Patent Publication Number: US-2023154531-A1

Title: 2t-1r architecture for resistive ram

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/338,494, filed Jun. 3, 2021, and entitled “2T-1R Architecture for Resistive RAM,” which is a continuation of U.S. patent application Ser. No. 16/796,428, filed Feb. 20, 2020, and entitled “2T-1R Architecture for Resistive RAM,” now U.S. Pat. No. 11,081,176, which is a continuation of U.S. patent application Ser. No. 16/224,206, filed Dec. 18, 2018, and entitled “2T-1R Architecture for Resistive RAM,” now U.S. Pat. No. 10,622,062, which is a continuation of U.S. patent application Ser. No. 16/043,688, filed Jul. 24, 2018, and entitled “2T-1R Architecture for Resistive RAM,” now U.S. Pat. No. 10,199,098, which is a divisional of U.S. patent application Ser. No. 15/039,784, filed May 26, 2016, and entitled “2T-1R Architecture for Resistive RAM,” now U.S. Pat. No. 10,037,801, which is a national stage application pursuant to 35 U.S.C. 371 of International Application No. PCT/US2014/068624, filed Dec. 4, 2014, and entitled “2T-1R Architecture for Resistive RAM,” which claims priority to U.S. Provisional Patent Application No. 62/010,923, filed Jun. 11, 2014, U.S. Provisional Patent Application No. 62/010,937, filed Jun. 11, 2014, and U.S. Provisional Patent Application No. 61/913,099, filed Dec. 6, 2013. The disclosures of all of the above-referenced applications are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to semiconductor memory and, more particularly, to resistive random access memory (RRAM) with a two transistor, one resistive element (2T-1R) memory cell architecture. 
     BACKGROUND 
     Non-volatile memory devices that retain stored data in the absence of power are pervasively used in many consumer electronic products including cell phones, tablets, personal computers, personal digital assistants, and the like. Unfortunately, many non-volatile memory devices have limitations that make them unsuitable for use as primary storage for these products including higher cost and lower performance when compared to volatile memory devices such as dynamic random access memory (DRAM). Examples of older technology non-volatile memory devices include read-only memory (ROM) and flash memory. Examples of newer technology non-volatile memory devices include resistive random access memory (RRAM), phase change memory (PCM), spin-transfer torque magneto resistive random access memory (STT-MRAM), ferroelectric random access memory (FRAM), and many others. RRAM operates on the basis that a typically insulating dielectric may be made to conduct through formation of a conduction path or filament upon application of a sufficiently high voltage. Formation of the conduction path may occur through different mechanisms, including defects and metal migration. Once the conduction path or filament forms, the filament may be reset (broken, resulting in high resistance) or set (reformed, resulting in lower resistance) by an appropriately applied voltage. Recent data suggests that the conduction path may include many current paths, rather than a single path through a single filament. 
     RRAM memory devices including conductive bridge RAM (CBRAM) and transition metal oxide RRAM are a focal point for current development. In CBRAM devices, metal filaments between two electrodes form the conduction path, where one of the electrodes participates in the reaction. In transition metal oxide RRAM, oxygen vacancy filaments in a transition metal such as hafnium oxide or tantalum oxide form the conduction path. 
     RRAM memory devices are often in use to store data or executable code in embedded applications having logic circuitry including core transistors. The voltage required to write data in RRAM memory devices may be higher than that required to operate the core transistors. A challenge to the use of RRAM memory devices in embedded applications, therefore, is to find a select transistor configured to select a cell in the RRAM memory device whose operational parameters are consistent with that of core transistors. 
     Input/output (I/O) transistors common in logic circuitry may be used as select transistors since I/O transistors may handle the high voltage requirements of RRAM memory devices. I/O transistors are disadvantageous as select transistors, however, because they have a large footprint that increases the cost of manufacture. A need remains, therefore, for an improved RRAM memory device including area efficient select transistors capable of handling higher voltages for use in embedded applications. 
    
    
     
       BRIEF DRAWINGS DESCRIPTION 
       The present disclosure describes various embodiments that may be understood and fully appreciated in conjunction with the following drawings: 
         FIG.  1    is a diagram of an embodiment of a 1T-1R memory cell; 
         FIG.  2    is a diagram of an embodiment of a portion of a 1T-1R memory array; 
         FIG.  3    is a diagram of an embodiment of a layout for a select transistor; 
         FIG.  4 A  is a diagram of an embodiment of a 2T-1R memory cell; 
         FIG.  4 B  is a diagram of an embodiment of a 2T-1R memory system including the 2T-1R memory cell shown in  FIG.  4 A ; 
         FIG.  5 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  5 B  is a table with exemplary biases for a form operation for the 2T-1R memory array shown in  FIG.  5 A ; 
         FIG.  6 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  6 B  is a table with exemplary biases for a reset operation for the 2T-1R memory array shown in  FIG.  6 A ; 
         FIG.  7 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  7 B  is a table with exemplary biases for a set operation for the 2T-1R memory array shown in  FIG.  7 A ; 
         FIG.  8 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  8 B  is a table with exemplary biases for a read operation for the 2T-1R memory array shown in  FIG.  8 A ; 
         FIG.  9    is a diagram of an embodiment of a layout of an embodiment of the select transistors shown in  FIG.  4   ; 
         FIG.  10    is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  11 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  11 B  is a table with exemplary biases for a form operation for the 2T-1R memory array shown in  FIG.  11 A ; 
         FIG.  12 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  12 B  is a table with exemplary biases for a reset operation for the 2T-1R memory array shown in  FIG.  11 A ; 
         FIG.  13 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  13 B  is a table with exemplary biases for a set operation for the 2T-1R memory array shown in  FIG.  11 A ; 
         FIG.  14 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  14 B  is a table with exemplary biases for a read operation for the 2T-1R memory array shown in  FIG.  11 A ; 
         FIG.  15 A  is a diagram of an embodiment of a 2T-1R memory array including an embodiment of current limiting selector circuit; 
         FIG.  15 B  is a diagram illustrating bit line and word line pulses applied to selected memory cells in the 2T-1R memory array shown in  FIG.  15 A ; 
         FIG.  16    is a diagram of an embodiment of a 2T-1R memory array including another embodiment of current limiting selector circuit; 
         FIG.  17    is a diagram of an embodiment of a 2T-1R memory array including yet another embodiment of current limiting selector circuit; 
         FIG.  18    is a diagram of an embodiment of a 2T-1R memory array including yet another embodiment of current limiting selector circuit; 
         FIG.  19 A  is a diagram of an embodiment of a 2T-1R memory array including yet another embodiment of current limiting selector circuit; 
         FIG.  19 B  is a diagram of an embodiment of a 2T-1R memory array including yet another embodiment of current limiting selector circuit on vertical source lines; 
         FIGS.  20 A- 20 E  are diagrams of embodiments of current limiting selector circuit; 
         FIG.  21    is a diagram of an embodiment of a 2T-1R memory array including yet another embodiment of current limiting selector circuit; 
         FIG.  22 A  is a diagram of an embodiment of a 2T-1R memory array having shared vertical source lines; 
         FIG.  22 B  is a diagram of an embodiment of a 2T-1R memory array with row drivers and exemplary biases for various operations; 
         FIG.  22 C  is a diagram of an embodiment of a 2T-1R memory array with row drivers and exemplary biases for various operations; 
         FIG.  23 A  is a diagram of an embodiment of a voltage signal sequence applied to a 2T-1R memory device during a form operation; 
         FIG.  23 B  is a timing diagram of an embodiment of the voltage signal sequence applied to a 2T-1R memory device shown in  FIG.  23 A  during a form operation; 
         FIG.  23 C  is a diagram of an embodiment of a portion of a 2T-1R memory array  2300  during a reset operation; 
         FIG.  23 D  is a timing diagram of an embodiment of a voltage signal sequence applied to the 2T-1R memory array shown in  FIG.  23 C  during the reset operation; 
         FIG.  24 A  is a diagram of an embodiment of a biasing scheme for various operations performed on a 2T-1R memory array; 
         FIG.  24 B  is a diagram of an embodiment of row driver/decoder circuits configured to generate the signals shown in  FIG.  24 A ; 
         FIG.  24 C  is a diagram of an embodiment of a voltage level shift circuit; 
         FIG.  25 A  is a diagram of an embodiment of a portion of 2T-1R memory array; 
         FIG.  25 B  is a diagram of layout of an embodiment of a 2T-1R memory cell included in the 2T-1R memory array shown in  FIG.  25 A ; 
         FIG.  26 A  is a diagram of an embodiment of a 2T-1R memory array with shared source lines; 
         FIG.  26 B  is a diagram of an embodiment of a layout of a 2T-1R memory cell included in the 2T-1R memory array shown in  FIG.  26 A ; 
         FIG.  26 C  is a diagram of an embodiment of a byte-level flash reset to avoid the gate oxide breakdown shown in  FIG.  26 C ; 
         FIG.  27 A  is a diagram of an embodiment of row driver circuit  2755 ; 
         FIGS.  27 B and  27 C  are diagrams of embodiments of row decoder circuit  2765 ; 
         FIG.  28    is a diagram of an embodiment of a 2T-1R memory array having shared vertical source lines; 
         FIG.  29 A  is a diagram of an embodiment of a 2T-1R memory array; 
         FIG.  29 B  is a table listing various measured currents for an unselected cell of a 2T-1R memory array shown in  FIG.  29 A ; 
         FIG.  30    is a diagram illustrating the effect of halo doping on the leakage current; 
         FIG.  31    is a table listing various measured currents for an unselected cell having a mix of doping levels for select transistors T top  and T bottom ; 
         FIG.  32    is a diagram of an embodiment of a 2T-1R memory array having shared vertical source lines; 
         FIG.  33 A  is a diagram of an embodiment of a hierarchical floor plan for a 2T-1R memory array; 
         FIG.  33 B  is a diagram of an embodiment of local row generators; 
         FIG.  33 C  is a diagram of an embodiment of a hierarchical row path; and 
         FIG.  33 D  is a diagram of another embodiment of a hierarchical row path. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes embodiments with reference to the drawing figures listed above. Persons of ordinary skill in the art will appreciate that the description and figures illustrate rather than limit the disclosure and that, in general, the figures are not drawn to scale for clarity of presentation. Such skilled persons will also realize that many more embodiments are possible by applying the inventive principles contained herein and that such embodiments fall within the scope of the disclosure which is not to be limited except by the claims. 
