Patent Publication Number: US-9424385-B1

Title: SRAM cell layout structure and devices therefrom

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Non-Provisional patent application Ser. No. 13/776,917, filed Feb. 26, 2013 and entitled “SRAM Cell Layout Structure and Devices Therefrom”, which claims priority to U.S. Provisional Application No. 61/615,166, entitled “Improved SRAM Cell Layout Structure and Related Methods”, filed Mar. 23, 2012, the contents of both of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to SRAM cell design, and more specifically to a method for fabricating integrated circuits including an improved SRAM cell layout structure and devices therefrom. 
     BACKGROUND 
     There are several interrelated design parameters that must be considered during SRAM cell design. These include, for example, static noise margin (“hereinafter” SNM), write margin, bit line speed, and data retention voltage. SNM is defined as the minimum DC noise voltage necessary to flip the state of the SRAM cell. An SRAM cell can have different SNM during read and write operations, referred to as read SNM and write SNM, respectively. Read SNM is also an indicator of cell stability and is sometimes simply referred to as cell stability. A higher read SNM indicates that it is more difficult to invert the state of the cell during a read operation. Write margin is defined as the minimum bit line voltage necessary to invert the state of an SRAM cell. A higher write margin indicates that it is easier to invert the state of the cell during a write operation. Read speed is defined as the bit line slew rate in response to a high word line voltage, typically the time from the rising edge assertion of word line until some differential between the high and falling bit line is obtained. Data retention voltage is defined as the minimum power supply voltage required to retain a logic state of either “0” or “1” data in the SRAM cell in standby mode. 
     As process technology has scaled, it has become increasingly difficult to control the variation of transistor parameters because of a variety of sources of systemic mismatch. These sources of systemic mismatch can also include geometric sources of mismatch that arise from variation in alignment and additional lithographic effects such as corner rounding. For example, the jogs or notches in the active silicon region, used to achieve a desired ratio between the strengths of the pull-down to pass-gate transistors (represented by the width to length ratio of each of these transistors) for cell stability during a read access, can be subject to significant corner rounding. Similarly, the jogs or notches in the gate structures, used to achieve a desired pull down transistor size, can also be subject to significant corner rounding. 
     Threshold voltage variations become a limiting factor in transistor design as process technology is optically scaled downward while voltage cannot be similarly scaled. Threshold voltage variations between neighboring MOSFETs can have significant impact on the SNM, cell stability, write margin, read speed, and data retention voltage of the SRAM cell. Threshold voltage variations between pass-gate and pull-down transistors of the SRAM cell can degrade cell stability. During a read, the read current discharging the bit line flows through the series connection of the pass-gate and pull-down NMOS transistors. The voltage divider formed by these transistors raises the low voltage in the cell, thereby contributing to the degradation of cell stability. Variations in the threshold voltage of the pass-gate or pull-down transistor can result in a large variation in the voltage divider ratio of the pass-gate transistors to pull down transistors, increasing the likelihood of inverting the SRAM cell during a read operation, i.e., upsetting the stored state. Other SRAM cell design parameters such as write margin, bit line speed (as measured by slew rate) or read current, and data retention voltage can also be affected by threshold voltage variations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment of an Enhanced Body Effect (EBE) transistor having an enhanced body coefficient, along with the ability to set threshold voltage with enhanced precision, according to certain described embodiments; 
         FIG. 2  shows an integrated circuit device according to an embodiment; 
         FIG. 3  shows an annotated SRAM cell, according to an embodiment, that illustrates some of the EBE transistor characteristics that can provide an SRAM cell with enhanced performance characteristics; 
         FIG. 4  is a plot of the drain current as a function of the drain voltage for an EBE transistor according to an embodiment and a conventional transistor. 
         FIG. 5A  is a flow diagram illustrating a method for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with an embodiment; 
         FIG. 5B  is a layout f a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method of  FIG. 5A ; 
         FIG. 6A  is a flow diagram illustrating a method for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with an embodiment; 
         FIG. 6B  is a layout of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method of  FIG. 6A ; 
         FIG. 7A  is a flow diagram illustrating a method for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with an embodiment; and 
         FIG. 7B  is a layout of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method. 
         FIG. 8A  is a flow diagram illustrating a method for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with an embodiment; and 
         FIG. 8B  is a layout of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method  800 ; and 
         FIG. 8C  illustrates the layout of the target SRAM cell after the resizing described above with reference to  FIG. 8A  and  FIG. 8B , in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the embodiments. Several embodiments are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. Embodiments not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement an embodiment. 
     Various embodiments are directed to circuits and methods related to an improved SRAM cell having transistors that have an enhanced body coefficient to improve against variation that limits SRAM designs and can rid the layout of notches and jogs. The various embodiments include circuits and methods in which the improved SRAM cell results from the use of transistors that have reduced variability of threshold voltage. By using such transistors, the SRAM cell can have an enhanced stability. Specifically, the various embodiments utilize Enhanced Body Effect (EBE) transistors. 
       FIG. 1  shows an embodiment of an EBE transistor  100  having an enhanced body coefficient, along with the ability to set threshold voltage Vt with enhanced precision, according to certain described embodiments. The EBE transistor  100  includes a gate electrode  102 , source  104 , drain  106 , and a gate dielectric  128  positioned over a substantially undoped channel  110 . As used herein, the term “substantially undoped” refers to a doping of less than 5×10 17  atoms/cm 3 . Lightly doped source and drain extensions (SDE)  132 , positioned respectively adjacent to source  104  and drain  106 , extend toward each other, setting the transistor channel length. 
     In  FIG. 1 , the EBE transistor  100  is shown as an N-channel transistor having a source  104  and drain  106  made of N-type dopant material, formed upon a substrate such as a P-type doped silicon substrate providing a P-well  114  formed on a substrate  116 . In addition, the N-channel EBE transistor in  FIG. 1  includes a highly doped screening region  112  made of P-type dopant material, and a threshold voltage set region  111  made of P-type dopant material. However, it will be understood that, with appropriate changes to dopant materials, a P-channel EBE transistor can be formed. 
     In one embodiment, a process for forming the EBE transistor begins with forming the screening region  112 . In certain embodiments, the screening region is formed by implanting dopants into the P-well  114 . In alternative embodiments the screening region is formed on the P-well using methods such as in-situ doped epitaxial silicon deposition, or epitaxial silicon deposition followed by dopant implantation. The screening region formation step can be before or after STI (shallow trench isolation) formation, depending on the application and results desired. Boron (B), Indium (I), or other P-type materials may be used for P-type implants, and arsenic (As), antimony (Sb) or phosphorous (P) and other N-type materials can be used for N-type implants. In certain embodiments, the screening region  112  can have a dopant concentration between about 1×10 19  to 5×10 20  dopant atoms/cm 3 , with the selected dopant concentration dependent on the desired threshold voltage as well as other desired transistor characteristics. A germanium (Ge), carbon (C), or other dopant migration resistant layer can be incorporated above the screening region to reduce upward migration of dopants. The dopant migration resistant layer can be formed by way of ion implantation, in-situ doped epitaxial growth or other process. 
