JPH097890A - Variable capacitor - Google Patents

Abstract

PURPOSE: To obtain a variable capacitor which is even a single element but high in withstand voltage, high in Q value which indicates the figure of merit of a capacitor, small in size, and large in variable rate of capacitance. CONSTITUTION: A counter electrode 15 serving as a movable electrode is supported with long and thin beams 17B and 17B long in cross section and set movable, and a pair of board surface electrodes 12 are provided confronting the counter electrode 15 interposing a prescribed space between them, and a stationary electrode 6 is provided confronting the movable electrode. An external bias voltage is applied between the movable electrode and the stationary electrode, a coulomb force is induced between the movable electrode and the stationary electrode, and on the other hand, an elastic force (restoring force) of the beams acts on the counter electrode 15 to return it to an original position, so that the counter electrode is displaced up to a point where the coulomb force and the elastic force of the beams are balanced. The overlap of the counter electrode with the board surface electrodes is changed in area with the displacement of the counter electrode, so that the capacitance between the counter electrode and the pair of board surface electrodes is charged. This electrostatic capacity is less affected by the inner resistance of a movable electrode than that in a case where capacitance is obtained between a board surface electrode and a movable electrode, so that a variable capacitor of this constitution is much improved in Q value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable capacitance capacitor which is a kind of voltage variable capacitance element.

[0002]

2. Description of the Related Art Conventionally, as a variable capacitance element, a stator composed of a plurality of fixed plates and a rotor composed of a rotating plate which is provided so as to face the fixed plates and rotates by rotating an axis without touching the fixed plates. A variable capacitor configured by is known. The stator and the rotor are arranged with a certain space therebetween, and when the rotor is rotated, the area where they face each other changes and the electrostatic capacity changes.

There is also known a variable capacitance diode whose electrostatic capacitance changes when an external bias voltage is applied to a space charge region of a semiconductor surface surrounded by an insulating layer.

Further, a variable capacitor disclosed in Japanese Patent Laid-Open No. 5-74655 is known. As shown in FIG. 12, the variable capacitor includes a fixed electrode 1 and a movable electrode 2, both of which are formed as a thin film body, and these are supported opposite to each other via a space 4 provided in an insulating support base 3. It has a different structure. The insulating support base 3 is, for example, a silicon substrate, and the fixed electrode 1 formed by vapor deposition of aluminum or the like is provided on the bottom surface of the space portion 4 which is a recess formed by engraving on one surface side thereof. In addition, the movable electrode 2 similarly formed on the opening edge of the recess.
Are provided in a floating state via the space portion 4, and an external bias voltage is applied between terminal portions (not shown) formed by being drawn from one end of each of the fixed electrode 1 and the movable electrode 2. It has become. When an external bias voltage is applied between the fixed electrode 1 and the movable electrode 2, the fixed electrode 1
The action of the Coulomb force between the movable electrode 2 and the movable electrode 2 increases or decreases the distance between the two and changes the electrostatic capacitance.

[0005]

However, the variable capacitor requires a rotating mechanism for rotating the rotor using a motor or the like, and thus it is difficult to reduce the size of the variable capacitor.

Further, the variable capacitance diode can change the electrostatic capacitance by a single element, but it is necessary to increase the internal resistance in order to improve the electrical withstand voltage. When the internal resistance is increased, 1 / 2πfcr
(However, f is the frequency, c is the electrostatic capacity, r is the internal resistance.) The Q factor, which indicates the figure of merit of the capacitor, is small, and the frequency stability is poor, and the carrier noise is large. was there.

In the case of a variable capacitor, the movable electrode 2
Is to be displaced by ⅓ or more of the distance between the fixed electrode 1 and the movable electrode 2 in the state where the external bias voltage is not applied, the thin plate having the movable electrode 2 formed on its surface returns to its original position. There is a drawback in that the elastic force (restoring force) that occurs and the Coulomb force generated between the fixed electrode 1 and the movable electrode 2 are no longer balanced, and the movable electrode 2 is attracted to the fixed electrode 1. Therefore, the variable rate of the electrostatic capacity cannot be increased.

Therefore, although the present invention is a single element,
An object of the present invention is to provide a variable capacitor having excellent electrical withstand voltage, high frequency stability, small size, and a large variable rate of electrostatic capacitance.

