Patent Publication Number: US-8995177-B1

Title: Integrated circuits with asymmetric transistors

Description:
This application is a division of patent application Ser. No. 13/110,823, filed May 18, 2011, which is a continuation-in-part of application Ser. No. 12/629,831, filed Dec. 2, 2009, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     Integrated circuits such as programmable integrated circuits may contain volatile memory elements in the form of static random access memory (SRAM) cells. In programmable integrated circuits, SRAM cells may serve as configuration random access memory (CRAM) cells. Programmable integrated circuits are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. CRAM cells are used to store configuration data supplied by the user. Once loaded, CRAM cells supply control signals to transistors to configure the transistors to implement the desired logic function. 
     Volatile memory elements such as SRAM and CRAM cells are typically based on cross-coupled inverters (latches). In each memory element, the cross-coupled inverters may be connected to an address transistor that is turned on when data is being read from or written into the memory element. When no data is being read from or written into the memory element, the address transistor is turned off to isolate the memory element. 
     There is a trend with each successive generation of integrated circuit technology to scale transistors to smaller sizes, smaller threshold voltages, and smaller power supply voltages. Lower power supply voltages and smaller devices may lead to decreased read/write margins for volatile memory elements. This can pose challenges for reliable device operation. 
     With conventional techniques, memory cell stability is ensured by adjusting transistor sizes and by forming transistors with asymmetric pocket implants. Redundant resources may also be provided so that defects can be bypassed by switching redundant circuitry into use. Although these approaches are sometimes satisfactory, they can lead to undesirable area overhead and increased manufacturing cost. 
     SUMMARY 
     Integrated circuits may include memory elements. A memory element may include a latching circuit (e.g., a storage circuit based on cross-coupled inverters). The latching circuit may be coupled to data lines through access transistors. The access transistors may be used to read data from and write data into the memory element. 
     The access transistors may be asymmetric transistors. In particular, the access transistors may be transistors with asymmetric source-drain resistances. For example, a memory element may include a latching circuit and first and second access transistors. The latching circuit may have first and second data storage nodes that store true and complementary versions of a single data bit. 
     The first access transistors may have a first source-drain terminal that is coupled to a first data line and a second source-drain terminal that is coupled to the first data storage node. The second access transistor may have a first source-drain terminal that is coupled to a second data line and a second source-drain terminal that is coupled to the second data storage node. The first source-drain terminals of the first and second access transistors may have resistance values that are less than the resistance values of the second source-drain terminals of the first and second access transistors (e.g., access transistor source-drain regions coupled to the first and data storage nodes may have higher contact/series resistance than the access transistor source-drain regions coupled to the first and second data lines). 
     Access transistors formed in this way may exhibit a first drive strength during a forward mode and a second drive strength during a reverse mode. During the forward mode, an access transistor may be used to pass a high voltage into a corresponding data storage node or may be used to expose the data storage node to a high precharge voltage. During the reverse mode, the access transistor may be used to pass a low voltage into the data storage node. Asymmetric access transistors operating in the forward mode may be weaker than asymmetric access transistors operating in the reverse mode. Access transistors with asymmetric source-drain resistances may therefore be used to improve read/write margins of memory elements. 
     Asymmetric access transistors may include transistors formed using asymmetric source-drain implants, asymmetric source-drain silicide deposition, asymmetric source-drain pre-amorphization implants, stress layers of different types (e.g., tensile-stress-inducing layers and compressive-stress-inducing layers), source-drain contact vias of different sizes, source-drain spacers of different lengths, asymmetric pocket implants, etc. Other arrangements such as adjusting the magnitude of power supply voltages and address signals, adjusting transistor sizing, and including read buffers may be used in conjunction with asymmetric access transistors to increase memory element read/write performance. 
     Further features of the present invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative integrated circuit that may contain transistor structures in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram of an illustrative array of memory cells in accordance with an embodiment of the present invention. 
         FIG. 3  is a circuit diagram of an illustrative memory cell formed using asymmetric access transistors in accordance with an embodiment of the present invention. 
         FIG. 4  is a cross-sectional side view of an illustrative asymmetric access transistor that is biased in a forward mode in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional side view of an illustrative asymmetric access transistor that is biased in a reverse mode in accordance with an embodiment of the present invention. 
         FIG. 6  is a cross-sectional side view of an illustrative access transistor with asymmetric source-drain implant doping in accordance with an embodiment of the present invention. 
         FIG. 7  is a cross-sectional side view of an illustrative access transistor with asymmetric silicide formation in accordance with an embodiment of the present invention. 