     Referring to  FIG.  1   , a 1T-1R memory cell  100  includes a memory element  101  coupled to a select transistor  102  at a first end and coupled to a bit line terminal at a second end that receives a bit line signal BL. Select transistor  102  receives a word line signal WL on a gate terminal and a source line signal SL on a source terminal. Memory cell  100 , therefore, operates in response to bit line signal BL, word line signal WL, and source line signal SL. 
     Memory element  101  may include any kind of memory technology known to a person of ordinary skill in the art that changes resistance as a function of applied voltage or current, e.g., Resistive Random Access Memory (RRAM), Phase Change Memory (PCM), Spin-Transfer Torque Magnetic Random Access Memory (STT-MRAM), and the like. 
       FIG.  2    is a diagram of a portion of a 1T-1R memory array  200  including a plurality of memory cells, e.g., cells  201 ,  202 ,  203 , and  204 , arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction. Memory cells  201 ,  202 ,  203 , and  204  may have a construction similar to that of memory cell  100  shown in  FIG.  1   . As with memory cell  100 , each of memory cells  201 ,  202 ,  203 , and  204  is coupled to receive a bit line signal BL, word line signal WL, and source line signal SL. Memory cells  201 ,  202 ,  203 , and  204  may include any type of memory technology known to a person of ordinary skill in the art that changes resistance as a function of applied voltage or current, e.g., RRAM, PCM, STT-MRAM, and the like. 
     In an embodiment of array  200 , a column of memory cells is coupled to receive a common bit line signal while a row of memory cells is coupled to receive a common word line signal and a common source line signal. For example, memory cells  204  and  203  arranged on a first column commonly receive a first bit line signal BL1 while memory cells  201  and  202  arranged on a second column commonly receive a second bit line signal BL2. Memory cells  204  and  201  arranged on a first row commonly receive a first word line signal WL1 at corresponding gate terminals and receive a first source line signal SL1 at corresponding source terminals. Likewise, memory cells  203  and  202  arranged on a second row commonly receive a second word line signal WL2 at corresponding gate terminals and receive a second source line signal SL2 at corresponding source terminals. 
     In an embodiment in which memory cell  202  is selected for a form operation before switching memory cell  202  between set and reset operations, an exemplary bias scheme is as follows:
         Bit line signal BL2=3.5V   Source line signal SL2=0V   Word line signal WL2=1.5V   All other bias signals BL1, BL3, SL1, SL3, WL1, WL3,=0V       

     In such a configuration, a gate of select transistor  205  in memory cell  202  may have a voltage across it of 3.5V, which is higher than the supply voltage of 1.1V or less applied during typical operations. The higher voltage drop may cause gate dielectric breakdown in select transistor  205 , which, in turn, may lead to device failure. To avoid gate dielectric breakdown, an I/O transistor may function as select transistor  205  since most I/O transistors are rated to operate at higher voltages, e.g., 1.5V or 1.8V in 28 nm technologies, because of thicker gate dielectrics. 
       FIG.  3    is a diagram of an I/O select transistor  300  which includes contacts  301 , a gate  302 , an oxide isolation area  303 , and a diffusion area  304 . By using I/O select transistor  300 , a length of gate  302 , a distance between contacts  301  and gate  302 , and a width of select transistor  300  may all be larger than necessary for optimally operating memory cell  101 . Since an area of I/O select transistor  300  may determine the minimum area of memory cell  101 , using I/O select transistor  300  in array  200  shown in  FIG.  2    may increase the overall size of the logic chip with embedded non-volatile memory present. Thus, an alternative to using I/O select transistor  300  may be beneficial especially if the alternative results in decreasing die size without adding lithography steps. 
     Referring to  FIG.  4 A , a 2T-1R memory cell  400  includes a memory element  401  serially-coupled to select transistor  402  and to select transistor  403  at a first end. Memory element  401  is coupled to a bit line terminal at a second end that receives a bit line signal BL. Select transistor  402  receives a word line signal WL1 on a corresponding gate terminal, Select transistor  403  receives a word line signal WL2 on a corresponding gate terminal and a source line signal SL on a corresponding source terminal. Memory cell  400 , therefore, operates in response to bit line signal BL, word line signals WL1 and WL2, and source line signal SL. The two select transistors  402  and  403  allow minimizing the gate oxide breakdown, punch-through and other deleterious effects by reducing the voltage across the gate of select transistors  402  and  403 . For example, memory element  401  may require 3.5V and up to 1 us of current during a form operation. By biasing select transistors  402  and  403  as shown in e.g.,  FIG.  5 B , a maximum voltage across the gate of any of unselected memory cells is about 2V compared to the 3.5V drop across the gate dielectric observed in memory cell  100 . Because memory cell  400  includes two select transistors  402  and  403 , each being independently controlled and biased with corresponding word line signals WL1 and WL2, select transistors  402  and  403  may be manufactured using any type of transistor technology, including lower voltage rated core logic transistors instead of using much larger, higher voltage rated I/O transistors. The lower voltage rated core logic transistors may reduce cell size. 
     A person of ordinary skill in the art will recognize that use of core transistors may provide advantages including smaller die size and improved optimization over I/O transistors particularly in embedded non-volatile memory applications since core transistors are the focus of process optimization by logic technology manufacturers. 
     Like memory element  101 , memory element  401  may include any kind of memory technology known to a person of ordinary skill in the art that changes resistance as a function of applied voltage or current, e.g., RRAM, PCM, STT-MRAM, and the like. 
     Referring to  FIG.  4 B , a 2T-1R memory device  460  includes a control circuit  450  coupled to a 2T-1R memory array  440  (only a portion of array  400  is shown). In an exemplary embodiment, 2T-1R memory array  440  includes a plurality of memory cells, e.g., cells  411 ,  412 ,  421 , and  422 , arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction and having a construction like that of memory cell  400  shown in  FIG.  4   . As the 2T-1R name implies, each of memory cells  411 ,  412 ,  421 , and  422  in array  440  includes two select transistors and a memory element. Each memory cell in array  440  including memory cells  411 ,  412 ,  421 , and  422  is coupled to receive a bit line signal BL, two word line signals WL1 and WL2, and a source line signal SL. For example, memory cell  411  is coupled to receive bit line signal BL1, word line signals WL11 and WL12, and source line signal SL1. 
     A control circuit  450  is coupled to 2T-1R memory array  400  and configured to generate voltage signals, e.g., word line signals, bit line signals, and source line signals, necessary for the various operations performed on memory array  400 . In an embodiment, control circuit  450  avoids voltage or high current stresses on the select transistors T1 and T2 that would result in damage, wear out, reduced life, or the like, by applying the necessary voltage signals in predetermined levels and/or in a predetermined sequence as further described below. 