[0009]

The variable capacitance capacitor of the present invention is configured as follows in order to achieve the above object. That is, first, a movable electrode having a counter electrode portion movably supported by a beam, a fixed electrode provided to face the movable electrode, and a pair of substrate surfaces provided to face the counter electrode portion. An electrode, and a means for applying an external bias voltage between the movable electrode and the fixed electrode, the means for displacing the counter electrode portion, and between the counter electrode portion and the pair of substrate surface electrodes. The change in capacitance is taken out from the pair of substrate surface electrodes, and secondly,
In the first invention, the counter electrode portion is composed of a plurality of counter electrode constituent portions arranged in parallel and a support portion that connects both ends of the counter electrode constituent portion to each other, and the pair of substrate surface electrodes is the plurality of counter electrode constituent electrodes. A plurality of pairs of substrate surface electrodes provided facing each other to the opposing electrode constituent parts are connected to each other by a different extraction electrode for each one substrate surface electrode and each other substrate surface electrode. Yes, and third,
In the first invention, in a state in which an external bias voltage is not applied, the pair of substrate surface electrodes are arranged at a position where the facing area overlapping with the facing electrode portion is the largest, and fourthly, in the first invention, In the state where the external bias voltage is not applied, the pair of substrate surface electrodes are arranged at the position where the facing area overlapping with the facing electrode portion is the smallest.

[0010]

In the first aspect of the invention, since the fixed electrode and the movable electrode are provided so as to face each other, when an external bias voltage is applied between the fixed electrode and the movable electrode, a Coulomb force is generated between them. As a result, the movable electrode is attracted to the fixed electrode. Therefore, the counter electrode portion is displaced to a position where the elastic force of the beam that movably supports the counter electrode portion and the Coulomb force balance.
When the magnitude of the applied external bias voltage is changed, the magnitude of the Coulomb force changes and the magnitude of the displacement distance of the counter electrode portion also changes. As a result, the facing area where the counter electrode portion and the pair of substrate surface electrodes overlap with each other changes, and the value of the capacitance between the counter electrode portion and the pair of substrate surface electrodes changes. The capacitance between the counter electrode portion and the pair of substrate surface electrodes is connected in series in terms of an equivalent circuit, and this combined capacitance is extracted from the pair of substrate surface electrodes.

In the second invention, the counter electrode portion is formed in a ladder shape by the plurality of counter electrode constituent portions and the support portion. Further, the plurality of pairs of substrate surface electrodes facing the counter electrode constituent portion and the lead-out electrodes are formed in a comb shape. As a result, the electrostatic capacitances between the counter electrode configuration portion and the pair of substrate surface electrodes are connected in series in an equivalent circuit manner and are connected in parallel in an equivalent circuit configuration including a plurality of pairs of substrate surface electrodes. It becomes the combined capacitance. Therefore, a relatively large capacitance can be obtained.

In the third aspect of the invention, when an external bias voltage is applied between the fixed electrode and the movable electrode, the opposing electrode portion is displaced in a direction in which the opposing area where the opposing electrode portion and the pair of substrate surface electrodes overlap each other decreases. To do. As a result, as the value of the external bias voltage increases, the value of the capacitance between the counter electrode portion and the pair of substrate surface electrodes decreases.

In the fourth aspect of the invention, when an external bias voltage is applied between the fixed electrode and the movable electrode, the counter electrode portion is displaced in a direction in which the counter area where the counter electrode portion and the pair of substrate surface electrodes overlap each other increases. To do. As a result, as the value of the external bias voltage increases, the value of the capacitance between the counter electrode portion and the pair of substrate surface electrodes increases.

[0014]

【Example】

(Embodiment 1) An embodiment of a variable capacitor according to the present invention will be described with reference to FIGS.

The variable capacitor comprises a substrate 5, a fixed electrode 6 and a movable electrode 7.

The substrate 5 is a rectangular plate made of an insulating material such as quartz glass or alumina, and a fixed electrode pedestal 8 and a pair of movable electrode pedestals 9A and 9B are provided on the surface thereof. The fixed electrode pedestal 8 is a thin rectangular plate having a thickness of t1, and is placed near the central portion on one long side of the substrate 5 so that the long side of the fixed electrode pedestal 8 and the long side of the substrate 5 are parallel to each other. Will be placed. The pair of movable electrode pedestals 9A and 9B are thin rectangular plates having the same thickness t1 as the fixed electrode pedestal 8. The movable electrode pedestals 9A and 9B are arranged with their short side surfaces facing each other so as to be symmetrical with respect to the central axis L of the fixed electrode pedestal 8 perpendicular to the long side surfaces of the fixed electrode pedestal 8.