         FIGS. 8A and 8B  are cross-sectional side views of an illustrative access transistor formed using asymmetric pre-amorphization implants in accordance with an embodiment of the present invention. 
         FIG. 9  is a cross-sectional side view of an illustrative access transistor with stress layers of different types in accordance with an embodiment of the present invention. 
         FIG. 10  is a cross-sectional side view of an illustrative access transistor with source-drain contacts of different sizes in accordance with an embodiment of the present invention. 
         FIG. 11  is a cross-sectional side view of an illustrative access transistor with asymmetric spacers in accordance with an embodiment of the present invention. 
         FIG. 12  is a circuit diagram of an illustrative memory cell formed using asymmetric access transistors in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative integrated circuit that may contain asymmetric transistors is shown in  FIG. 1 . As shown in  FIG. 1 , integrated circuit  10  may include memory elements (cells)  20 . 
     Memory elements  20  can be used in any suitable integrated circuits that use memory. These integrated circuits may be memory chips, digital signal processing circuits with memory arrays, microprocessors, application specific integrated circuits with memory arrays, programmable integrated circuits such as programmable logic device integrated circuits in which memory elements are used for configuration memory, or other suitable integrated circuit. For clarity, the use of memory elements  20  is sometimes be described in the context of programmable integrated circuits such as programmable logic device integrated circuits. This is, however, merely illustrative and not intended to limit the scope of the present invention. Memory cells  20  and the asymmetric transistors in such memory cells may be used in any suitable circuits. 
     On integrated circuits such as memory chips or other circuits in which memory is needed to store processing data, memory elements  20  can be used to perform the functions of static random-access memory (RAM) cells and are sometimes referred to as SRAM cells. In the context of programmable logic device integrated circuits, memory elements  20  can be used to store configuration data and are therefore sometimes referred to in this context as configuration random-access memory (CRAM) cells. 
     As shown in  FIG. 1 , device  10  may have input/output (I/O) circuitry  12  for driving signals off of device  10  and for receiving signals from other devices via input/output pins  14 . Interconnection resources  16  such as global and local vertical and horizontal conductive lines and busses may be used to route signals on device  10 . Interconnection resources  16  include fixed interconnects (conductive lines) and programmable interconnects (i.e., programmable connections between respective fixed interconnects). Programmable logic  18  may include combinational and sequential logic circuitry. The programmable logic  18  may be configured to perform a custom logic function. The programmable interconnects associated with interconnection resources may be considered to be a part of programmable logic  18 . 
     Integrated circuit  10  may contain memory elements  20  that are volatile. For example, integrated circuit  10  may be a programmable integrated circuit such as a programmable logic device integrated circuit that can be loaded with configuration data (also called programming data) using pins  14  and input/output circuitry  12 . Once memory elements  20  are loaded in this way, the memory elements each provide a corresponding static control output signal that controls the state of an associated logic component in programmable logic  18 . If desired, memory elements  20  may be used in SRAM-type memory arrays (e.g., to store data for processing circuitry during operation of device  10 ). 
     Each memory element  20  may be formed from a number of transistors configured to form a bistable circuit (i.e., a latch-type circuit). True and complement data storage nodes in the bi-stable circuit element can store corresponding true and complement versions of a data bit. 
     A bi-stable circuit element may be based on any suitable number of transistors. For example, the bi-stable portion of each memory element may be formed from cross-coupled inverters, from groups of multiple inverter-like circuits (e.g., in a distributed configuration that provides enhanced immunity from soft-error-upset events, etc.). Arrangements with bi-stable elements formed from cross-coupled inverter pairs are sometimes described herein as an example. This is, however, merely illustrative and not intended to limit the scope of the present invention. Memory elements  20  may be formed using any suitable memory cell architecture. 
     With one suitable approach, complementary metal-oxide-semiconductor (CMOS) integrated circuit technology is used to form the memory elements  20 , so CMOS-based memory element implementations are described herein as an example. If desired, other integrated circuit technologies may be used to form the memory elements and the integrated circuit in which the memory elements are used to form memory arrays. 
     The memory elements may be loaded from any suitable source of data. As an example, memory elements  20  may be loaded with configuration data from an external erasable-programmable read-only memory and control chip or other suitable data source via pins  14  and input/output circuitry  12 . Loaded CRAM memory elements  20  may provide static control signals that are applied to the terminals (e.g., gates) of circuit elements (e.g., metal-oxide-semiconductor transistors) in programmable logic  18  to control those elements (e.g., to turn certain transistors on or off) and thereby configure the logic in programmable logic  18 . The circuit elements may be transistors such as pass transistors, parts of multiplexers, look-up tables, logic arrays, AND, OR, NAND, and NOR logic gates, etc. 
