Patent Publication Number: US-9899389-B2

Title: Two-transistor SRAM semiconductor structure and methods of fabrication

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional of U.S. patent application Ser. No. 14/607,025, filed Jan. 27, 2015, issued as U.S. Pat. No. 9,564,441 on Feb. 7, 2017, which is a continuation of U.S. patent application Ser. No. 14/590,852 entitled “Cross-Coupled Thyristor SRAM Semiconductor Structures and Methods of Fabrication,” filed Jan. 6, 2015. It is related to U.S. patent application Ser. No. 14/607,023, filed of even date and entitled, “Two-Transistor SRAM Circuit and Methods of Operation,” all of which claim the benefit of U.S. Provisional Patent Application No. 62/055,582, filed Sep. 25, 2014, which is incorporated by reference along with all other references cited in this application. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention is related to integrated circuit devices having memory functions and, in particular, to static random access memory (SRAM) devices. 
     From the invention of the integrated circuit in the late 1950&#39;s, circuit designs have been in constant development, particularly for integrated devices, in keeping with the developing semiconductor technologies. An early technology was bipolar technology which, compared to later integrated circuit technologies, occupied much space on a semiconductor substrate surface and required large amounts of electrical current with resulting high power consumption. Later field-effect technologies, particularly MOS (Metal-Oxide-Semiconductor) technology, used transistors which were much smaller than their bipolar counterparts with lower currents and consequently lower power consumption. CMOS (Complementary MOS) technology lowered currents and power consumption in an integrated circuit even further. Currently nearly all large-scale integrated circuits have turned to complementary metal oxide semiconductor (CMOS) technology. 
     Bipolar technology for semiconductor memory has been investigated over the years. But this research has typically focused upon the individual memory cell and has stopped at the conclusion that the memory cell could be part of an array. Further research and development of a bipolar memory cell array has been hampered by the longstanding belief that the CMOS memory cells occupied less space and consumed less power than any bipolar memory cell and that any bipolar memory cell array would necessarily be inferior to a CMOS array. Progress in recent years has relied upon relentless scaling in semiconductor processing technology, thus shrinking memory cell dimensions for greater circuit densities and higher operational speeds. 
     One integrated circuit implemented in CMOS technology is the SRAM, a circuit which employs bistable latching circuitry in its memory cells, enabling an SRAM memory cell to stay in a logic “1” or a logic “0” state as long as power is applied to the cell. Two cross-coupled inverters, each of which includes an active transistor and a complementary load transistor, and two select transistors, form the six-transistor CMOS SRAM cell which has been used for decades. Many integrated circuits in use today require a combination of CMOS logic circuits and on-chip high performance memories. Modern high performance processors and System-on-Chip (SoC) applications demands more on-chip memory to meet the performance and throughput requirements. For example, one integrated circuit can include 32 megabytes of CMOS SRAM as a cache memory on the chip. With a V DD  of 0.9 volts and a leakage current of 25 nanoamperes per memory cell, such a circuit consumes 7 amperes just from the memory array, without considering the power consumption of the logic portion of the chip. In addition, as the size of such circuits shrink with continued scaling in process technology used to manufacture the circuits, the stability and power consumption of the memory cells have become one of the limiting factors in process cost and circuit complexity, making the designers of such chips reluctant to use the latest process technology. 
     The CMOS SRAM in such devices typically has an access time on the order of 200 picoseconds with a standard deviation of 30 picoseconds. Thus to obtain 6−σ sigma reliability an additional allowance of 6×30 picoseconds is necessary, resulting in a requirement to allow 380 picoseconds for access to the memory cells. The use of faster bipolar technology in such devices is typically limited to driver circuits in the SRAM memory, and even when used there, a more complicated bipolar CMOS (BiCMOS) fabrication process is used, requiring additional thermal cycles and making the fabrication of the MOS devices more difficult and expensive. 
     As semiconductor processes shrink down to nanometer generations, however, both leakage and active currents through MOS transistors are particularly susceptible to wide variations compared to the currents comparably sized bipolar transistors. Projections indicate that with increased packing densities and statistical deviations in electrical current the operation of future CMOS SRAM devices is problematical. It is desirable that an alternative approach be found. 
     The present invention provides for an SRAM memory cell that is based upon a thyristor, one form of bipolar technology and often represented by two coupled bipolar transistors. The memory cell is highly adaptable with many variations and the resulting SRAM integrated circuit can be designed for high-speed operation, or for lower speed operation if less power is required, or even for higher integration if a tightly packed SRAM integrated circuit is required. Furthermore, the SRAM memory cell can be manufactured with conventional CMOS technologies to avoid the development costs of a new technology. 