     In certain embodiments, a threshold voltage set region  111  is positioned above the screening region  112 . The threshold voltage set region  111  can be either adjacent to, incorporated within or vertically offset from the screening region. In certain embodiments, the threshold voltage set region  111  is formed by delta doping, controlled in-situ deposition, or atomic layer deposition. In alternative embodiments, the threshold voltage set region  111  can be formed by way of controlled outdiffusion of dopant material from the screening region  112  into an undoped epitaxial layer, or by way of a separate implantation into the substrate following formation of the screening region  112 , before the undoped epitaxial layer is formed. Setting of the threshold voltage for the transistor is implemented by suitably selecting dopant concentration and thickness of the threshold voltage set region  111 , as well as maintaining a separation of the threshold voltage set region  111  from the gate dielectric  128 , leaving a substantially undoped channel layer directly adjacent to the gate dielectric  128 . In certain embodiments, the threshold voltage set region  111  can have a dopant concentration between about 1×10 18  dopant atoms/cm 3  and about 5×10 19  dopant atoms per cm 3 . In alternative embodiments, the threshold voltage set region  111  can have a dopant concentration that is approximately less than half of the concentration of dopants in the screening region  112 . In certain embodiments, the final layer of the channel is formed above the screening region  112  and threshold voltage set region  111  by way of a blanket or selective EPI deposition, to result in a substantially undoped channel region  110  of a thickness tailored to the technical specifications of the device. As a general matter, the thickness of the substantially undoped channel region  110  ranges from approximately 5-25 nm, with the selected thickness based upon the desired threshold voltage for the transistor. Preferably, a blanket EPI deposition step is performed after forming the screening region  112 , and the threshold voltage setting region  111  is formed by controlled outdiffusion of dopants from the screening region  112  into a portion of the blanket EPI layer, as described below. Dopant migration resistant layers of C, Ge, or the like can be utilized as needed, especially in the NMOS regions, to prevent dopant migration from the threshold voltage set region  111  into the substantially undoped channel region  110 , or alternatively from the screening region  112  into the threshold voltage set region  111 . STI structures are preferably formed after the blanket EPI deposition step, using a process that remains within a thermal budget that effectively avoids substantial change to the dopant profiles of the previously formed screening region  112  and threshold voltage setting region  111 . 
     In addition to using dopant migration resistant layers, other techniques can be used to reduce upward migration of dopants from the screening region  112  and the threshold voltage set region  111 , including but not limited to low temperature processing, selection or substitution of low migration dopants such as antimony or indium, low temperature or flash annealing to reduce interstitial dopant migration, or any other technique to reduce movement of dopant atoms can be used. 
     As described above, the substantially undoped channel region  110  is positioned above the threshold voltage set region  111 . Preferably, the substantially undoped channel region  110  has a dopant concentration less than 5×10 17  dopant atoms per cm 3  adjacent or near the gate dielectric  128 . In some embodiments, the substantially undoped channel region  110  can have a dopant concentration that is specified to be approximately less than one tenth of the dopant concentration in the screening region  112 . In still other embodiments, depending on the transistor characteristics desired, the substantially undoped channel region  110  may contain dopants so that the dopant concentration is elevated to above 5×10 17  dopant atoms per cm 3  adjacent or near the gate dielectric  128 . Preferably, the substantially undoped channel region  110  remains substantially undoped by avoiding the use of halo or other channel implants. 
     A gate stack may be formed or otherwise constructed above the substantially undoped channel region  110  in a number of different ways, from different materials including polysilicon and metals to form what is known as “high-k metal gate”. The metal gate process flow may be “gate 1 st ” or “gate last”. Preferably, the metal gate materials for NMOS and PMOS are selected to near mid-gap, to take full advantage of the EBE transistor. However, traditional metal gate work function band-gap settings may also be used. In one scheme, as a way to attain the desired work functions for given devices, metal gate materials can be switched between NMOS and PMOS pairs. Following formation of the gate stack, source/drain portions may be formed. Typically, the extension portions are implanted, followed by additional spacer formation and then implant or, alternatively, selective epitaxial deposition of deep source/drain regions. 
     The source  104  and drain  106  can be formed preferably using conventional processes and materials such as ion implantation and in-situ doped epitaxial deposition. Source  104  and drain  106  may further include stress inducing source/drain structures, raised and/or recessed source/drains, asymmetrically doped, counter-doped or crystal structure modified source/drains, or implant doping of source/drain extension regions according to LDD (lightly doped drain) techniques, provided that the thermal budget for any anneal steps be within the boundaries of what is required to keep the screening region  112  and threshold voltage setting region  111  substantially intact. The channel  110  contacts and extends between the source  104  and the drain  106 , and supports movement of mobile charge carriers between the source and the drain. In operation, when gate electrode voltage is applied to the EBE transistor  100  at a predetermined level, a depletion region formed in the substantially undoped channel  110  can extend to the screening region  112 , since channel depletion depth is a function of the integrated charge from dopants in the doped channel lattice, and the substantially undoped channel  110  has very few dopants. The screening region  112 , if fabricated according to specification, effectively pins the depletion region to define the depletion zone width. The threshold voltage in conventional field effect transistors (FETs) can be set by directly implanting a “threshold voltage implant” into the channel, raising the threshold voltage to an acceptable level that reduces transistor off-state leakage while still allowing speedy transistor switching. 
     The threshold voltage in conventional field effect transistors (FETs) can be set by directly implanting a “threshold voltage implant” into the channel, raising the threshold voltage to an acceptable level that reduces transistor off-state leakage while still allowing speedy transistor switching. Alternatively, the threshold voltage (V t ) in conventional FETs can also be set by a technique variously known as “halo” implants, high angle implants, or pocket implants. Such implants create a localized, graded dopant distribution near a transistor source and drain that extends a distance into the channel. Halo implants are often required by transistor designers who want to reduce unwanted source/drain leakage conduction or “punch through” current, but have the added advantage of adjusting threshold voltage. Unfortunately, halo implants introduce additional process steps thereby increasing the manufacturing cost. Also, halo implants can introduce additional dopants in random, unwanted locations in the channel. These additional dopants increase the variability of threshold voltage between transistors, and decrease mobility and channel transconductance due to the adverse effects of additional and unwanted dopant scattering centers in the channel. Eliminating or greatly reducing the number of halo implants is desirable for reducing manufacture time and making more reliable wafer processing. By contrast, the techniques for forming the EBE transistor  100  use different threshold voltage setting techniques that do not rely on halo implants (i.e. haloless processing) or channel implants to set the threshold voltage to a desired range. By maintaining a substantially undoped channel near the gate, the EBE transistor further allows for greater channel mobility for electron and hole carriers with improved variation in threshold voltage from device to device. 
     As will also be appreciated, position, concentration, and thickness of the screening region  112  are important factors in the design of the EBE transistor. In certain embodiments, the screening region is located above the bottom of the source and drain junctions. To dope the screening region so as to cause its peak dopant concentration to define the edge of the depletion width when the transistor is turned on, methods such as delta doping, broad dopant implants, or in-situ doping is preferred, since the screening region  112  should have a finite thickness to enable the screening region  112  to adequately screen the well therebelow while avoiding creating a path for excessive junction leakage. When transistors are configured to have such screening regions, the transistor can simultaneously have good threshold voltage matching, high output resistance, low junction leakage, good short channel effects, and still have an independently controllable body due to a strong body effect. In addition, multiple EBE transistors having different threshold voltages can be easily implemented by customizing the position, thickness, and dopant concentration of the threshold voltage set region  111  and/or the screening region  112  while at the same time achieving a reduction in the threshold voltage variation. In one embodiment, the screening region is positioned such that the top surface of the screening region is located approximately at a distance of Lg/1.5 to Lg/5 below the gate (where Lg is the gate length). In one embodiment, the threshold voltage set region has a dopant concentration that is approximately 1/10 of the screening region dopant concentration. In certain embodiments, the threshold voltage set region is thin so that the combination of the threshold voltage set region and the screening region is located approximately within a distance of Lg/1.5 to Lg/5 below the gate. 