       FIG.  5 A  is a diagram of a portion of a 2T-1R memory array  500  including a plurality of memory cells, e.g., memory cells  501 ,  502 ,  503 , and  504 . In an embodiment, each of memory cells  501 ,  502 ,  503 , and  504  have a construction like that of 2T-1R memory cell  400  shown in  FIG.  4    and may be arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction. As the 2T-1R name implies, each of memory cells  501 ,  502 ,  503 , and  504  in array  500  includes two select transistors and a memory element. Each memory cell in array  500  including memory cells  501 ,  502 ,  503 , and  504  is coupled to receive a bit line signal BL, two word line signals WL1 and WL2, and a source line signal SL from a control circuit, e.g., control circuit  450  shown in  FIG.  4 B . 
     In an embodiment, memory cells arranged on a row of the array  500  are configured to receive the same two word line signals and the same source line signal. For example, memory cells  504  and  501  receive word line signals WL11 and WL12 at corresponding gate terminals, respectively, and source line signal SL1 at corresponding source line terminals. Likewise, memory cells  503  and  502  receive word line signals WL21 and WL22 at corresponding gate terminals, respectively, and source line signal SL2 at corresponding source terminals. 
     Memory cells arranged on a column of memory array  500  are configured to receive the same bit line signal. For example, memory cells  504  and  503  receive bit line signal BL1 and memory cells  501  and  502  receive bit line signal BL2. A person of ordinary skill in the art should recognize that other arrangements of array  500  are possible and come within the scope of the inventive principles disclosed herein. 
     Memory cells  501 ,  502 ,  503 , and  504  may include any type of memory technology known to a person of ordinary skill in the art that changes resistance as a function of applied voltage or current, e.g., RRAM, PCM, STT-MRAM, and the like. 
     As is well known, memory cells  501 ,  502 ,  503 , and  504  may be subject to three types of operations: form, reset, and set. A form operation may create or form a conduction path through one or more filaments formed in the memory cell after application of a sufficiently high voltage. A form operation may occur just after manufacture of the memory cell but before actual data storage. The conduction path or filament is reset (broken) or set (re-created) after a form operation. 
       FIG.  5 B  is an exemplary bias scheme for a form operation performed on memory cell  502  of array  500 . A person of ordinary skill in the art should recognize that other bias schemes for a form operation are possible. In the exemplary bias scheme shown in  FIG.  5 B , a maximum voltage across the gate of any of unselected memory cells  501 ,  503 , or  504  is about 2V compared to the 3.5V drop across the gate dielectric observed in memory cell  100 . The lower voltage drop across the gate of select transistors for unselected memory cells  501 ,  503 , and  504  allows for the use of smaller transistors, e.g., core logic transistors, which, in turn, reduces cell size. 
       FIG.  6 A  is a diagram of a portion of a 2T-1R memory array  600  including a plurality of memory cells, e.g., memory cell  601 . In an embodiment, the plurality of memory cells including memory cell  601  have a construction like that of 2T-1R memory cell  400  shown in  FIG.  4    and may be arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction. 
       FIG.  6 B  is an exemplary bias scheme for a reset operation performed on memory cell  601  of array  600 . A person of ordinary skill in the art should recognize that other bias schemes for a reset operation are possible. In the exemplary bias scheme shown in  FIG.  6 B , a maximum voltage across the gate of any of unselected memory cells is less than 2V, compared to the 3.5V drop across the gate dielectric observed in memory cell  100 . 
       FIG.  7 A  is a diagram of a portion of a 2T-1R memory array  700  including a plurality of memory cells, e.g., memory cell  701 . In an embodiment, the plurality of memory cells including memory cell  701  have a construction like that of 2T-1R memory cell  400  shown in  FIG.  4    and may be arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction. 
       FIG.  7 B  is an exemplary bias scheme for a set operation performed on memory cell  701  of array  700 . A person of ordinary skill in the art should recognize that other bias schemes for a set operation are possible. In the exemplary bias scheme shown in  FIG.  7 B , a maximum voltage across the gate of any of unselected memory cells is less than 2V, compared to the 3.5V drop across the gate dielectric observed in memory cell  100 . 
       FIG.  8 A  is a diagram of a portion of a 2T-1R memory array  800  including a plurality of memory cells, e.g., memory cell  801 . In an embodiment, the plurality of memory cells including memory cell  801  have a construction like that of 2T-1R memory cell  400  shown in  FIG.  4    and may be arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction.  FIG.  8 B  is an exemplary bias scheme for a read operation performed on memory cell  801 . 
       FIG.  9    is a diagram of select transistors  402  and  403  of memory cell  400  shown in  FIG.  4   . Transistors  402  and  403  include contact area  901 , gate areas  902  and  905 , oxide isolation area  903 , and diffusion area  904 . Various dimensions in  FIG.  9    may potentially be smaller than corresponding dimensions in the prior art I/O select transistor approach shown in  FIG.  3   , which, in turn, may lead to the possibility of lower cell size and other advantages. A person of ordinary skill in the art should recognize that various alternatives to laying out select transistors  402  and  403  are possible, including the addition of vertical source lines instead of horizontal source lines as shown in e.g.,  FIGS.  5 A,  6 A,  7 A, and  8 A . A person of ordinary skill in the art should likewise recognize that bit lines, source lines, or other lines may be shared or common to two or more adjacent memory cells. A person of ordinary skill in the art should recognize that hierarchical wiring schemes may be used to reduce area and that one of select transistors  402  or  403  may be shared between more than a single memory cell. 
       FIG.  10    is a diagram of a portion of a 2T-1R memory array  1000  including a plurality of memory cells, e.g., memory cells  1001 ,  1002 ,  1003 , and  1004 . In an embodiment, each of the plurality of memory cells including memory cells  1001 ,  1002 ,  1003 , and  1004  have a construction like that of 2T-1R memory cell  400  shown in  FIG.  4 A  and may be arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction. In an embodiment, each of memory cells  1001 ,  1002 ,  1003 , and  1004  is coupled to receive a bit line signal BL, two pairs of word line signals WL1(1,2) and WL2(1,2), and a source line signal SL. 
     Memory cells arranged on rows of array  1000  are configured to receive the same two word line signals and memory cells arranged on adjacent rows of array  1000  share a same source line signal. For example, memory cells  1003  and  1001  receive word line signals WL11 and WL12 and memory cells  1004  and  1002  receive word line signals WL21 and WL22. Memory cells  1003  and  1001  arranged on a row adjacent to memory cells  1004  and  1002  share source line signal SL1. 
     Memory cells arranged on a column of memory array  1000  are configured to receive the same bit line signal. For example, memory cells  1003  and  1004  arranged on a first column receive bit line signal BL1 and memory cells  1001  and  1002  arranged on a second column receive bit line signal BL2. A person of ordinary skill in the art should recognize that other arrangements of array  1000  are possible and come within the inventive principles disclosed herein. For example, in some embodiments, memory cells formed in adjacent rows or adjacent columns may share source lines, bit lines, source contacts, drain contacts, and the like depending on the architecture implemented. 
     Memory cells  1001 ,  1002 ,  1003 , and  1004  may include any type of memory technology known to a person of ordinary skill in the art that changes resistance as a function of applied voltage or current, e.g., RRAM, PCM, STT-MRAM, and the like. 
     Like previously-described memory cells, memory cells  1001 ,  1002 ,  1003 , and  1004  may be subject to three types of operations: form, reset, and set. A form operation may create or form a conduction path through one or more filaments formed in the memory cell after application of a sufficiently high voltage. A form operation may occur just after manufacture of the memory cell but before actual data storage. The conduction path or filament is reset (broken) or set (re-formed) after a form operation. 
       FIGS.  11 A,  12 A,  13 A, and  14 A  are diagrams of portion of 2T-1R memory array  1000  (shown in  FIG.  10   ) including a plurality of memory cells each having a construction similar to that of memory cell  400  shown in  FIG.  4 A . Memory array  1000  may include a plurality of memory cells that are arranged in a plurality of columns extending in a first direction and a plurality of rows extending in a second direction perpendicular to the first direction. Memory cells arranged on adjacent rows may share a common source line signal. 
       FIG.  11 B  is an exemplary bias scheme for a form operation performed on memory cell  1101  shown in  FIG.  11 A . A person of ordinary skill in the art should recognize that other bias schemes for a form operation are possible. Referring to  FIG.  11 B , a maximum voltage across the gate of any of unselected memory cells is less than 2V compared to over 3.5V drop across the gate dielectric observed in memory cell  100 . 
