MEMRISTIVE STRUCTURE AND METHOD FOR PROVIDING MODULATION BETWEEN RESISTANCE STATES OF A MEMRISTIVE STRUCTURE

Abstract

Method for providing a symmetrical modulation between resistance states of a memristive structure, wherein the method comprises: Forming a first electrode and a second electrode, each of which comes into direct contact with an insulating substrate contained therein (102); Forming an anode that is in contact with the first and second electrodes (104); Formation of an ion conductor across the anode (106); Forming a cathode over the ion conductor (108); Forming a third electrode above the cathode (110); and Enabling bidirectional transport of ions between the anode and the cathode, resulting in a resistance adjustment of the memristive structure, with the anode and the cathode being formed from metastable materials (112).

Description

TECHNICAL AREA

[0001] The present invention relates generally to semiconductor units and more specifically to a memristive structure of a memristive unit. BACKGROUND

[0002] A memristive unit is an electronic device capable of changing its conductivity. For example, a memristive unit can provide a high conductivity state when a first voltage is applied, and a low conductivity state when a second voltage is applied. A memristive unit can be made from a memristive material that, after electroforming or conditioning, provides two or more conductivity states.A memristive unit can be used in a variety of electronic applications, such as in non-volatile memory, memory arrays, 3D memory, switching operations, reconfigurable and fast-tunable bandpass and stopband filters, reversibly field-programmable fuse arrays, sample-hold circuits, programmable resistor elements within a variable-gain amplifier, analog-to-digital converters, and the like. Furthermore, a memristive unit can be integrated with other electronic components.

[0003] In US 2014 / 0030593 A1, an active electrode material consisting of an organic radical polyimide (organic radical polyimide electrode active material) is described. WO 2016 / 171700 A1 discloses optical computing that incorporates spectrally programmable memristor devices. In US 2011 / 0309321 A1, memristors with a switching layer comprising a multi-phase composite are taught. A publication by FULLER, ELLIOT J., et al.: Li-Ion Synaptic Transistor for Low Power Analog Computing, Adv. Mater. 2017, 29, 1604310, deals with a Li-ion-based transistor. US 2013 / 0140648 A1 describes an electrochemical transistor and US 2011 / 0248381 A1 discloses memristive devices that are multilayered. SUMMARY

[0004] According to one embodiment, a method for producing a memristive structure for symmetrical modulation between resistance states is provided.

[0005] The process involves forming a first electrode (e.g., a source) and a second electrode (e.g., a drain) over an insulating substrate, forming an anode in contact with the first and second electrodes, forming an ion conductor over the anode, forming a cathode from the same material as the anode over the ion conductor, forming a third electrode (e.g., a gate electrode) over the cathode, and enabling bidirectional transport of ions between the anode and the cathode, which are formed from the same mixed-conducting material, resulting in a resistance adjustment of the original mixed-conducting layer, with the anode and the cathode being formed from metastable mixed-conducting materials with a conductivity that depends on the ion concentration.

[0006] According to one embodiment, a memristive structure is provided for symmetrical modulation between resistance states. The memristive structure includes a first electrode and a second electrode formed over an insulating substrate, an anode in contact with the first and second electrodes, an ionic conductor formed over the anode, a cathode made of the same material as the anode and formed over the ionic conductor, and a third electrode formed over the cathode. The anode and cathode are formed from metastable materials with embedded ions, thus enabling bidirectional ion transport between the anode and cathode, resulting in a resistance adjustment of the original mixed conductor layer.

[0007] According to one embodiment, a memristive structure is provided for symmetrical modulation between resistance states. The memristive structure includes an ion conductor formed between a metastable anode and a metastable cathode, as well as electrodes located adjacent to the metastable anode and the metastable cathode. Symmetrical modulation between resistance states results from a bidirectional transfer of ions between the metastable anode and the metastable cathode, which contain the same mixed-conducting material.

[0008] It should be noted that the exemplary embodiments are described with reference to various topics. In particular, some embodiments are described with reference to method claims, while other embodiments are described with reference to apparatus claims. However, a person skilled in the art will understand from the foregoing and the following description that, unless otherwise indicated, in addition to any combination of features belonging to one type of topic, any combination of features relating to other topics, in particular features of the method claims and features of the apparatus claims, is also to be considered as if it were described in this document.