     Modifying threshold voltage by use of a threshold voltage set region  111  positioned above the screening region  112  and below the substantially undoped channel  110  is an alternative technique to conventional threshold voltage implants for adjusting threshold voltage. Care must be taken to prevent dopant migration into the substantially undoped channel  110 , and use of low temperature anneals and anti-migration materials, such as carbon or germanium, are recommended for many applications. More information about the formation of the threshold voltage set region  111  and the EBE transistor is found in pending U.S. patent application Ser. No. 12/895,785 filed Sep. 30, 2010 and entitled ADVANCED TRANSISTORS WITH THRESHOLD VOLTAGE SET DOPANT STRUCTURES, the entirety of which disclosure in herein incorporated by reference. 
     Yet another technique for modifying threshold voltage relies on selection of a gate material having a suitable work function. The gate electrode  102  can be formed from conventional materials, preferably including, but not limited to, metals, metal alloys, metal nitrides and metal silicides, as well as laminates thereof and composites thereof. In certain embodiments the gate electrode  102  may also be formed from polysilicon, including, for example, highly doped polysilicon and polysilicon-germanium alloy. Metals or metal alloys may include those containing aluminum, titanium, tantalum, or nitrides thereof, including titanium containing compounds such as titanium nitride. Formation of the gate electrode  102  can include silicide methods, chemical vapor deposition methods and physical vapor deposition methods, such as, but not limited to, evaporative methods and sputtering methods. Typically, the gate electrode  102  has an overall thickness from about 1 to about 500 nanometers. In certain embodiments, metals having a work function intermediate between band edge and mid-gap can be selected. As discussed in pending U.S. patent application Ser. No. 12/960,266, filed Dec. 3, 2010 and entitled SEMICONDUCTOR STRUCTURE AND METHOD OF FABRICATION THEREOF WITH MIXED METAL TYPES, the entirety of which disclosure is herein incorporated by reference, such metal gates simplify swapping of PMOS and NMOS gate metals to allow a reduction in mask steps and different required metal types for systems on a chip or other die supporting multiple transistor types. 
     Applied bias to the screening region  112  is yet another technique for modifying threshold voltage of the EBE  100 . The screening region  112  sets the body effect for the transistor and allows for a higher body effect than is found in conventional FET technologies. For example, a body tap  126  to the screening region  112  of the EBE transistor can be formed in order to provide further control of threshold voltage. The applied bias can be either reverse or forward biased, and can result in significant changes to threshold voltage. Bias can be static or dynamic, and can be applied to isolated transistors, or to groups of transistors that share a common well. Biasing can be static to set threshold voltage at a fixed set point, or dynamic, to adjust to changes in transistor operating conditions or requirements. Various suitable biasing techniques are disclosed in pending U.S. patent application Ser. No. 12/708,497 filed Feb. 18, 2010 now U.S. Pat. No. 8,273,617, the entirety of which disclosure is herein incorporated by reference. 
     Advantageously, EBE transistors have a reduced mismatch arising from scattered or random dopant variations as compared to conventional MOS transistors. In certain embodiments, the reduced variation results from the adoption of structures such as the screening region, the optional threshold voltage set region, and the epitaxially grown channel region. In certain alternative embodiments, mismatch between EBE transistors can be reduced by implanting the screening layer across multiple EBE transistors before the creation of transistor isolation structures, and forming the channel layer as a blanket epitaxial layer that is grown before the creation of transistor epitaxial structures. In certain embodiments, the screening region has a substantially uniform concentration of dopants in a lateral plane. The EBE transistor can be formed using a semiconductor process having a thermal budget that allows for a reasonable throughput while managing the diffusivities of the dopants in the channel. Further examples of transistor structure and manufacture suitable for use in EBE transistors are disclosed in U.S. patent application Ser. No. 12/708,497, filed on Feb. 18, 2010 and entitled ELECTRONIC DEVICES AND SYSTEMS, AND METHODS FOR MAKING AND USING THE SAME now U.S. Pat. No. 8,273,617, as well as U.S. patent application Ser. No. 12/971,884, filed on Dec. 17, 2010 and entitled LOW POWER SEMICONDUCTOR TRANSISTOR STRUCTURE AND METHOD OF FABRICATION THEREOF and U.S. patent application Ser. No. 12/971,955 filed on Dec. 17, 2010 and entitled TRANSISTOR WITH THRESHOLD VOLTAGE SET NOTCH AND METHOD OF FABRICATION THEREOF, the respective contents of which are incorporated by reference herein. 
     Referring initially to  FIG. 2 , an integrated circuit device according to an embodiment is shown in a block diagram and designated by the general reference character  200 . Integrated circuit  200  is a Static Random Access Memory (SRAM) device that may include a number of SRAM cells, including SRAM cells arranged in multiple rows and columns. For ease of discussion, however, only two SRAM cells  205  and  210  are illustrated and discussed along with the associated column power supplies  215  and  220  to generate the applied power supply voltages. The SRAM cells  205  and  210  are implemented using EBE transistors. 
     In  FIG. 2 , the SRAM cell  205  includes a pair of pass-gate EBE transistors  225  and  230 , a pair of pull-up EBE transistors  235  and  240 , and a pair of pull-down EBE transistors  245  and  250 . The pass-gate EBE transistors  225  and  230  and the pull-down EBE transistors  245  and  250  are typically NMOS transistors. The pass-gate EBE transistors  225  and  230  couple a pair of data lines BL 0  and BLN 0 , also referred to as “bit lines”, to storage nodes SN 1  and SN 2  respectively, where the voltages at nodes SN 1  and SN 2  are inversely related. The pull-down EBE transistors  245  and  250  couple a power supply VSS, usually the ground voltage of the circuit, to the storage nodes SN 1  and SN 2  respectively. The pull-up EBE transistors  235  and  240  are typically PMOS transistors that couple the positive power supply VDDCOL 0  to the storage nodes SN 1  and SN 2  respectively. The substrates of the NMOS transistors are connected to the ground voltage of the circuit, and the substrates for the PMOS transistors are connected to a power supply voltage VBPCOL 0 . SRAM cell  210  is similar, and includes a pair of pass-gate EBE transistors  255  and  260 , a pair of pull-up EBE transistors  265  and  270 , a pair of pull-down EBE transistors  275  and  280 , storage nodes SN 3  and SN 4 , bit lines BL 1  and BLN 1 , and power supplies VDDCOL 1  and VBPCOL 1  (forward and reverse) body bias voltages required in different modules or circuit blocks in the SoC. 
     In  FIG. 2 , each column of the SRAM  200  includes a column power supply block that supplies the power supply voltage for the corresponding column. The column power supply block  215  supplies the power supply voltage VDDCOL 0  and body bias voltage VBPCOL 0 , and the column power supply block  220  supplies the power supply voltage VDDCOL 1  and body bias voltage VBPCOL 1 . Each column power supply block independently controls the power supply voltage and PMOS pull-up transistor body bias voltage supplied to each column such that each column can receive different power supply and body bias voltages. In addition, the column power supply block can provide different power supply voltages and body bias voltages to the same column at different times, or during different modes of operation. For example, as described in more detail below, the column power supply block can supply different power supply voltages and/or body bias voltages to the corresponding column during read and write operations. 
     The SRAM cell shown in  FIG. 2  can retain its state indefinitely as long as the supplied power is sufficient to operate the cell correctly. The SRAM cell  205  includes two cross-coupled inverters consisting of the pair of transistors  235  and  245 , and  240  and  250 . The two inverters operate to reinforce the stored charge on storage nodes SN 1  and SN 2  continuously, such that the voltages at each of the two storage nodes are inverted with respect to one another. When SN 1  is at a logical “1”, usually a high voltage, SN 2  is at a logical “0”, usually a low voltage, and vice versa. 
     Referring to  FIG. 2 , a write operation can be performed to store data in a selected SRAM cell, and a read operation can be performed to access stored data in a selected SRAM cell. In one embodiment, data is stored in a selected SRAM cell, e.g. SRAM cell  205 , during a write operation by placing complementary write data signals on the two bit lines BL 0  and BLN 0 , and placing a positive voltage VWL on the word line WL connected to the gate of the pass-gate transistors  225  and  230 , such that the two bits lines are coupled to the storage nodes SN 1  and SN 2  respectively. The write operation is successful when the write data signals on the two bit lines overcome the voltages on the two storage nodes and modify the state of the SRAM cell. The cell write is primarily due to the bit line driven low overpowering the PMOS pull-up transistor via the pass-gate transistor. Thus the relative strength ratio of the NMOS pass-gate transistor to the PMOS pull-up transistor (represented by the width to length ratio of these transistors) is important to maximizing the write margin. Data is accessed from a selected SRAM cell, e.g. SRAM cell  205 , during a read operation by placing a positive voltage VWL on the word line WL such that the pass-gate transistors  225  and  230  allow the storage nodes SN 1  and SN 2  to be coupled to the bit lines BL 0  and BLN 0  respectively. During the read operation the SRAM cell  205  drives complementary read data signals onto the bit lines BL 0  and BLN 0 . The differential voltage on the bit lines BL 0  and BLN 0  can be sensed using a differential sense amplifier (not shown) that senses and amplifies the differential voltage signal on the bit lines. The output of the sense amplifier is subsequently output as the read data for the selected SRAM cell. 