     BRIEF SUMMARY OF THE INVENTION 
     In an integrated circuit having at least one logic circuit operating within a logic circuit voltage range and connected to a plurality of memory cells arranged in an array on a substrate interconnected by a plurality of pairs of complementary bit lines and word lines, the present invention provides for each memory cell comprising a pair of cross-coupled thyristors; a pair of bipolar transistors, each bipolar transistor respectively having a base region connected to a word line, an emitter region connected to one of the thyristors and a collector region connected to one of the pair of complementary bit lines; and an electrically isolated tub holding the pair of cross-coupled thyristors, and the tub in the substrate and electrically biased so that voltages on the word line within the logic circuit voltage range turn the pair of bipolar transistors on and off to connect and disconnect the cross-coupled thyristors to the pair of complementary bit lines. 
     A process for forming a pair of cross-coupled bipolar transistors to form the thyristors described above includes steps of forming an annular region of insulating material extending into the upper surface of a first conductivity type semiconductor substrate to define a tub in the substrate, and introducing opposite conductivity type dopant to form a buried layer at the bottom of the tub. First conductivity type dopant is then implanted into the tub to form a deep well region extending to the buried layer. Next opposite conductivity type dopant is implanted to form a shallow well. Then in the same process used elsewhere on the integrated circuit to form gates for field effect transistors, two gates are formed over the tub and used as a mask to implant dopants into the tub to form emitters and collectors for the pair of cross-coupled bipolar transistors. Finally, electrical connections are provided to each of the pair of cross-coupled bipolar transistors, but not to the gate electrodes. 
     The resulting semiconductor structure provides a cross-coupled pair of first type and opposite type bipolar transistors in which the annular insulating region extends into the substrate to surround a first portion of the upper surface of the substrate with a buried layer of opposite conductivity type to the first conductivity type disposed in the substrate beneath the first portion of the upper surface. A connecting region of first conductivity type extends to the buried layer to provide an electrical connection to it. Inside the tub, a shallow well region of opposite conductivity type extends from the upper surface into the substrate over a second portion of the upper surface smaller than the first portion. A base contact of first conductivity type extends into the tub outside the first portion of the upper surface. A dummy field effect transistor gate is provided over the shallow well region to enable self-aligned implanting of emitter and collector regions adjacent the gate. In the structure, the buried layer provides an emitter of the first type bipolar transistor and is coupled to a first voltage supply. The first conductivity type region adjacent one side of the gate provides an emitter region for the opposite type bipolar transistor and is coupled to a second voltage supply. The part of the tub beneath the shallow well provides a base for the first type bipolar transistor and a collector for the opposite type bipolar transistor, and the shallow well provides a base for the opposite type bipolar transistor and a collector for the first type bipolar transistor. 
     Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Herein the same reference numerals are often used in different drawings where the numbered element serves the identical or similar function and helps the reader&#39;s understanding of the described subject matter. 
         FIG. 1A  is a circuit diagram of a thyristor; and  FIG. 1B  shows a circuit diagram of cross-coupled thyristors that form a memory cell. 
         FIG. 2  illustrates an array of SRAM memory cells. 
         FIG. 3A  is a circuit diagram of a cross-coupled thyristor memory cell with bipolar select transistors; and  FIG. 3B  is circuit diagram of a memory cell with MOS select transistors. 
         FIG. 4A  is a circuit diagram of a cross-coupled memory cell without select transistors; 
         FIG. 4B  illustrates a cross point array of  FIG. 4A  memory cells with voltages on bit and word lines for a Write operation; and  FIG. 4C  illustrates the cross point array of  FIG. 4A  memory cells with voltages on bit and word lines for a Read operation;  FIG. 4D  shows a low-power Write operation for a cross-coupled thyristor memory cell array connected to complementary bit lines and word lines in one configuration;  FIG. 4E  is a representative timing diagram at various points of the memory cell in  FIG. 4D ;  FIG. 4F  shows a low-power Write operation for a cross-coupled thyristor memory cell array connected to complementary bit lines and word lines in a reverse configuration to that of  FIG. 4D ;  FIG. 4G  is a representative timing diagram at various points of the memory cell in  FIG. 4F ; and  FIG. 4H  is a representative diagram of circuit blocks supporting an SRAM memory array for one embodiment of the invention. 
         FIG. 5A  is a block diagram of an array of SRAM cells with current sources to maintain the memory cells in Standby mode by row;  FIG. 5B  is a circuit diagram of the current sources in  FIG. 5A ;  FIG. 5C  is a circuit diagram of Standby current sources for SRAM cells with cross-coupled thyristors in a cross point configuration in which the cathodes of the thyristors are connected to the complementary bit lines and the anodes are connected to the word line; and  FIG. 5D  is a circuit diagram in which the connection of the cross-coupled thyristors to the bit and word lines are reversed. 
         FIG. 6  is a circuit diagram of the current source for a block of SRAM cells to maintain the memory cells in Standby mode. 