     The memory elements  20  may be arranged in an array pattern. In a typical modern integrated circuit such as a programmable integrated circuit, there may be millions of memory elements  20  on each chip. During programming operations, the array of memory elements is provided with configuration data by a user (e.g., a logic designer). Once loaded with configuration data, the memory elements  20  produce static control signals at their outputs that selectively control portions of the circuitry in the programmable logic  18  and thereby customize its functions so that it will operate as desired. 
     The circuitry of device  10  may be organized using any suitable architecture. As an example, the logic of device  10  may be organized in a series of rows and columns of larger programmable logic regions each of which contains multiple smaller logic regions. The logic resources of device  10  may be interconnected by interconnection resources  16  such as associated vertical and horizontal conductors. These conductors may include global conductive lines that span substantially all of device  10 , fractional lines such as half-lines or quarter lines that span part of device  10 , staggered lines of a particular length (e.g., sufficient to interconnect several logic areas), smaller local lines, or any other suitable interconnection resource arrangement. If desired, the logic of device  10  may be arranged in more levels or layers in which multiple large regions are interconnected to form still larger portions of logic. Still other device arrangements may use logic that is not arranged in rows and columns. 
     When memory elements  20  are arranged in an array, horizontal and vertical conductors and associated loading circuitry may be used to load the memory elements with configuration data. Any suitable memory array architecture may be used for memory elements  20 . One suitable arrangement is shown in  FIG. 2 . 
     As shown in  FIG. 2 , control circuitry  24  may be used to read and write data from memory cells  20  in memory cell array  28 . When array  28  is being used as regular SRAM, data write operations may be performed when it is desired to store processing results and data read operations may be performed when it is desired to retrieve stored data. When array  28  is being used as CRAM, data write operations may be performed to load configuration data and data read operations may be performed to confirm that configuration data loading operations have been performed successfully. 
     In CRAM environments and certain SRAM environments, read speeds may be less critical than in conventional SRAM environments, because read operations may be performed relatively infrequently (e.g., mostly when confirming that configuration data has been written properly into a CRAM array). During normal operation of a circuit contains an array of CRAM cells (i.e., when CRAM cells are neither being written to or read from), the static output of each CRAM cell can be used to control a respective programmable logic component such as a transistor. For example, the output signal OUT from each cell  20  may be provided to the gate G of a corresponding pass transistor such as n-channel metal-oxide-semiconductor transistor  40  (i.e., programmable logic transistors such as transistor  40  in blocks of programmable logic  18 ). 
     Array  28  may include rows and columns of memory cells  20 . In the example of  FIG. 2 , there are three rows and three columns of memory elements  20  in array  28 . This is merely illustrative. Integrated circuits such as integrated circuit  10  may have any suitable number of memory elements  20 . A typical memory array might, as an example, have thousands or millions of memory elements  20  arranged in hundreds or thousands or rows and columns. 
     Lines such as lines  32 ,  34 ,  36 , and  38  may be used to distribute signals in array  28 . One or more lines per column such as lines  34  may be used to load data into cells  20  and may be used to read data out from cells  20 . Lines  34  may sometimes be referred to as data lines or bit lines. One or more lines  32  per row may be used to convey address signals and may sometimes be referred to as address lines or word lines. In some array configurations, the cells of the array may be cleared (e.g., during power-up operations). Array  28  may be cleared by writing zeros into the array using through the data lines or by asserting one or more global clear signals using a global (or nearly global) network of clear lines such as clear lines  36 . 
     During data writing operation, write drivers in circuitry  24  may supply data to array  28  on lines  34  (e.g., in appropriate columns of the array) while appropriate address lines are asserted to define the desired array location (i.e., the appropriate rows of the array) to which the data is to be written. During read operations, appropriate address lines are asserted to define the desired array location from which data is to be read (i.e., appropriate rows) while the outputs of appropriate data lines  34  are monitored (e.g., using sense amplifiers). Single-ended and differential schemes may be used for reading and/or writing. In differential write schemes, a pair of true and complement data lines are used. In differential read schemes, a differential sense amplifier may be used in reading signals from a pair of true and complement data lines. 
     Memory cells  20  may include asymmetric transistors (e.g., transistors with asymmetric source-drain resistance values). Memory cells  20  that include asymmetric transistors may exhibit improved read/write performance. For example, a memory cell  20  may include a storage element (e.g., a latching circuit) and at least one access transistor coupled to the storage element. During read operations, the access transistor may be turned on to read data from the storage element. During write operations, the access transistor may be turned on to load data into the storage element. 