     It is noted that one or more of the cells of the SRAM  200  can include a plurality of word lines and bit lines, even though only one word line and two sets of bits lines have been shown in  FIG. 2 . Therefore, even though only two SRAM cells  205  and  210  are shown in  FIG. 2 , other SRAM cells (not shown) can be placed at intersections of the plurality of word lines and bit lines. In some embodiments, the SRAM  200  can have 8, 16, 32, 64, 128 or more columns that can be arranged in word widths of 8, 16, 32, 64, 128, 256, or more cells. In some embodiments, each column of the SRAM  200  has an associated column power supply block that independently controls the column power supply voltages provided to the corresponding column. In alternative embodiments, each column of the SRAM  200  can be sub-divided into column sub-groups, where each column sub-group has an associated column power supply block that independently controls the column power supply voltages provided to corresponding column subgroup. In certain other embodiments, one column power supply block can be associated with more than one column or column subgroup. In addition, power supply and body bias voltages other than the ones described above may be applied to the SRAM cells of SRAM  200  during read and write operations. Such power supply voltages can be selected based on the design of the SRAM cell, and the electrical characteristics of the EBE transistors used in the SRAM cell. 
     Embodiments of SRAM cells using EBE transistors for at least some of the transistors have enhanced performance characteristics as compared to SRAM cells using all conventional MOSFETs. Some of the reasons for the enhanced performance characteristics are (1) the EBE transistors have lower threshold voltage variation, i.e., lower σV T , and (2) the EBE transistors have higher I eff  and higher body coefficient. As a result, SRAM cells using EBE transistors can have (1) enhanced read stability that can be measured as enhanced read static noise margin, as well as lower SRAM minimum operating voltage V DDmin ; (2) enhanced write margin; (3) faster SRAM operation resulting from lower read current variability; and (4) lower SRAM cell leakage resulting from lower σV T . 
       FIG. 3  shows an annotated SRAM cell that illustrates some of the EBE transistor characteristics that can provide an SRAM cell with enhanced performance characteristics. The EBE transistors can exhibit a higher current drive as compared to conventional transistors, when a low voltage is being applied to the gate and the drain to source voltage is less than V GS -V T  of the transistor, i.e., such that the transistor is operating in the linear mode. This higher current drive is illustrated in  FIG. 4 , which shows the drain current as a function of the drain voltage for an EBE transistor, curve  305 , and a conventional transistor, curve  310 . As shown in  FIG. 4 , the EBE transistor drain current can be 1.5-2 times the drain current of the conventional transistor when the transistor is operating in the linear mode and reduced V GS , which may occur due to the circuit operating at reduced VDD. The drain to source voltage on NMOS pull-down transistors of the SRAM cell is diminished during a read operation (this voltage directly relates to the read SNM as described previously), and therefore, these transistors operate in the linear region during a read operation. This voltage V CN  can be less than 0.2 V in certain embodiments. This voltage can be as low as 0.1 volts in certain alternative embodiments. The NMOS pass-gate transistor connected to the NMOS pull-down transistor is operating in saturation during the important portions of the read operation, and therefore, does not benefit from this enhanced current drive capability. However, the NMOS pass-gate transistor has an increased body bias voltage that results from the rise in the storage node voltage during the read operation. Therefore, the enhanced body coefficient of the EBE transistor results in a NMOS pass-gate transistor with reduced current drive capability. The combination of the enhanced drive capability of the pull-down transistor, and the reduced drive capability of the pass-gate transistor result in an increased read SNM and increased cell stability. This is evident qualitatively by the better voltage divider ratio obtained by weakening the pass-gate and strengthening the pull-down NMOS transistors, respectively. The increase in the read SNM and cell stability can be determined from butterfly curves obtained from SPICE simulations of the SRAM cell using EBE transistors. 
       FIG. 3  also shows that the SRAM cell using EBE transistors may not have a significant impact on the write ability of the SRAM cell when the bitline BL is driven to VSS because the pass-gate NMOS transistor has no body bias voltage under these conditions. However, during a write margin measurement the bitline BL is swept from VDD to VSS, and therefore, there is a body effect that has an impact on the measured write margin. In certain embodiments of the SRAM cell, the write margin is not a limitation as the read SNM can be the dominant limitation that determines the design of the SRAM cell. In addition, the lower variability of the threshold voltage of the EBE transistor, resulting from the lower σV T , results in higher worst case read current. The enhanced body coefficient of the EBE transistor can also be used to provide SRAM cells having a lower standby power, and better column level margin controls. Additional benefits of an SRAM cell using EBE transistors are discussed in pending U.S. patent application Ser. No. 13/471,353, filed May 14, 2012 and entitled INTEGRATED CIRCUIT DEVICES AND METHODS, the entirety of which disclosure is herein incorporated by reference. As discussed above, EBE transistors having a screening region have enhanced threshold voltage matching, in addition to having an enhanced body coefficient. Therefore, SRAMs using EBE transistors have reduced threshold voltage variations between the transistors used in different cells of the SRAM, as well as between the transistors used within a particular SRAM cell. An SRAM cell using EBE transistors also has increased read SNM and cell stability as a result of the reduced threshold voltage variations. Reduction of threshold voltage variation between the pass-gate transistors and the pull-down transistors within an SRAM cell contributes in part to the increase in read SNM. In addition, reduction in the threshold voltage variations of PMOS transistors in SRAM cells also contributes to the increase in read SNM, as well as less variability in write margin, i.e., an increase in worst-case as fabricated write margin. 
     Now turning to  FIGS. 5A-8C , various methodologies for porting a source or existing SRAM cell using conventional transistors to a target SRAM cell using EBE transistors are described. In general, each of these process flows includes the following steps:
         1) The design layout data for an integrated circuit is obtained and the source SRAM cells to be converted for use with EBE transistors are identified or selected.   2) The active area patterns (NMOS and PMOS) for these SRAM cells are extracted to form EBE active area layers or layouts (PMOS and NMOS). This involves removing these patterns from the corresponding conventional active areas or layouts. Thus, during subsequent processing to form conventional transistors, for instance, the EBE active areas in the SRAM are masked to prevent the introduction of additional dopants which would affect the EBE channel structure.   3) The NMOS EBE active area is adjusted to provide at least one of the following:
           a. A width of the EBE NMOS pull-down transistor that substantially matches the width of the EBE NMOS pass-gate transistor. For example, referring back to  FIG. 2 , the EBE NMOS active areas defining the widths of transistors  225 ,  230 ,  245 ,  250  are selected so that these transistors have the same width. In some embodiments, the resizing is based on reducing the width of the pull-down transistor, which is commonly larger in conventional SRAM cells, as discussed below.   b. A reducing in at least one lateral dimension of the SRAM cell.   
           4) The gate layer is adjusted so that the length of the EBE PMOS pull-up transistors and the length of the EBE NMOS pull-down transistors are substantially the same. That is, referring back to  FIG. 2 , transistors  235 ,  240 ,  245 , and  250  are selected so that these transistors have the same width. In some embodiments, this is done by reducing the length of the EBE PMOS pull-up transistors. In other embodiments, this is done by increasing the length of the EBE NMOS pull-down transistors.   5) Based on some criteria for the electrical parameters for the EBE transistors, the new layout for the gate layer, and the locations for the EBE and conventional active regions, the process parameters for the EBE transistors and conventional transistors built on EBE active areas are selected.   6) Appropriate masks can be produced and the integrated circuit can be formed.
 