       FIG.  12 B  is an exemplary bias scheme for a reset operation performed in parallel on memory cells  1201  and  1202  shown in  FIG.  12 A . In an embodiment, memory cells other than memory cells  1201  and  1202  may be reset in parallel with memory cells  1201  and  1202 . A person of ordinary skill in the art should recognize that other bias schemes for a reset operation are possible. The reset operation may be performed in parallel on a portion of the plurality of memory cells in array  1200  such that all bits in a certain section of the memory array  1200  may be reset and only a few selected cells may then be set to store the desired bit pattern. 
       FIG.  13 B  is an exemplary bias scheme for a set operation performed on memory cell  1301  shown in  FIG.  13 A . A person of ordinary skill in the art should recognize that other bias schemes for a set operation are possible. Referring to  FIG.  13 B , a maximum voltage across the gate of any of unselected memory cells is less than 2V allowing for the use of smaller select transistors than is otherwise possible.  FIG.  14 B  is an exemplary bias scheme for a read operation performed on memory cell  1401  shown in  FIG.  14 A . A person of ordinary skill in the art should recognize that other bias schemes for a read operation are possible. 
     In resistive memory devices including arrays such as those described above, it becomes important to control the current flowing through the memory devices during certain operations, e.g., during form and set operations. Doing so allows for the production of a conduction path or filament having a controlled cross-sectional area. Capacitance in the bit line and source line buses, the sense amplifier, and surge currents produced in the memory cell itself result in a difficult current control. If for example, the current flowing through the memory device is high during set or form operations, the operable reset current may problematically increase, which, in turn, may cause an increase in die size and an increase in power requirements due to the need for wider select and periphery transistors. If, for another example, the current flowing through the memory device is low due to the existence of an ineffective current limiting circuit, data retention issue may exist. 
     U.S. patent publication 2013/0215669 discloses a current flowing through a memory element in association with switching the memory element from the high resistance state to the low resistance state. Namely, in response to the switching, a magnitude of the current flowing through the memory element increases by the ratio of the resistance change. This current flow may cause the filament in the conduction path to grow in size, which can increase a threshold or operable reset current required to switch back to the high resistance state. U.S. patent publication 2013/0215669, which is herein incorporated by reference in its entirety, discloses biasing the select MOS transistor in a 1T-1R memory cell into saturation to act as a current source in order to limit the RRAM element current in a set operation when the element switches from a high resistance state to a low resistance state. 
       FIG.  15 A  is a diagram of a portion of 2T-1R memory array  1000  (shown in  FIG.  10   ) including an embodiment of current limiting selector circuit  1510 .  FIG.  15 B  is a diagram illustrating bit line and word line pulses applied to a selected memory cell in the 2T-1R memory array shown in  FIG.  15 A . Referring to  FIG.  15 A , current limiting selector circuit  1510  may optionally include a first unity gain amplifier  1511  and a second unity gain amplifier  1512  to isolate the gate of select transistors T1 and T2 from the circuitry applying the word line voltage. The word line voltage is chosen such that the current flowing through the memory element is limited. 
     Referring to  FIGS.  15 A and  15 B , a control circuit (not shown) applies a bit line voltage as a bit line signal to the memory array including selected memory cell  1520 . The control circuit also applies an initial word line voltage V write  as a word line signal to one or both select transistors, e.g., transistors T1 or T2, of selected memory cell  1520  through current limiting selector circuit  1510 . The control circuit may perform a verify read operation on the selected memory cell  1520  to verify a predetermined target resistance value for the memory cell  1520  by applying a read bit line voltage and a read word line signal. If selected memory cell  1520  has not yet reached the target resistance value, control circuit may increase the initial word line voltage V write  and apply the increased word line voltage V write  to one or both select transistors, e.g., transistors T1 or T2 through the current limiting circuit  1510 . In other embodiments, a same value of V write  may be re-applied to one or both select transistors, e.g., transistors T1 or T2 to allow for filament formation. The control circuit may perform another verify read operation on selected memory cell  1520  to verify a target resistance value. The control circuit may cycle through increasing the word line voltage V write  applied to select transistors T1 and/or T2 through current limiting selector circuit  1510  and performing a verify read operation until selected memory cell  1520  reaches the target resistance value. Current limiting selector circuit  1510  limits the current through the memory cell  1520  by limiting the voltage across the gate of transistors T1 and T2. 
       FIG.  16    is a diagram of a portion of 2T-1R memory array  1000  (shown in  FIG.  10   ) including an embodiment of current limiting selector circuit  1610 . Referring to  FIG.  16   , current limiting selector circuit  1610  may include a current mirror  1611  coupled to optional first unity gain amplifier  1612  and second unity gain amplifier  1613  that provides a current-limited word line signal to select transistors, e.g., transistors T1 or T3, of selected memory cell  1620 . As is well known to a person of ordinary skill in the art, a current mirror  1611  will provide an approximately (or relatively) constant current at its output regardless of loading. Thus, the current flowing through transistors T2 and T4 of current mirror  1611  will control the current flowing through the resistive memory element of selected cell  1620  through select transistors T2 and T3. 
       FIG.  17    is a diagram of a portion of a 2T-1R memory array  1000  (shown in  FIG.  10   ) including an embodiment of current limiting selector circuit  1710 . Current limiting selector circuit  1710  includes independent current mirror circuits for each of select transistors T1 and T3 that could provide a better output swing than the current limiting selector circuit  1610  shown in  FIG.  16   . 
       FIG.  18    is a diagram of a portion of a 2T-1R memory array  1000  (shown in  FIG.  10   ) including an embodiment of a current limiting selector circuit  1810 . Referring to  FIG.  18   , current limiting selector circuit  1810  combines the application of incremental voltage pulses to one of the two select transistors, e.g., select transistor T1, and the use of a current mirror to the other of the two select transistors, e.g., select transistor T2. 
       FIG.  19 A  is a diagram of a portion of a 2T-1R memory array  1000  (shown in  FIG.  10   ) including an embodiment of current limiting selector circuits  1910 A,  1910 B, and  1910 C. In an embodiment, memory array  1000  is divided into blocks, e.g., Block  1 , Block  2 , and Block  3 , each with its own source line current limiting circuits  1910 A,  1910 B, and  1910 C. 
     As shown in  FIGS.  20 A- 20 E , a source line may be pulled low by a Metal Oxide Semiconductor (MOS) transistor biased into saturation to act as a constant current source. The gate of a cell select Field-Effect Transistor (FET), e.g. the word line, may be biased higher at greater than, e.g. a little more than, a threshold voltage above the gate of the source line pull-down transistor. The bit lines and/or word lines of the remaining cells on the source line may be biased such that they do not conduct any current. 
     The biasing corresponding to the selected memory cell may cause the transistor in the selected cell to pull the source line up to less than, e.g. a little less than, one voltage threshold below the word line voltage and to act as a cascode stage positioned between the source line and the memory element of the selected cell. This voltage may be sufficient to maintain the source line pull-down transistor in saturation such that it may continue to act as a constant current source independent of small fluctuations in the source-line voltage. 
     In this configuration, the specific voltage that will appear on the source line may vary as a function of the voltage threshold of the select transistor in the selected cell. If the voltage threshold is low, the source line may rise (charged by the cell current) until the normalized saturation drain current (I DSAT ) of the select transistor matches the I DSAT  of the source line pull-down transistor. According, the variation of a threshold voltage of the cell select transistor (in an advanced Complementary Metal Oxide Semiconductor (CMOS) process where the variation in transistor threshold voltage from device to device can be relatively large, for example 150-200 mV) may be addressed by the self-compensating effect of the cascade configuration between the select transistor and the source line pull-down transistor. 
       FIG.  19 B  is a diagram of 2T-1R memory array  1000  (shown in  FIG.  10   ) including yet another embodiment of a current limiting selector circuit, in which the array  1000  is divided into vertical strings of memory cells, each source line signal in the string being current controlled with a corresponding current limiting selector circuit, e.g., circuits  1950 A and  1950 B. Embodiments of current limiting selector circuits  1910 A,  1910 B,  1910 C,  1950 A, and  1950 B are shown in  FIGS.  20 A- 20 E . 
       FIG.  21    is a diagram of a 2T-1R memory array  1000  (shown in  FIG.  10   ) including yet another embodiment of a current limiting selector circuits  2210 A and  2210 B. Referring to  FIG.  22   , select transistors between the local bit line signal LBL and the global bit line signal GBL in hierarchical bit line architecture may be used to form part of a current mirror selector circuit  2110 B. 
