JP2008506265A - Metal-insulator varactor device - Google Patents

Abstract

  The varactor is composed of first and second conductive layers that are spaced apart from each other such that a given voltage is applied across the first and second conductive layers. Furthermore, the insulating structure includes at least one insulating layer disposed between the first and second conductive layers, and a device capacitance value between the first and second conductive layers changes according to a given voltage. As such, it is configured to cooperate with the first and second conductive layers to create a charge pool that varies in response to changes in a given voltage. The insulating structure may include at least one layer, two separate layers, or more separate layers. One or more of these layers may be an amorphous material. A zero bias voltage version of the varactor is also described.

Description

Related applications

  This application is a continuation-in-part of US patent application Ser. No. 11 / 113,587 entitled “THIN-FILM TRANSISTORS BASIC ON TRUNNING STRUCTURES AND APPLICATIONS” filed on April 25, 2005. This application claims priority from US Provisional Patent Application No. 60 / 586,493, entitled “METAL-INSULATOR VARACTOR DEVICES” of Japanese application. The 11 / 113,587 application is a continuation-in-part of US patent application Ser. No. 10 / 877,874, itself entitled “HIGH SPEED ELECTRON TUNELING DEVICES” filed on June 26, 2004, It claims the priority of US Provisional Patent Application No. 60 / 565,700 entitled "PRACTICAL THIN-EILM TRANSISTORS BASSED ON METAL-INSULATOR TUNELING STRUCIURES AND THEIR APPLICATIONS". No. 10 / 887,874 application is a continuation of US patent application Ser. No. 10 / 347,534, entitled “HIGH SPEED ELECTRON TUNELING DEVICES”, filed Jan. 20, 2003. Is also a continuation of US patent application Ser. No. 09 / 860,972 entitled “HIGH SPEED ELECTRON TUNELING DEVICE AND APPLICATIONS” filed on May 21, 2001. All of the above are incorporated herein by reference in their entirety.

  Today's electronic designs often use electronic components whose capacitance varies with the bias voltage and will continue to be used in the future. Such devices are often referred to as “varactors”. High speed varactors (ie, varactors whose capacitance changes very rapidly with high frequency changes in voltage) are used as low loss frequency multipliers and tuning elements in high frequency circuits.

  Prior art varactor designs typically use semiconductor material to create a pn junction that is biased to change the capacitance of the junction. Although the use of a semiconductor-based design is effective for its intended purpose, the present application has a significant effect on device speed, for example, rather than using a general prior art pn junction design, A new design intended to provide yet another effect is disclosed.

  As described in more detail below, disclosed herein are highly effective varactors and associated methods.

  In one embodiment of the invention, the varactor is composed of first and second conductive layers spaced apart from each other such that a given voltage is applied between the first and second conductive layers. Furthermore, the insulating structure includes at least one insulating layer disposed between the first and second conductive layers, and a device capacitance value between the first and second conductive layers changes according to a given voltage. As such, it is configured to cooperate with the first and second conductive layers to create a charge pool that varies in response to changes in a given voltage.

  In one aspect, the insulating structure includes at least two separate layers.

  In another feature, the insulating structure includes a single material layer.

  In related features, at least one of the insulator layers is an amorphous material.

  In another aspect of the invention, the MIIM device is configured to set the threshold bias voltage to zero volts, at least approximately zero volts, so that the varactor can be used without requiring a bias voltage.

  The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Accordingly, the present invention is not intended to be limited to the embodiments shown herein, but is consistent with the principles and features described herein, including alternatives, modifications, and equivalents as defined in the appended claims. It follows the widest range. It should be noted that the drawings are not to scale, and are schematic so that, by their nature, the features of interest can be best illustrated. Also, throughout this disclosure, the same reference numerals have been applied to the same elements where they are useful.

A highly advantageous metal-insulator device structure is disclosed that forms a variable capacitance device (hereinafter varactor) in which the capacitance of the two-terminal device varies as a function of the voltage across the device. The disclosed varactor has several advantages over competing semiconductor-based varactors:
1. 1. Large change in capacitance 2. High capacitance per unit area High differential capacity Compatible with a wide range of substrate materials

  In one implementation, the basic varactor in this disclosure has a metal-insulator-insulator-metal (MIIM) structure, which is a U.S. patent by Eliasson and Model, which is hereby incorporated by reference in its entirety. No. 6,534,784 (hereinafter 784 patent) of the type first disclosed. In this regard, it should be recognized that the device of the 784 patent relates specifically to solar energy conversion.