Various exemplary flows are illustrated below.
       

       FIG. 5A  is a flow diagram illustrating a method  500  for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with a first embodiment.  FIG. 5B  is a layout  550  of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method  500 . 
     With reference to the method  500  of  FIG. 5A , in step  505 , the size of the transistors in the source SRAM cell that uses conventional transistors is determined. In step  510 , an intermediate SRAM cell is designed using EBE transistors. In one embodiment, the intermediate SRAM cell is a drop in replacement for the source SRAM cell and uses EBE transistors of substantially the same size (i.e. substantially the same width and length) as the source SRAM cell. Since the intermediate SRAM cell as a drop-in replacement preserves the width and length of the EBE NMOS pull-down transistor and the EBE NMOS pass-gate transistor, the NMOS active area of the intermediate SRAM cell can include a jog or a notch resulting from differing widths for the NMOS pull-down transistor and the NMOS pass-gate transistor. Similarly, there can be a jog in the gate layer for the EBE NMOS pull-down transistor and the EBE PMOS pull-up transistor resulting from the differing lengths of these two transistors relative to each other in the source SRAM cell. These jogs or notches in the NMOS active area and the gate layer are illustrated in the SRAM cell layout  550  ( FIG. 5B ). 
     Referring again to method  500  ( FIG. 5A ), in step  515 , the width of the EBE NMOS pull-down transistor is resized to substantially the same width as the EBE NMOS pass-gate transistor, where the resizing may include substantially eliminating the jog or notch in the NMOS active area. In step  520 , the length of the EBE PMOS pull-up transistor is resized to substantially the same length as the EBE NMOS pull-down transistor, where the resizing may include substantially eliminating the jog or notch in the gate layer. The extent of resizing in steps  515  and  520  can depend on the process used to fabricate the source SRAM cell and the target SRAM cell, and the layout of the source SRAM cell. In certain embodiments, the resizing performed in steps  515  and  520  can be approximately in the range of 2-30 nm. In step  525 , the NMOS EBE transistor threshold voltage VTN and the PMOS EBE transistor threshold voltage VTP are selected. In one embodiment, VTN and VTP for the transistors in the SRAM cell are substantially matched to the threshold voltages of the EBE NMOS and EBE PMOS transistors used in logic gates in the same integrated circuit. In an alternative embodiment, the threshold voltages V TN  and V TP  for the EBE transistors used in the SRAM cell are set to provide enhanced and preselected performance characteristics for the target SRAM cell, such as, read margin, write margin, and cell leakage current. In step  530 , EBE transistor process parameters to result in the selected VTN and VTP (step  525 ) are determined. Such process parameters can include one or a combination of the thickness of the blanket epitaxial layer, the position of the screening region, the position of the threshold voltage tuning region, and/or the dopant concentration of the threshold voltage tuning region or screening region or both. 
     With reference to  FIG. 5B , the layout  550  corresponds to the intermediate SRAM cell using EBE transistors resulting from step  510  ( FIG. 5A ), in accordance with one embodiment. The annotations in  FIG. 5B  illustrate the resizing of the EBE transistors in the intermediate SRAM cell that are performed to substantially reduce or eliminate the jogs or notches in the NMOS active area and the gate layer, in accordance with the first embodiment. In layout  550 , the width of the EBE NMOS pull-down transistor is reduced to substantially the same width as the EBE NMOS pass-gate transistor by moving the edge  560  of the diffusion region such that it is substantially aligned with the edge  555 , as shown in  FIG. 5B . In addition, the length of the EBE PMOS pull-up transistor is reduced to substantially the same length as the EBE NMOS pull-down transistor by moving the edge  570  of the gate layer such that it is substantially aligned with the edge  565 , as shown in  FIG. 5B . As a result of these changes the diffusion region between the gate of the EBE NMOS pass-gate transistor (labeled NPG in  FIG. 5B ) and the gate of the EBE NMOS pull-down transistor (labeled NPD in  FIG. 5B ) is substantially rectangular in the layout  550  of the target SRAM cell, and the edges of this rectangular region are substantially aligned with the NPD and NPG areas. In addition, the gate layer extending between areas PPU and NPD (corresponding to the gates of the EBE PMOS pull-up and the EBE NMOS pull-down transistors, respectively) is substantially rectangular for the target SRAM cell, and the edges of this rectangular gate layer are substantially aligned with the edges of the PPU and NPD areas. It is noted that even though these changes are illustrated with respect to one of the NMOS active area and one of the gates in FIG.  5 B, the corresponding changes can also be made to the other NMOS active area  580  and the gate layer  575  to obtain a target SRAM cell that has substantially no jogs or notches in the NMOS active area and the gate layer. 
       FIG. 6A  is a flow diagram illustrating a method  600  for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with a second embodiment.  FIG. 6B  is a layout  650  of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method  600 . 
     With reference to the method  600  of  FIG. 6A , in step  605  the size of the transistors in the source SRAM cell that uses conventional transistors is determined. In step  610 , an intermediate SRAM cell is designed using EBE transistors. In one embodiment, the intermediate SRAM cell is a drop in replacement for the source SRAM cell and uses EBE transistors of substantially the same size (i.e. substantially the same width and length) as the source SRAM cell. Since the intermediate SRAM cell as a drop-in replacement preserves the width and length of the EBE NMOS pull-down transistor and the EBE NMOS pass-gate transistor, the NMOS active area of the intermediate SRAM cell can include a jog or a notch resulting from differing widths for the NMOS pull-down transistor and the NMOS pass-gate transistor. Similarly, there can be a jog or notch in the gate layer for the EBE NMOS pull-down transistor and the EBE PMOS pull-up transistor resulting from differing lengths of these two transistors in the source SRAM cell. These jogs or notches in the NMOS active area and the gate layer are illustrated in the SRAM cell layout  650  ( FIG. 6B ). 
     Referring again to method  600  ( FIG. 6A ), in step  615 , the width of the EBE NMOS pull-down transistor is resized to substantially the same width as the EBE NMOS pass-gate transistor, to substantially eliminate the jog or notch in the NMOS active area. In step  620 , the length of the EBE PMOS pull-up transistor is resized to be substantially the same length as the EBE NMOS pull-down transistor, which may result in substantially eliminating the jog in the gate layer. In step  625 , the width of the NMOS active area is reduced, thereby reducing the width of both the EBE NMOS pull-down and pass-gate transistors. This results in a reduction of the overall width of the SRAM cell. For example, if the width of the NMOS active area is reduced by 10 nm, then the width of the SRAM cell is reduced by 20 nm as a result of the reduced width of the two NMOS active areas in the SRAM cell. The extent of resizing in steps  615 ,  620 , and  625  can depend on the process used to fabricate the source SRAM cell and the target SRAM cell, and the layout of the source SRAM cell. In certain embodiments, the resizing performed in steps  615 ,  620 , and  625  can be approximately in the range of 2-30 nm. In step  630 , the NMOS EBE transistor threshold voltage VTN and the PMOS EBE transistor threshold voltage VTP are selected. In one embodiment, VTN and VTP for the transistors in the SRAM cell are selected to substantially match the threshold voltages of the EBE NMOS and EBE PMOS transistors used in logic gates in the same integrated circuit. In an alternative embodiment, the threshold voltages VTN and VTP for the EBE transistors used in the SRAM cell are set to provide enhanced and preselected performance characteristics for the target SRAM cell, such as, read margin, write margin, and cell leakage current. In step  635 , EBE transistor process parameters for the selected VTN and VTP (selected in step  630 ) are determined. Such process parameters can include one or a combination of the thickness of the blanket epitaxial layer, the position of the screening region, the position of the threshold voltage tuning region, and/or the dopant concentration of the threshold voltage tuning region or screening region or both. 