         FIG. 7A  is a circuit diagram of a cross-coupled thyristor SRAM memory cell with write-assist transistors and capacitors;  FIG. 7B  illustrates a cross point array of  FIG. 7A  memory cells with voltages on bit lines and word lines for a Read operation;  FIG. 7C  illustrates a cross point array of  FIG. 7A  memory cells with voltages on bit lines and word lines for a Write operation;  FIG. 7D  is a circuit diagram of a cross-coupled thyristor SRAM memory cell with PMOS write-assist transistors;  FIG. 7E  illustrates a cross point array of  FIG. 7D  memory cells with voltages on bit lines and word lines for a Read operation;  FIG. 7F  illustrates a cross point array of  FIG. 7F  memory cells with voltages on bit lines and word lines for a Write operation. 
         FIG. 8A  is a circuit diagram of a single thyristor SRAM memory cell with a select transistor; and  FIG. 8B  is a circuit diagram of a single thyristor SRAM memory cell with no select transistor. 
         FIG. 9A  illustrates a single thyristor SRAM memory cell with a PMOS and NMOS Write assist transistors for a cross point array;  FIG. 9B  shows another single thyristor memory cell similar to that of  FIG. 9A  with anode and cathode connections reversed; and  FIG. 9C  is a circuit schematic for a standby current source for a single thyristor SRAM memory cell in a cross point array,  FIG. 9D  shows a circuit diagram of a single thyristor SRAM memory cell having its anode connected to the bit line and its cathode connected to the word line with a PMOS Write assist transistor;  FIG. 9E  shows a cross point array of  FIG. 9D  memory cells with voltages on bit lines and word lines for a Read operation;  FIGS. 9F and 9G  show a cross point array of  FIG. 9D  memory cells with voltages on bit lines and word lines for a two-stage Write operation;  FIG. 9H  shows a circuit diagram of a single thyristor SRAM memory cell having its anode connected to the word line and its cathode connected to the bit line with a PMOS Write assist transistor;  FIG. 9I  shows a cross point array of  FIG. 9H  memory cells with voltages on bit lines and word lines for a Read operation;  FIGS. 9J and 9K  show a cross point array of  FIG. 9H  memory cells with voltages on bit lines and word lines for a two-stage Write operation;  FIG. 9L  shows a low-power Write operation for a single thyristor memory cell array connected to bit lines and word lines in one configuration;  FIG. 9M  is a representative timing diagram at various points of the memory cell in  FIG. 9L ; and  FIG. 9N  shows a low-power Write operation for a single thyristor memory cell array connected to bit lines and word lines in a reverse configuration to that of  FIG. 9M ; and  FIG. 9O  is a representative timing diagram at various points of the memory cell in  FIG. 9N . 
         FIG. 10  illustrates a prior art complementary metal oxide semiconductor (CMOS) process. 
         FIGS. 11A and 11B  illustrates a process for forming shallow trench isolation regions. 
         FIG. 12  illustrates a vertical bipolar transistor. 
         FIG. 13  is a flowchart of a process for fabricating the structure illustrated in  FIG. 12 . 
         FIGS. 14A-14D  illustrate some of the steps of  FIG. 13  in more detail. 
         FIG. 15  illustrates a lateral bipolar transistor. 
         FIG. 16  is a flowchart of a process for fabricating the structure illustrated in  FIG. 15 . 
         FIGS. 17A-17D  illustrate a process for forming a merged NMOS-NPN structure. 
         FIGS. 18A and 18B  illustrates cross-sections of an SRAM cell. 
         FIG. 19  is a top view of the structures shown in  FIGS. 18A and 18B . 
         FIGS. 20A-20D  illustrate portions of the process for forming the SRAM cell of  FIGS. 18 and 19 . 
         FIGS. 21A and 21B  illustrates cross-sections of an SRAM cell with bipolar select transistors. 
         FIG. 22  is a top view of the structure shown in  FIGS. 21A and 21B . 
         FIGS. 23A-23E  illustrate process steps for making the structure of  FIGS. 21 and 22 . 
         FIG. 24  illustrates a cross-section of an SRAM cell structure formed using a deep N-well. 
         FIG. 25  is a cross-section of a three-transistor SRAM cell. 
         FIG. 26  is a top view of the layout of the three-transistor SRAM cell shown in  FIG. 25 . 
         FIGS. 27A and 27B  illustrate cross-sectional views of a four-transistor SRAM cell. 
         FIG. 28  is a top view of the structure shown in  FIGS. 27A and 27B . 
         FIGS. 29A and 29B  illustrate cross-sectional view of a four-transistor SRAM cell with write assist FETs. 
         FIG. 30  is a top view of the structure shown in  FIGS. 29A and 29B . 
         FIG. 31  is a cross-sectional view of a two-transistor SRAM cell. 
         FIG. 32  illustrates a detail of a manufacturing process. 