[0009] These and other features and advantages will become apparent from the following detailed description of illustrative embodiments, which should be read in conjunction with the accompanying drawings. Brief description of the multiple views of the drawings

[0010] In the following description of preferred embodiments, the invention provides details with reference to the following figures, in which: Fig. 1 a cross-sectional view of a memristive unit with three connections according to the present invention; Fig. 2 a cross-sectional view of the memristive unit of Fig. 1 according to the present embodiment, in which electrical impulses are applied between an upper electrode and a lower electrode of the three-terminal memristive unit to enable a writing operation; Fig. 3 a cross-sectional view of the memristive unit of Fig. 1 according to the present invention, wherein electrical impulses are applied between the lower electrodes of the memristive unit with three terminals to enable a reading process; Fig. 4 a physical arrangement of the memristive unit with three connections of Fig. 1 according to the present invention for an application relating to a resistive processing unit (RPU); Fig. 5 is a graphical representation showing a change of resistors for the RPU application according to the present invention; Fig. 6 is a graphical representation showing an asymmetric and a symmetric modulation between resistance states according to the present invention; Fig. Figure 7 shows a block diagram / flowchart of a method for producing a memristive structure for symmetrical modulation between resistance states according to the present invention.

[0011] In the drawings, identical or similar reference symbols consistently represent the same or similar elements. DETAILED DESCRIPTION

[0012] Embodiments according to the present invention provide methods and units for a three-terminal memristive unit. Generally, a memristive unit is a hypothetical, non-linear, passive electrical component relating to the interaction of electric charge and magnetic flux. The electrical resistance of the memristor is not constant but depends on the history of the current that has previously flowed through the unit; for example, its instantaneous resistance depends on how much electric charge has flowed through it in the past and in which direction. An external switch is typically required to prevent reverse discharge in the memristive unit. The main features of a memristive unit are analog modulation, decoupled read / write operations, bidirectional modulation, and symmetrical modulation.The memristive unit with three terminals of the exemplary embodiments is formed with metastable materials or layers of the same conductive material such that a symmetrical modulation between resistance states is achieved without the use of an external switch, since a resistance adjustment or change occurs due to the transfer or transport of ions between the metastable layers without the incorporation of an EMF / voltage.

[0013] Embodiments according to the present invention provide methods and units for a three-terminal memristive unit that achieves symmetrical modulation between resistance states. Symmetrical resistance modulation is necessary for machine learning to occur in a backpropagation-trained neural network composed of these units. For symmetrical modulation to occur, the memristive unit must exhibit metastability, that is, phase separation, which is necessary to maintain a constant chemical potential of mobile ions between storage elements (e.g., metastable anode and cathode layers). The exemplary embodiments introduce a three-terminal memristive unit in which an electrically insulating electrolytic layer is formed between a metastable anode and a metastable cathode.Ions are reversibly moved from the cathode into / out of the anode, and a current is generated by the source and / or drain (S / D) voltage, depending on the ion concentration in the channel. The movement of ions between the metastable anode and the metastable cathode causes a change in resistance within the memristive unit, resulting in symmetrical modulation. Furthermore, the chemical potential of the ions between the metastable anode and the metastable cathode is kept close to zero.

[0014] Embodiments according to the present invention provide methods and units for a memristive unit with three terminals, wherein the read and write operations are separate in the memristive unit, so that simultaneous writing and reading operations are enabled, while preventing the write operation from reading the unit.

[0015] Memory cells are common components of an integrated circuit. A single memory cell contains a unit that exists in two or more stable memory states. The act of "writing" to the unit involves placing the unit into a desired memory state, and the act of "reading" the unit involves determining which of the memory states the unit is in.

[0016] Writing to the unit may involve transmitting a programming voltage to the unit, where the programming voltage is sufficient to cause a change from one memory state to another. Reading the unit may involve measuring an electrical parameter that is affected by the unit's memory state, such as measuring a current flowing through the unit. It may be desirable for the reading to be performed under conditions that do not change the unit's memory state, so that the reading process does not "write" to the unit. The three-terminal memristive unit of the exemplary embodiments prevents such undesired processes and thus prevents reverse discharge.

[0017] The term "exemplary" is used herein to mean "serving as an example, case, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Similarly, the term "embodiments" does not require that all embodiments include the discussed feature, advantage, or mode of operation.

