Abstract:
A read circuit for sensing a resistance state of a resistive switching device in a crosspoint array utilizes a transimpedance equipotential preamplifier connected to a selected column line of the resistive switching device in the array. The equipotential preamplifier delivers a sense current while maintaining the selected column line at a reference voltage near a biasing voltage applied to unselected row lines of the array. A reference resistor is selectively connected to the equipotential preamplifier for setting a reference current, wherein the equipotential preamplifier is set to produce a preamplifier output voltage having a magnitude depending on whether the sense current is smaller or greater than the reference current. A voltage comparator is connected to the equipotential preamplifier to compare the preamplifier output voltage with a setup reference voltage and generate a comparator output voltage indicative of the resistance state of the resistive switching device.

Description:
BACKGROUND 
       [0001]    Memristive devices, or memristors, are a new type of switching devices with an electrically switchable device resistance. Memristive devices are both scientifically and technically interesting, and hold promise for non-volatile memory (NVM) and other fields. With today&#39;s flash memory technology reaching its scaling limit, there is an urgent need for new memory technologies that can meet the storage capacity and speed demanded by future applications. Memories using resistive switching devices, such as memristors, are a promising candidate for meeting that need. For NVM applications, many nanoscale resistive switching devices can be formed in a two-dimensional array, such as a crossbar structure, to provide a very high storage capacity. Nevertheless, it has been a major challenge to reliably read the resistance state of a selected resistive switching device in an array, due that existence of other switching devices in the array that may form paths for leakage current, which can significantly reduce the signal/noise ratio of the read operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a schematic cross-sectional view of an example of a memristive device as one type of resistive switching device; 
           [0003]      FIG. 2  is a schematic view of a crossbar structure containing multiple resistive switching devices; 
           [0004]      FIG. 3  is a schematic diagram representing an abstraction of a crossbar of resistive switching devices; 
           [0005]      FIG. 4  is a schematic diagram of an electronic circuit with a transimpedance equipotential preamplifier for reading a selected resistive switching device in a crossbar; 
           [0006]      FIG. 5  is a flowchart showing a process of reading a selected resistive switching device in a crossbar using the circuit of  FIG. 4 ; and 
           [0007]      FIG. 6  is a schematic diagram of an implementation of the electronic circuit of  FIG. 4  for reading a selected resistive switching device in a crossbar. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    The following description provides a circuit having a transimpedance pre-amplifier for reading the resistance state of a resistive switching device in an array of switching devices, and a corresponding method for performing the read operation. In some embodiments, the reading circuit may provide a digital output to represent the resistance state of switching device. For instance, a digital “0” may indicate that the device is in a high resistance state, or an “OFF” state, while a digital “1” may indicate that the device is in a low resistance state, or an “ON” state. 
         [0009]    In some embodiments, the resistive switching device may be a bipolar memristive device (or memristor). As used herein, a memristive device is a switching device with its resistance representing its switching state, and the resistance depends on the history of the voltage and current applied to the device. The term “bipolar” means that the device can be switched from a low-resistance state (“LRS”) to a high-resistance state (“HRS”) by applying a switching voltage of one polarity, and from a high-resistance state to a low-resistance state by applying a switching voltage of the opposite polarity. 
         [0010]      FIG. 1  shows, in a schematic form, an example of a bipolar memristive device  100 . In the embodiment shown in  FIG. 1 , the memristive device is a two-terminal device, with a top electrode  120  and a bottom electrode  110 . An active region  122 , where the switching behavior takes place, is disposed between the two electrodes. The active region  122  of the switching device  100  includes a switching material that may be electronically semiconducting or nominally insulating, as well as a weak ionic conductor. The switching material contains dopants that may be driven under a sufficiently strong electric field to drift through the switching material, resulting in changes in the resistance of the memristive device. The memristive device  100  can be used, for example, as a non-volatile memory cell, for storing digital information. Such a memory cell may be incorporated into a crossbar structure to provide a high storage capacity, as illustrated in  FIG. 2 . 
         [0011]    Many different materials with their respective suitable dopants can be used as the switching material. Materials that exhibit suitable properties for switching include oxides, sulfides, selenides, nitrides, carbides, phosphides, arsenides, chlorides, and bromides of transition and rare earth metals. Suitable switching materials also include elemental semiconductors such as Si and Ge, and compound semiconductors such as III-V and II-VI compound semiconductors. The listing of possible switching materials is not exhaustive and do not restrict the scope of the present invention. The dopant species used to alter the electrical properties of the switching material depends on the particular type of switching material chosen, and may be cations, anions or vacancies, or impurities as electron donors or acceptors. For instance, in the case of transition metal oxides such as TiO 2 , the dopant species may be oxygen vacancies. For GaN, the dopant species may be nitride vacancies or sulfide ions. For compound semiconductors, the dopants may be n-type or p-type impurities. 
