JPS6260662B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a measurement circuit for an ion sensor using an insulated gate field effect transistor (hereinafter abbreviated as FET).

Conventionally, as this type of ion sensor, one having a structure shown in FIG. 1, for example, is known. In the figure, 1 is a p-type silicon substrate, on which an n-type common drain 2 and a comparison electrode source 3 are provided.
and a sensor source 4 are formed, and a reference electrode 5 necessary to keep the potential of the measurement liquid constant
is formed by vapor deposition of gold. 6,7,
8, 9, and 10 are the common drain layer 2, the comparison electrode source 3, the sensor source 4, the reference electrode layer 5, and each electrode part of the silicon substrate 1; 11 is the sensor gate part; 12 is the comparison electrode gate part; Figure 2 shows its cross section.

As is clear from FIG. 2, each gate section 1
1 and 12 have a two-layer structure of a silicon nitride layer 13 and a silicon oxide layer 14 formed on a silicon substrate 1, the sensor gate part 11 has an ion sensitive layer 15, and the comparison electrode gate part 12 has a two-layer structure. is coated with a hydrophobic organic film 16.

The ion sensor with the above configuration is immersed in the test liquid 18 in the container 17, as shown in the equivalent circuit of FIG. 3, and the common drain 2 is connected to the constant pressure power source 19.
The comparison electrode source 3 and the sensor source 4 are connected to differential amplifiers 20 and 21, respectively, and the difference between their outputs is determined by a subtraction circuit 22.
The output corresponds to the ion concentration of the test liquid 18.

In the above measurement, the source/drain current flowing through each source 3, 4 is kept constant by constant current devices 23, 24. That is, by operating with a constant drain voltage Vd and respective source-drain currents Id ₃ and Id ₄ , outputs V _g3 and V _g4 are obtained. These outputs V _g3 and V _g4 are expressed by the gate potentials E _g3 and E _g4 of each gate 3 and 4 and the interface potential E _p of the reference electrode 5, as follows: V _g3 = E _g3 − _{E p} ……(1) V _g4 = E _g4 - E _p ...(2).

Although each of these potentials E _g3 , E _g4 , and E _p has a component that depends on temperature, both outputs V _g3 , V
The difference output ΔV of _g4 becomes ΔV=V _g3 −V _g4 =E _g3 −E _g4 (3), and the term related to the interface potential E _p of the reference electrode 5 is eliminated.

However, since each of the gate potentials E _g3 and E _g4 depends on temperature, and the temperature dependence characteristics differ for each FET, accurate ion measurement of the test liquid 18 is impossible.

This invention was made to improve the above-mentioned drawbacks, and provides a measurement circuit for an ion sensor that is capable of accurately measuring ions in a sample liquid by eliminating the temperature-dependent characteristics of the gate potential at the sensor gate and the comparison electrode gate. The purpose is to

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is an electric circuit diagram showing an example of the measurement circuit for an ion sensor according to the present invention. In the figure, the same parts as in FIG.・Drain current I _d3 , I _d4
Current controllers 25 and 26 are provided to control the current, and the current values are displayed on ammeters 27 and 28. Moreover, FIG. 5 shows the temperature dependence characteristics of each gate potential E _g3 , E _g4 with respect to the source-drain current I _d (I _d3 , I _d4 ) in each gate 3, 4 ∂E _g3 /∂T, ∂E _g
FIG. 4 is a diagram showing ₄ /∂T.

Now, operate the current controllers 25 and 26 to set the source-drain current I _d3 of the sensor gate 3 to the current value at point a in FIG. ₅ , and set the source-drain current I _d4 of the comparison electrode gate 4 b ₁ in Figure 5
When set to the current value at the point, each gate potential E
Temperature dependent characteristics of _g3 , E _g4 ∂E _g3 /∂T and ∂E _g
4 /∂T becomes zero.

Alternatively, the above current I _d3 is set to the current value at ₂ points a in Fig. 5, and the above current I _d4 is set to the current value at 2 points a in Fig.
b When the current value is set at _two points, the temperature dependence characteristics of each gate potential E _g3 , E _g4 ∂E _g3 /∂T, ∂E _g
Although neither ₄ /∂T becomes zero, the value C of each temperature-dependent characteristic becomes equal, so this temperature-dependent characteristic value C is eliminated from the difference output ΔV shown by equation (3). Therefore, when measuring the test liquid 18, accurate ion measurement is possible regardless of temperature.

By the way, each of the above source/drain currents I _d3 ,
If the value of I _d4 at point a ₁ and b ₁ in Fig. 5 is too low, noise will easily enter, that is, the signal-to-noise ratio (SN
If it is too high, it will cause danger in biological ion measurement, heat generation, and shorten battery life, so it is preferably 5 μA to 1 mA.
In particular, for long-term stable use in vivo, 20
μA to 600 μA is optimal. Also, in Figure 5
When measuring by setting the values at _two points a ₂ and b, the value at point C does not need to be extremely large;
It is sufficient if it is 5mV/℃.

As described in detail above, the present invention provides a measurement circuit for an ion sensor that can eliminate the temperature dependence characteristics of the gate potential at the sensor gate and the reference electrode gate, and can accurately measure the sample liquid or gas. can.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing an example of an ion sensor, FIG. 2 is a sectional view taken along the line AA in FIG. 1, FIG. 3 is a circuit diagram showing an example of a conventional measurement circuit for an ion sensor, and FIG. The figure is an electric circuit diagram showing an example of the measurement circuit for an ion sensor according to the present invention, and FIG. 5 is a characteristic diagram for explaining the operation of FIG. 4. 2... Common drain, 3... Reference electrode source,
4...Sensor source, 5...Reference electrode, 11...
Sensor gate, 12... Comparison electrode gate, 22...
...Subtraction circuit, 25, 26...Current controller.

Claims (1)

[Claims] 1. An ion sensor comprising an ion sensor having a sensor gate of an insulated gate field effect transistor structure, a comparison electrode gate, and a reference electrode, and a subtraction circuit for extracting the difference output between the gate potential of each gate and the interface potential of the reference electrode. In the measurement circuit for
A current controller is provided to control the source-drain current flowing through each source of the sensor, and the current is controlled so that the temperature dependence characteristic of the sensor with respect to the reference electrode is equal to the temperature dependence characteristic of the comparison electrode with respect to the reference electrode. 1. A measurement circuit for an ion sensor, characterized in that it is configured such that its temperature dependent characteristics can be erased by operating the device.