Resistive switches

Resistive switches and related methods are provided. Such a resistive switch includes an active material in contact with opposite end electrodes. The active material defines electron traps that capture or release charges in accordance with applied switching voltages. Resistive switches are characterized by ON state and OFF state resistance curves. Resistance ratios of ten times or more are exhibited. The state of a resistive switch is determined using sensing voltages lesser then the switching threshold.

BACKGROUND

Resistive switches exhibit a plurality of distinct electrical resistances respectively corresponding to various operating states. Sometimes, two distinct “ON” and “OFF” states are defined at (or near) opposite ends of the resistive range for a given resistive switch. No moving parts are involved due to their solid state construction, and switching between states is achieved by way of control signaling.

However, most of the known resistive switches require undesirably high switching and reading (i.e., state sensing) currents. Additionally, such relatively high currents cause excessive heating, resulting in undesirably short operating life spans. The present teachings are directed to the foregoing and other related concerns.

DETAILED DESCRIPTION

Introduction

Means and methods related to resistive switches are provided by the present teachings. A resistive switch includes an active material in contact with opposite end electrodes. The active material defines electron traps that capture or release charges in accordance with applied switching voltages. Resistive switches are characterized by respectively different ON-state and OFF-state resistance curves. Resistance ratios of ten times or more are exhibited. The state of a resistive switch is determined using sensing voltages lesser then the switching threshold.

In one embodiment, an apparatus includes a resistive switch. The resistive switch includes a first electrode and an active material layer in contact with the first electrode. The resistive switch further includes a second electrode disposed opposite the first electrode. The active material layer is also in contact with the second electrode. The resistive switch is characterized by a first electrical resistance curve in response to an applied forward switching voltage. The resistive switch further is characterized by a second electrical resistance curve in response to an applied reverse switching voltage. The second electrical resistance curve is different than the first electrical resistance curve.

In another embodiment, a method includes operating a resistive switch characterized by a first electrical resistance curve. The resistive switch is characterized by a first end electrode and a second end electrode, and an active material layer disposed between and in contact with the first and second end electrodes. The method also includes switching the resistive switch so as to change the character of the resistive switch from the first electrical resistance curve to a second electrical resistance curve distinct from the first. The method further includes operating the resistive switch characterized by the second electrical resistance curve.

First Illustrative Embodiment

Reference is now directed toFIG. 1, which depicts an isometric view of a resistive switch (switch)100according to the present teachings. The switch100is illustrative and non-limiting with respect to the present teachings. Thus, other switches can be configured and/or operated in accordance with the present teachings,

The resistive switch100includes a first end electrode102. The end electrode102includes a lead portion104extending away from an end area portion106. In one embodiment, the end electrode102is formed from (or includes) platinum (Pt). Other suitable metals or materials can also be used. In one embodiment, the end electrode102is defined by a thickness dimension “T1” of about nine nanometers (one nanometer =1×10−9meters). Other suitable thicknesses can also be used.

The switch100also includes an active material layer (AML)108. The AML108is in contact with the first end electrode102and is defined by a thickness dimension “T2”, and across-sectional area “A1” normal to the thickness dimension T2. In one embodiment, the AML108is formed from (or includes) titanium dioxide (TiO2) having a thickness T2in the range of four-to-100 nanometers and a cross-sectional area A1of 1 micrometer square to 100 nanometers square. Other suitable materials, thicknesses or cross-sectional areas can also be used. In one embodiment, the AML108is formed by sputtering at ambient temperature.

The resistive switch100further includes a second end electrode110. The end electrode110includes a lead portion112extending away from an end area portion114. In one embodiment, the end electrode110is formed from (or includes) platinum (Pt). Other suitable metals or materials can also be used. In one embodiment, the end electrode110is defined by a thickness dimension “T3” of about nine nanometers. Other suitable thicknesses can also be used.

The AML108is contact with the second end electrode110. Thus, the active material layer108is disposed between, and in contact with, the end electrodes102and110, respectively. In this way, the resistive switch100is also referred to as a two-terminal device or two-terminal switch. The end electrodes102and110, respectively, are configured to electrically couple the switch100to another entity or entities (e.g., electronic circuitry, various components, etc.).