Referring now to FIGS. 1a and 1b, energy band diagrams of MIIM varactors are illustrated and are generally indicated by reference numerals 10 and 12, respectively. FIG. 1a shows a device with zero bias shown by V = 0, and FIG. 1b shows a device with a bias voltage (forward bias) applied in the positive direction shown by V = V ⁺ . In these figures, the first metal layer M ₁ and the second metal layer M ₂ are arranged in spaced relation. Insulation structure 20 is disposed between the first and second metal layers _{M 1} and _{M 2.} In this example, the insulating structure 20 includes a first insulating layer _{I 1} and the second insulating layer _{I 2.} The varactor device of the present disclosure differs from the MIIM tunnel device of the 784 patent in that the device functions as a capacitor rather than a non-linear resistance device or diode. In the MI784 diode of the 784 patent, the insulator material and insulator thickness are selected so that electrons can tunnel the device from one metal to the other. This unexpected and surprising behavior was discovered in connection with device characteristics and design using complex modeling techniques aimed at the development of nonlinear devices. Based on this notable discovery, the MIIM structure has been changed to function in a completely different way as a completely different type of device, but this is not common or obvious. In particular, the first insulator I ₁ is configured to have a very high barrier height (>> 1 eV) and the second insulator I ₂ has a low barrier height (typically <0.3 eV). Therefore, electrons from the _second metal (M ₂ ) can easily tunnel I ₂ but cannot tunnel I ₁ . Thus, under forward bias, free electron charges are pooled in an approximately triangular potential well 30 between the two insulators. More specifically, the potential well 30 is formed at the boundary 32 near between _{I 1} and _{I 2} in _{I 2.} It is further understood that this charge pooling causes a change in capacitance depending on the applied voltage, forming a varactor device.

  In the following, the theoretical calculation of the capacitance of the MIIM varactor shown in FIGS. 1a and 1b is made up of (i) applied voltage, (ii) C (V), (iii) alternative varactor configuration, (iv) frequency response and performance of the varactor. This will be explained according to the factors that affect

[Theory-Calculate C (V)]
The following analysis is presented with the intent of helping the reader to understand and is not intended to be limited by the theoretical concepts presented.

Referring to FIGS. 2a-2c, to understand the operation of the MIIM varactor and to estimate the capacitance as a function of applied voltage, we can start by solving the electrostatic problem of charge and electric field distribution shown in the figure. it can. 2a-2c are plots of potential E, charge ρ, and electric field strength ε, respectively, for distance x. A first (high) I ₁ barrier of thickness d ₁ , an electron affinity χ ₁ , and a dielectric constant ε ₁ , and a second (low) I ₂ barrier of thickness d ₂ , an electron affinity χ ₂ , and a dielectric constant ε ₂ Starting with a MIIM structure comprising M ₁ , I ₁ , I ₂ , and M ₂ above. These barriers are bounded by a left metal M ₁ having a work function Φ _M1 and a right metal M ₂ having a work function Φ _M2 . The barrier heights shown in FIG. 2 are all positive numbers, and are obtained by the following equations.

ここで、ｈ（バー）はプランク定数であり、ｍは電子質量であり、Ｅは伝導帯端上のエネルギーである。 Finally, a voltage of magnitude _{V b} between _M 1, _{M 2} of MIIM. This analysis makes several important assumptions. First, assume that the first (high) barrier is high enough and / or wide enough to ignore the electrons tunneling through it. Otherwise, it is necessary to take into account the tunnel speed between between, and a charge pool and a second metal M ₂ of the first metal M ₁ and the charge pool. In such a three-level system, the electron density and pseudo-Fermi energy of the charge pool change with changes in tunnel velocity, so this complexity is avoided in the initial analysis. Similarly, it is assumed that the free charge density in the first barrier is negligible. Next, assume that the density of states of the second (low) barrier material is not modified by quantum mechanical reflection. In other words, quantum confinement and triangular quantum well formation are ignored. This assumption seems to be accurate because the usual barrier material is an amorphous oxide and it is believed that any quantum confinement effect is destroyed by the lack of long-range electron coherence. Third, although these barriers have an amorphous nature, we ignore the band tail state in the density of states and simply use a parabolic band model with a single effective electron mass. This density of states is obtained by the following equation.