[0018] As used herein, the term "approximately," which modifies the quantity of an ingredient, component, or reactant of the invention, refers to a variation in the numerical quantity that may occur, for example, due to typical measurement procedures and procedures for handling liquids used to prepare concentrates or solutions. Furthermore, a variation may occur due to an unintentional error in measurement procedures, differences in the manufacture, source, or purity of the ingredients used to prepare the compositions or to carry out the processes, and the like. In one aspect, the term "approximately" means that a value is within 10% of the stated numerical value. In another aspect, the term "approximately" means that a value is within 5% of the stated numerical value.In yet another aspect, the term "approximately" means that a value lies within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2% or 1% of the specified numerical value.

[0019] It is understood that the present invention is described with respect to a given illustrative setup; however, other setups, structures, substrate materials, process features, process steps, and / or blocks may be varied within the scope of the present invention. It should be noted that, for the sake of clarity, certain features may not be shown in all figures. This should not be interpreted as a limitation of any particular embodiment or representation, or of the scope of the claims.

[0020] Various illustrative embodiments of the invention are described below.

[0021] For the sake of clarity, this description does not include all features of a real-world application. It is obvious that developing any such real-world embodiment requires numerous application-specific decisions to achieve the developers' specific goals, such as compatibility with system-related and economic constraints, which vary from one application to another. Furthermore, it is evident that such development effort can be complex and time-consuming, but for a person skilled in the art who benefits from this invention, it is a routine process.

[0022] Fig. Figure 1 is a cross-sectional view of a memristive unit with three connections according to the present invention.

[0023] A structure 5 comprises a substrate 10. Within the substrate 10, a first electrode 12 (or a source) and a second electrode 14 (or a drain) are formed. An anode 16 is formed over a region of the first and second electrodes 12 and 14. An ionic conductor 18 (or an ionic conductive layer 18) is formed over the anode 16. The ionic conductor 18 can, for example, be an electrolyte. A cathode 20 is then formed over the ionic conductor 18. The anode 16 and the cathode 20 can be formed from metastable materials. The anode 16 and the cathode 20 can be formed from the same metastable materials. A third electrode 22 (or a gate electrode) is formed over the cathode 20. Thus, the structure 5 is a memristive unit with three terminals. A person skilled in the art may consider a reversal of the electrode order.The first and second electrodes can, for example, be positioned or arranged adjacent to the cathode 20, and the third electrode can be positioned or arranged adjacent to the anode 16. The anode 16 and the cathode 20 can be considered metastable mixed-conducting layers. The anode and cathode layers 16, 20 are dependent on the concentration of a mobile, intercalated ion. The anode and cathode layers 16, 20 consist of the same metastable material.

[0024] In one or more embodiments, the substrate 10 may, for example, be an insulator or an insulating material.

[0025] The first, second, and third electrodes 12, 14, and 22 can be made of inert metals. These inert metals can be, for example, platinum (Pt), gold (Au), iridium (Ir), etc. The inert metals do not oxidize during a tempering process in air at approximately 500 °C. In an exemplary embodiment, Pt is preferred when lithium cobalt oxide (LiCoO2) is used as the metastable material in the anode 16 and the cathode 20, since this reduces the temperature required for the formation of metastable phase-separated layers of mixed ionic-electronic conductors (MIECs) and high-temperature LiCoO2.

[0026] The anode 16 and the cathode 20 can be formed from metastable materials, such as MIECs. A metastable phase is a phase that is locally stable with respect to small fluctuations. Metastable materials exhibit a spinodal stability limit between a bimodal equilibrium curve that defines the metastable region. Within this stability limit, there is a miscibility gap that defines thermodynamically unstable compositions for the material; thus, the material forms two phases with varying concentrations, defined by stability and temperature. Metastable MIECs can include, for example, lithium cobalt oxide (Li₂CO₃). x CoO2), Lithium niobate (Li x NbO3), doped Li 4+x Ti5O 12 (LTO), Lithium titanate (Li x TiO2), lithium samarium nickelate (Li xSmNiO3), etc. The thickness of the anode 16 and the cathode 20 can be, for example, approximately 50 nm to approximately 1000 nm. The anode 16 and the cathode 20 are selected such that their resistance changes based on the ion concentration, as described further below. The initial ion concentrations of the anode 16 and the cathode 20 are selected or tuned to ensure the metastability of the anode 16 and the cathode 20. Chemical or electrical delitiation can be used to tune or adjust the initial concentrations of the metastable anode 16 and the metastable cathode 20. Metastability allows for symmetrical switching between states and is important for non-volatility.In other words, the symmetrical modulation between resistance states results from the bidirectional transfer of ions between the metastable anode and the metastable cathode, which contain the same mixed-conducting material.