The active material layer108is configured to define a plurality of “electron traps” that release electrons under the influence of a first (“forward”) applied electrical potential, and capture electrons under the influence of a second (“reverse”) applied electrical potential. These forward and reverse potentials are also referred to as forward (Le., “ON”) and reverse (i.e., “OFF”) switching voltages, respectively. In one embodiment, the electron traps are defined by oxygen vacancies in the titanium dioxide (TiO2-x).

It is noted that the forward and reverse switching voltages are of opposite polarity to one another as applied to the end electrodes102and110of the switch100. Thus, for purposes of the present teachings, a forward potential is applied as positive to electrode102and negative to electrode110, while a reverse potential is applied as negative to electrode102and positive to electrode110. Applied forward potential is depicted by the polarity arrow “VF” inFIG. 1.

It is further noted that the resistive switch100is characterized by a nominal electrical resistance in the ON state, and a different nominal electrical resistance in the OFF state. Such ON and OFF electrical resistances are retained for at least a day when electric potential is completely removed from the resistive switch100. In particular, these ON and OFF resistances are defined by respective resistance curves that are non-linear in nature, and which are described in illustrative and non-limiting terms hereinafter.

The Table 1 below summarizes parameters and characteristics of an illustrative and non-limiting embodiment of the resistive switch100in accordance with the present teachings:

Attention is now directed toFIG. 2, which depicts a flow diagram of a method according to one embodiment of the present teachings. The method ofFIG. 2includes particular operations and order of execution. However, other methods including other operations, omitting one or more of the depicted operations, and/or proceeding in other orders of execution can also be used according to the present teachings. Thus, the method ofFIG. 2is illustrative and non-limiting in nature. Reference is also made toFIG. 1in the interest of understanding the method ofFIG. 2.

At200, a forward switching voltage is applied to a resistive switch so as to set an “ON” state. For purposes of non-limiting example, it is assumed that about two volts direct-current (DC) potential is applied to resistive switch100. Such potential is applied as positive to electrode102and negative to electrode110, thus defining a forward switching potential. In so doing, electrons are driven from traps within the active material layer108and the electrical resistance of the switch100is set to about ten-million Ohms (nominal) value.

At202, the resistive switch is operated in the ON state. For purposes of the ongoing example, it is assumed that the instantaneous resistance value of the switch100is sensed from time to time by way of an applied sensing voltage. The sensing voltage is of lesser magnitude than either a forward or reverse switching voltage, and thus does not cause switching (i.e., change in state) of the switch100. The sensing voltage is thus considered a “reading” voltage. The sensing voltage can be applied in either the forward or reverse orientation, as desired.

At204, a reverse witching voltage is applied to a resistive switch so as to set an “OFF” state. For purposes of the ongoing example, it is assumed that about minus two volts direct-current (DC) potential is applied to resistive switch100. Such potential is applied as negative to electrode102and positive to electrode110, thus defining a reverse switching potential. In so doing, electrons are captured (i.e., accumulated) within the traps of the active material layer108and the electrical resistance of the switch100is set to about five-billion Ohms (nominal) value.

At206, the resistive switch is operated in the OFF state. For purposes of the ongoing example, it is assumed that the instantaneous resistance value of the switch100is sensed from time to time by way of an applied sensing voltage. The sensing voltage is of insufficient magnitude to cause switching of the switch100, and is considered a “reading”voltage. The sensing voltage can be applied in either the forward or reverse orientation. The method can then (eventually) return to step200above. In this way, the method depicts “toggling” a resistive switch between ON and OFF states, as desired.

First Illustrative Device

Reference is now made toFIG. 3, which depicts a block diagram of a device300according to another embodiment of the present teachings. The device300is illustrative and non-limiting in nature. Thus, other devices, circuits and systems are contemplated that include one or more aspects of the present teachings.

The device300includes a resistive switch (switch)302. The switch302can be defined by any embodiment consistent with the present teachings. In one embodiment, the switch302is materially and operationally equivalent to the switch100as described above. Other embodiments can also be used.