         FIG. 33  is a graph illustrating typical dopant concentrations for vertical bipolar transistors. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Description of Circuits and their Operation 
     a. Description and Operation of a Thyristor 
     As observed earlier, as semiconductor processes shrink the statistical variations of MOS transistors increase. This is not the case with bipolar transistors. In fact, at a certain point, which is believed to be processes having a critical dimension of 55 nm, often termed “55 nm processes,” many of the traditional advantages of MOS transistors over bipolar transistors disappear. That is, bipolar transistors have a comparable size to MOS transistors, and carry current with less statistical variation than their MOS transistor counterparts. Hence one observation of the present invention is that with 55 nm processes or smaller, SRAM memory arrays are better suited with bipolar technology. 
     The SRAM memory cells of the present invention are based upon a thyristor. A thyristor, also termed a silicon-controlled rectifier, are normally used for power applications. A thyristor is a four-layer (PNPN), three-junction device typically with two terminals, an anode and a cathode. In power applications there is typically a third terminal, called a control terminal for controlling the currents between the anode and cathode.  FIG. 1A  is a circuit diagram of a thyristor as represented by two bipolar transistors, a PNP transistor  10  and a NPN transistor  11 , which when merged form a PNPN thyristor. The emitter terminal  12  of the PNP transistor  10  forms the anode, and the emitter terminal  13  of the NPN transistor  11  forms the cathode. For purposes of completeness, a control terminal is shown in the drawing by a dotted line. 
     With the thyristor appropriately powered between two voltage supplies, e.g., at voltages V DD  and V SS , the thyristor can be in one of two states, either “ON” and conducting current, or “OFF” and non-conducting. If the thyristor in  FIG. 1A  is conducting, the base region of the NPN transistor  11 /collector region of PNP transistor  10  (indicated by reference numeral  14 ) is high, i.e., the base-emitter junction of the transistor  11  is forward-biased. The collector region of the NPN transistor  11 /base region of the PNP transistor  10  (indicated by the reference numeral  15 ) is low, i.e., the base-emitter junction of the transistor  10  is also forward-biased. Both transistors  10  and  11  are in saturation mode and current flows through the thyristor. If the thyristor is “off,” the base region of the NPN transistor  11 /collector region of PNP transistor  10  is low, i.e., the base-emitter junction of the transistor  11  is not forward-biased. The collector region of the NPN transistor  11 /base region of the PNP transistor  10  is high, i.e., the base-emitter junction of the transistor  10  is also not forward-biased. Both transistors  10  and  11  are off mode and no current flows through the thyristor (except for leakage currents). 
     At the scale of integrated circuits the thyristor is compatible with current conventional CMOS manufacturing technologies, as explained below, and is surprising adaptable to different circuit configurations to emphasize speed, power and integration as demanded by a user&#39;s application of an SRAM memory, whether as an SRAM integrated circuit or as a part of an integrated circuit. 
     b. Description and Operation of Cross-Coupled Thyristors 
     In one preferred embodiment of the present invention a pair of thyristors, each thyristor  16 A,  16 B formed by a PNP transistor  10 A,  10 B and an NPN transistor  11 A,  11 B, are cross-coupled as shown in  FIG. 1B  to form a memory cell. Where the memory cell includes select transistors, the P-type emitter terminals  12 A,  12 B of both transistors  10 A,  10 B are connected to a voltage supply line at V DD , the memory high power supply voltage, while the N-type emitter terminals  13 A,  13 B of the transistors  11 A,  11 B are connected to a second power line at V SS , the memory low power supply voltage. The two thyristors  16 A and  16 B are cross-coupled by the connection of the merged N-type base of the transistor  10 A and collector of the transistor  11 A to the merged P-type collector of the transistor  10 B and the base of the transistor  11 B, and the connection of the merged N-type collector of the transistor  11 B and base of the transistor  10 B to the merged P-type collector of the transistor  10 A and base of the transistor  11 A. A terminal  17 A to the merged N-type base of the transistor  10 A and collector of the transistor  11 A is connected to a select transistor for the memory cell (not shown in the drawing), and a second terminal  17 B to the merged N-type base of the transistor  10 B and collector of the transistor  11 B is connected to a second select transistor for the memory cell. 
     Operationally, the described cross-coupled thyristor memory cell core can be in one of two logic states, one arbitrarily termed “1” and the other “0”. Using the circuit of  FIG. 1B  as an example, if the thyristor  16 A is ON, with the transistor  10 A in saturation mode, the merged P-type collector of the transistor  10 A and base of the transistor  11 A is high (to forward-bias the base-emitter junction). By the cross-coupling, the collector of the transistor  11 B and base of the transistor  10  is high, and the base-emitter junction of the transistor  10 B in the thyristor  15 B cannot be forward-biased to turn the transistor  10 B off. The thyristor  15 B is OFF. Conversely, if the thyristor  15 B is ON, the thyristor  15 A is OFF. 