Here, h (bar) is the Planck constant, m is the electron mass, and E is the energy on the conduction band edge.

ここで、ｆ（Ｅ）は電子のフェルミ分布である。なお、図２ａに示すエネルギー仕様を用いており、ここで第１の金属のフェルミ準位はゼロであり、電位エネルギーＥは縦軸の下に行くにつれて正の方向に増加する。障壁Ｉ_２の電荷密度が電位の関数であることがわかっているので、電位分布Ｖ（χ）、障壁Ｉ_２の電荷密度Ｑ_ｃ、および２つの金属の界面における電荷密度Ｑ_１およびＱ_２を解くのに適当な境界状態を有するポアソン方程式を用いてもよい。なお、図２ｂは、これらの各種の電荷密度値を説明するためにｘの関数としての電荷密度をプロットする。アノードＭ_１電荷Ｑ_１（Ｖ_ｂ）のバイアス電圧依存性より、キャパシタンスをｄＱ_１／ｄＶ_ｂとして計算することができる。 To calculate the free electron charge density at a given point χ of the second (low) I ₂ barrier, it is only necessary to know the barrier potential V (χ). At this time, the charge density is as follows.

Here, f (E) is the Fermi distribution of electrons. Note that the energy specification shown in FIG. 2a is used, where the Fermi level of the first metal is zero, and the potential energy E increases in the positive direction as it goes down the vertical axis. Since the charge density of the barrier I ₂ is known to be a function of the potential, the potential distribution V (χ), the charge density Q _c of the barrier I ₂ , and the charge densities Q ₁ and Q ₂ at the interface of the two metals are A Poisson equation with appropriate boundary states to solve may be used. Note that FIG. 2b plots the charge density as a function of x to illustrate these various charge density values. From the bias voltage dependency of the anode M ₁ charge Q ₁ (V _b ), the capacitance can be calculated as dQ ₁ / dV _b .

上記式６の両辺に２・ｄＶ／ｄχをかけることにより、次を得ることができる。

両辺を積分することにより、次を得ることができる。

Next, the Poisson relationship between the electric charge and the potential in the barrier I ₂ is obtained by the following equation.

By applying 2 · dV / dχ to both sides of Equation 6, the following can be obtained.

By integrating both sides, the following can be obtained:

これらの境界条件により、積分定数は以下のとおりとなる。

すると、電界を求める式は以下のとおりとなる。

Next, the integration constant C can be solved using the boundary condition of the I ₂ -M ₂ interface. This point (ie, the I ₂ -M ₂ interface) is called χ = 0, and a specification is used in which χ increases in the positive direction toward the left and toward I _{1 as} seen in FIGS. The potential at χ = 0 is known. Simply the -φ _2. If φ ₂ >> kT so that the charge density at this time can be ignored, the electric field at χ = 0 is given by Q ₂ / ε ₂ . Then, the χ = 0 boundary condition is as follows.

Due to these boundary conditions, the integration constant is as follows.

Then, the equation for obtaining the electric field is as follows.

ここで、ｈは有限要素幅（ｄχ）であり、±はＶ（χ）の勾配によって決まる。Ｖ’（χ）＞０のとき符号は負であり、それ以外の場合は符号は正である。 Since the Poisson equation is a highly nonlinear second-order differential equation, it is necessary to use a numerical method to solve V (χ) and Q _c . The known Taylor method works well because dV / dχ is known (see, for example, Schaum's “Outline of Theory and Problems of Differential Equations” McGraw Hill (1973), which is incorporated herein by reference. ). Divide the second barrier into finite elements along the x-axis. Starting from χ = 0, which is known to be V (χ) and V ′ (χ), we work towards χ = d ₂ . To begin this process must assume the value of Q _2. Later, boundary conditions will be used to find the correct value of Q ₂ for a given bias voltage. Using the Taylor method, continuous potential values are obtained from the following equation:

Here, h is a finite element width (dχ), and ± is determined by the gradient of V (χ). The sign is negative when V ′ (χ)> 0, and the sign is positive otherwise.