     During a read operation, it may be desirable for the access transistor to be weak relative to transistors in the storage element so that the enabled access transistor does not inadvertently flip the contents of the storage element. During a write operation, it may be desirable for the access transistor to be strong relative to the transistors in the storage element so that the enabled access transistor can overwrite the contents of the storage element. One way of providing an access transistor operable to exhibit a first drive strength during a first mode (e.g., during a read operation) and a second drive strength that is stronger than the first drive strength during a second mode (e.g., during a write operation) is to form an asymmetric access transistor. Memory cells  20  that include asymmetric access transistors may exhibit improve read/write stability. 
       FIG. 3  is a circuit diagram of an illustrative memory cell  20  that may be included in device  10 . As shown in  FIG. 3 , memory cell  20  may include access transistors PG 1  and PG 2  and a storage element formed from a pair of cross-coupled inverters INV 1  and INV 2 . Inverter INV 1  may include p-channel transistor PU 1  and n-channel transistor PD 1  coupled in series between a positive power supply line  42  (e.g., a positive power supply line on which positive power supply voltage Vcc is provided) and a ground power supply line  44  (e.g., a ground power supply line on which ground power supply voltage Vss is provided). 
     Inverters INV 1  and INV 2  may each have an input and an output. The output of INV 1  may be coupled to the input of INV 2 , whereas the output of INV 2  may be coupled to the input of INV 1 . The output of INV 1  may serve as a first internal data storage node X, whereas the output of INV 2  may serve as a second internal data storage node nX (see, e.g.,  FIG. 3 ). Data storage nodes X and nX store true and complementary versions of a single data bit. When enabled, transistors PU 1  and PU 2  serve to pull corresponding data storage nodes of cell  18  high and are therefore sometimes referred to as pull-up transistors. When enabled, transistors PD 1  and PD 2  serve to pull corresponding data storage nodes of cell  18  low and are therefore sometimes referred to as pull-down transistors. Data storage node nX of memory cell  20  may supply a static control signal OUT to a corresponding pass transistor over path  38 . 
     An access transistor such as n-channel access transistor PG 1  may be coupled between a first data line  34  (e.g., a first bit line on which bit line signal BL is provided) and data storage node X. N-channel access transistor PG 2  may be coupled between a second data line  34 ′ (e.g., a second bit line on which bit line signal /BL is provided) and data storage node nX). Access transistors PG 1  and PG 2  may each have a gate that is controlled by a word line signal WL (sometimes referred to as an address signal) provided over line  32 . Access transistors PG 1  and PG 2  may be used to read data from cell  20  and to load data into memory cell  20 . Access transistors PG 1  and PG 2  may sometimes be referred to as address transistors, read/write pass transistors, read/write pass-gate transistors, etc. 
     A metal-oxide-semiconductor (MOS) transistor has four terminals: a source, a drain, a gate that overlies a channel region, and a body. The source and drain are sometimes collectively referred to as source-drain terminals. By convention, the drain of an MOS transistor is typically the source-drain terminal that is biased high, whereas the source is grounded or biased at a lower voltage. Because the labels “source” and “drain” may therefore be context-sensitive, it may sometimes be clearest to refer to the both the source and the drain of a MOS transistor as being “source-drain” terminals or “source-drains.” 
     In a symmetric transistor, the source-drain terminals of the transistors are substantially identical. It therefore does not matter whether the source-drain terminals of a symmetrical transistor are reversed, as performance will not significantly change. In an asymmetric transistor, however, the resistance at one of the source-drain terminal is substantially greater than the resistance at the other of the source-drain terminals. This leads to different performance characteristics depending on how the transistor is operated. 
     The access transistors (e.g., transistors PG 1  and PG 2  of  FIG. 3 ) may each include symmetric pocket implants P (e.g., p-type pocket implants). Symmetric pocket implants may serve to increase the energy barrier at the respective source-drain terminals of an access transistor, thereby raising the threshold voltage associated with that access transistor. Pocket implants P need not be present in the access transistors. 
     As shown in  FIG. 3 , the access transistors may be asymmetric transistors (e.g., the access transistors may exhibit asymmetric source-drain resistance values). For example, transistor PG 1  may have higher source-drain resistance at the source-drain terminal that is connected to data storage node X (as indicated by symbol R) compared to the source-drain resistance at the source-drain terminal that is connected to data line  34 . Similarly, transistor PG 2  may exhibit higher source-drain resistance at the source-drain terminal that is connected to data storage node nX (as indicated by symbol R) compared to the source-drain resistance at the source-drain terminal that is connected to data line  34 ′. 