     With reference to  FIG. 6B , the layout  650  corresponds to the intermediate SRAM cell using EBE transistors resulting from step  610 , in accordance with one embodiment. The annotations in  FIG. 6B  illustrate the resizing of the EBE transistors in the intermediate SRAM cell that are performed to obtain the target SRAM cell, in accordance with the second embodiment. In layout  650 , the width of the EBE NMOS pull-down transistor is reduced to substantially the same width as the EBE NMOS pass-gate transistor by moving the edge  660  of the diffusion region such that it is substantially aligned with the edge  655 , as shown in  FIG. 6B . Also, the length of the EBE PMOS pull-up transistor is reduced to substantially the same length as the EBE NMOS pull-down transistor by moving the edge  670  of the gate layer such that it is substantially aligned with the edge  665 , as shown in  FIG. 6B . In addition to these changes, the width of the NMOS active area is reduced by reducing the width of the diffusion region to reduce the width of the SRAM cell, as shown in  FIG. 6B . As a result of these changes, the diffusion region between the gate of the EBE NMOS pass-gate transistor (labeled NPG in  FIG. 6B ) and the gate of the EBE NMOS pull-down transistor (labeled NPD in  FIG. 6B ) is substantially rectangular for the target SRAM cell, and the edges of this rectangular region are substantially aligned with the edges of the NPG and NPD areas. In addition, the gate layer extending between the areas PPU and NPD (corresponding to the gates of the EBE PMOS pull-up and the EBE NMOS pull-down transistors, respectively) is substantially rectangular for the target SRAM cell, and the edges of this rectangular gate layer are substantially aligned with the edges of the PPU and NPD areas. It is noted that even though these changes are illustrated with respect to one of the NMOS active area and one of the gate layers in  FIG. 6B , the corresponding changes can also be made to the other NMOS active area  675  and the gate layer  680  to obtain a target SRAM cell that has substantially no jogs or notches in the NMOS active area and the gate layer. In one embodiment, where the NMOS active area width is reduced by 10 nm, the target SRAM cell using EBE transistors has an overall width that is reduced by 20 nm compared to the source SRAM cell from which the EBE transistor-based SRAM cell is derived. 
       FIG. 7A  is a flow diagram illustrating a method  700  for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with a third embodiment.  FIG. 7B  is a layout  750  of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method  700 . 
     With reference to the method  700  of  FIG. 7A , in step  705 , the size of the transistors in the source SRAM cell that uses conventional transistors is determined. In step  710 , an intermediate SRAM cell is designed using EBE transistors. In one embodiment, the intermediate SRAM cell is a drop in replacement for the source SRAM cell and uses EBE transistors of substantially the same size (i.e. substantially the same width and length) as the source SRAM cell. Since the intermediate SRAM cell as a drop-in replacement preserves the width and length of the EBE NMOS pull-down transistor and the EBE NMOS pass-gate transistor, the NMOS active area of the intermediate SRAM cell can have a jog or notch resulting from differing widths for the NMOS pull-down transistor and the NMOS pass-gate transistor. Similarly, there can be a jog or notch in the gate layer for the EBE NMOS pull-down transistor and the EBE PMOS pull-up transistor resulting from differing lengths of these two transistors in the source SRAM cell. These jogs or notches in the NMOS active area and the gate layer are illustrated in the SRAM cell layout  750  ( FIG. 7B ). 
     Referring again to method  700  ( FIG. 7A ), in step  715 , the length of the EBE NMOS pull-down transistor is resized to substantially the same length as the EBE PMOS pull-up transistor, where the resizing may include substantially eliminating the jog or notch in the gate layer. In step  720 , the width of the NMOS active area is reduced thereby reducing the width of both the EBE NMOS pull-down and pass-gate transistors. This results in a reduction of the overall width of the SRAM cell. For example, if the width of the NMOS active area is reduced by 20 nm, then the width of the SRAM cell is reduced by 40 nm as a result of the reduced width of the two NMOS active areas in the SRAM cell. The extent of resizing in steps  715 , and  720  can depend on the process used to fabricate the source SRAM cell and the target SRAM cell, and the layout of the source SRAM cell. In certain embodiments, the resizing performed in steps  715  and  720  can be approximately in the range of 2-30 nm. In step  725 , the NMOS EBE transistor threshold voltage VTN and the PMOS EBE transistor threshold voltage VTP are selected. In one embodiment, VTN and VTP for the transistors in the SRAM cell are selected to substantially match the threshold voltages of the EBE NMOS and EBE PMOS transistors used in logic gates in the same integrated circuit. In an alternative embodiment, the threshold voltages VTN and VTP for the EBE transistors used in the SRAM cell are set to provide enhanced and preselected performance characteristics for the target SRAM cell, such as, read margin, write margin, and cell leakage current. In step  730 , EBE transistor design parameters to obtain the VTN and VTP are determined. Such process parameters can include the thickness of the blanket epitaxial layer, the position of the screening region, the position of the threshold voltage tuning region, and/or the dopant concentration of the threshold voltage tuning region. 
     With reference to  FIG. 7B , the layout  750  corresponds to the intermediate SRAM cell using EBE transistors resulting from step  710 , in accordance with one embodiment. The annotations in  FIG. 7B  illustrate the resizing of the EBE transistors in the intermediate SRAM cell that are performed to obtain the target SRAM cell, in accordance with the third embodiment. In layout  750 , the length of the EBE NMOS pull-down transistor is increased to substantially the same length as the EBE PMOS pull-up transistor by moving the edge  765  of the gate layer such that it is substantially aligned with the edge  770 , as shown in  FIG. 7B . In addition to these changes, the width of the NMOS active area is reduced by reducing the width of the diffusion region to reduce the overall width of the SRAM cell, as shown in  FIG. 7B . As a result of these changes the gate layer between areas PPU and NPD (corresponding to the gates of the EBE PMOS pull-up and the EBE NMOS pull-down transistors, respectively) is substantially rectangular for the target SRAM cell, and the edges of this rectangular gate layer are substantially aligned with the edges of the PPU and NPD areas. It is noted that even though these changes are illustrated with respect to one of the NMOS active area and one of the gate layers in  FIG. 7B , the corresponding changes can also be made to the other NMOS active area  775  and the gate layer  780  to obtain a target SRAM cell that has substantially no jogs or notches in the gate layer. In one embodiment, where the NMOS active area width is reduced by 10 nm, the target SRAM cell using EBE transistors has a width that is reduced by 20 nm. 