Since V (χ) is solved from the temporary value of Q ₂ , Q _c can be solved as follows.

ここで、φ_ｃ＝Ｖ_ｎｍａｘである。Ｑ_２（Ｖ_ｂ）を解くためには、Ｑ_ｃについての先の２つの式を同等とみなしてＱ_２を解くことができる。 The boundary condition that can solve the correct value of Q _{2 for} a given bias voltage is obtained from the following equation:

Here, φ _c = V _nmax . To solve Q _{2 (V b)} it can be solved _{Q 2} is regarded as equivalent to two equations above for _{Q c.}

Since Q ₂ (V _b ) has been obtained, it is inserted into Equation 15 to obtain Q ₁ , and Q ₁ (V _b ) is obtained. Finally, the differential capacity of MIIM is obtained from the following equation.

FIG. 3 includes a coordinate system generally designated by reference numeral 40 and shows the capacitance plotted against voltage for a given MIIM device. The inserted coordinate system 42 shows an energy band diagram 44, where d ₁ = 2 nm, d ₂ = 8 nm, ε ₁ = 10ε ₀ , ε ₂ = 60ε ₀ , φ ₁ = 1.0 eV, φ ₂ = 0. For a given MIIM device with 1 eV and φ ₁₂ = 0.9 eV, the energy in electron volts versus distance in angstroms is shown. The plots are given for four different temperatures 150 ° K, 200 ° K, 250 ° K, and 300 ° K, indicated by reference numbers 45, 46, 47, and 48, respectively.

図３のＭＩＩＭ構造について、Ｖ_ｔｈはほぼ０．２５Ｖである。実際の利用においては、すぐ後でより詳細に説明するように、ゼロバイアスにおけるキャパシタンスの最大変化が生じてバイアスの供給が必要とならないように、Ｖ_ｔｈ＝０とすることが有利である場合がある。 In light of FIG. 3, the general trend is clear. In the negative bias voltage, and the threshold voltage V _th voltage lower than the capacitance of MIIM approaches the series of the two geometry capacitance of the dielectric I ₁ and I _2. Above the threshold, the capacitance approaches that of the first (higher) I ₁ barrier only. As the temperature decreases toward T = 0K, the varactor will switch between these two capacitance values more rapidly. Threshold voltage is a positive voltage applied to the insulator wells in the Fermi energy level of M _2, is obtained from the following equation.

For the MIIM structure of FIG. 3, _Vth is approximately 0.25V. In practical use, it may be advantageous to have V _th = 0 so that the maximum change in capacitance at zero bias occurs and no bias supply is required, as will be described in more detail later. is there.

Turning now to FIGS. 4a and 4b, which are energy band diagrams generally indicated by reference numbers 50 and 60, respectively, for a “zero bias” MIIM device that is very advantageous, the bias supply voltage for the varactor to operate. _Vth is moved to zero volts so that is not required. FIG. 4a shows an energy band configuration with a bias of V = 0, and FIG. 4b shows an energy band configuration with a bias of V = V ⁺ . To achieve this, φ ₂ is set equal to zero by selecting Φ _M2 = χ ₂ . That is, the work function of M ₂ is equal to the electron affinity χ ₂ of I ₂ . This change Compared with Figure 2a to Figure 4b, for Figure 2a shows a non-zero value phi _2, it is clearly visible. Furthermore, as shown in FIG. 4b, phi ₁ is set equal to phi _12.

From the above analysis and taking into account all of the above devices and examples, in order to maximize the capacitance swing, the first barrier I ₁ layer is made very thin and very high and the second barrier it can be seen that must layer I ₂ rather broadly and lower. It may be advantageous to make ε ₂ <ε ₁ if possible, in order to distribute the electric field more strongly so as to favor the charge storage region. Of course, practical limits on materials and device speed (discussed below) will limit the achievable performance.

[Alternative varactor configuration]
Alternative metal-insulator varactor structures can be considered and it is understood that the present invention is not limited to the MIIM structure described above. Next, such an alternative structure will be described.