The device300also includes switching circuitry304. The switching (or control) circuitry304is electrically coupled to the switch302and is configured to apply forward and reverse switching voltages thereto. The switching circuitry304is thus configured to cause controlled changes in the electrical resistance of the switch302. The switching circuitry304is further configured to sense the instantaneous electrical resistance (i.e., ON or OFF state) of the switch304by way of appropriate sensing voltage(s). The switching circuitry304can be defined by or include any suitable components or configuration such as, but not limited to, a microprocessor, a microcontroller, digital circuitry, analog circuitry, a state machine, etc.

The device300further includes other circuitry306that is electrically coupled to (or includes) the switch302. The other circuitry306can be defined by any suitable circuitry configured to perform one or more normal operations using the switch302. Non-limiting examples of such operations include cellular communication, environmental sensing, instrumentation and control, radio communication, data storage and retrieval, etc. Thus, the device300can be generally defined by any one or more suitable areas of application (e.g., a cellular telephone, a data storage array, etc.).

Second Illustrative Device

Reference is now made toFIG. 4, which depicts a block diagram of a device400according to another embodiment of the present teachings. The device400is illustrative and non-limiting in nature. Thus, other devices, circuits and systems are contemplated that include one or more aspects of the present teachings.

The device400includes a memory array controller (controller)402. The controller402is configured to address individual switches404of the device400. Such addressing is performed by way of row control lines406and column control lines408. The controller402is also configured to apply forward and reverse switching voltages, as well as reading voltages, to the switches404by way of the controls lines406and408.

The device400further includes a plurality of switches404. Each switch404is defined, configured and operative in accordance with the present teachings. In one embodiment, one or more of the switches404is are materially and operationally equivalent to the resistive switch100described above. Other embodiments can also be used.

The switches404are arranged as an X-by-Y array, with each switch404being individually addressable by the controller402. Each switch404can be operated as a storage cell representing a digital bit, etc.FIG. 4depicts a total of four switches404arranged as an array. However, it is to be understood that other arrays including any suitable number of switches can also be defined and operated in accordance with the present teachings.

Illustrative Operating Theory

Attention is now directed toFIG. 5, which depicts energy level diagrams500and520, respectively corresponding to an illustrative and non-limiting resistive switch540according to the present teachings. The energy level diagrams500and520are general in nature, and are depicted for purposes of better understanding the present teachings.

As a preliminary matter, it is noted that the illustrative resistive switch540includes a first platinum electrode542, a titanium dioxide (active material) layer544, and a second platinum electrode546. Relative dimensions of the electrodes542and546and the AML544are exaggerated withinFIG. 5in the interest of clarity.

The energy level diagram500corresponds to an ON state for the resistive switch540, having a nominal electrical resistance of ten million Ohms from electrode542to electrode546. It is noted that the electrodes542and546, being formed of platinum, are respectively characterized by electron energy levels within (or substantially so) the conduction band “CB”. It is also noted that active material layer544is characterized by illustrative electron traps502. The electron traps502are “empty” or devoid of trapped charges, respectively. In this ON state, the electron energy level of the AML544corresponds to an illustrative barrier height504.

In turn, the energy level diagram520corresponds to an OFF state for the resistive switch540, having a nominal electrical resistance of five billion Ohms from electrode542to electrode546. It is noted that active material layer544is characterized by negative charges522held within respective electron traps502. In this OFF state, the electron energy level of the AML544corresponds to an illustrative barrier height524that is greater than the barrier height504. The distinct ON and OFF state (nominal) resistances are attributable to these respectively different barrier heights504and524

Illustrative Signal Diagrams

FIG. 6is a signal diagram600according to one embodiment of the present teachings. The diagram600is general and illustrative in nature. It is to be understood that the diagram600is provided in the interest of understanding the present teachings. As such, varying embodiments of resistive switch according to the present teachings correspond to respectively varying signal diagrams. For purposes of non-limiting illustration, it is assumed that the signal diagram600corresponds to electrical behavior of an embodiment of resistive switch100as described above.

The signal diagram600includes a first curve portion602corresponding to ON switching behavior for the illustrative resistive switch. Specifically, the portion602plots applied forward voltage versus switch current. It is assumed that applied forward voltage begins at zero and increases in magnitude, with corresponding switch current beginning on the lower branch of the curve602and increasing as shown by arrow604. The switch current increases with increases in applied forward voltage, along the lower branch of the curve602, until a “fold-over” point606is reached. The resistive switch is now in an ON state.