A lead electrode 10 is formed on the surface of the fixed electrode pedestal 8. Also, the movable electrode pedestals 9A and 9B
The extraction electrodes 11A and 11B are formed on the surface of the. Further, on the surface of the substrate 5 on the line connecting the extraction electrodes 11A and 11B, a pair of rectangular substrate surface electrodes 12A, 12B are arranged so that their short sides face each other so as to be symmetrical with respect to the central axis L. Is formed.

From the center of the long side of the substrate surface electrodes 12A, 12B, the fixed electrode pedestal 8 is arranged so that the extraction electrodes 13A, 13B connected to the substrate surface electrodes 12A, 12B are symmetrical with respect to the central axis L. It is formed on the surface of the substrate 5 on the opposite side.

The substrate surface electrode 12A and the extraction electrode 13A thus arranged and formed, and the substrate surface electrode 12
Since B and the extraction electrode 13B are arranged with a constant space therebetween, they are electrically insulated from each other. Fixed electrode pedestal 8 and pair of movable electrode pedestals 9A, 9B
Is a fixed electrode base 8 and a pair of movable electrode bases 9.
It is formed by removing the surface of the substrate 5 other than the portions where A and 9B are formed by means such as chemical etching. That is, the convex portions remaining on the surface of the substrate 5 after chemical etching or the like become the fixed electrode pedestal 8 and the pair of movable electrode pedestals 9A and 9B. In addition, the extraction electrode 1
0, 11A, 11B, 13A, 13B and the substrate surface electrodes 12A, 12B are formed of a thin film of aluminum or gold. Upon formation, a means such as sputtering or vapor deposition using a mask pattern of a predetermined shape is used.

The fixed electrode 6 has a notch 14 having a concave shape perpendicular to the long side surface on one long side surface of a rectangular plate having a thickness of t2.
It is an E-shaped plate provided with A and 14B. The fixed electrode 6 is
The cutout portions 14A and 14B are directed toward the movable electrode 7 described later,
Part of the fixed electrode base 8 is overlapped. At this time, the fixed electrode 6 is arranged so as to be symmetrical with respect to the central axis L. After that, the fixed electrode 6 is fixed to the fixed electrode pedestal 8 by means such as anodic bonding or a conductive adhesive. As a result, the cutouts 14A and 14B sides of the fixed electrode 6 are fixed in a floating state on the surface of the substrate 5.

The movable electrode 7 includes a counter electrode portion 15 and a protrusion 1.
6A and 16B, beams 17A and 17B, and fixing portions 18A,
18B is a plate integrally formed with 18B and having a thickness t2. The counter electrode portion 15 is a rectangular plate, and one long side surface is
Square columnar protrusions 16A and 16B are provided upright in a direction perpendicular to the long side surfaces. The protrusion 16A loosely fits with the notch 14A provided in the fixed electrode 6. Further, the protrusion 16B loosely fits into the notch portion 14B provided in the fixed portion 6. At this time,
The gap between the side surfaces of the protrusion 16A and the facing side of the cutout portion 14A and the gap of the side surface of the facing side between the projection 16B and the cutout portion 14B are equal.

At the center of both side surfaces of the short side of the counter electrode portion 15, one ends of beams 17A and 17B, which are elongated rods having a vertically long rectangular cross section in the extension direction, are vertically provided. The other ends of the beams 17A and 17B are fixed portions 1 of a square plate.
It is connected vertically to the side surfaces of 8A and 18B. Fixed part 18
A and 18B are superposed on the surfaces of the movable electrode pedestals 9A and 9B so as to be symmetrical with respect to the central axis L. After that, the fixed portions 18A and 18B are fixed to the movable electrode pedestals 9A and 9B by means such as anodic bonding or a conductive adhesive. As a result, the counter electrode portion 15 has beams 17A and 17B.
Supported on the substrate 5 and held in a floating state on the surface of the substrate 5. By making the cross-sectional shape of the beams 17A and 17B in the extending direction a vertically long rectangle, the beams 17A and 17B are easily bent in the short piece direction of the cross section, but are hard to be bent in the long piece direction. Therefore, the counter electrode portion 15 has the central axis L
, But not in the direction perpendicular to the surface of the substrate 5. Therefore, the counter electrode portion 15 includes the beams 17A and 1A.
Despite being supported by 7B, the gap with the substrate 5 is kept constant without sagging.