       FIG.  22 A  is a diagram of an embodiment of a 2T-1R memory array having shared vertical source lines. Referring to  FIG.  22 A , memory array  2200  includes vertical source lines  2208  and source contacts—source contacts are not shown separately from source lines  2208 —shared between adjacent memory cells  2202 . During form or set operations shown at (A) and (B), voltage signals applied to word lines may be determined using the current mirror circuits as described earlier. During a reset operation shown at (C), two adjacent memory cells  2202  may be written in parallel by applying appropriate voltage signals to bit lines  2204  and  2206  and source lines  2208 . In an embodiment, memory cells formed in adjacent rows or adjacent columns may share bit lines, source lines, source contacts, drain contacts, and the like. 
     In an exemplary embodiment, a reset operation may be performed in parallel on a portion of the plurality of memory cells in array  2200  such that all bits in a certain section of the memory array  2200  may be reset and only a few selected cells may then be set to store the desired bit pattern. When a certain byte has to be written, bits are reset during a reset operation optionally performed on several memory cells in parallel followed by setting just the bits needed during a set operation performed subsequent to the reset operation. Exemplary bias voltage signals are shown for memory array  2200  during a read operation at (D). 
       FIG.  22 B  is a diagram of an embodiment of a 2T-1R memory array with row drivers and exemplary biases for various operations. Referring to  FIG.  22 B , 2T-1R memory array  2200  includes a plurality of memory cells  2202  memory cells  2411 ,  2412 ,  2421 , and  2422 . Note that only a portion of memory array  2400  is shown for simplicity: memory array  2400  may include many more memory cells than just the four memory cells  2411 ,  2412 ,  2421 , and  2422  shown. The tables to the right and below memory array  2400  list voltage signal levels for various signals provided to memory array  2400  by row driver/decoder circuits  2455  and  2465  ( FIG.  22 B (A)) during the various indicated operations. 
     Note that only a portion of memory array  2200  is shown for simplicity: memory array  2200  may include many more memory cells than just those shown in  FIG.  22 B (A). The tables below memory array  2200  list voltage signal levels for various signals provided to memory array  2200  by row driver/decoder circuits  2255  and  2265  ( FIG.  22 B (A)) during the various indicated operations. 
     Row driver circuit  2255  may generate word line, bit line, and source line signals to drive memory array  2200  in response to signals generated by row decoder circuit  2265 . During a form operation on memory cell  2202 , for example, bit line signals BL SEL , BL UNSEL  and BL HALFSEL  are set to 4V, 0V, and 0V, respectively, while source line signals SL SEL  and SL UNSEL  are both set to 0V. Word line signals N SL_SIDE_WL  and N BL_SIDE_WL  are set to 0V and 2V, respectively. In an embodiment, row driver/decoder circuits  2255  and  2265  may include any of the current mirror circuits described above with reference to  FIG.  15 A,  15 B,  16 - 19   , or  20 A-E. During form or reset operations, row driver/decoder circuit  2455  may provide word line signals P SL_SIDE_WL  and P BL_SIDE_WL  corresponding to a selected memory cell to memory array  2200 . 
     Row driver circuit  2255  may be driven by row decoder circuit  2265  as shown. In an embodiment, row decoder circuit  2265  may include core transistors and, at a final stage, a voltage level shift circuit that is capable of shifting a first voltage level to a second voltage as is well known to a person of skill in the art. In an embodiment, row decoder circuit  2265  may include non-core transistors capable of sustaining the larger voltage signals necessary to drive selected word line signals without need for a voltage level shift circuit. Row driver/decoder circuits  2255  and  2265  may generate the voltage signals shown at (B) by including any of a variety of circuits, e.g., digital to analog converters (DACs) that operate off voltage values set in registers, charge pumps, bootstrapping circuits to drive the higher voltages and reduce requirements for charge pumps, unity gain amplifiers to drive the voltage signals for vertical lines, and the like. A person of ordinary skill in the art would understand that other circuits and variations are possible including using hierarchical word line, bit line, and source line architectures. 
       FIG.  22 C  is a diagram of an embodiment of a 2T-1R memory array with row drivers and exemplary biases for various operations. Referring to  FIG.  22 C , memory array  2200  may be coupled to a current limiting circuit to limit the current delivered to memory cell  2202  to avoid gate oxide breakdown of memory cell  2202 . In an embodiment, a current limiting circuit may be coupled to the bit line driver  2270  and to the select transistors in memory cell  2202  and may include transistors  2290 ,  2292 , and  2294 . The current limiting circuit may optionally include unity gain amplifiers  2282 ,  2284 , and  2286  to drive the large capacitive load of the shared wires. 
     Referring back to  FIG.  4 B , control circuit  450  provides the word line signals, bit line signals, and word line signals to the memory array  400 . In an embodiment, control circuit  450  avoids voltage or current stresses on select transistors T1 and T2 that would result in damage, wear out, reduced life, or the like, by applying the necessary voltage signals in predetermined levels and/or in a predetermined sequence as further described below. 
       FIG.  23 A  is a diagram of an embodiment of a portion of a 2T-1R memory array  2300  during a fatal operation.  FIG.  23 B  is a timing diagram of an embodiment of a voltage signal sequence applied to the 2T-1R memory array shown in  FIG.  23 A  during the form operation. Referring to  FIGS.  23 A and  23 B , a portion of memory array  2300  includes a 3×3 matrix of 2T-1R memory cells  2311 ,  2312 ,  2313 ,  2321 ,  2322 ,  2323 ,  2331 ,  2332 , and  2333 . Each of the memory cells  2311 ,  2312 ,  2313 ,  2321 ,  2322 ,  2323 ,  2331 ,  2332 , and  2333  may have a construction like that of memory cell  400  shown in  FIG.  4 A  including, in some embodiments, using thin oxide devices for select transistors  402  and  403 . In an embodiment including 512 bits per bit line, memory array  2300  is configured to receive a plurality of voltage signals, e.g., bit line signals BL0:511, pairs of first and second word line signals WL0:511_0, 1, and source line signals S_0:511 from a control circuit, e.g., control circuit  450  shown in  FIG.  4 B . For simplicity of explanation, the portion of memory array  2300  including 2T-1R memory cells  2311 ,  2312 ,  2313 ,  2321 ,  2322 ,  2323 ,  2331 ,  2332 , and  2333  shown in  FIG.  23 A  is configured to receive bit line signals BL0:2, pairs of first and second word line signals WL0:2_0, 1, and source line signals S_0:2. 
     A form operation is typically performed once on a memory device during its manufacture. In an embodiment in which a form operation is performed on selected memory cell  2322 , control circuit  450  ( FIG.  4 B ) may apply the sequence of voltage signals to memory array  2300  as follows: 
     At step 0, maintain first word line signals WL0_0, WL1_0, and WL2_0 for unselected memory cells  2311 ,  2312 ,  2313 ,  2331 ,  2332 , and  2333 , and all source line signals S_0, S_1, and S_2 to a ground voltage. 
     At step 1, charge second word line signals WL0_1 and WL2_1 corresponding to unselected memory cells  2311 ,  2312 ,  2313 ,  2331 ,  2332 , and  2333  to a first word line voltage, e.g., 1.5V. 
     At step 2, set second word line signal WL1_1 corresponding to selected memory cell  2322  to a second word line voltage higher than a first word line voltage, e.g., 2V. 
     At steps 3a and 3b, provide an intermediate bit line voltage, e.g., 1.5V, to bit line signal BL1 (step 3a) corresponding to selected memory cell  2322 , and boost the intermediate bit line voltage, e.g., 1.5V, provided to the bit line signal BL1 to boosted bit line voltage, e.g., 3.5V (step 3b) using, e.g., a charge pump circuit. 
     At step 4, set first word line signal WL1_0 corresponding to selected memory cell  2322  to a second word line voltage, e.g., 2V to coincide with boosted bit line signal BL1 approaching the 3.5V level. This step begins formation of the memory element included in selected memory cell  2322 . 
     At step 5a, discharge bit line signal SL1 corresponding to selected memory cell  2322  to ground voltage to end forming of the memory element included in selected memory cell  2322  (Voltage Sequence Option 1). 
     Alternatively, at step 5b, discharge first word line signal WL1_0 corresponding to selected memory cell  2322  to a ground voltage to end the forming the memory element included in selected memory cell  2322  (Voltage Sequence Option 2). 
     Steps 6a to 6d more particularly describe step 5b transition to step 7 for the Voltage Sequence Option 2, which alternative operation provides for lower voltage pump loading. 
     At step 6a, disconnect pumped voltage source from bit line BL1 after first word line signal WL1_0 is discharged to a ground voltage. 
     At step 6b, set bit line equalization signal EQ_0 to a boosted voltage 3.5V and charge share bit line signals BL1 and BL2 such that remaining charge on bit line signal BL1 is shared by bit line signal BL2 resulting in a voltage of e.g., 1.75V. 