       FIGS. 4 and 5  illustrate a transistor such as access transistor PG 1  with asymmetric source-drain resistance (sometimes referred to herein as source-drain contact resistance or source-drain series resistance).  FIG. 4  is a cross-sectional side view of transistor PG 1  biased under forward mode (e.g., a mode in which the access transistor exposes a corresponding data storage node to a high voltage). As shown in  FIG. 4 , transistor PG 1  may be formed in an integrated circuit substrate  202 . Substrate  202  may be formed, for example, from a silicon wafer. Portion  204  may be a p-type substrate layer (as an example), whereas portion  206  may be a doped well (e.g., a p-well). Well  206  may form part of a well layer in the integrated circuit. If desired, other semiconductor materials (e.g., epitaxial silicon germanium sources and drains, etc.) may be used in forming the transistor structures of integrated circuit  10 . 
     Transistor PG 1  may have a conductive gate structure GC disposed over the surface of substrate  202 . Gate GC may be formed from polysilicon or other gate conductors (e.g., metal, etc.). Dielectric spacers SP may be formed on opposing sides of gate GC. During ion implantation processes, source-drain regions SD 1  and SD 2  (sometimes referred to as heavily-doped drain regions) and pocket implants P may be formed in substrate  202 . There may be a lightly doped portion in the source-drain regions such as lightly-doped drain (LDD) regions LDD 1  and LDD 2  under spacers SP. Gate insulator GI (e.g., silicon oxide or other dielectric) may be formed under gate GC. Transistor PG 1  may have a channel region formed at the surface of well  206  that lie beneath gate insulator GI. 
     Source-drain terminal SD 1  of transistor PG 1  may be coupled to first data line  34 , whereas source-drain terminal SD 2  of transistor PG 1  may be coupled to data storage node X. The series resistance between the channel region and the first data line may be represented by resistance R1, whereas the series resistance between the channel region and data storage node X may be represented by resistance R2. Resistance R2 may be substantially greater than resistance R1 (e.g., R2 may be at least 500 ohms greater than R1). As an example, resistance R1 may be 100 ohms, whereas resistance R2 may be 600 ohms or greater than 600 ohms. 
     In the forward mode, transistor PG 1  may expose data storage node X to a high voltage level. As shown in  FIG. 4 , terminal SD 1  may be driven high (e.g., signal BL may be precharged to Vcc during read operations or driven to Vcc during write operations), whereas terminal SD 2  may be driven low (e.g., data storage node X may be held at ground by transistor PD 1 ). Address signal WL may be asserted to turn on transistor PG 1 . Transistor PG 1  may have a drive strength that is based on a gate-to-source voltage Vgs (e.g., a voltage difference between the voltage level at gate GC and terminal SD 2 ). 
     A drain-to-source current may flow from drain SD 1  to source SD 2  when transistor PG 1  is turned on. The voltage level of source SD 2  may be proportional to the product of the drain-to-source current and resistance R2 (i.e., voltage is equal to current multiplied by resistance). The voltage level of source SD 2  may therefore be high (relative to the source voltage of a transistor having symmetrically low resistance values), because resistance R2 is high (relative to nominal transistor source-drain resistance values). A high source voltage level at source SD 2  will decrease gate-to-source voltage Vgs of transistor PG 1 . Reducing Vgs using this approach may lower the drive strength of transistor PG 1  and decrease the drain-to-source current. 
     Weakening an access transistor when it is in the forward mode may be desirable, because cell  20  needs to be able to retain its stored content when its data storage nodes are exposed to high voltage levels through the access transistor (e.g., during read operations). Weakening access transistors PG 1  and PG 2  during read operations may improve the ability of the storage element of cell  20  to hold its content (e.g., weakening the access transistors during read operations may increase read margin). 
     In the reverse mode, transistor PG 1  may be used to load a “0” into data storage node X. As shown in  FIG. 5 , terminal SD 1  may be driven low (e.g., signal BL may be driven to Vss by write drivers during write operations), whereas terminal SD 2  may be driven high (e.g., data storage node X may be held at Vcc by transistor PU 1 ). 