The pair of substrate surface electrodes 12A and 12B are formed so as to face and overlap the counter electrode portion 15. The extraction electrode 10 is electrically connected to the fixed electrode 6 via the bottom surface of the fixed electrode 6. In addition, the extraction electrode 11
A is electrically connected to the fixed portion 18A via the bottom surface of the fixed portion 18A, and the extraction electrode 11B is electrically connected to the fixed portion 18B via the bottom surface of the fixed portion 18B.

Assuming that the facing area where the pair of substrate surface electrodes 12A and 12B and the counter electrode portion 15 overlap with each other is S1 and the dielectric constant of the air interposed between the pair of substrate surface electrodes 12A and 12B and the counter electrode portion 15 is ε, A pair of substrate surface electrodes 1
Between 2A and 12B and the counter electrode portion 15, ε
A capacitance C1 of S1 / t1 is generated. Therefore, two capacitances C1 are connected in series between the extraction electrodes 13A and 13B in terms of an equivalent circuit.
The capacitance C2 between 3A and 13B is (1/2) × ε · S
It becomes 1 / t1. The capacitance C2 is taken out from the extraction electrodes 13A and 13B.

The fixed electrode 6 and the movable electrode 7 are made of a conductor such as a metal, a conductor in which conductive thin films are provided on the surface of an insulator to improve the high frequency skin effect, or a semiconductor having a low resistance. It is formed by applying means such as chemical etching.

Extraction electrode 10 and extraction electrode 11A
Alternatively, when an external bias voltage is applied between the fixed electrode 6 and the movable electrode 7 via 11B, a Coulomb force is generated between the two and the counter electrode portion 15 is attracted to the fixed electrode 6. On the other hand, the counter electrode portion 15 is acted upon by the beams 17A and 17B that try to pull the counter electrode portion 15 back to its original position. As a result, the counter electrode portion 15 is displaced to a position where the Coulomb force and the elastic force are balanced, and is stationary.

When the counter electrode portion 15 is displaced by the distance x from the state in which the external bias voltage V is not applied, the overlapping facing area S of the pair of substrate surface electrodes 12A, 12B and the counter electrode portion 15 is a value proportional to the distance x. Decrease. That is, the pair of substrate surface electrodes 12A and 12B and the counter electrode portion 1
The electrostatic capacitance C1 and the electrostatic capacitance C2 between 5 decrease in the counter electrode portion 15 by a value proportional to the distance x. From the balance between the Coulomb force generated in proportion to the square of the external bias voltage V and the elastic force generated in proportion to the displacement x of the counter electrode portion 15, the displacement x of the counter electrode portion 15 is It is proportional to the square of the external bias voltage V. Therefore, as shown in FIG. 6, when the change of the external bias voltage V is plotted on the abscissa and the change of the electrostatic capacitance C2 is plotted on the ordinate, the obtained curve becomes a downwardly convex curve.

Incidentally, as shown in FIG. 7, in a state where the external bias voltage V is not applied to the counter electrode portion 15, it is arranged so as not to face the pair of substrate surface electrodes 12A and 12B.
The pair of substrate surface electrodes 12A and 12B may be formed so as to be offset from the surface of the substrate 5 near the fixed electrode 6. In this case, in the state where the external bias voltage V is not applied, the facing area S1 where the pair of substrate surface electrodes 12A and 12B and the facing electrode portion 15 overlap is the smallest, and the external bias voltage is applied between the fixed electrode 6 and the movable electrode 7. When V is applied, the facing area S1 where the pair of substrate surface electrodes 12A and 12B and the facing electrode portion 15 overlap with each other increases. That is, as shown in FIG. 8, the capacitance C2
When the change of is plotted on the vertical axis, the obtained curve is a downwardly convex increasing curve.