         [0012]    The nanoscale switching device  100  can be switched between ON and OFF states by controlling the concentration and distribution of the oxygen vacancies in the switching material in the active region  122 . When a DC switching voltage is applied across the top and bottom electrodes  120  and  110 , an electric field is created across the active region  122 . The switching voltage and current may be supplied by a switching circuit  200 . The electric field across the active region  122 , if of a sufficient strength and proper polarity, may drive the oxygen vacancies to drift through the switching material towards the top electrode  120 , thereby turning the device into an ON state. 
         [0013]    By way of example, as shown in  FIG. 1 , in one embodiment the switching material may be TiO 2 . In this case, the dopants that may be carried by and transported through the switching material are oxygen vacancies (Vo 2+ ). The active region  122  of the switching device has two sub-regions or layers: a primary region  124  and a secondary region  126 . The primary region  124  is the main place where the switching behavior occurs. In the originally formed state of the device, the primary region  124  has a relatively low dopant concentration, while the secondary region  126  has a relatively high dopant level. The secondary region  126  functions as a dopant source/drain. During a switching operation, dopants may be driven from the secondary region  126  into the primary region  124 , or from the primary region to the secondary region, to change the distribution of dopants in the primary region, thereby changing the conductivity across the primary region. 
         [0014]    If the polarity of the electric field is reversed, the dopants may drift in an opposite direction across the switching material and away from the top electrode  120 , thereby turning the device into an OFF state. In this way, the switching is reversible and may be repeated. Due to the relatively large electric field needed to cause dopant drifting, after the switching voltage is removed, the locations of the dopants remain stable in the switching material. The switching is bipolar in that voltages of opposite polarities are used to switch the device on and off. The state of the switching device  100  may be read by applying a read voltage to the bottom and top electrodes  110  and  120  to sense the resistance across these two electrodes. The read voltage is typically much lower than the threshold voltage required to induce drifting of the ionic dopants between the top and bottom electrodes, so that the read operation does not alter the resistance state of the switching device. 
         [0015]    Memristive switching devices may be formed into an array for various applications that benefit from having a high density of switching devices.  FIG. 2  shows an example of a two-dimensional array  160  of memristive switching devices. The array  160  has a first group  161  of generally parallel nanowires  162  running in a first direction, and a second group  163  of generally parallel nanowires  164  running in a second direction at an angle, such as 90 degrees, from the first direction. One group of the nanowires may be labeled as the row lines, and the other group may be labeled as the column lines. The two layers of nanowires  162  and  164  form a two-dimensional lattice which is commonly referred to as a crossbar structure, with each nanowire  162  in the first layer intersecting a plurality of the nanowires  164  of the second layer, and vice versa. A memristive switching device  166  may be formed at each intersection of the nanowires  162  and  164 . The switching device  166  has a nanowire of the second group  163  as its top electrode and a nanowire of the first group  161  as the bottom electrode, and an active region  172  containing a switching material between the two nanowires. Each memristive device  166  in the two-dimensional array can be uniquely addressed by selecting the row line and column line that form the electrodes of the memristive device. 
         [0016]    As mentioned above, one challenge that results from the use of a crossbar memory structure is that it can be difficult to reliably read the resistance state of a selected device in the array. To sense the resistance state of the selected device, a sensing voltage may be applied to the device via the row line and column line of the device, and the current flowing through the selected device may be monitored to determine the resistance of the device. There are, however, other switching devices connected to the selected row line or the selected column line. Those devices, referred to as “half-selected” devices, can form paths for leakage current, and it can be difficult to isolate the current flowing through the selected device from the leakage current, which can be rather large if there are many devices on each row line or column line. 
