JP3390756B2 - Field effect transistor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field effect transistor which uses a liquid electrolyte as a gate and uses a hydrogen-terminated surface of diamond as a channel.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field,
Some are listed below.

(1) H. Kawarada, Surfa
ce Science Reports 26 (199
6) 205 (2) G.I. W. Swain, Advanced Mat
erials, 6, (1994) 388 (3) Fujishima A; Chemistry and Industry 51, (1998) 207 ISFET (Ion Sensitive Field Effect Transistor) has been actively studied due to its merit of integration and miniaturization. Those using Si are commercially available. Since diamond is physically and chemically stable, it is expected as a biocompatible biosensor in the future.

Boron-doped diamond exhibits p-type semiconducting conductivity. Undoped diamond whose surface is terminated with hydrogen also has a p-type conductive layer on the surface.
This hydrogen-terminated surface conductive layer exhibits a high surface carrier density (10 ¹³ / cm ² ) even at room temperature and exhibits almost no temperature dependence. Furthermore, most carriers are in the shallow region from the surface (~ 10 nm). Since such a structure is advantageous for the operation of the FET, the inventors of the present invention have used IS using undoped hydrogen-terminated diamond.
We are studying FETs.

Further, the hydrogen-terminated structure has an as grown pattern by the microwave plasma CVD method for synthesizing diamond.
Therefore, p-type semiconductor conductivity can be obtained more easily than boron doping. Boron-doped diamond electrodes have a wide potential window, are less affected by dissolved oxygen, and have a small background current. Therefore, electrode research has advanced in a wide range. The inventors of the present application have already confirmed that undoped hydrogen-terminated diamond has a wide potential window like boron-doped diamond,
This was used to develop the diamond ISFET for the first time in the world.

[0006]

Further, as a result of further research by the present inventors, H of liquid electrolyte as a gate
In an aqueous solution of Cl, KCl, etc., the drain current of the FET was found to increase as the concentration of HCl, KCl increased, so that it is possible to sensitively react with the negative ions of the solution, particularly Cl ⁻ ions. An object is to provide a field effect transistor.

[0007]

In order to achieve the above object, the present invention provides [1] a field effect transistor, a channel in which a hydrogen-terminated surface of diamond is exposed between a source electrode and a drain electrode, and a channel of this channel. PH 1-pH 14 containing negative ions in contact with the hydrogen-terminated surface of exposed diamond
And a gate consisting of the solution of
It is characterized by detecting the concentration of negative ions in the solution.

[2] The field effect transistor described in [1] is characterized in that a change in threshold voltage is detected with respect to the concentration of Cl ions in the solution of pH1 to pH14.

[3] In the field effect transistor described in [2] above, the solution is KCl.

[4] In the field effect transistor described in [2], the solution is HCl.

[5] In the field effect transistor described in [2], the solution is NaCl.

[6] In the field effect transistor described in [1] above, the channel is a p-channel.

[7] In the field effect transistor described in [6], the p channel is pinched off.

[8] In the field-effect transistor described in [1], the diamond is formed of an undoped hydrogen-terminated single crystal or polycrystalline diamond thin film.

[0015]

[9] In the field-effect transistor according to [1], the hydrogen-terminated surface of the diamond has a wide potential window, and operates accurately in the range of this potential window.

[10] The field effect transistor described in [1] above is characterized in that the threshold voltage has a characteristic that it does not substantially depend on the pH of the liquid electrolyte.

[11] The field effect transistor according to the above [1] is characterized in that the threshold voltage has a characteristic that is not influenced by the environment.

The hydrogen-terminated diamond surface is chemically stable and has attracted attention as an electrochemical electrode having a wide potential window as a part of a secondary battery and a biosensor. According to the present invention, a field effect transistor is expected as a highly sensitive ion sensor for negative ions such as chlorine in a solution.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below.

FIG. 1 is a sectional view of a liquid electrolyte gate diamond FET (p-channel FET) according to the present invention, FIG.
Is a plan view thereof.

In these figures, 1 is polycrystalline diamond as a substrate, 2 is a channel (polycrystalline diamond termination surface: p-type surface conductive layer), 3 is a source electrode (Au electrode), 4 is a gate (liquid electrolyte: For example, KCl aqueous solution), 5 is a gate electrode, 6 is a drain electrode (Au electrode), 7 is a protective film, and 8 is a liquid tank (insulator).