Further, although the rectangular substrate surface electrode 12 and the counter electrode portion 15 of the rectangular plate have been described, the pair of substrate surface electrodes 12A and 12B and the counter electrode portion 15 may have other shapes. In this case, the counter electrode portion 15 and the pair of substrate surface electrodes 12 are not proportional to the displacement x of the counter electrode portion 15 when the external bias voltage V is applied.
The value of the facing area of A and 12B can be changed. Therefore, the change in the case where the change of the electrostatic capacitance C2 is plotted on the vertical axis and the change of the external bias voltage V is plotted on the horizontal axis can be freely changed, for example, by making it linear.

(Embodiment 2) The present invention is not limited to the embodiment described above, and the fixed electrode, the movable electrode and the substrate surface electrode may have other shapes. Other fixed electrodes, movable electrodes, and substrate surface electrodes will be described with reference to FIGS. 9 to 11. The same parts as those in the above-described embodiment are designated by the same reference numerals, and the description thereof will be omitted.

The fixed electrode 19 has a plurality of notches 20 which are concave on one rectangular side surface of the rectangular plate and are perpendicular to the long side surface.
It is a comb-toothed plate provided with A, 20B, ..., 20N.

The movable electrode 21 includes a counter electrode portion 22, projections 23A, 23B, ..., 23N, beams 24A, 24B, 2
It is composed of 4C and 24D.

The counter electrode portion 22 is a ladder-like structure in which a plurality of rectangular counter electrode constituent portions 25A, 25B, ..., 25N are arranged in parallel and both ends are connected to each other by support portions 26A, 26B. A plurality of protrusions 23A are provided on the long side surface of the counter electrode constituent portion 25N facing the fixed electrode 19.
23B are vertically formed. The projections 23A, 23B, ..., 23N loosely fit with the notches 20A, 20B, ..., 20N formed in the fixed electrode 19.

Each of the beams 24A, 24B, 24C, and 24D is formed by integrally forming an elongated rod having a vertically long rectangular cross-section in the extending direction into a U-shape. One of the U-shaped parallel elongated rods is formed shorter than the other. The ends of the short elongated rods of the beams 24A and 24B are fixed to the movable electrode pedestal 9A with their side surfaces facing each other by means such as anodic bonding or a conductive adhesive. The ends of the elongated rods of the beams 24A and 24B are attached to the support 26
It is connected to both ends of A, respectively. Similarly, the beam 24C,
24D is fixed to the movable electrode pedestal 9B while being coupled to the support portion 20B.

Counter electrode constituent parts 25A, 25B, ..., 25
N pairs of substrate surface electrodes 27A and 27B are formed on the surface of the substrate 5 facing each N. The end portions of the N substrate surface electrodes 27A on the movable electrode pedestal 9A side are connected to each other by the extraction electrode 28A, and similarly, the respective end portions of the N substrate surface electrodes 27B on the movable electrode pedestal 9A side are connected to each other. The lead electrodes 28B are connected to each other.
The substrate surface electrode 27A and the extraction electrode 28A, and the substrate surface electrode 27B and the extraction electrode 28B, which are arranged and formed in this manner, are arranged at a constant interval, and thus are electrically insulated from each other.

25N of the counter surface forming electrodes 25A, 25B, ..., 25N facing each other, and the counter surface overlapping areas of the substrate surface electrodes 27A, 27B and the counter electrode forming sections 25A, 25B ,. Assuming that the permittivity of air interposed therebetween is ε, the substrate surface electrode 12A,
Capacitance C3 of ε · S2 / t1 is generated between 12B and the counter electrode constituent portions 25A, 25B, ..., 25N. Therefore, between the extraction electrodes 28A and 28B, a series connection made up of two capacitances C3 is equivalent to N in terms of an equivalent circuit.
It means that they are connected in parallel. That is, the capacitance C4 between the extraction electrodes 28A and 28B is N · (1
/2).epsilon.S2/t1 and a relatively large capacitance C4 can be obtained. The capacitance C4 is taken out via the lead electrodes 28A and 28B.