         [0017]    To facilitate a better understanding of the issue of leakage current in a crossbar and how it can complicate the operation of reading a selected resistive switching device (or the “target device”),  FIG. 3  shows an abstraction of a crossbar  210  in a simplified form. The target device  202  (shown in electronic circuit symbol of a memristor) to be read is at the intersection of a selected row line SR and a selected column line SC. The unselected row UR in  FIG. 3  represents all rows in the crossbar  210  other than the selected row SR, and the unselected column line UC represents all columns of the crossbar  210  other than the selected column line. The device  204  represents all other resistive switching devices connected in parallel to the selected column line SC, and the device  206  represents all other resistive switching devices connected in parallel to the selected row line SR. The device  208  represents all resistive switching devices in the crossbar  210  that are not connected to either the selected column or the selected row. When a read voltage is applied across the selected column SC and the selected row SR, the devices  204  and  206  become half-selected. If there is a voltage difference between the selected row or column line and the unselected lines, the half-selected devices will pass leakage currents due to their finite resistance values. Such leakage currents are a form of noise for the read operation. If there are many switching devices connected to each row or column line in the crossbar, the magnitude of the leakage current can become rather large, and can swamp the real signal of the read operation, which is the current passing through the target device under the read voltage. 
         [0018]    An effective solution to the leakage current problem is to bias all the unselected row lines in the crossbar to substantially the same voltage that is applied to the selected column line during the read operation. As illustrated in  FIG. 3 , when the unselected row line UR is biased to substantially the same voltage as the selected column line, the leakage current passing through the half-selected device  204  will be zero or very small. Thus, the sensing current I_Sense flowing through the selected column SC can have a very small noise component and be mostly the read current I_R_Device flowing through the target device  202 . This approach, termed “equipotential sensing,” provides an effective way to achieve a reasonably high signal/noise ratio for the read operation. To maintain the selected column line SC at substantially the same voltage of the unselected row lines, an equipotential preamplifier  220  may be used. The equipotential preamplifier  220  is connected to the selected column SC, and has a reference voltage input. For the read operation, the reference voltage V_Ref is set to be substantially the same as sense voltage V_S to which the unselected row lines are biased. The equipotential preamplifier holds the selected column line SC to the reference voltage V_Ref while allowing the sensing current I_Sense to flow to the crossbar  210  through the selected column line SC. The effectiveness of the equipotential sensing technique depends on the proper setting of the reference voltage for the equipotential preamplifier. The reference voltage V_Ref is set not only to be close to the biasing voltage V_S on the unselected row lines so as to reduce the leakage current, but also to enable the equipotential preamplifier to operate in a linear range. Moreover, it is desirable to have a convenient and effective way to determine the resistance state of the target device and to indicate the state in an easy-to-read format. 
         [0019]      FIG. 4  shows an embodiment of an “equipotential sensing” circuit  250  which includes an equipotential preamplifier  260 . The equipotential preamplifier  260  is a transimpedance amplifier, and is represented in  FIG. 4  as an operational amplifier  262  with a feedback resistor  268  connecting the negative input  266  and output  270  of the operational amplifier. Thus, the equipotential preamplifier  260  converts a current output into a voltage output. The reference voltage V_Ref goes to the positive input  264  of the operational amplifier  262 . The output of the operational amplifier  262  is connected to the negative input of a voltage comparator  280 . The voltage comparator  280  has a voltage V_SetUpRef connected to its positive input. The output of the voltage comparator  280  is connected to the input of a sense latch  288 , which provides a digital output indicative of the resistance of the target device  202  being read by the circuit  250 . 
         [0020]    For setting up the reference voltage V_Ref, the circuit  250  has reference voltage setting components which include a feedback switch  272  and a sample-and-hold capacitor  274 . The circuit utilizes feedback to set the reference voltage V_Ref. The reference setting components further includes a reference resistor  276  and a transistor  278 . The transistor  278  functions as a switch for connecting the reference resistor  276  to the ground or breaking that electrical connection under the control of a control pulse pSetUp. The resistance of the reference resistor  276 , designated R_HRS_min, is selected to be of a value smaller than the range of resistance for a resistive switching device in a high-resistance state (or OFF state), but distinguishably larger than the low-resistance state (or ON state) value of the device. As explained in greater detail below, the use of the reference resistor  276  in a set-up stage allows the equipotential preamplifier  260  to be set up such that the ON or OFF state of the target device can be determined in the sensing stage in a very simple and convenient way. 
         [0021]    The process of reading the target device  202  in the crossbar  210  using the read circuit  250  is now described with reference to the flowchart in  FIG. 5 . First, the circuit  250  is initialized for setting up the circuit for the read operation (step  300 ). To that end, the row lines (SR and UR) of the array are all connected to the read voltage V_S, which may be provided by an external voltage source. The unselected column lines UC are left floating. A setup reference voltage V_SetUpRef is applied to the positive input of the voltage comparator  280 . In this embodiment, the value of the setup reference voltage is set to be the same as the read voltage V_S. The selected column line SC of the target device  202  to be read is connected to the negative input  266  of the operational amplifier  262 . 