     Arranging a plurality of SRAM memory cells into an array provides an integrated circuit memory that can be combined with other circuits, for example, logic circuits. The memory cells are interconnected by sets of electrically conducing lines running in perpendicular directions.  FIG. 2  illustrates one such general arrangement with an array  20  of SRAM cells  27 , each cell having a cross-coupled thyristor core. A set of horizontal word lines  24  and a set of vertical bit lines  23 A,  23 B interconnect the SRAM cells. Each cell  27  is connected to a word line  24 , which in the drawing runs horizontally, and a bit line  23 A and its complementary bit line  23 B that run vertically in the drawing. Conventionally, a bit line is understood to carry data, a bit of information, to and from a memory cell. A word line is understood to activate a memory cell for the data to be carried into the memory cell, a Write operation, or for data to carried from the memory cell, a Read operation. In a Write operation the bit lines  23 A and  23 B carry complementary voltages, representing either a digital logic “1” or a “0” state, to be written into the selected memory cell  31  for storage. In a Read operation the bit lines  23 A and  23 B start at equal voltages and then tip high or low depending upon the digital signal stored in the memory cell  27 . The bit line  23 B carries the signal complementary to that carried by the bit line  23 A so that if the bit line  23 A is at a “high” voltage, the bit line  23 B is at a “low” voltage, and vice versa. If the memory cells  27  are neither being written nor being read, the memory cells are in Standby by which they are kept in a steady state condition to maintain their respective stored logic states. 
     c. SRAM Cell with Cross-Coupled Thyristors and Select Transistors 
     In one arrangement select transistors are used to connect the cross-coupled thyristors to the bit lines. This is illustrated in  FIG. 3A  in which the same reference numerals as in  FIG. 1B  are used for the cross-coupled thyristors  16 A and  16 B. Bipolar transistors  32 A and  32 B are used for select transistors to the cross-coupled thyristors  16 A and  16 B to form an SRAM cell. The emitter region of the NPN select transistor  32 A is connected to the merged collector region of the NPN transistor  10 A/base region of the PNP transistor  11 A of the thyristor  16 A and also to the merged base region of the NPN transistor  11 B/collector region of the PNP transistor  10 B of the thyristor  16 B. The collector region of the NPN select transistor  32 A is connected to the bit line  23 A and the base region of the transistor  32 A is connected to the word line  24 . In a similar fashion the emitter region of the NPN select transistor  32 B is connected to the merged collector region of the NPN transistor  11 B/base region of the PNP transistor  10 B of the thyristor  16 B and also to the merged base region of the NPN transistor  11 A/collector region of the PNP transistor  10 A of the thyristor  16 A. The collector region of the NPN select transistor  32 B is connected to the bit line  23 B and the base region of the transistor  32 B is connected to the word line  24 . To power the memory cell, the emitter regions of the PNP transistors  10 A and  10 B are connected to the upper power supply at voltage V DD  and the emitter regions of the NPN transistors  11 A and  11 B are connected to the lower power supply at voltage V SS . 
     For the SRAM cell to function properly, the memory cell requires a voltage higher than standard logic core voltage, i.e., the voltage used by the logic circuits of the integrated circuit to which the SRAM cell belongs. Standard core logic circuits, which are CMOS, typically operate in a range from 0 to 1 volt. Such SRAM memory supply voltage requirements can be met by using a higher voltage for the SRAM power supply, for example, the voltage used by the Input/Output circuits of the integrated circuit, V ddIO , or a voltage derived from V ddIO . But in one embodiment of the present invention, the requirement for extra circuitry can be avoided. In the construction of the memory cell, the SRAM memory cell can be placed in P-type tub in a deep N-type well in the semiconductor substrate. See  FIG. 31  and related description of the structure and process for the deep N-type well. The P-type tub is biased negatively so that the bipolar select transistors  32 A and  32 B can use the standard logic core voltages. Thus where the word lines are typically driven between the upper supply voltage V DD  and the lower supply voltage V SS  for the SRAM cells, the voltage swing for the word lines connected to the select bipolar transistors  32 A and  32 B is limited to 0.5V above V SS (=0V or ground) and 1.5V with respect to V DD  in one embodiment of the invention. This arrangement enables the circuits which drive the word lines to be constructed directly from core logic circuitry, i.e., no intervening voltage translation circuitry is required between the core logic and the bit lines. 