Here, the metal-insulator-metal (MIM) varactor energy band diagrams are respectively shown for the case where the bias generally indicated by reference numeral 60 is V = 0 and the bias indicated generally by reference numeral 62 is V = V ^+. Attention is now directed to FIGS. First and second metal layers are each indicated as M ₁ and M _2, indicating an insulator and I. In the fabrication of the MIM varactor, the negative barrier height is formed by selecting the metal M ₂ having the electron affinity χ and work function Φ _M2 of the insulator I as follows.

Therefore, the negative barrier height is formed at the boundary 64 between the insulator I and the metal M _2. As can be seen from a comparison of FIGS. 5a and 5b, the in this MIM structure, the negative barrier between the insulation I and the metal M ₂ form a charge well 66 width is modulated by an applied voltage, the applied voltage The capacitance changes according to.

Another alternative varactor structure is shown in the energy band diagram of a metal-insulator-metal (MIIIM) varactor for the case where the bias, generally indicated by reference numeral 70, is V = 0, and generally by reference numeral 72, respectively. 6a and 6b illustrate the case where the bias to be applied is at V = V ⁺ . The first and second metal layers are denoted as M ₁ and M ₂ , respectively, and are denoted as an insulating layer structure 74 including a first insulating layer I ₁ , a second insulating layer I ₂ , and a third insulating layer I ₃ . The device is configured to create a charge well 76 at I ₂ near the boundary 78 between I ₁ and I ₂ . Note that the devices of FIGS. 6a and 6b represent zero bias devices. Since the MIIIM structure is quite similar in operation and spirit to the MIIM varactor, detailed analysis is not performed for convenience. Furthermore, those skilled in the art will readily be able to create MIIM devices from the above description.

[Frequency response and performance]
The speed or frequency response of the varactors described herein is determined by the speed of charge movement in and out of the charge pool. In the following, studies will be limited to the MIIM varactor structure, but it is considered that alternative structures can be fully understood by this study.

Referring to Figures 1a and 1b again, when a positive voltage is applied to increase the varactor, electrons by tunneling from the metal M ₂ satisfy the charge pool 30. When the voltage decreases again, excess charge flows out of the pool through the following two processes. (1) returns to the metal _{M 2} Tunneling (2) returns to the metal _{M 2} in band transport across the conduction band of the insulator _{I 2.} Moving speed of these electrons move between the charge pool and the metal M ₂ determines the frequency response of the varactor.

ここで各種の要素は式１９と同じ符号が付される。 If the band transport across the insulator I ₂ although the too late in the case of amorphous insulating material, ignore it, for example, Blake J. Using current tunnel model as described in Eliasson doctoral thesis "METAL-INSULATOR-METAL DIODES FOR SOLAR ENERGY CONVERSION " (University of Colorado (2001)) calculates the tunnel current between the charge pool and the metal _{M 2} can do. This article is incorporated herein by reference. This calculation results in a differential resistance R ₂ (V) for the tunneling electrons. The title Here, the voltage V is a voltage between the charge pool and a metal M _2. Note that R ₂ (V) is not equal to R ₂ (−V) because the tunnel probability at V = 0 is not necessarily symmetric. Adding the capacitance C ₂ between the charge pool and the metal M ₂ , the simple small signal model of FIG. 7 can be constructed that can calculate the frequency response as follows:

Here, various elements are denoted by the same reference numerals as in Expression 19.

Referring to FIG. 2a in conjunction with FIG. 7, high speed varactor design requires R ₂ and C ₂ to be minimized. When I ₂ thickness _{(d 2)} and the barrier height of the (phi ₂₎ to minimize, but _{R 2} is reduced it is known _{that C 2} is increased. However, although R ₂ depends exponentially on the barrier height and thickness, but C ₂ depends only linearly, the net gain of the frequency response can be seen by reducing d ₂ and φ ₂ . The disadvantage of reducing these values is that the capacitance swing with respect to _Vth is reduced (see FIG. 3).

When the material is actually considered, the frequency response is lower than the ideal RC limit value. In particular, for amorphous insulator materials, localized band tail states, deep trap states, and common flat surface states are likely to occur. Electrons in this state extending below the conduction band “edge” of the insulator will cause long-term charging to the insulator I ₂ near the interface of the insulator 1 and will be partially applied. The voltage is shielded and the C (V) curve is changed.