     At step 6c, set bit line equalization signal EQ_1 to a ground voltage and isolate bit line signal BL1 from bit line signal BL2. 
     At step 6d, discharge bit line signal BL1 corresponding to selected memory cell  2322  to a ground voltage and charge bit line signal BL2 from e.g., 1.75V to the boosted bit line voltage, e.g., 3.5V, saving load on the bit line 3.5V charge pump. 
     At step 7, for Voltage Sequence Option 1, both word line signals WL1_0,1 have remained at 2V after bit line signal BL1 was discharged to ground. The forming for memory element  2323  begins when bit line signal BL2 is charged to 3.5V as per steps 3a and 3b. For Voltage Sequence Option 2, after bit line signal BL2 has been charged to 3.5V, set first word line signal WL1_0 corresponding to selected memory cell  2323  is set to a second word line voltage, e.g., 2V. This begins formation of the memory element of selected memory cell  2323  for Option 2. 
     The result is that the exemplary sequences (Voltage Sequence Options 1 and 2) limit voltage or current stresses by controlling the signals provided to select transistors corresponding to memory cells selected for a form operation, e.g., memory cell  2322  or memory cell  2323 , since no current flows through the select transistor corresponding to the selected memory cell until the first word line signal, e.g., WL1_0, is active. Note that the exemplary sequence allows for control of the memory element formation time t form  by either turning off the bit line signal corresponding to the selected memory cell, e.g., bit line signal BL1 (Option 1), or turning off the first word line signal WL1_0 corresponding to the selected memory cell (Option 2). 
     In Voltage Sequence Option 1, the formation of the memory element for the selected memory cell ends by turning off (or discharging to a ground voltage) the corresponding bit line signal BL1. The number of memory elements in array  2300  that can be formed in parallel may depend on the capacity of the charge pump (not shown) to maintain forming currents while charging the bit lines from an intermediate bit line voltage of, e.g., 1.5V, to a boosted bit line voltage of e.g., 3.5V. 
     In Voltage Sequence Option 2, the formation of the memory element corresponding to the selected memory cell ends by control of the corresponding first word line signal WL1_0. The sequencing of bit line equalization signals EQ_0 and EQ_1 to share charge between bit line signals BL1 and BL2, such that bit line signal BL2 is charged up to an intermediate bit line voltage, e.g., 1.5V or 1.75V, before being boosted to a boosted bit line voltage of, e.g., 3.5V reduces noise of the charge pump supply voltage line to provide consistent forming currents to the memory element for the selected memory cells. 
     An alternative sequence may include pre-charging the bit line signal BL2 corresponding to a memory cell selected to be formed next to the intermediate bit line voltage, e.g., 1.5V, sharing charge between the bit line signal BL1 corresponding to the memory cell currently being formed and the bit line signal BL2 corresponding to the memory cell to be formed next by sequencing the bit line equalization signals EQ_0 and EQ_1, and finishing charging the bit line signal BL2 from the shared charge bit line voltage, e.g., 2V to the boosted bit line voltage of 3.5V, which may further reduce charge pump loading. 
     After completing the form operation, discharge the last bit line signal and first and second word line signals WL0_0, WL1_0, WL2_0, WL0_1, WL1_1, WL2_1 to ground. 
       FIG.  23 C  is a diagram of an embodiment of a portion of a 2T-1R memory array  2300  during a reset operation.  FIG.  23 D  is a timing diagram of an embodiment of a voltage signal sequence applied to the 2T-1R memory array shown in  FIG.  23 C  during the reset operation. Referring to  FIGS.  23 C and  23 D , in an embodiment in which the reset operation is performed on memory cell  2322 , the sequence of voltage signals is applied as follows: 
     At step 1, set first word line signal WL1_0 corresponding to selected memory cell  2322  and second word line signals WL0_1, WL1_1, WL2_1 corresponding to unselected memory cells  2311 ,  2312 ,  2313 ,  2321 ,  2323 ,  2331 ,  2332 , and  2333  to a first word line voltage, e.g., 1.5V. 
     At step 2, set source line signals S_1, S_2, S_3 to a first source line voltage, e.g., 1.5V. 
     At step 3, set bit line signals BL1, BL2, and BL3 to a first bit line voltage, e.g., 1.5V. 
     At step 4, boost the voltage at bit line signals BL0, BL2, and BL3 from a first bit line voltage, e.g., 1.5V, to a boosted bit line voltage, e.g., 3.0V using, e.g., a charge pump device. 
     At step 5, increase the voltage applied to the source line signal S_1 corresponding to selected memory cell  2322  from the first source line voltage, e.g., 1.5V, to a second source line voltage, e.g., 3V. 
     At step 6, increase the voltage applied to first and second word line signals WL1_0 and WL1_1 corresponding to selected memory cell  2322  from first word line voltage, e.g., 1.5V, to a second word line voltage, e.g., 4V. 
     At step 7a, set bit line signal BL1 corresponding to selected memory cell  2322  to a ground voltage GND (only time current flows through element) for a reset time t reset . This step begins reset of the memory element of selected memory cell  2322 . 
     At step 7b, set bit line signal BL1 corresponding to selected memory cell  2322  to a boosted bit line voltage, e.g., 3.5V. This step ends reset of the memory element of selected memory cell  2322 . 
     At step 8, disconnect drivers for first and second word line signals WL1_0 and WL1_1 corresponding to selected memory cell  2322  to float first and second word line signals WL1_0 and WL1_1. 
     At step 9, discharge source line signal S_1 corresponding to selected memory cell  2322  and bit line signals BL0, BL1, and BL2. 
     At step 10, discharge source line signals S_0 and S_2 corresponding to unselected memory cells  2311 ,  2312 ,  2313 ,  2331 ,  2332 , and  2333 . 
     At step 11, discharge second word line signals WL0_1 and WL1_1 corresponding to unselected memory cells  2311 ,  2312 ,  2313 ,  2331 ,  2332 , and  2333  and discharge first and second word line signals WL1_0 and WL1_1 to selected memory cell  2322  to a ground voltage. 
     The sequence of voltage signals applied to the memory array  2300  to reset selected memory cell  2322  results in moderating the instantaneous charging currents from a drain voltage, e.g., voltage Vdd. The voltage drop of 2.25V across the memory element of selected memory cell  2322  (until the filament opens) insures that the voltage between the gate receiving the second word line voltage signal WL1_1 and the drain coupled to the memory element is limited to 1.75V (4V-2.25V) until the filament opens. 
     After completing the reset operation, disconnect the word line driver to float the first and second word line signals WL1_0 and WL1_1 corresponding to selected memory cell  2322 . At this time, discharge bit line signals BL0, BL2, and BL2 and source line signal S_1 to a ground voltage, e.g., Vss, coupling the first and second word line signals WL1_0 and WL1_1 corresponding to selected memory cell  2322  toward 1.5V. Next, discharge source line signals S_0 and S_2 corresponding to unselected memory cells. 
     The above-described voltage sequences are designed to minimize stresses induced on memory cells of the 2T-1R array in preparation for forming and reset operations. The sequence insures that the applied voltage signals produce currents that only flow through the target element of the selected memory cell during its programming, resulting in increased reliability and consistent filament control. 
     Referring back to  FIG.  4 B , control circuit  450  provides the word line signals, bit line signals, and word line signals to the memory array  400 . In an embodiment, control circuit  450  includes a row driver/decoder circuit configured to generate the voltage signals applied to memory array  400  during various operations including the predetermined sequence of voltage signals described with reference to  FIGS.  23 A- 23 D  and the voltage signals applied to memory array  400  to select two adjacent rows of memory array  400  for a reset operation and a single row of memory array  400  for set, form, or read operations. 
       FIG.  24 A  is a diagram of an embodiment of a biasing scheme for various operations performed on a 2T-1R memory array  2400 .  FIG.  24 B  is a diagram of an embodiment of row driver/decoder circuits  2455  and  2465  configured to generate the signals shown in  FIG.  24 A . 
     Referring to  FIGS.  24 A and  24 B , 2T-1R memory array  2400  includes memory cells  2411 ,  2412 ,  2421 , and  2422 . Note that only a portion of memory array  2400  is shown for simplicity: memory array  2400  may include many more memory cells than just the four memory cells  2411 ,  2412 ,  2421 , and  2422  shown. The tables to the right and below memory array  2400  list voltage signal levels for various signals provided to memory array  2400  by row driver/decoder circuits  2455  and  2465  ( FIG.  24 B ) during the various indicated operations. 