The drain current is controlled by the gate voltage and can be completely pinched off. The threshold voltage is within a range of ± 0.2 V with respect to changes in pH 1-14 of the liquid electrolyte, does not make a Nernst response, and can be said to have little pH dependence. The operating voltage range of the FET is a single crystal or polycrystal of diamond corresponding to the range of the potential window when diamond is used as the electrochemical electrode, and the surface is covered with hydrogen atoms.

The liquid electrolyte gate diamond FET (p-channel FET) will be described in detail below.

Si was formed by the microwave plasma CVD method.
Polycrystalline diamond was deposited on the substrate. First, a Si substrate was subjected to ultrasonic treatment with a diamond bower, and hydrogen was used as a carrier gas to obtain a methane concentration of 1% (total flow rate: 200 sc).
cm) for 20 hours. The formed diamond film exhibits stable p-type conductivity in the atmosphere. Hall effect measurements were performed to evaluate the electrical properties of this film. Then, cyclic voltammetry (CV) measurement was performed to evaluate the electrochemical characteristics. The reference electrode is Ag /
A spiral Pt wire was used as the counter electrode using an AgCl electrode. The electrode area is 1.15 cm ² .

The structure of the FET is similar to that shown in FIG. The source / drain electrodes are formed of Au by vacuum evaporation using a metal mask. The region except the gate and the metal electrode part is isolated by Ar ⁺ ion irradiation, and the part other than the channel part is protected with epoxy to complete the process. In the FET thus manufactured, the channel is exposed, and the insulating film and the special sensitive film are not deposited. This FET is directly immersed in an aqueous electrolyte solution, and static characteristics are measured using a source ground circuit. For the I _ds -V _ds characteristic, the potential of the reference electrode (V _gs ) is kept constant and V _ds is changed to measure I _ds . I _ds -V
_{For the gs} characteristic, V _ds is kept constant, V _gs is changed, and I _ds is measured. The threshold voltage of the FET is calculated from the obtained data, and the pH
We investigated the dependency of such as.

The results will be described below.

(1) Electrical characteristics As a result of the Hall effect measurement, the sheet resistance is 10 to 20.
It has a kΩ and a sheet carrier density of 5 × 10 ¹³ / cm ² and exhibits p-type conductivity. The mobility is about 10 cm ² / V · s. From this, it can be seen that even if undoped, the surface has sufficient conductivity.

(2) CV measurement The CV measurement result in a 0.1 M (pH 12.6) KOH solution is shown in FIG.

From this, the hydrogen generation potential is -1.8 V, the oxygen generation potential is +0.9 V, and the potential window thereof is about 2.7 V. From this result, it can be seen that a sufficient potential window is obtained even with undoped diamond. In addition, as a result of performing bubbling with nitrogen gas and measuring again, it was found that the influence of dissolved oxygen was small, and it was found that the same result as that of boron-doped diamond was obtained. Fabricated FET
Since the bias point of (1) does not pass through the gate insulating film, it must be set to the potential within this potential window in order to suppress the gate leakage current. Therefore, it can be said that the wider the potential window is, the more advantageous for the FET operation.

(3) FET characteristics When I _ds -V _ds measurement was performed to confirm the FET operation of the manufactured device, complete pinch-off of the channel was confirmed.

FIG. 4 is a diagram showing static characteristics of an HCl solution at pH 1. Hydrogen-terminated diamond has a low surface state density because the surface dangling bonds are terminated by hydrogen,
It is considered that the carrier applied to the surface conduction layer could be controlled by the potential applied to the solution.

When the I _ds -V _gs characteristics were measured with a KOH aqueous solution using pH as a parameter, the measurement results shown in FIG. 5 were obtained.

From this, the threshold voltage of the FET is pH8-pH.
12 shows a substantially constant value of 0.2V. The threshold voltage was a gate voltage with a drain current of 1 μA.

Next, in order to investigate the threshold voltage in the acidic region, measurement was carried out with an HCl solution.