         [0022]    Thereafter, a setup pulse pSetUp is applied to the transistor  278 , thereby connecting the reference resistor  276  to ground (step  302 ). The feedback switch  272  is closed to close the feedback loop (step  304 ). As a result, the output of the voltage comparator  280  is applied to positive input  264  of the operational amplifier  262  via the feedback switch  272 . Due to the application of the voltage, a current I_Sense flows through the feedback resistor  268  from the output  270  to the negative input  266  of the operational amplifier. The output voltage of the operational amplifier  262  is fed to the voltage comparator  280  as an input. The voltage comparator  280  compares the output voltage of the operational amplifier  262  with the setup voltage V_SetUpRef, and changes its output voltage accordingly. The changed output of the voltage comparator then goes through the feedback path to the positive input of the operational amplifier  262 . This feedback process is left on for a sufficient time until the voltages and current transients settle (step  306 ). At the end of this feedback-controlled process, the equipotential preamplifier reference voltage V_Ref on the positive input of the operational amplifier  264  is close to the sense voltage V_S applied to the row lines, but with a slight difference such that the preamplifier  260  is in its linear operating range and is passing a pre-selected amount of current. Specifically, the current I_Sense flowing through the feedback resistor  268  is the current output of the preamplifier  260 . It includes two components: a current flowing through the reference resistor  276 , and a leakage current component flowing to the crossbar  200  via the selected column SC. The magnitude of the current flowing through the reference resistor  276  is designated I_HRS_max, and is close to V_S divided by R_HRS_min. By means of the selection of R_HRS_min, this amount of current is set to be higher than the current that would go through an HRS (high resistance state) device under V_S, but lower than the current through an LRS (low resistance state) device under V_S. 
         [0023]    After the reference voltage V_Ref is set, the feedback loop is opened by opening the switch  272  (step  308 ). The reference voltage V_Ref is held by the sample-and-hold capacitor  274  and applied to the positive input of the operational amplifier  262 . The setup pulse pSetUp is de-asserted to turn the transistor  278  off, thereby isolating the reference resistor  276  from the ground. This stops the current flowing through the reference resistor  276 . As a result, the current I_Sense flowing through the feedback resistor  268  now is smaller than I_HRS_max. This causes the output V_Sense of the voltage comparator  280  to swing up to the supply voltage Vdd. This comparator output voltage is converted by the sense latch  288  to provide a digital signal Vout indicating a Logic 1. Now the circuit  250  has been set up and is ready for the read operation. 
         [0024]    To initiate the read operation, the selected row SR of the target device  202  is connected to ground (step  310 ). Due to the equipotential approach, the current I_Sense generated by the preamplifier  260  and flowing to the crossbar is mainly the read current I_R_Device flowing through the target device  202 , plus a small leakage component. The magnitude of I_Sense determines the voltage output V_Pre of the operational amplifier  262 . The voltage comparator  280  takes V_Pre as an input and compares it to V_SetUpRef (step  312 ). If I_Sense is less than I_HRS_max, the output of the voltage comparator  280 , V_Sense, goes toward Vdd. On the other hand, if I_Sense is greater than I_HRS_max, V_Sense goes toward ground. In this regard, a condition of I_Sense&lt;I_HRS max indicates that the target device is in a high-resistance or OFF state, while a condition of I_Sense&gt;I_HRS_max indicates that the target device is in a low-resistance or ON state. The output of the voltage comparator  280  is buffered by the sense latch  288  and converted into a digital output signal. If V_Sense goes up, the sense latch produces an output of Logic 1 (step  314 ). If V_Sense goes down, the sense latch produces an output of Logic 0 (step  316 ). 
         [0025]      FIG. 6  shows implementation features of some components in the embodiment of the read circuit shown in  FIG. 4 . These implementation features facilitate the fabrication of the read circuit  250  using semiconductor fabrication techniques. Specifically, the sample-and-hold capacitor  274  may be implemented as a PMOS transistor. The drain and source of the transistor are connected together, and the gate is connected to the positive input of the operational amplifier  262 . Thus, the capacitance utilized for the sample-and-hold function is the gate capacitance of the transistor. The feedback switch  272  is implemented as a PMOS transistor and an NMOS transistor tied together to form a transmission gate switch. Also shown in  FIG. 6 , the sense latch  288  may be implemented as a 1-bit A/D converter that outputs either a Logic 1 or a Logic 0 state. 
         [0026]    In the foregoing description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details. While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.