     During a form operation, for example, memory array  2400  may receive from row driver/decoder circuits  2455  and  2465  word line signals WL unsel_BL_side  and WL unsel_SL_side  set to 1.5V and 0V, respectively, source line signal set to 0V, bit line signals BL unsel  and BL sel  set to 0V and 3.5V, respectively, and source line signals SL sel  set to 0V. 
     In an embodiment, row driver/decoder circuits  2455  and  2465  may include any of the current mirror circuits described above with reference to  FIG.  15 A,  15 B,  16 - 19   , or  20 A-E. During form or reset operations, row driver/decoder circuit  2455  may provide word line signals WL sel_BL_side  and WL sel_ST_side  Corresponding to a selected memory cell to memory array  2400 . 
     Row driver circuit  2455  may be driven by row decoder circuit  2465  as shown. In an embodiment, row decoder circuit  2465  may include core transistors and, at a final stage, a voltage level shift circuit, e.g., voltage level shift circuit  2480  shown in  FIG.  24 C . The voltage level shift circuit may include any circuit capable of shifting a first voltage level to a second voltage that is known to a person of skill in the art. In an embodiment, the voltage level shift circuit may have a construction similar to voltage level shift circuit  2480 , which is also well known to a person of ordinary skill in the art. In another embodiment, row decoder circuit  2465  may include non-core transistors capable of sustaining the larger voltage signals necessary to drive selected word line signals without need for a voltage level shift circuit. Row driver/decoder circuits  2455  and  2465  may generate the voltage signals shown in table  2470  by including any of a variety of circuits, e.g., digital to analog converters (DACs) that operate off voltage values set in registers, charge pumps, bootstrapping circuits to drive the higher voltages and reduce requirements for charge pumps, unity gain amplifiers to drive the voltage signals for vertical lines, and the like. A person of ordinary skill in the art would understand that other circuits and variations are possible including using hierarchical word line, bit line, and source line architectures. 
       FIG.  25 A  is a diagram of an embodiment of a portion of 2T-1R memory array and  FIG.  25 B  is a diagram of layout of an embodiment of a 2T-1R memory cell included in the 2T-1R memory array shown in  FIG.  25 A .  FIG.  26 A  is a diagram of an embodiment of a 2T-1R memory array having shared source lines and  FIG.  26 B  is a diagram of layout of an embodiment of the 2T-1R memory cell shown in  FIG.  26 A . The portions of the memory arrays shown in  FIGS.  25 A and  26 A  have been fully described previously with relation to various drawings, including FIGS.  4 ,  9 , and  1000 . Each memory cell in array  2500  has a corresponding source line while memory cells in adjacent rows of array  2600  share a same or common source line. As shown by comparing  FIGS.  25 B and  26 B , sharing source lines between memory cells in adjacent rows may result in a reducing a size of a memory cell from e.g., a memory cell having dimensions 328 nm×140 nm ( FIG.  25 B ) to a memory cell having dimensions 278 nm×140 nm ( FIG.  26 B ). 
     2T-1R memory arrays having shared source line architectures like memory array  2600  may subject memory cells during reset operations to voltage stresses that cause oxide breakdown in thin oxide devices. 
     Byte-level flash reset, as we explain below, prevents high voltage stress conditions that may cause oxide breakdown in thin oxide devices. Referring to  FIG.  26 C , oxide breakdown of unselected memory cells may be avoided by using a byte-level flash reset of adjacent rows of memory cells. None of the select transistors in either of memory cells  2702  and  2704  are subject to voltage stresses that could result in oxide breakdown. Following the byte level flash reset, set operations may be performed as necessary. 
       FIG.  27 A  is a diagram of an embodiment of row driver circuit  2755  configured to generate the flash reset signals shown in  FIG.  26 C .  FIGS.  27 B and  27 C  are diagram of embodiments of a row decoder circuit  2765 , whose outputs change based on the operation to be performed on the memory array  2700 . 
     Referring to  FIGS.  26 C,  27 A,  27 B, and  27 C  during a reset operation both D sel  and D sel_  are 0. During set, read, or form operations, only one of D sel  and D sel_  is 0. D sl_sel  is 0 when any of the two rows are selected. Row decoder circuit  2765  may enable the functionality of row driver circuit  2755  where values for D sel , D sel_  and D sl_sel  change based on the operation to be performed on the memory array  2700 , e.g., reset or some other operation. An embodiment of row decoder circuit  2765  may include voltage level shift circuits similar in function to voltage level shift circuit  2480  shown in  FIG.  24 C . 
     Referring back to  FIG.  24 B , the portion of row driver  2455  that drives the source lines of memory array  2400  may need voltage passed through the select transistor T1 during read, set, and form operations. Since select transistor T1, however, is a PMOS device that does not allow 0V to pass through it in the presence of a positive voltage signal at the gate, biasing the driver would necessitate passing a voltage higher than a threshold voltage for transistor T1, e.g., 0.5V, to the source line. This, in turn, would necessitate the bit line voltage to be raised by, e.g., 0.5V, which could raise power and other concerns. A solution to this problem may involve using row decoder  2766  shown in  FIG.  27 B  in which D sl_sel =HIGH is output during read, set, and form operations and in which D sl_sel =LOW is output during reset operations for the selected row. Doing so allows passing 0V to the source line during read, set, and form operations using the NMOS transistor T2 shown in  FIG.  24 B . 
     It should be apparent to a person of ordinary skill in the art that the concepts described above involving 2T-1R memory arrays and corresponding row decoder/driver circuits are equally applicable to vertical shared source line architectures for 2T-1R memory arrays in which the source lines are aligned vertically as shown in  FIG.  28   . 
     Leakage currents, produced by operating core or thin oxide transistors at high voltages, should to be carefully considered and minimized. In the following description and drawings, the present disclosure details various techniques to reduce static power in 2T-1R memory arrays.  FIG.  29 A  is a diagram of an embodiment of a 2T-1R memory array with a biasing scheme during a reset operation.  FIG.  29 B  is a table listing measured junction leakage current, sub-threshold leakage current, and maximum write current during the reset operation performed on a selected memory cell  2902 . 
     Referring to  FIGS.  29 A and  29 B , in an embodiment, select transistors T top  and T bottom  may be core FET transistors operated at high voltages that generate leakage currents at various junctions, e.g., the drain junction of transistor T top  for unselected memory cell  2904 . In a large memory array having a large number of memory cells that are similarly biased, the junction leakage current may be multiplied many times over creating reliability issues during operation. As shown in  FIG.  29 B , junction leakage currents (0.9 mA) during a reset operation, for example, may be of concern while sub-threshold leakage current (30 uA) is small for devices having regular voltage thresholds Vt. Non-uniform channel doping or halo doping may reduce junction leakage currents and, therefore, improve reliability. 
     Select transistors T top  or T bottom  may be halo doped meaning that either may be more heavily doped near the source and drain terminals to reduce the size of the depletion region in the vicinity of these junctions. At short channel lengths the halo doping of the source overlaps that of the drain, increasing the average channel doping concentration, and thus increasing the threshold voltage. 
       FIG.  30    is a diagram illustrating that halo doping levels are an important factor in determining junction leakage currents for unselected cell  2904  shown in  FIG.  29 A . By tailoring the halo implant of the drain junction of select transistor Trop, junction leakage may be reduced by two orders of magnitude, while increasing sub-threshold leakage. As shown in  FIG.  29 B , sub-threshold leakage current during a reset operation is less of a concern since it is relative small, and the drain junction of select transistor T top  is only one of many contributors to sub-threshold leakage of unselected cells including unselected cell  2904 . The tradeoff, therefore, may be acceptable. In an embodiment, halo doping may occur on just the drain side of select transistor Trop or may vary at other junctions of the memory cell as well. Other embodiments could change halo doping by using additional masks or process optimizations similar to those described by T. B. Hook, et al., IEEE Transactions on Electron Device, September 2002, which is incorporated herein in its entirety. 
     Select transistor T top  may determine junction leakage of unselected memory cell memory cell  2904  (and all other unselected memory cells). Select transistor T bottom  may have a stronger body effect. Memory array  2900  may be optimized by building select transistor T top  as a low junction leakage transistor and building select transistor T bottom  as a high drive current transistor.  FIG.  31    tabulates the junction leakage current, sub-threshold leakage current, total static leakage current and maximum write current for three different doping combinations for select transistors T top  and T bottom .  FIG.  31    demonstrates that the total static leakage using a mix of select transistor types for T top  and T bottom  is similar, but drive current could be higher than architectures where the two select transistors T top  and T bottom  are similarly or identically doped. A person of ordinary skill in the art should realize that various other types of asymmetric transistors could be used in a 2T-1R memory array architecture. 