       FIG. 8A  is a flow diagram illustrating a method  800  for porting a source SRAM cell using conventional transistors to a target SRAM cell using EBE transistors, in accordance with a first embodiment.  FIG. 8B  is a layout  850  of a 6T SRAM cell using EBE transistors illustrating the transistor size modifications that are performed to obtain the target SRAM cell, in accordance with the method  800 .  FIG. 8C  is a layout  890  of the target SRAM cell obtained as a result of method  800 . 
     With reference to the method  800  of  FIG. 8A , in step  805 , the size of the transistors in the source SRAM cell that uses conventional transistors is determined. In step  810 , an intermediate SRAM cell is designed using EBE transistors. In one embodiment, the intermediate SRAM cell is a drop in replacement for the source SRAM cell and uses EBE transistors of substantially the same size (i.e. substantially the same width and length) as the source SRAM cell. Since the intermediate SRAM cell as a drop in replacement preserves the width and length of the EBE NMOS pull-down transistor and the EBE NMOS pass-gate transistor, the NMOS active area of the intermediate SRAM cell can have a jog or a notch resulting from differing widths for the NMOS pull-down transistor and the NMOS pass-gate transistor. Similarly, there can be a jog or notch in the gate layer for the EBE NMOS pull-down transistor and the EBE PMOS pull-up transistor resulting from differing lengths of these two transistors in the source SRAM cell. These jogs or notches in the NMOS active area and the gate layer are illustrated in the SRAM cell layout  850  ( FIG. 8B ). 
     Referring again to method  800  ( FIG. 8A ), in step  815 , the width of the EBE NMOS pull-down transistor is resized to substantially the same width as the EBE NMOS pass-gate transistor, where the resizing may substantially eliminate the jog or notch in the NMOS active area. In step  820 , the length of the EBE NMOS pull-down transistor is resized to substantially the same length as the EBE PMOS pull-up transistor, where the resizing may substantially eliminate the jog in the gate layer. The extent of resizing in steps  815  and  820  can depend on the process used to fabricate the source SRAM cell and the target SRAM cell, and the layout of the source SRAM cell. In certain embodiments, the resizing performed in steps  815  and  820  can be approximately in the range of 2-30 nm. In step  825 , the NMOS EBE transistor threshold voltage VTN and the PMOS EBE transistor threshold voltage are selected. In one embodiment, the threshold voltage of the EBE NMOS and EBE PMOS transistors in the SRAM cell are selected to have substantially the same value as the corresponding EBE NMOS and EBE PMOS transistors used in logic gates in the same integrated circuit. In an alternative embodiment, the threshold voltages VTN and VTP for the EBE transistors used in the SRAM cell are set to provide enhanced and preselected performance characteristics for the target SRAM cell, such as, read margin, write margin, and cell leakage current. In step  830 , EBE transistor process parameters to result in the selected VTN and VTP are determined. Such process parameters can include the thickness of the blanket epitaxial layer, the position of the screening region, the position of the threshold voltage tuning region, and/or the dopant concentration of the threshold voltage tuning region. 
     With reference to  FIG. 8B , the layout  850  corresponds to the intermediate SRAM cell using EBE transistors resulting from step  810 , in accordance with one embodiment. The annotations in  FIG. 8B  illustrate the resizing of the EBE transistors in the intermediate SRAM cell that are performed to obtain the target SRAM cell, in accordance with the fourth embodiment. In layout  850 , the width of the EBE NMOS pull-down transistor is reduced to substantially the same width as the EBE NMOS pass-gate transistor by moving the edge  860  of the diffusion region such that it is substantially aligned with the edge  855 , as shown in  FIG. 8B . In addition, the length of the EBE NMOS pull-down transistor is increased to substantially the same length as the EBE PMOS pull-up transistor by moving the edge  865  of the gate layer such that it is substantially aligned with the edge  870 , as shown in  FIG. 8B . As a result of these changes the diffusion region between the gate of the EBE NMOS pass-gate transistor (labeled NPG in  FIG. 8B ) and the gate of the EBE NMOS pull-down transistor (labeled NPD in  FIG. 8B ) is substantially rectangular for the target SRAM cell, and the edges of this rectangular region are substantially aligned with the edges of the NPG and NPD areas. In addition, the gate layer between area PPU and NPD (corresponding to the gates of the EBE PMOS pull-up and EBE NMOS pull-down transistors, respectively) is substantially rectangular for the target SRAM cell, and the edges of this rectangular gate layer are substantially aligned with the edges of the PPU and NPD areas. It is noted that even though these changes are illustrated with respect to one of the NMOS active areas and one of the gate layers in  FIG. 8B , the corresponding changes can also be made to the other NMOS active area  875  and the gate layer  880  to obtain a target SRAM cell that has substantially no jogs or notches in the NMOS active area and the gate layer. 
       FIG. 8C  illustrates the layout  890  of the target SRAM cell after the resizing described above with reference to  FIG. 8A  and  FIG. 8B , in accordance with one embodiment. 
     Table I lists the enhanced performance characteristics of the four target SRAM cell embodiments using EBE transistors described above as compared to a source SRAM cell using conventional transistors. Table I provides the performance characteristics for six types of SRAM cells—(1) a SRAM cell using conventional transistors; (2) an intermediate SRAM cell obtained as a result of replacing each conventional transistor with an EBE transistor of substantially similar width and length; (3) a SRAM cell in accordance with the first embodiment discussed with reference to  FIGS. 5A-C  above; (4) a SRAM cell in accordance with the second embodiment discussed with reference to  FIGS. 6A, 6B  above; (5) a SRAM cell in accordance with the third embodiment discussed with reference to  FIGS. 7A, 7B  above; and (6) a SRAM cell in accordance with the fourth embodiment discussed with reference to  FIGS. 8A-C  above. The SRAM performance characteristics listed in Table I were obtained from Monte Carlo simulations performed with 1000 trials at a power supply voltage or 1.2 V, and at a temperature of 25° C. The values of VTN and VTP were 0.608 V and −0.761 V, respectively, for the SRAM cell using conventional transistors. The values of VTN and VTP were 0.488 V and −0.481 V, respectively, for the intermediate SRAM cell, and the four SRAM cell embodiments using EBE transistors. Each cell of Table I has three numbers with the first number corresponding to −5σ, the second number corresponding to the mean, and the third number corresponding to +5σ, respectively. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                   
                 Read 
                 Read 
                 Write 
                   