[0027] The solid electrolyte layer 18 can be configured to be in contact with the anode layer 16 and the cathode layer 20, and it can be configured to electrically insulate the anode layer 16 from the cathode layer 20. For example, the electrolyte layer 18 could be lithium phosphorus oxynitride (LiPON).

[0028] Suitable materials for layer 18 of an electrolyte can also include lithium-ion complexes containing ethylene carbonate and diethyl carbonate, and other (e.g., acidic or alkaline) electrolytes that exhibit suitable ion transport properties. In lithium-ion applications in microbatteries, the electrolyte 18 is typically non-aqueous to prevent reaction with lithium metal components in the anode 16 and the cathode 20, and also to prevent evaporation during cathode annealing.

[0029] Suitable materials for the anode 16 and the cathode 20 include lithium, lithium cobalt oxide, lithium iron phosphate and other lithium metal phosphates, lithium manganese oxide, carbon and graphite or graphite infused with lithium ions.

[0030] Each of the layers (e.g., the anode 16, the cathode 20, the electrolyte 18) can be formed using conventional vacuum deposition techniques, thus enabling direct encapsulation of reactive layers before exposure to any environment. Exemplary methods include chemical or physical vapor deposition, flash evaporation, laser ablation, and co-evaporation. Physical vapor deposition (PVD) methods can include, for example, reactive or non-reactive sputtering processes. In sputtering, any conventional power supply can be used to generate the ion current (e.g., from Ar). + to generate ions) to the target, for example magnetron, DC, or pulsed DC power supplies. Suitable sputtering targets for the various barrier layer compositions can be designed as molten powder or pressed powder targets.

[0031] Fig. Figure 2 is a cross-sectional view of the memristive unit of Fig. 1 according to the present invention, wherein electrical impulses are applied between an upper electrode and a lower electrode of the memristive unit with three terminals to enable a writing operation.

[0032] In various embodiments, a voltage source 24 is connected between the second electrode 14 and the third electrode 22 to provide electrical pulses between them, enabling a writing operation. The voltage source 24 can also be connected between the first electrode 12 and the third electrode 22 to provide electrical pulses between them, enabling a writing operation. The electrical pulses cause a transfer of ions (or interstitial ions) between the anode 16 and the cathode 20. Due to the three-terminal design of the memristive unit, the read and write operations are separated, thus allowing simultaneous reading and writing while preventing unintended reading of the unit.

[0033] Fig. Figure 3 is a cross-sectional view of the memristive unit of Fig. 1 according to the present invention, wherein electrical impulses are applied between the lower electrodes of the memristive unit with three terminals to enable a reading process.

[0034] In various embodiments, a voltage source 26 is connected between the first electrode 12 and the second electrode 14 to provide electrical pulses between them, enabling a reading operation. These electrical pulses cause a transfer of ions (or interstitial ions) between the anode 16 and the cathode 20. The relative concentration of ions in the anode 16 controls the read resistance of the memristive unit. Again, the read and write operations of the unit are separated due to the three-terminal design of the memristive unit, thus allowing simultaneous reading and writing while preventing unintended reading of the unit.

[0035] Therefore, in the Fig. 1, Fig. 2 to Fig. 3. No external field-effect transistor (FET) switch is necessary to prevent reverse discharge. Instead, bidirectional ion movement between anode 16 and cathode 20 results in a resistance setting of the memristive structure 5. This resistance setting is accompanied by resistive switching to maintain symmetrical modulation between the resistance states. Furthermore, the chemical potential of the ions between anode 16 and cathode 20 is kept close to zero.

[0036] Fig. 4 is a physical arrangement of the memristive unit with three terminals of Fig. 1 according to the present invention for an application relating to a resistive processing unit (RPU).