     Address signal WL may be asserted to turn on transistor PG 1 . A drain-to-source current may flow from terminal SD 2  to terminal SD 1  when transistor PG 1  is turned on. The voltage level of source terminal SD 1  may be proportional to the product of the drain-to-source current and resistance R1. The voltage level of source terminal SD 1  may therefore be low because resistance R1 is low (e.g., the product of the drain-to-source current and R1 is less than 50 mV). Such low source voltage at source SD 1  will only affect gate-to-source voltage Vgs of transistor PG 1  by a negligible amount. The drive strength of transistor PG 1  is therefore relatively unaffected during the reverse mode compared to the noticeably weaker drive strength during the forward mode. 
     Having an access transistor with non-reduced drive strength when it is in the reverse mode may be desirable, because the access transistor needs to be sufficiently strong to overwrite cell  20  (e.g., during write operations). Maintaining the Vgs of access transistors PG 1  and PG 2  close to Vcc may help maximize write margin. 
     Other access transistors such as transistor PG 2  may be formed with asymmetric source-drain resistances. For example, transistor PG 2  may have source-drain terminal SD 1  that is coupled to the second data line and source-drain terminal SD 2  that is coupled to data storage node nX. The series resistance from the channel region of PG 2  to the second data line may be equal to R1, whereas the series resistance from the channel region of PG 2  to data storage node nX may be equal to R2. Resistance R2 may be substantially greater than resistance R1 for transistor PG 2  (as an example). 
     Incorporating asymmetric address transistors in memory cell  20  improves read stability without degrading write stability (e.g., the drive strength of an asymmetric address transistor is lowered during the forward mode, whereas the drive strength of the asymmetric address transistor remains unaffected during the reverse mode). Forming memory cells with address transistors having asymmetric source-drain resistances may therefore be an effective way of optimizing the tradeoff between read margin and write margin. The use of asymmetric address transistors to provide improved read/write performance may therefore reduce the need for redundant resources for bypassing defective memory cells, thereby freeing up valuable integrated circuit real estate and reducing cost. 
     Asymmetric transistors may be formed by adjusting the sizes, shapes, and materials of the structures that make up the transistor. With one suitable arrangement, an asymmetric transistor can be formed by implanting the source and drain regions (sometimes referred to as oxide definition regions or source-drain diffusion regions) with different amounts of dopant. 
     In  FIG. 6 , for example, heavily-doped drain implant region SD 1  may have a first dopant concentration ranging from 10 20  atoms/cm 3  to 10 21  atoms/cm 3 , whereas heavily doped drain implant region SD 2  may have a second dopant concentration ranging from 10 18  atoms/cm 3  to 10 19  atoms/cm 3 . Region LDD 1  may have a third dopant concentration ranging from 10 18  atoms/cm 3  to 10 19  atoms/cm 3 , whereas region LDD 2  may have a fourth dopant concentration ranging from 10 17  atoms/cm 3  to 10 18  atoms/cm 3 . Regions SD 1 , SD 2 , LDD 1 , and LDD 2  may be implanted using n-type dopant atoms (sometimes referred to as electron donors) such as arsenic, phosphorous, etc. Doped in this way, region SD 1  may have a corresponding resistivity that is equal to Ra, region SD 2  may have a corresponding resistivity that is equal to Rb, region LDD 1  may have a corresponding resistivity that is equal to Rc, and region LDD 2  may have a corresponding resistivity that is equal to Rd (see, e.g.,  FIG. 6 ). 
     In general, regions with lower dopant concentrations exhibit higher resistivity compared to regions with higher dopant concentrations. Transistor PG 1  may have a first overall source-drain series resistance (Ra+Rc) that is associated with the source-drain terminal connected to the first data line. Transistor PG 1  may have a second overall source-drain series resistance (Rb+Rd) that is associated with the source-drain terminal connected to data storage node X. The second overall series resistance may be greater than the first overall series resistance, because regions SD 2  and LDD 2  have lower dopant concentrations than regions SD 1  and LDD 1 . If desired, any number of access/address transistors may be formed using asymmetric source-drain doping to form transistors with asymmetric source-drain resistance. 
     In another suitable arrangement, an asymmetric transistor can be formed by using a silicide blocking mask to prevent silicide formation at the surface of a selected one of the source-drain regions in the asymmetric transistor. 
     In  FIG. 7 , for example, a first source-drain contact CO may form an electrical connection with region SD 1 , whereas a second source-drain contact CO may form an electrical connection with region SD 2 . First and second contacts CO may be metal contacts made from tungsten (as an example). First contact CO may be coupled to the first bit line  34 , whereas second contact CO may be coupled to data storage node X. 