  In broad terms, this document defines: The varactor is configured with first and second conductive layers spaced from each other so that a given voltage can be applied across the first and second conductive layers. The insulating structure includes at least one insulating layer disposed between the first and second conductive layers, and the charge varies in response to a change in the given voltage in cooperation with the first and second conductive layers. Configured to create a pool, the device capacitance value between the first and second conductive layers changes in response to the change in the given voltage. The insulating structure may include one layer, two separate layers, or two or more separate layers. One or more of the layers may be an amorphous material. A zero bias voltage version of this varactor is also described.

  While physical embodiments having various elements with the above specific orientations have been described, respectively, the present invention is not limited to various embodiments having various elements arranged in a wide range of arrangements and mutual orientations. Can be taken into account. For example, as discussed in detail in the incorporated US Pat. No. 6,563,185, other materials can be used in place of the metal layer, including but not limited to semiconductors and semi-metals. . Furthermore, the method described herein can be modified without limitation, for example, the order of the various procedures comprising the method can be changed. Accordingly, the examples are to be understood as illustrative and not restrictive, and the invention is not limited to the details described herein, It can be modified in scope.

The invention may be understood with reference to the following detailed description in conjunction with the drawings briefly described below. For clarity of explanation, some elements in the drawings are not drawn to scale. Furthermore, descriptive terms such as vertical and horizontal used in the various drawings are used for illustrative purposes only and are not intended to limit the useful orientation of the described structure or device. Absent.
FIG. 4 is an energy band diagram of the MIIM device of the present invention, showing an unbiased state. FIG. 4 is an energy band diagram of the MIIM device of the present invention, showing a biased state. FIG. 2 is an energy band diagram of the MIIM device of FIGS. 2b is a plot of charge versus lateral distance for the biased device of FIG. 2a. 2b is a plot of field strength versus lateral distance for the biased device of FIG. 2a. FIG. 5 is a plot of capacitance versus bias voltage for an exemplary MIIM varactor device made in accordance with the present invention, including an insertion plot of an energy band diagram for this exemplary MIIM. FIG. 4 is an energy band diagram of the MIIM device of the present invention, showing an unbiased state, where the device is configured to be used without the need for a zero bias voltage. FIG. 4 is an energy band diagram of the MIIM device of the present invention, showing a biased state, where the device is configured to be used without the need for a zero bias voltage. FIG. 4 is an energy band diagram of a first alternative MIM device of the present invention, showing an unbiased state. FIG. 4 is an energy band diagram of a first alternative MIM device of the present invention, showing a biased state. FIG. 4 is an energy band diagram of a second alternative MIIM device of the present invention, showing an unbiased state. FIG. 4 is an energy band diagram of a second alternative MIIM device of the present invention, showing a biased state. It is a schematic diagram of a small signal model derived from the MIIM device structure.

Claims (22)