[0037] The physical array 30 is a right-angled array of memristive units 5, where a set of x-lines and a set of y-lines are connected at each xy-intersection by a memristive unit 5. The array can be an mx n array configured to be modified according to the developer's requirements. This forms a resistive processing unit (RPU) for use in a hardware implementation, such as a neural network.

[0038] The first electrode 12 is connected to a current line 34, the second electrode 14 is connected to a voltage line 32, and the third electrode 22 is connected to a voltage line 36. Applying a low reading bias to the voltage line 32 induces a current in line 34.

[0039] Fig. Figure 5 is a graphical representation illustrating a resistance change for the RPU application according to the present invention.

[0040] Figure 40 depicts electrical pulses applied between the electrodes of the memristor unit 5. Arrows 41 represent the memristive unit 5 in a first resistance state, and arrows 43 represent the memristive unit 5 transitioning to a second resistance state. Thus, a constant ion movement between the anode 16 and the cathode 20 results in a change in resistance, maintaining a chemical potential difference between the ions at or near zero. The reversible adjustment of the ion concentrations between layers 16 and 20 to zero EMF allows symmetrical resistance states to be maintained during operation of the memristive units 5.

[0041] Fig. Figure 6 is a graphical representation showing an asymmetric and a symmetric modulation between resistance states according to the present invention.

[0042] The graphical representation on the left shows an asymmetric modulation 52, and the graphical representation on the right shows a symmetric modulation 54. An asymmetric curve 51 is shown for the asymmetric modulation 52. Symmetric curves 55 and 57 are shown for the symmetric modulation 54. The exemplary embodiments of the present invention achieve symmetric modulation. For a machine learning process to take place in the memristive unit, the resistance must be modulated symmetrically. In other words, if n positive pulses are applied to the unit, n negative pulses will return the unit to the same resistance. To achieve symmetric modulation between resistance states, each state of the memristive unit must have the same potential.Therefore, the memristive unit must include metastability, meaning that phase separation is necessary to achieve a constant chemical potential of mobile ions between storage elements (e.g., the anode 16 and the cathode 20).

[0043] Fig. Figure 7 is a block diagram / flowchart for a method for producing a memristive structure for symmetrical modulation between resistance states according to the present invention.

[0044] In Block 102, a first electrode and a second electrode are formed over an insulating substrate. The first and second electrodes can be made of inert metals.

[0045] In block 104, an anode is formed which is in contact with the first and second electrodes, the anode being a channel forming a metastable MIEC.

[0046] In block 106, an ionic conductor is formed across the channel that forms a MIEC. This ionic conductor could, for example, be an electrolyte.

[0047] In block 108, a cathode made of the same material as the anode is formed above the ion conductor, with the cathode being a metastable cathode.

[0048] In block 110, a third electrode is formed above the cathode. The third electrode can, for example, be made of an inert metal.

[0049] In Block 112, bidirectional ion movement is enabled between the metastable anode and the metastable cathode, resulting in a resistance adjustment of the memristive structure. This resistance adjustment is accompanied by a change in resistance to maintain symmetrical modulation between the resistance states. The difference in the chemical potential of the ions between the metastable anode and the metastable cathode is kept at or near zero to provide this symmetrical modulation. The anode and cathode can also be considered reservoirs for mobile ions, allowing them to drift / diffusion in both directions (e.g., towards and away from the anode / cathode). The equal potential and the presence of the electrolyte prevent back-diffusion. The ions are moved from the anode to the cathode by drift in an applied electric field.It should be noted that only ions are transferred or transported between the metastable anode layer and the metastable cathode layer. The three-terminal memristive structure does not require an external FET switching mechanism to provide non-volatile resistance states. The three-terminal memristive unit can, for example, be used as a neuron in a neural network.

[0050] It is also understood that when an element, such as a layer, area, or substrate, is described as lying "on" or "over" another element, it may be located directly on top of the other element, or there may be intermediate elements. Conversely, when an element is described as lying "directly on" or "directly above" another element, there are no intermediate elements. It is also understood that when an element is described as "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or there may be intermediate elements. Conversely, when an element is described as "directly connected" or "directly coupled" to another element, there are no intermediate elements.