     As shown in  FIG. 7 , silicide layer  300  may be formed at the surface of substrate  202  in region SD 1  (e.g., silicide layer  300  may cover an entire surface of diffusion region SD 1 ). Silicide layer  302  may be formed at the surface of substrate  202  to cover only a portion of the surface of region SD 2 . A portion of diffusion region SD 2  may not be covered by silicide layer  302  (see, e.g., portion  50  in  FIG. 7 ). A silicide-block mask may be used during manufacturing processes of device  10  to prevent formation of silicide in region  50 . 
     A silicide layer is typically used to reduce the contact resistance (e.g., an ohmic resistance at the junction of metal contact CO and associated source-drain region). A lack of silicide in region SD 2  may therefore increase the contact resistance associated with second metal contact CO relative to the contact resistance associated with first metal contact CO. If desired, any number of access transistors with asymmetric source-drain silicide formation may be used in cell  20 . 
     In another suitable arrangement, an asymmetric transistor can be formed by adjusting the co-implant (sometimes referred to as a pre-amorphization implant) dosage in the respective source-drain regions. Pre-amorphization implants may be applied prior to implanting dopant atoms. Pre-amorphization implants may help amorphize substrate  202  before implanting the dopants so that the dopants will not channel through substrate  202 . The concentration of the pre-amorphization implants may affect the resistivity of the source-drain regions. 
     In  FIG. 8A , for example, pre-amorphization dopants D 1  (e.g., a combination of arsenic with silicon, germanium, indium, carbon, or argon) may be implanted into region SD 1 , as indicated by arrows  56 . Regions on device  10  that are covered by mask layer  52  will not receive dopants D 1 . Dopants D 1  may be implanted into the surface of substrate  202  in exposed region SD 1  down to a depth delineated by dotted line  60  (see, e.g.,  FIG. 8A ). Dopants D 1  may have a first co-implant dopant concentration ranging from 10 20  atoms/cm 3  to 10 21  atoms/cm 3  (as an example). 
     In another step as shown in  FIG. 8B , pre-amorphization dopants D 2  (e.g., a combination of arsenic with silicon, germanium, indium, carbon, or argon) may be implanted into region SD 2 , as indicated by arrows  58 . Regions on device  10  that are covered by mask layer  54  will not receive dopants D 2 . Dopants D 2  may be implanted into the surface of substrate  202  in exposed region SD 2  down to a depth as delineated by dotted line  62  (see, e.g.,  FIG. 8B ). Dopants D 2  may have a second co-implant dopant concentration ranging from 10 18  atoms/cm 3  to 10 19  atoms/cm 3  (as an example). If desired, the partial deposition of silicide layers  300  and  302  in  FIG. 7  may also be formed using masks that cover at least portion  50 . 
     As described in connection with  FIG. 6 , regions with lower dopant concentrations will have higher resistivity compared to regions with higher dopant concentrations. The resistance associated with region SD 2  will therefore be greater than the resistance associated with region SD 1 , because region SD 2  has been implanted with fewer dopant atoms than region SD 1  (i.e., the second co-implant dopant concentration is less than the first co-implant dopant concentration). If desired, any number of access transistors may be formed using asymmetric source-drain pre-amorphization implants to form transistors with asymmetric source-drain resistance. 
     In another suitable arrangement, an asymmetric transistor may include different types of stress layers formed over transistor PG 1 . As shown in  FIG. 9 , for example, stress layer  70  of a first type may be selectively formed over region  300  (e.g., a region associated with region SD 1  of the access transistor), whereas stress layer  72  of a second type may be selectively formed over region  302  (e.g., a region associated with region SD 2  of the access transistor). Layers  70  and  72  may serve as etch stop layers and may be formed using silicon nitride and other types of etch stop layer. Interlayer dielectric material ILD (e.g., oxide) may be formed over layers  70  and  72  and around contacts CO (see, e.g.,  FIG. 9 ). 
     In some arrangements, layers  70  and  72  may be removed before forming the ILD layers. If layers  70  and  72  are removed, the stress induced by layers  70  and  72  may remain even after the layers are remove. 
     For example, layer  70  may induce tensile stress on the channel region of the transistor, whereas layer  72  may induce compressive stress on the channel region of the transistor. Layer  70  need not provide any particular type of stress, if desired. 
     Forming different types of stress-inducing layers such as layers  70  and  72  over an access transistor may cause the access transistor to exhibit asymmetric behaviors. In particular, compressive stress layer  72  formed over region SD 2  of n-channel transistor PG 1  and tensile stress layer  70  formed over region SD 1  of n-channel transistor PG 1  may cause transistor PG 1  to be stronger in the reverse mode (e.g., when passing a “0” from SD 1  to SD 2 ) and weaker in the forward mode (e.g., when exposing SD 2  to a high precharge voltage). If desired, any number of access transistors may be formed using stress layers of different types to form transistors with asymmetric source-drain resistance. 