     Field effect transistors can also be used for the select transistors as shown in  FIG. 3B . NMOS(FET) transistors  33 A and  33 B respectively connect the word lines  23 A and  23 B to the cross-coupled thyristors  16 A and  16 B, as illustrated in  FIG. 3B  with many of the same reference numerals as in  FIG. 3A . A source/drain region of the NMOS select transistor  33 A is connected to the merged collector region of the NPN transistor  11 A/base region of the PNP transistor  10 A of the thyristor  16 A and also to the merged base region of the NPN transistor  11 B/collector region of the PNP transistor  10 B of the thyristor  16 B. The other source/drain region of the NMOS select transistor  33 A is connected to the bit line  23 A and the gate of the transistor  33 A is connected to the word line  24 . In a similar fashion one source/drain of the NMOS select transistor  33 B is connected to the merged collector region of the NPN transistor  11 B/base region of the PNP transistor  10 B of the thyristor  16 B and also to the merged base region of the NPN transistor  11 A/collector region of the PNP transistor  10 A of the thyristor  16 A. The other source/drain region of the transistor  33 B is connected to the complementary bit line  23 B and gate of the transistor  33 B is connected to the word line  24 . To power the memory cell, the emitter regions of the PNP transistors  10 A and  10 B are connected to the upper power supply at voltage V DD  and the emitter regions of the NPN transistors  11 A and  11 B are connected to the lower power supply at voltage V SS . Below ( FIG. 26  et seq.) we describe how the circuit of  FIG. 3B  is implemented and manufactured. 
     Whether bipolar or field effect transistors are used as select transistors for the memory cell is dependent upon various design “trade-offs.” The field effect transistor is smaller than the bipolar transistor with a resulting desirable smaller cell size. But there are major disadvantages to using FETs as the select devices. (1) They limit the access speed of the memory cell to being the same as a CMOS counter-part. In addition, FETs as select devices limit the amount of current that can be provided by a memory cell to raise or lower the voltages on the bit line capacitance. The bipolar NPN select devices can provide much more current and thus swing the voltage on the bit line much faster. (2) FETs manufactured with advanced processing are subject to a high degree of random variation from cell to cell, as described earlier. This degrades the operating margins of the memory cell which often must be countered by either exotic circuit design approaches in the word line and bit line drive circuits or by increasing transistor sizes in the memory cell, thus increasing the cell size. The random variations in the NPN select devices are much smaller. (3) The standby current of the unselected memory cells is controlled by the undesired leakage of the “OFF” FET devices in the memory cell. In advanced processed devices this leakage gets very large. Using an MOS select transistor with the thyristor-based memory cell introduces this source of unwanted leakage current into the cell design, whereas an NPN select device avoids this unwanted leakage current. 
     d. SRAM Cell with Cross-Coupled Thyristors and No Select Transistors 
     In another embodiment of the present invention, the SRAM memory cell has no select transistors to access the memory cell. Rather, cross-coupled thyristors  46 A and  46 B are connected directly to the word line  44  and the bit lines  43 A and  43 B, as shown in  FIG. 4A  to form a cross point memory cell array. In this arrangement, where nominally connected to an upper SRAM supply voltage V DD , the anodes, the emitter regions of the PNP transistors  40 A and  40 B, of the thyristors  35 A and  35 B are connected respectively to the bit lines  43 A and  43 B and where nominally connected to the lower SRAM supply voltage V SS , the cathodes, the emitter regions of the NPN transistors  41 A and  41 B, of the thyristors  35 A and  35 B are connected to the word line  44 . Since the word and bit lines provide power to the memory cell, the voltage operations on these lines in an array of such memory cells must be carefully choreographed. These operations are described with reference to an array of SRAM memory cells  47  with no select transistors. The cells are interconnected by bit lines  43 A,  43 B and word line  44 , as shown in  FIGS. 4B and 4C . 
     It should also be noted that voltage and current values are given in this text to better explain the invention and the values should be considered approximate. The voltages and currents can be varied or changed to accommodate the electrical characteristics of the specific devices being used on the integrated circuit. As an example,  FIG. 4H  shows a thyristor-based SRAM integrated circuit at the block-level. An array of SRAM cells is addressed by signals from a Row Decoder block, conventionally for the word lines of the array, and signals from a Column Decoder, Write &amp; Sense Amplifier block, conventionally for the bit line(s) of the array. These address signals and control signals are carried by an Address Block address, Control and I/O Bus, which passes the signals to a Row Pre-decode block and a Column Pre-decoder &amp; Control block. These blocks process the signals and send their signals to the Row Decoder block and Column Decoder, Write &amp; Sense Amplifier block, as is well known to practitioners in the art. The arrows in  FIG. 4H  are shown as pointing toward the array because this example operation is an addressing operation where particular memory cells in the array are selected, whether for a Read or Write operation. 
     To turn a thyristor on, the voltage imposed across the thyristor, i.e., between the anode and the cathode, should be at least the sum of a forward-biased PN junction plus the emitter-collector voltage of a PNP transistor in saturation. Thus the imposed voltage can vary quite a lot, starting from about 0.8V and larger. The larger the imposed voltage, the quicker the thyristor turns fully on. As shown in  FIG. 4H , 1.4V is selected as the best compromise for optimum thyristor response at a fairly low voltage. But the blocks surrounding the array operate with CMOS logic circuits and current CMOS logic voltage range is approximately from 0 to 1.0 volts. One way to allow the CMOS logic circuits in the Row Decoder block and the Column Decoder, Write &amp; Sense Amplifier block to interface properly is to raise or shift the CMOS logic voltage range in the Row Decoder block upwards 0.4 volts. That is, while the CMOS logic circuits still operate in a 1.0 volt range, the range extends between 0.4 to 1.4 volts. This is achieved by a voltage Level Shift block between the Row Pre-decoder block and the Row Decoder block. The Level Shift block can use the special voltages from the other blocks, such as the I/O (Input/Output) block. 