[0051] The present embodiments may include a design for an integrated circuit chip that can be created in a graphical computer programming language and stored on a computer storage medium (such as a floppy disk, tape, physical hard disk, or virtual hard disk, such as in a storage access network). If the designer does not manufacture chips or the photolithographic masks used to manufacture chips, the designer may send the resulting design directly or indirectly to such facilities by physical means (e.g., by providing a copy of the storage medium in which the design is stored) or electronically (e.g., via the Internet). The stored design is then converted into the appropriate format (e.g.,GDSII) is converted for the production of photolithographic masks containing multiple copies of the relevant chip design, which are to be formed on a wafer. The photolithographic masks are used to define areas of the wafer (and / or the layers on it) that are to be etched or otherwise processed.

[0052] Methods such as those described herein can be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the manufacturer in raw wafer form (that is, as a single wafer containing multiple unpacked chips), as a bare chip, or in a packed form. In the latter case, the chip is mounted in a single-chip pack (such as a plastic substrate with leads attached to a mainboard or other higher-level support) or is mounted in a multi-chip pack (such as a ceramic substrate having either surface interconnects or buried interconnects, or both).In any case, the chip is then integrated with other chips, discrete circuit elements, and / or other signal processing units as part of either (a) an intermediate product, such as a motherboard, or (b) a final product. The final product can be any product that incorporates integrated circuit chips, ranging from toys and other low-end applications to sophisticated computer products that include a display, keyboard or other input device, and a central processing unit.

[0053] It is also understood that material compounds are described in terms of the listed elements, e.g., SiGe. These compounds contain varying proportions of the elements within the compound; for example, SiGe contains Si x Ge 1-x, where x is less than or equal to 1, etc. Furthermore, the compound may contain additional elements and may still perform its function according to the present embodiments. Compounds with additional elements are referred to herein as alloys.

[0054] A reference in the description to "one embodiment" or "an embodiment," as well as to further variations, means that a specific feature, structure, property, and so on, described in connection with the embodiment, is included in at least one embodiment of the present invention. Thus, the various forms of the phrase "in one embodiment" or "in an embodiment," as well as any other variations that appear at different points in the description, do not necessarily all refer to the same embodiment.

[0055] It is evident that the use of any of the following “ / ”, “and / or”, and “at least one of”—for example, in the cases “A / B”, “A and / or B”, and “at least one of A and B”—is intended to include the selection of only the first listed option (A), the selection of only the second listed option (B), or the selection of both options (A and B). As a further example, such wording in the cases “A, B and / or C” and “at least one of A, B and C” is intended to include the selection of only the first listed option (A), the selection of only the second listed option (B), the selection of only the third listed option (C), the selection of only the first and second listed options (A and B), the selection of only the first and third listed options (A and C), the selection of only the second and third listed options (B and C), or the selection of all three options (A, B, and C).This can be extended to as many points as listed, as is readily apparent to an expert in this and related fields.

[0056] The terminology used herein serves only to describe specific embodiments and is not intended to be limited to exemplary embodiments. As used herein, the singular forms "a," "an," "an," and "the" are to be understood as including the plural forms unless the context clearly indicates otherwise. It is further understood that the terms "indicates," "indicating," "includes," and / or "including," when used herein, specify the presence of specified features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more further features, integers, steps, operations, elements, components, and / or groups thereof.

[0057] Spatially relative terms, such as "below," "under," "lower," "above," "upper," and the like, may be used herein, for the sake of simplicity, to describe the relationship of one element or feature to another element (other elements) or feature (other features), as illustrated in the figures. It is understood that the spatially relative terms are intended to encompass various orientations of the unit in use or operation in addition to the orientation shown in the figures. For example, if the unit in the figures is inverted, the elements described as "below" or "underneath" other elements or features are then oriented "above" them. Thus, the term "below" can encompass both an orientation above and below.The unit may be oriented differently (rotated by 90 degrees or in other orientations), and the spatially relative descriptors used herein may be interpreted accordingly. Furthermore, it is understood that when a layer is described as lying "between" two layers, it may be the only layer between the two layers, or there may be one or more intervening layers.

[0058] It is understood that, although the terms first / first / first, second / second / second, etc., may be used herein to describe different elements, these elements are not limited to these terms. These terms are merely used to distinguish one element from another. Thus, a first element discussed below may be referred to as a second element without deviating from the scope of the present concept.