[0037]

The present invention having the above-mentioned structure has the following effects. That is, from the balance between the Coulomb force generated by the external bias voltage applied between the movable electrode and the fixed electrode and the elasticity of the beam to return the counter electrode portion forming the movable electrode to the original position, the counter electrode portion is It is displaced by a predetermined distance, and the electrostatic capacitance between the counter electrode portion and the pair of substrate surface electrodes changes. Since the electrostatic capacitance between the counter electrode portion and the pair of substrate surface electrodes is extracted from the pair of substrate surface electrodes, for example, when the electrostatic capacitance is extracted via the extraction electrode connected to the substrate surface electrode and the movable electrode. Compared with, the influence of the internal resistance of the movable electrode is reduced. Further, the series resistance becomes extremely small as compared with the varactor diode. Therefore, 1 / 2πfcr (however, f
Is a frequency, c is a capacitance, and r is an internal resistance), and a very high Q value is obtained, and the frequency stability is improved. Since the opposing electrode portion and the pair of substrate surface electrodes are insulated by air, the withstand voltage of the variable capacitor increases. Further, since the movable electrode is not attracted to the fixed electrode and the inter-electrode distance does not become zero and the variable capacitor does not stop functioning, the electrostatic capacitance can be changed over a wide range.

[Brief description of drawings]

FIG. 1 is a perspective view of a variable capacitor according to the present invention.

FIG. 2 is an exploded perspective view of a variable capacitor according to the present invention.

FIG. 3 is a part of a cross section taken along the line AA ′ in FIG. 1 of the variable capacitor according to the present invention.

FIG. 4 is a part of a cross section taken along the line BB ′ in FIG. 1 of the variable capacitor according to the present invention.

FIG. 5 is a diagram showing an equivalent circuit of a variable capacitor according to the present invention.

FIG. 6 is a diagram showing a relationship between an external bias voltage to be applied and an electrostatic capacitance of the variable capacitance capacitor according to the present invention.

FIG. 7 shows another variable capacitance capacitor according to the present invention, FIG.
Is a part of a cross section of a place corresponding to AA ′ in FIG.

FIG. 8 is a diagram showing a relationship between an external bias voltage to be applied and an electrostatic capacitance of another variable capacitance capacitor according to the present invention.

FIG. 9 is a perspective view of another variable capacitor according to the present invention.

FIG. 10 is a diagram showing a substrate surface electrode and a lead electrode of another variable capacitance capacitor according to the present invention.

FIG. 11 is a diagram showing an equivalent circuit of another variable capacitor according to the present invention.

FIG. 12 is a cross-sectional view of a conventional variable capacitance capacitor.

[Explanation of symbols]

 5 substrate 6 fixed electrode 7 movable electrode 8 fixed electrode pedestal 9A, 9B movable electrode pedestal 10, 11A, 11B, 13 lead-out electrode 12 substrate surface electrode 14A, 14B notch 15 counter electrode 16A, 16B protrusion 17A, 17B beam 18A, 18B Fixed part 19 Fixed electrode 20A, 20B, ..., 20N Cutout part 21 Movable electrode 22 Counter electrode part 23A, 23B, ..., 23N Protrusion 24A, 24B, 24C, 24D Beam 25A, 25B, ..., 25N Counter electrode configuration Part 26A, 26B Support part 27A, 27B Substrate surface electrode 28A, 28B Extraction electrode

Claims (4)

[Claims] 1. A movable electrode having a counter electrode portion movably supported by a beam, a fixed electrode provided so as to face the movable electrode, and a pair of substrate surface electrodes provided so as to face the counter electrode portion. And a means for applying an external bias voltage between the movable electrode and the fixed electrode, the means for displacing the counter electrode portion so that the static electrode between the counter electrode portion and the pair of substrate surface electrodes is disposed. A variable capacitor, wherein a change in capacitance is taken out from the pair of substrate surface electrodes. 2. The counter electrode part comprises a plurality of counter electrode constituent parts arranged in parallel and a supporting part for connecting both ends of the counter electrode constituent part to each other, and a pair of substrate surface electrodes comprises the plurality of counter electrode parts. One of the plurality of pairs of substrate surface electrodes provided facing each other in the electrode configuration portion is connected to each other by a different extraction electrode for each substrate surface electrode and for the other substrate surface electrode. The variable capacitor according to claim 1. 3. The variable capacitor according to claim 1, wherein a pair of substrate surface electrodes are arranged at positions where a facing area where the facing electrode portion and the facing electrode portion are overlapped with each other is maximized when no external bias voltage is applied. 4. The variable capacitance capacitor according to claim 1, wherein a pair of substrate surface electrodes are arranged at positions where a facing area overlapping with the facing electrode portion is minimized in a state where an external bias voltage is not applied.