     Another way for the thyristor-based memory cell array to interface properly with the CMOS logic circuits in the Row Decoder block and the Column Decoder, Write &amp; Sense Amplifier block is to bias the array at a desired voltage. In the exemplary voltages of  FIG. 4H , the array and the Column Decoder, Write &amp; Sense Amplifier block can be biased by a negative 0.4 volts in the manner described above with respect to the cross-coupled thyristor cell with select transistors and the material related to  FIG. 31 . With the proper bias the Level Shift block shown in  FIG. 4H  is not required, but another level shift block for the column signals to shift the voltage swing in the described 0 to +1.0V range down to the −0.4 to +0.6V range. These alternatives consume different amounts of circuit resources, such as layout area, which must be considered and balanced by the design architect of the SRAM integrated circuit. 
     Returning to the operations of the cross-coupled thyristor memory cell array itself, in Standby the bit lines  43 A and  43 B and word line  44  are held at voltages so that the memory cells of the array maintain their stored logic states indefinitely, i.e., each of the memory cells are “kept alive” and none switches states. In a Write operation the complementary bit lines and the word line of the selected memory cell(s) are driven high and low to place the selected cell(s) in the desired logic state(s). The bit and word lines of the unselected memory cells are kept at the Standby voltages to remain unchanged. Difficulties occur for the memory cells which are “half-selected,” i.e., memory cells which have either their bit lines or their word lines (but not both) connected to the selected memory cells. The voltages on the selected memory cells for the Write operation must be sufficiently large so as to drive the selected cells into the desired logic states, yet not so large so to change the logic states of the half-selected cells. 
     In a Read operation the complementary bit lines of the selected memory cell(s) are driven high and the word line of the selected memory cell(s) are driven low for the selected memory cell(s) to allow the state of the cell to tip the voltages of the complementary bit lines for reading of the memory cell state by a differential amplifier. The bit and word lines of the unselected memory cells, on the other hand, are kept at the Standby voltages to remain unchanged. For the half-selected memory cells, the voltages on the selected memory cells for the read operation must be sufficiently large so that the state of a selected memory cell can affect the voltages on the bit lines. But the voltages should not be so high so as to affect the logic states of the other memory cells, the half-selected cells, which have either their bit lines or their word lines (but not both) connected to the selected memory cells. 
     A Write operation is illustrated by  FIG. 4B  with an array of memory cells  47 , such as connected to the word and bit lines shown in  FIG. 4A . In this example, the first and third memory cells in the first row are to be written and the bit lines  43 A and  43 B for the selected memory cells are respectively driven to 1.6V (or 0.8V) and 0.8V (or 1.6V), depending upon the states to be written. The word line  44  for the selected memory cells is driven low from 0.4V to 0V so that a bias of 1.6V is applied across one thyristor in each selected memory cell while the second thyristor sees a bias of 0.8V. These voltages drive the memory cell into the desired state accordingly. The unselected memory cells see the Standby voltages of 0.8V on their bit lines  43 A,  43 B and 0.4V on their word line  44 . The resulting voltage of 0.4V maintains the unselected memory cells in their present state. The half-selected memory cells, on the other hand, see a voltage of 0.8V for cells connected to the same word line as the selected cells, or a voltage of 1.2V (1.6-0.4V) or 0.4V (0.8-0.4V) for the cells connected to the same bit lines as the selected cells. These voltages are insufficient to “flip” or change the state of a half-selected memory cell. 
     In a Read operation the bit lines  43 A and  43 B for the selected memory cell are both driven to 1.3V and the word line  44  for the selected memory cell is driven to 0V. This is illustrated in the example of  FIG. 4C . The state of the selected memory cell “tips” the voltages on the bit lines  43 A and  43 B one way or the other so that the memory cell can be read. The bit lines  43 A and  43 B for the unselected memory cells are left at 0.8V and the word line  44  for the unselected memory cells 0.4V to maintain these memory cells in their present state. The half-selected memory cells that are connected to the same word line as the selected memory cells see a voltage of 0.8V, while the half-selected memory cells which are connected to the same bit lines as the selected cells, see a voltage of 0.9V. Neither of these voltages is sufficient to “flip” or change the state of a memory cell. 