During typical normal operation, forward voltage is not increased beyond point606, as such is unnecessary to either switching or reading the resistive switch. Furthermore, increase in applied forward voltage beyond point606result in excess heating and premature damage to the resistive switch.

From point606, forward voltage is decreased back toward zero. In response, the current follows along the upper branch of signal curve602as shown by arrow608. As the forward voltage approaches zero, the current through the resistive switch decreases toward a point610. This current value at point610is a greater value than the original staring point. The upper branch of the curve portion602represents the non-volatile resistance curve of the resistive switch while it is in the ON state.

The signal diagram600also includes a second curve portion612corresponding to OFF switching behavior for a resistive switch of the ongoing illustration. Specifically, the portion612plots applied reverse voltage versus switch current. It is assumed that applied reverse voltage begins at zero and increases in magnitude (i.e., increasing absolute value) with corresponding switch current beginning at about the point610and increasing as shown by arrow614. The switch current increases with applied reverse voltage, along the upper branch of the curve612. Eventually, a point616is reached, and the resistive switch is now in an OFF state.

During typical normal operation, reverse voltage is not increased beyond point616, as such is unnecessary to either switching or reading the resistive switch. The current response curve generally flattens beyond the point616—that is, the resistive switch exhibits diode-like behavior.

From point616, reverse voltage is decreased back toward zero. In response, the current follows along the lower branch of curve portion612as shown by arrow618. As the reverse voltage approaches zero, the current through the resistive switch decreases to a lesser value than the original staring point. The lower branch of the curve portion612represents the non-volatile resistance curve of the resistive switch while It is in the OFF state.

Attention is now directed toFIG. 7, which is a signal diagram700according to one embodiment of the present teachings. The diagram700is general and illustrative in nature, and corresponds to the signal diagram600. It is to be understood that the diagram700is provided in the interest of understanding the present teachings. The signal diagram700depicts electrical behavior for an illustrative resistive switch (e.g., switch100) when subject to sensing (state reading) voltages in the ON and OFF states; respectively.

The signal diagram700includes a first curve portion702corresponding to ON reading (i.e., sensing) behavior for an illustrative resistive switch. As applied sensing voltage increases or decreases away from zero, corresponding switch current begins to flow as depicted the curve portion702. Thus, ON state resistance can be determined with either a forward or reverse sensing voltage.

The signal diagram700includes a second curve portion704corresponding to OFF reading behavior for an illustrative resistive switch. As applied sensing voltage increases or decreases away from zero, corresponding switch current begins to flow as depicted the curve portion704. The OFF state resistance of the switch can be determined with either a forward or reverse sensing voltage. A differential706between the curves702and704corresponds to the difference between the OFF state and ON state resistances for some particular sensing voltage.

Straightforward sampling and testing can determine the respective OFF and ON resistance curves for a particular embodiment of resistive switch. Those results can then be used to determine a sensing voltage corresponding to the greatest OFF-to-ON resistance ratio. This “optimized” sensing voltage value can then be used with corresponding embodiments of resistive switch. In one embodiment, a forward sensing voltage of about zero-point-five volts is used. Other values can also be used.

In general, and without limitation, the present teachings contemplate various resistive switches including opposite end electrodes and an active material layer in contact there between. Such resistive switches are also referred to as two-terminal devices. The active material layer defines a number of electron traps. A forward switching voltage can be applied to the end electrodes causing a release of charges from the electron traps. Conversely, a reverse switching voltage respectively can be applied to the end electrodes causing an accumulation of charges within the electron traps. In this way, the resistive switch can be driven in to ON and OFF states, respectively.

Respective, resistance curves correspond to the ON and OFF states of the resistive switch. Additionally, the present state of the switch (ON or OFF) can be determined by way of a sensing (or reading) voltage applied in either a forward or reverse polarity. The sensing voltage is notably lower in magnitude then either the ON or OFF switching voltage, and thus does not cause switching of the resistive switch. Resistive switches of the present teachings can be used within data storage arrays, less communications equipment, instrumentation and control apparatus, computers, and numerous other devices and systems.