       FIG.  32    is a diagram of an embodiment of a 2T-1R memory array having shared vertical source lines. Referring to  FIG.  32   , a biasing scheme used with 2T-1R memory array  3000  having vertical source lines may be tuned to minimize leakage currents. Note that the bit line and source line of unselected memory cell C1 are at a same voltage thus eliminating sub-threshold leakage from these cells. The result is more flexibility for tuning the halo implants to reduce leakage. 
       FIG.  33 A  is a diagram of an embodiment of a hierarchical floor plan for a 2T-1R memory device  3300 . Referring to  FIG.  33 A , 2T-1R memory device  3300  comprises a plurality of array tiles, e.g., memory array tiles  3302 A and  3302 B. For simplicity, only memory array tile  3302 A is described in further detail. It should be apparent to a person of ordinary skill in the art that other memory array tiles, e.g., memory array tile  3302 B, are similarly constructed. 
     Memory array tile  3302 A includes two bit cell tiles  3320 A and  3320 B and each bit cell tile, in turn, includes two 2T-1R memory cells  3324 A and  3324 B. Bit cell tiles  3320 A and  3320 B are divided by the local word line and source line buffers  3326 . 
     Each of 2T-1R memory cells  3324 A and  3324 B (after  FIG.  10   ) may also have a construction like that of 2T-1R memory cell  400  shown in  FIG.  4 A . As the 2T-1R name implies, each of 2T-1R memory cells  3324 A and  33248  includes two select transistors and a memory element. Each of 2T-1R memory cells  3324 A and  3324 B is coupled to receive a bit line signal, two word line signals, and a source line signal. For example, memory cell  3324 A is coupled to receive bit line signal BL, local word line signals LWL1_BL and LWL1_SL, and local shared source line signal LSL. For another example, memory cell  3324 B is coupled to receive bit line signal BL, local word line signals LWL0_BL and LWL0_BL, and local source line signal LSL. 
     Note that the memory cells  3324 A and  3324 E are shown as having a shared source line LSL although other memory cell architectures, as described above, in which a source line is not shared between adjacent memory cells, come within the scope of the present disclosure. 
     Memory array tile  3302 A further includes sense amplifiers  3312 A and  3312 B coupled to bit cell tile  3320 A through corresponding bit line signals BLA and BLB and sense amplifiers  3314 A and  3314 B coupled to bit cell tile  3320 B. Local row generator  3310 A is coupled between sense amplifiers  3312 A and  3314 A and local row generator  3310 E is coupled between sense amplifiers  3312 B and  3314 B. Local word line and source line buffers  3326  are coupled between bit cell tiles  3320 A and  3320 B. Local row generators  3310 A and  3310 E are configured to generate local word lines LWLs and local source line LSL signals. 
     Sense amplifiers  3312 A and  3314 A are coupled to receive row control signals from row control circuit  3306 A, which is coupled in turn to row pre-decoder  3304 A. Similarly, sense amplifiers  3312 B and  3314 B are coupled to receive row control signals from row control circuit  3306 B, which is coupled in turn to row pre-decoder  3304 B. Row pre-decoders  3304 A and  3304 B are coupled to master word line and source line decoder  3308 , which is configured to provide master word lines MWLs and master source lines MSL_shr to local word line and source line buffer  3326 . 
     A voltage realm in the present disclosure refers to a range of voltage values in which particular circuits, components, and the like may be capable of reliably indicating high and low signals. Logic devices operate in any number of voltage realms, e.g., 5V, 3.3V, 3V, 2.5V, 1.8V, 1.5V, and the like. To communicate reliably between logic voltage levels, level shifting between signals may be desirable. 
     Memory array tile  3302 A may be controlled using a global signals bus powered at a first voltage in a first voltage realm, e.g., 1V, to allow saving power and circuit area. Pre-decoders  3304 A and  3304 B may pre-decode the address signals from the global address bus and provide them to row control circuits  3306 A and  3306  and master word line and source line buffer  3308 . Pre-decoders  3304 A and  3304 B may voltage level shift the global signals from the first voltage (first voltage realm) to a second voltage (second voltage realm), e.g., from 1V to 1.5V, using any manner of voltage level shift circuitry known to a person of ordinary skill in the art. Pre-decoders  3304 A and  3304 B may provide the local row pre-decoded address bus signals to row control circuits  3306 A and  3306 B and master word line and source line buffer  3308 . Master word line and source line buffer  3308  may decode and level shift the master word line MWL and the master source line MSL_shr to the second voltage (second voltage realm) to a third voltage (third voltage realm), e.g., from 1.5V to 3V, to interface to the local word line and source line buffer  3326 . Master source line MSL_shr may be pulled low only during a reset operation. 
     Local row control generators  3310 A and  3310 E may voltage level shift row control signals from the second voltage (second voltage realm) to a third voltage (third voltage realm), e.g., from 1.5V to 3V, to interface to local word line and source line buffer  3326 . 
     Array tile  3302 A is illustrated with the local word line and local source line buffer  3326  located in the center in  FIG.  33 A  although other arrangements are possible. Another embodiment may involve locating the local word line and source line buffer  3326  to the left or right of the bit cell tiles  3320 A or  3320 B may be optimal for some memory device technologies or architectures. For example, it may be beneficial to drive the local word line signal LWL0 (even) from the left side while driving the local word line signal LWL1 (odd) from the right side, with driving the local source line from one or both sides. 
       FIG.  33 B  is a diagram of an embodiment of local row generator  3310 . Referring to  FIG.  33 B , local row generator  3310  is configured to generate local word line signals LWL and local source line signal LSL for interfacing with local word line and source line buffer  3326  in response to decoded signals received from a corresponding row control circuits, e.g.,  3306 A or  3306 B. The decoded signal received from row control circuits  3306 A or  3306 B may indicate any of the various operations to be performed on array tiles  3302 A and  3302 B, e.g., set, read, reset, or form. In an embodiment, local row generator  3310  may include thick oxide devices capable of sustaining 3V biased signals over the product&#39;s lifetime. 
       FIG.  33 C  is a diagram of an embodiment of a hierarchical row path to illustrate a voltage level shift operation of row pre-decoder  3304 A and master word line and source line decoder  3308  as they operate on bit cells  3324 A and  3324 B. Referring to  FIG.  33 C , row local pre-decoder  3304 A is configured to pre-decode row addresses at a first voltage in the first voltage realm, e.g., 1V, and provides the local pre-decoded addresses to master word line and source line decoder  3308 , which, in turn, is configured to level shift the pre-decoded master word line MWL and master source line MSL from the first voltage in the first voltage realm, e.g., 1V, to the second voltage in the second voltage realm, e.g., 3V, using any voltage level shift circuit known to a person of ordinary skill in the art. Doing so, allows a reduction in the depth of the master word line and source line decoder  3308 . Because voltage level shift circuits can be area intensive, in one embodiment the level shift decode state may be combined with the reset to control the master word line and source line decoder  3308 . In an embodiment, during activation, unselected master word line signals and master source line signals remain at 3V while selected master word line signal MWL is pulled to Vss. Only during a reset operation, the selected master source line MSL is also pulled to Vss. Local row generator  3310  is configured to control local word line and source line buffer  3326  that, in turn, generates the local word line signals LWL and local source line signals LSL. 
       FIG.  33 D  is a diagram of an embodiment of a hierarchical row path to illustrate a voltage level shift operation of row pre-decoder  3304 A and  3304 B and master word line and source line decoder  3308  as they operate on bit cells  3324 A and  3324 B, which share a source line. Referring to  FIG.  33 D , bit cells  3324 A and  3324 B share a source line signal LSL_shr, which enables a flash reset described in detail above. 
     In an embodiment, during activation, unselected master word line signals MWL0, MWL1 and master source line signals MSL_shr remain at 3V. Shared master source line signal MSL_shr is generated by decoding the level shifted inputs to the MWL0 and MWL1 buffers. As in Option 1, the selected master word line signal MWL0 or MWL1 is to be pulled to Vss. Only during a reset operation, the selected master source line signal MSL_shr is also pulled to Vss. The LWL and LSL are then controlled from the Local Row generator signals, as in Option 1 in  FIG.  33 C . An advantage to the circuit shown in  FIG.  33 D  is that five signals control two bit cells while the circuit shown in  FIG.  33 C  utilizes six signals to control the two bit cells. 
     It will also be appreciated by persons of ordinary skill in the art that the present disclosure is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present disclosure includes both combinations and sub-combinations of the various features described hereinabove as well as modifications and variations which would occur to such skilled persons upon reading the foregoing description. Thus the disclosure is limited only by the appended claims.