Claims

[1] Method for providing a symmetrical modulation between resistance states of a memristive structure, wherein the method comprises: Forming a first electrode and a second electrode, each of which comes into direct contact with an insulating substrate contained therein (102); Forming an anode that is in contact with the first and second electrodes (104); Formation of an ion conductor across the anode (106); Forming a cathode over the ion conductor (108); Forming a third electrode above the cathode (110); and Enabling bidirectional transport of ions between the anode and the cathode, resulting in a resistance adjustment of the memristive structure, with the anode and the cathode being formed from metastable materials (112). [2] Method according to claim 1, wherein the resistance setting is accompanied by a resistance change to maintain the symmetrical modulation between the resistance states. [3] Method according to claim 1, wherein the first, second and third electrodes are formed from inert metals. [4] Method according to claim 1, wherein the metastable materials are metastable phase-separated ionic-electronic mixed conductors (MIECs) whose conductivity depends on a concentration of an embedded mobile ion. [5] Method according to claim 1, wherein electrical impulses are applied between the first electrode (12) and the third electrode (22) or between the second electrode (14) and the third electrode (22) to enable a writing operation. [6] Method according to claim 1, wherein electrical impulses are applied between the first (12) and the second electrode (14) to enable a reading process. [7] Method according to claim 1, wherein movement of the ions is enabled by applying a voltage to allow simultaneous operation of read and write operations. [8] Method according to claim 1, wherein the difference in the chemical potential of ions between the anode and the cathode is kept at approximately zero. [9] Method according to claim 1, wherein the first and the second electrode are substantially planar. [10] Memristive structure (5) for providing symmetrical modulation between resistance states, wherein the structure has: a first electrode (12) and a second electrode (14), each of which comes into direct contact with an insulating substrate (10) located therein; an anode (16) which is in contact with the first (12) and the second electrode (14); an ion conductor (18) formed above the anode; a cathode (20) formed above the ion conductor; and a third electrode (22) which is formed above the cathode; wherein the anode (16) and the cathode (20) are formed from metastable materials which allow bidirectional transport of ions between the anode and the cathode, resulting in a resistance adjustment of the memristive structure. [11] Structure according to claim 10, wherein the resistance setting is accompanied by a resistance change to maintain the symmetrical modulation between the resistance states. [12] Structure according to claim 10, wherein the first, second and third electrodes are formed from inert metals, the inert metals being selected from one or more of the group consisting of platinum (Pt), gold (Au) and iridium (Ir). [13] Structure according to claim 10, wherein the metastable materials consist of metastable phase-separated ionic-electronic mixed conductors (MIECs) whose conductivity depends on a concentration of embedded mobile ions. [14] Structure according to claim 10, wherein electrical impulses are applied between the first electrode and the third electrode or between the second electrode and the third electrode to enable a writing process. [15] Structure according to claim 10, wherein electrical impulses are applied between the first and the second electrode to enable a reading process. [16] Structure according to claim 10, wherein the movement of the ions is enabled by applying a voltage, wherein the reading and writing process in a memristive unit of the memristive structure is separated due to the construction in the memristive unit with three terminals, so that a simultaneous reading and writing process is enabled in the memristive unit, while an unwanted reading of the unit is prevented. [17] Structure according to claim 10, wherein a constant ion movement between the anode (16) and the cathode (20) results in a change of resistance, such that a difference in the chemical potential of the ions in the structure is kept at or near zero. [18] Memristive structure (5) for providing symmetrical modulation between resistance states, wherein the structure has: an ionically conductive layer (18) formed between a metastable anode (16) and a metastable cathode (20); and a first electrode (12; 14) and at least one other electrode (22) that comes into direct contact with the metastable anode (16) or the metastable cathode (20); where a bidirectional transport of ions between the metastable anode and the metastable cathode results in a change in resistance to maintain the symmetrical modulation between the resistance states. [19] Structure according to claim 18, wherein the electrodes (12, 14, 16) are formed from inert metals and the metastable anode (16) and the metastable cathode (20) consist of metastable phase-separated ionic-electronic mixed conductors (MIECs) whose conductivity depends on a concentration of an embedded mobile ion, wherein the inert metals are selected from one or more of the group consisting of platinum (Pt), gold (Au) and iridium (Ir). [20] Structure according to claim 18, wherein electrical impulses are applied between the electrodes to enable a writing operation and a reading operation. [21] Structure according to claim 18, wherein the total chemical potential of all ions is kept at nearly zero.

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