                 Cell 
                 Cell  
               
               
                 SRAM Cell 
                 Current 
                 Margin 
                 Margin 
                 V ddmin   
                 Leakage 
                 Area 
               
               
                 Type 
                 (μA) 
                 (mV) 
                 (mV) 
                 (V) 
                 (pA) 
                 (μm 2 ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Source 
                 20.47 
                 25 
                 269 
                 730 
                 0 
                 No  
               
               
                 (conventional 
                 31.70 
                 183 
                 494 
                   
                 55.20 
                 Change 
               
               
                 transistors)  
                 42.87 
                 341 
                 719 
                   
                 226.2 
                   
               
               
                 Intermediate 
                 33.97 
                 105 
                 460 
                 410 
                 0 
                 No  
               
               
                 (EBE) 
                 37.20 
                 147 
                 510 
                   
                 101 
                 Change 
               
               
                   
                 40.45 
                 189 
                 560 
                   
                 212 
                   
               
               
                 First 
                 32.60 
                 97.5 
                 457 
                 390 
                 0 
                 No  
               
               
                 Embodiment 
                 35.70 
                 141 
                 507 
                   
                 103 
                 Change 
               
               
                   
                 38.80 
                 185 
                 557 
                   
                 218.5 
                   
               
               
                 Second 
                 29.75 
                 97 
                 448 
                 390 
                 0 
                 ~2% 
               
               
                 Embodiment 
                 32.70 
                 142 
                 503 
                   
                 105 
                 reduction 
               
               
                   
                 35.65 
                 187 
                 558 
                   
                 229 
                   
               
               
                 Third 
                 27.65 
                 105 
                 269 
                 410 
                 0 
                 ~4% 
               
               
                 Embodiment 
                 30.50 
                 150 
                 494 
                   
                 84.1 
                 reduction 
               
               
                   
                 33.35 
                 195 
                 719 
                   
                 182.1 
                   
               
               
                 Fourth 
                 31.80 
                 96 
                 269 
                 400 
                 0 
                 No  
               
               
                 Embodiment 
                 34.80 
                 139 
                 494 
                   
                 81.1 
                 Change 
               
               
                   
                 37.80 
                 182 
                 719 
                   
                 168.6 
               
               
                   
               
            
           
         
       
     
     Table II illustrates the enhanced performance characteristics of the four target SRAM cell embodiments using EBE transistors after setting VTN and VTP, as described above. Note that the tradeoffs, for instance, for V ddmin  and Cell Leakage depend on the specifications to support the application for the SRAM. Table II provides SRAM performance characteristics for the six types of SRAM cells listed in Table II after the determination of VTN and VTP. The SRAM performance characteristics listed in Table I were obtained from Monte Carlo simulations performed with 1000 trials at a power supply voltage or 1.2 V, and at a temperature of 25° C. The values of VTN are in the range of 0.538 V to 0.558 V, and the values of VTP are in the range of −0.381 V to −0.361 V for the intermediate SRAM cell, and the four SRAM cell embodiments using EBE transistors. The values of VTN and VTP were 0.608 V and −0.761 V, respectively, for the SRAM cell using conventional transistors. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE II 
               
               
                   
               
               
                   
                 Read 
                 Read 
                 Write 
                   
                 Cell 
                 Cell  
               
               
                 SRAM Cell 
                 Current 
                 Margin 
                 Margin 
                 V ddmin   
                 Leakage 
                 Area 
               
               
                 Type 
                 (μA) 
                 (mV) 
                 (mV) 
                 (V) 
                 (pA) 
                 (μm 2 ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Source 
                 20.47 
                 25 
                 269 
                 730 
                 0 
                 0.5408 
               
               
                 (conventional 
                 31.70 
                 183 
                 494 
                   
                 55.20 
                   
               
               
                 transistors) 
                 42.87 
                 341 
                 719 
                   
                 226.2 
                   
               
               
                 Intermediate 
                 28.30 
                 174 
                 357 
                 570 
                 0 
                 No 
               
               
                 (EBE) 
                 31.21 
                 216 
                 412 
                   
                 50.1 
                 Change 
               
               
                   
                 34.14 
                 258 
                 467 
                   
                 140.1 
                   
               
               
                 First 
                 27.04 
                 165 
                 348 
                 560 
                 0 
                 No 
               
               
                 Embodiment 
                 29.98 
                 208 
                 402 
                   
                 49.1 
                 Change 
               
               
                   
                 32.93 
                 251 
                 457 
                   
                 133.1 
                   
               
               
                 Second 
                 24.81 
                 164 
                 346 
                 560 
                 0 
                 ~2% 
               
               
                 Embodiment 
                 27.51 
                 208 
                 401 
                   
                 50.1 
                 reduction 
               
               
                   
                 30.21 
                 252 
                 456 
                   
                 140.1 
                   
               
               
                 Third 
                 24.30 
                 159 
                 361 
                 530 
                 0 
                 ~4% 
               
               
                 Embodiment 
                 27.00 
                 203 
                 416 
                   
                 55.8 
                 reduction 
               
               
                   
                 29.70 
                 247 
                 471 
                   
                 145.3 
                   
               
               
                 Fourth 
                 26.43 
                 167 
                 365 
                 540 
                 0 
                 No 
               
               
                 Embodiment 
                 29.18 
                 208 
                 415 
                   
                 48 
                 Change 
               
               
                   
                 31.93 
                 250 
                 465 
                   
                 136 
               
               
                   
               
            
           
         
       
     
     The SRAM cell embodiments and related methods described above can also be applied to hybrid target SRAM cells that use both EBE transistors and legacy transistors. For example, such hybrid target SRAM cell embodiments can use conventional PMOS pull-up transistors, and EBE NMOS pass-gate and pull-down transistors. 
     Together, the structures and methods of making the structures allow for EBE transistors having an enhanced body coefficient as compared to conventional nanoscale devices. With body bias voltage applied to the screening region, the EBE transistor can facilitate an even greater control over a wider range of device metrics, such as ON-current and OFF-current, compared to a conventional device. In addition, the EBE transistors have a better AVT, i.e., a lower σV T  than conventional devices. The lower σV T  enables a lower minimum operating voltage VDD and a wider range of available and reliable nominal values of V T . As will be understood, wafers and die supporting multiple transistor types, including those with and without the described dopant layers and structures are contemplated. Electronic devices that include the disclosed transistor structures or are manufactured in accordance with the disclosed processes can incorporate die configured to operate as “systems on a chip” (SoC), advanced microprocessors, radio frequency, memory, and other die with one or more digital and analog transistor configurations, and are capable of supporting a wide range of applications, including wireless telephones, communication devices, “smart phones”, embedded computers, portable computers, personal computers, servers, and other electronic devices. Electronic devices can optionally include both conventional transistors and transistors as disclosed, either on the same die or connected to other die via motherboard, electrical or optical interconnect, stacking or through used of 3D wafer bonding or packaging. According to the methods and processes discussed herein, a system having a variety of combinations of analog and/or digital transistor devices, channel lengths, and strain or other structures can be produced. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents. 
     Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.