[0035] When confirmed similarly FET operation from I _ds -V _ds measured, to measure the threshold voltage from the I _ds -V _gs characteristics,
It was confirmed that the threshold voltage shifted in the direction of increasing absolute value as the pH increased. It is considered that this is because the HCl solution contains not only hydrogen ions but also chlorine ions, and the diamond surface senses the chlorine ions. Therefore, FIG. 6 shows the result of measurement with the KCl solution using the concentration as a parameter.

From FIG. 6, it can be confirmed that in the KCl solution, the threshold voltage shifts in the direction in which the absolute value increases as the molar concentration decreases. From the results of the HCl solution, KCl
From the results of the solution (FIG. 6), it can be confirmed that the absolute value of the threshold voltage increases as the Cl ⁻ ion concentration decreases. This is because Cl ⁻ is selectively adsorbed on the diamond surface rather than H ⁺ (OH ⁻ ) ions and attracts holes in the diamond, or Cl ⁻ ions have some influence on the conductivity of the diamond surface directly. It is thought to be affecting.

From the above results, it was confirmed that the ISFET using the undoped hydrogen-terminated diamond surface is not affected by the change in pH, but has a dependency on Cl ^- ions, and the Cl ^- sensitive FET. It turns out to be promising.

The same effect can be obtained with a NaCl solution.

The present invention is not limited to the above embodiments, but various modifications can be made based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention.

[0040]

As described above in detail, according to the present invention, the field effect transistor has a remarkable practical effect as a highly sensitive ion sensor for negative ions such as chlorine in pH 1 to pH 14 solutions. is there. Especially KCl, N
It is expected as a high-sensitivity ion sensor ^- NaCl, HCl of Cl.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a liquid electrolyte gate diamond FET (p-channel FET) according to the present invention.

FIG. 2 is a plan view of a liquid electrolyte gate diamond FET (p-channel FET) according to the present invention.

FIG. 3 is a diagram showing a CV measurement result in a 0.1 M (pH 12.6) KOH solution.

FIG. 4 is a diagram showing static characteristics of an HCl solution at pH 1.

FIG. 5: I _ds − with pH as a parameter in KOH aqueous solution
It is a figure which shows the measurement result of a _Vgs characteristic.

FIG. 6 is a diagram showing the results of measurement with a KCl solution of a liquid electrolyte gate diamond FET (p-channel FET) according to the present invention using concentration as a parameter.

[Explanation of symbols]

1 Polycrystalline Diamond 2 Channel (Polycrystalline Diamond Termination Surface: p-Type Surface Conductive Layer) 3 Source Electrode (Au Electrode) 4 Gate (Liquid Electrolyte: For example, KCl Aqueous Solution) 5 Gate Electrode 6 Drain Electrode (Au Electrode) 7 Protective Film 8 Liquid tank (insulator)

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 27/414 G01N 27/416

Claims (11)

(57) [Claims] 1. A channel in which a hydrogen-terminated surface of diamond is exposed between (a) a source electrode and a drain electrode, and (b)
A gate composed of a solution of pH 1 to pH 14 containing negative ions in contact with the hydrogen-terminated surface of the exposed diamond of the channel, and (c) detecting the concentration of negative ions in the solution of pH 1 to pH 14. Field effect transistor to be. 2. The field effect transistor according to claim 1, wherein a change in the threshold voltage is detected with respect to the concentration of Cl ions in the solution of pH1 to pH14. 3. The field effect transistor according to claim 2, wherein the solution is KCl. 4. The field effect transistor according to claim 2, wherein the solution is HCl. 5. The field effect transistor according to claim 2, wherein the solution is NaCl. 6. The field effect transistor according to claim 1, wherein the channel is a p channel. 7. The field effect transistor according to claim 6, wherein the p channel is pinched off. 8. The field effect transistor according to claim 1, wherein the diamond comprises an undoped hydrogen-terminated single crystal or polycrystalline diamond thin film. 9. The field effect transistor according to claim 1, wherein the hydrogen-terminated surface of the diamond has a wide potential window, and operates accurately in the range of the potential window. 10. The field effect transistor according to claim 1, wherein the threshold voltage has a characteristic that it does not depend on the pH of the liquid electrolyte. 11. The field effect transistor according to claim 1, wherein the threshold voltage has a characteristic that the threshold voltage is not influenced by the environment.