     In another suitable arrangement, an asymmetric transistor can be formed by manufacturing source-drain contacts of different sizes. 
     As shown in  FIG. 10 , for example, the first contact (sometimes referred to as contact plugs or contact vias) coupled to data line  34  may have a first diameter d1, whereas the second contact coupled to data storage node X may have a second diameter d2. The contact resistance associated with the first contact may be represented by resistance Rx, whereas the contact resistance associated with the second contact may be represented by resistance Ry. 
     In general, contact plugs with larger diameters provide lower contact resistance values compared to contact vias with smaller diameters. In the example of  FIG. 10 , diameter d1 of the first contact may be greater than diameter d2 of the second contact (e.g., the first contact may contact SD 1  over a first area, whereas the second contact may contact SD 2  over a second area that is smaller than the first area). Diameter d2 may, for example, be no more than 90% of d1, 80% of d1, 70% of d1, etc. As a result, resistance Rx may be less than Ry. If desired, any number of access transistors may be formed using asymmetric source-drain contacts to form transistors with asymmetric source-drain resistance. 
     In another suitable arrangement, an asymmetric transistor may include spacers of different sizes. As shown in  FIG. 11 , for example, first spacer SP adjacent to region SD 1  may have a first length L1 (e.g., a horizontal distance from gate insulator GI to region SD 1 ), whereas second spacer SP adjacent to region SD 2  may have a second length L2 (e.g., a horizontal distance from gate insulator GI to region SD 2 ). The lengths of regions LDD 1  and LDD 2  may depend on the lengths of their respective spacers, because the lightly-doped drain (LDD) regions are formed under the spacers. The series resistance associated with region LDD 1  under the first spacer may be Rs, whereas the series resistance associated with region LDD 2  under the second spacer may be Rss (see, e.g.,  FIG. 11 ). 
     In general, LDD regions with greater lengths exhibit higher series resistance compared to LDD regions with smaller lengths. In the example of  FIG. 11 , length L2 may be at least 10% greater than L1, at least 20% greater than L1, etc. As a result, resistance Rss may be sufficiently higher than Rs to provide desired drive strengths during the forward and reverse modes. If desired, any number of access transistors may be formed using asymmetric spacers of different sizes to form transistors with asymmetric source-drain resistance. 
       FIG. 12  is a circuit diagram of memory cell  20  with asymmetric address transistors formed using asymmetric source-drain resistances and asymmetric pocket implants. As shown in  FIG. 12 , transistors PG 1  and PG 2  may each have higher series/contact resistance and pocket implant P at the source-drain terminal coupled to a data storage node. For example, transistor PG 1  may have a first source-drain coupled to first data line  34  and a second source-drain coupled to data storage node X. The second source-drain of PG 1  may exhibit higher resistivity than the first source-drain. Pocket implant P may only be implanted at the second source-drain region of PG 1 . Similarly, transistor PG 2  may have a first source-drain coupled to second data line  34 ′ and a second source-drain coupled to data storage node nX. The second source-drain of PG 2  may exhibit higher resistivity than the first source-drain of transistor PG 2 . Pocket implant P may only be implanted at the second source-drain region of transistor PG 2 . 
     Address transistors having asymmetric resistance values formed using this approach may provide a first drive strength when passing a “0” into corresponding data storage nodes (e.g., during reverse mode) and a second drive strength when exposing the data storage nodes to high precharge voltage levels (e.g., during forward mode). The first drive strength may be greater than the second drive strength (e.g., address transistors operating the forward mode are weaker than address transistors operating in the reverse mode). 
     Performing pocket implants only at the source-drain regions connected to the data storage nodes may elevate the energy barrier at those regions. Elevating energy barrier levels in this way may result in access transistors being more capable of passing a “0” into corresponding data storage nodes and less likely to cause a read disturb. Forming memory cells  20  with asymmetric source-drain resistances and asymmetric pocket implants may therefore exhibit improved read/write margins. If desired, other known techniques of improving read/write stability (e.g., adjusting the power supply voltages of cell  20 , adjusting the magnitude of control signal WL, adjusting the sizes of transistors in cell  20 , forming cell  20  using additional inverters or inverter-like circuits, etc.) may be used in conjunction with the asymmetric address transistors described in connection with  FIGS. 3-12 . 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.