First and second conductive layers spaced apart from each other such that a given voltage is applied across the first and second conductive layers;
The first and second conductive layers include at least one insulating layer disposed between the first and second conductive layers, and the device capacitance value between the first and second conductive layers changes according to a given voltage. And an insulating structure configured to cooperate with the second conductive layer to produce a charge pool that changes in response to a change in a given voltage;
A varactor with   The method of claim 1, wherein the given voltage is used to bias the second conductive layer in a more positive direction than the first conductive layer to produce a corresponding increase in device capacitance value. Varactor.   The insulating structure includes at least a first insulating layer adjacent to the first conductive layer and a second insulating layer adjacent to the second conductive layer and adjacent to the first insulating layer, with a boundary therebetween. The varactor according to claim 1, wherein a charge pool is generated in the second insulating layer of the plurality of insulating layers in the vicinity of the boundary between the plurality of insulating layers and the first insulating layer.   The first insulating layer includes a first barrier height, the second insulating layer includes a second barrier height, and the first and second barrier heights increase from the second conductive layer according to the given voltage. The varactor according to claim 3, wherein the varactor is selected to generate a tunnel of electrons passing through two insulating layers and to inhibit the tunneling of electrons across the first insulating layer to generate the charge pool.   The varactor according to claim 4, wherein the first barrier height is higher than the second barrier height.   The varactor of claim 5, wherein the first barrier height is much higher than 1 eV and the second barrier height is lower than 0.3 eV. The first insulating layer includes a first capacitance value, and the second insulating layer includes a second capacitance value;
Application of a given voltage below a given threshold of a given voltage causes the device capacitance value to become a first capacitance value that causes the first conductive layer to become increasingly positive with respect to the second conductive layer. Approaches the series combination of the second capacitance value and
Application of a given voltage above a given threshold for a given voltage causes the device capacitance value to be greater than the first insulating layer, causing the first conductive layer to become increasingly negative with respect to the second conductive layer. The varactor of claim 3, wherein the varactor is adapted to approach a first capacitance value of   The varactor of claim 7, wherein the varactor is configured to set a threshold of a given voltage to zero volts, at least approximately zero volts.   The insulating structure includes only one layer of insulating material between the first and second conductive layers, the insulating material cooperating with a second layer work function of the second conductive layer and the insulating material and the An insulating barrier height that generates a negative barrier height in response to a given voltage with the second conductive layer, wherein the charge pool is in the vicinity of the boundary with the second conductive layer in the insulator material; The varactor according to claim 1, which is formed.   The varactor of claim 9, wherein the charge pool has a width that is modulated by a change in a given voltage.   The varactor according to claim 1, wherein the insulating structure includes at least one layer made of an amorphous material. The first and second conductive layers are spaced apart from each other such that a given voltage is applied across the first and second conductive layers;
The first and second conductive layers include at least one insulating layer disposed between the first and second conductive layers, and the device capacitance value between the first and second conductive layers changes according to a given voltage. And an insulating structure configured to cooperate with the second conductive layer to generate a charge pool that changes in response to a change in a given voltage.
How to make a varactor.   Biasing the second conductive layer more gradually in the positive direction than the first conductive layer using the given voltage in cooperation with the first and second conductive layers. 13. The method of claim 12, wherein the method is configured to generate a corresponding increase in device capacitance value.   The insulating structure includes at least a first insulating layer adjacent to the first conductive layer and a second insulating layer adjacent to the second conductive layer and adjacent to the first insulating layer, with a boundary therebetween The method of claim 12, wherein the charge pool is defined and configured to generate a charge pool in a second insulating layer of the plurality of insulating layers in the vicinity of the boundary of the plurality of insulating layers with the first insulating layer.   Generating the charge pool by generating tunneling of electrons from the second conductive layer through the second insulating layer in response to the given voltage, forbidden tunneling of electrons across the first insulating layer, and 15. The method of claim 14, wherein the first insulating layer includes a first barrier height and the second insulating layer is selected to include a second barrier height.   The method of claim 15, wherein the first barrier height is higher than the second barrier height.   The method of claim 16, wherein the first barrier height is selected much higher than 1 eV and the second barrier height is selected lower than 0.3 eV. The first insulating layer includes a first capacitance value, and the second insulating layer includes a second capacitance value;
Application of a given voltage below a given threshold of a given voltage causes the device capacitance value to become a first capacitance value that causes the first conductive layer to become increasingly positive with respect to the second conductive layer. Approaches the series combination of the second capacitance value and
Application of a given voltage above a given threshold for a given voltage causes the device capacitance value to be greater than the first insulating layer, causing the first conductive layer to become increasingly negative with respect to the second conductive layer. The method of claim 14, wherein the method approaches a first capacitance value of:   The method of claim 18, wherein the varactor is configured to set a threshold of a given voltage to zero volts, at least approximately zero volts.   The insulating structure includes only one layer of insulating material between the first and second conductive layers, the insulating material cooperating with a second layer work function of the second conductive layer and the insulating material and the An insulating barrier height that creates a negative barrier height in response to a given voltage with the second conductive layer, and the charge pool is formed in the insulator material near the boundary with the second conductive layer The method of claim 12, configured to be performed.   21. The method of claim 20, wherein the charge pool has a width that is modulated by a change in a given voltage.   13. The method according to claim 12, wherein at least one layer of amorphous material is used as part of the insulating structure.

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