of the drain electrode D, and a reference resistor 25 (Rs) is inserted to the source electrode S and grounded. On the other hand, an input voltage Vi supplied from a voltage source (not shown) is applied to the input terminal 26.
is supplied to the input terminal of the operational amplifier 28 via the input resistor (Ri) 27, and a feedback resistor (Ri) is connected between the input terminal of the operational amplifier 28 and the source electrode S of the ion sensor 21.
Connect R ratio) 29. The output terminal of the operational amplifier 28 and the comparison electrode 30 immersed in the electrolytic solution 23 are connected, and a measurement terminal 31 used for directly reading the measured voltage is provided at this connection point. As mentioned above, a voltmeter, recorder, etc. may be connected to this measurement terminal 31, and an analog-to-digital (A/D) converter 32 may be connected via an amplifier (not shown) to convert the digital value. This measured value is connected to an input terminal of a display device 38 consisting of an LED, a liquid crystal, etc., and displayed digitally. Next, the operation of this circuit will be explained.

The potential V^ at point A of the source electrode S is VA:Rs・IDS
Also, Isoshiro stands v^=vessel/vi.

The potential V^ at point A at the feedback resistor (Rfb) 29, that is, the drain current 1.

s can be controlled. Potential V at point B on the output side of operational amplifier 28
out becomes equal to the gate-source voltage Vcs. The operational amplifier 28 controls the gate-source voltage Vcs so that the drain current los is as specified in the explanation of the basic principle. As mentioned above, the electrolyte 23
When the ion concentration (PH) of changes from xl to x2 by Δx, the interfacial potential of the electrolyte changes by 0x12 from xl to x2. Therefore, the gate and source potentials VGsl-V correspond to the ion concentration (PH) from xl to x2, respectively. s2 =△
VGs3Δ0×12, and the minute potential change x12 can be obtained. By operating the ion sensor with a constant current and measuring the gate-source voltage, changes in the interface potential can be obtained as a digital value on the display device 33. This invention is not limited to the embodiments described above, but can be modified in various ways.

That is, in the basic configuration circuit shown in FIG. 2b, a constant voltage Vo is connected to the drain electrode D of the ion sensor 11, and a reference resistor 15 and a potential control circuit 17 are connected to the source electrode S.
Even if the drain electrode D of the ion sensor 11 is connected to an electric wire via a reference resistor 15 and the potential control circuit 17 is connected to this electrode D, the same measurement as described above can be made even if the source electrode S is grounded. It is clear that the invention can also be applied to the embodiments of . As described above, according to the present invention, by operating the semiconductor ion sensor with a constant current, it is possible to measure the ion potential with a simple circuit configuration and with high precision based on minute changes in the interfacial potential appearing on the reference electrode. .

[Brief explanation of the drawing]

Figures 1a, b, and c show an example of a circuit for measuring the structure and ion activity of an insulated gate transistor having a multilayered gate insulation structure, and Figure 2a shows characteristics used to explain the basic principle of this invention. FIG. 3B shows an example of a basic circuit for measuring the ion potential of the present invention, and FIG. 3 shows another embodiment of the circuit for measuring the ion potential of the present invention. 11, 12... Ion sensor, 12, 22... Container, 13, 23... Electrolyte, 15, 25... Reference resistance, (Rs), 17... Potential control circuit, 16, 30
... Ratio electrode, 28 ... Operational amplifier, 29.
...Feedback resistor (Rfb), 27...Input resistance (Ri), 32...A/○ converter, 3
3...Display device. Figure 1 Figure 2 Figure 2 Figure 3

Claims (1)

[Claims] 1. Measuring ion potential using an ion sensor and a reference electrode, in which the gate insulating film of an insulated gate transistor is provided with an ion-selective membrane that is directly immersed in an electrolytic solution and generates an interfacial potential according to the amount of ion activity in the electrolytic solution. In the circuit where
The ion sensor is equipped with a reference resistor connected to the drain or source of the ion sensor, and a potential control circuit that detects a potential across the reference resistor and controls the potential of a comparison electrode, and operates the ion sensor with a constant current. An ion potential measuring circuit characterized in that it measures the potential of a reference electrode when