     The voltages above should be understood as compromises between operating speeds and power dissipation. The 0.8V voltage, for example, is slightly above the 0.7V silicon junction turn-on voltage and may result in some leakage current through the half-selected memory cells during read or write operations. However, speed is optimized. A higher bias results in higher speeds, but higher leakage (power dissipation) on the selected lines. The particular application for the SRAM helps determine the trade-off between speed and power dissipation. Furthermore, the relative biasing of the word lines and bit lines for the selected memory cells and for the unselected memory cells may be adjusted to minimize leakage or to optimize compatibility with other circuits. For example, during a Read operation the word line for the selected memory cells could be set at 0V and the bit lines at 1.3V, or the word line at 0.3V and the bit lines at 1.6V, while holding the word lines for the unselected memory cells at 0.4V and bit lines at 0.8V. Both of these operating parameters work but they result in different leakage currents through the word and bit lines for the selected memory cell. If multiple cells are read simultaneously, extra current is carried on the word and bit lines for the selected memory cells. The biases for the word and bit lines can be adjusted to minimize the voltage drop or the maximum current through the memory array lines. In conjunction with  FIG. 34  et seq. below we describe the structure and manufacture of a cell such as depicted in  FIG. 4A . 
     It should also understood that the description and voltage values above were given for one arrangement of the connections between the word and bit lines and the anodes and cathodes of a cross-coupled thyristor memory cell. The connections illustrated in  FIG. 4A  can be reversed, i.e., the anodes of the thyristors connected to the word line and the cathodes connected to the complementary bit line pair with the operations and voltages correspondingly changed to reflect the reversed connections. 
     e. Reduced Power Write Operations for Cross-Coupled Thyristor SRAM Cells 
     In the Write operation description above the bit line connected to the “OFF” thyristor in the memory cell which is to be written to is held high to guarantee that the thyristor receives the maximum Write voltage during the duration of the Write operation pulse to turn the thyristor “ON”. Once the memory cell, i.e., the “OFF” thyristor turns “ON” and its cross-coupled counterpart turns “OFF”, the newly “ON” thyristor conducts the maximum “ON” current while the thyristor&#39;s bit line is held high. This increases the power consumption during the Write operation. More importantly, the word line must carry away the current from all the cells on the row so that this current can be very large. This can cause a significant voltage drop along the word line due to the electrical resistance of the line and may result in instability in the memory array. 
     This situation can be ameliorated by pre-charging the bit line to the “OFF” thyristor high prior to the Write pulse in a fashion similar to a Read operation described above as illustrated in  FIG. 4D .  FIG. 4E  is a representation of the voltages at various locations of the  FIG. 4D  memory cell. After the pre-charge, the pre-charge is turned off to allow the bit line to “float” until the memory cell flips state. At that point the current through the newly “ON” thyristor pulls the bit line low (similar to a Read operation). As the bit line voltage falls, the voltage drop across the thyristor decreases and the current falls toward the minimum holding current. This has three benefits. First, the peak current is reduced because the bit line voltage starts to drop before the internal voltages in the memory cell turn the thyristor fully “ON”. Secondly, the now narrow current pulse (shown by the shape of I ThyL  in Figure E) reduces the power dissipated in the Write operation. Thirdly, since from a statistical standpoint the various cells in the row being written flip at different times, the peak current in the word line is reduced and spread over a larger time interval, further minimizing the undesirable voltage drop along the word line. 
       FIG. 4F  illustrates the low power Write operation for a cross-coupled thyristor cell in which the connections to the word and bit lines are reversed compared to that of  FIG. 4D . That is, the  FIG. 4F  memory cell has its anodes connected to the word line and its cathodes to respective bit lines.  FIG. 4G  is a representation of the voltages at various locations of the  FIG. 4F  memory cell. In this example, the bit line to the “OFF” thyristor is pre-charged low prior to the Write pulse (similar to a Read operation). Then the pre-charge path is turned off to allow the bit line to “float” until the memory cell flips state. At that point current through the newly “ON” thyristor pulls the bit line high (similar to a Read operation). As the bit line rises, the voltage drop across the thyristor decreases and the current drops toward the minimum holding current. The same benefits as described in the previous paragraph are achieved. First, the peak current is reduced because the bit line voltage starts to rise before the internal voltages in the memory cell turn the thyristor fully “ON”. Secondly, the now narrow current pulse (shown by the shape of I ThyL  in Figure G) reduces the power dissipated in the Write operation. Thirdly, since from a statistical standpoint the various cells in the row being written flip at different times, the peak current in the word line is reduced and spread over a larger time interval, further minimizing the undesirable voltage drop along the word line. 
     f. Reduced Power Standby Operation for Cross-Coupled Thyristor SRAM Cells 
     Rather than using fixed voltages (between 0.3 to 0.8V) to maintain the memory cells (see  FIG. 4A ) of the array in their various states between Read and/or Write operations, an alternative approach regulates the voltage with a current source to maintain the memory cell states in Standby mode. The current source provides enough current for all of the connected memory cells and sufficient current so that the weakest memory cell does not “flip.” The memory cell array is divided into partial arrays. Since typically multiple cells are read along a single word line, the array is divided along the word line direction as illustrated in  FIG. 5A . In the memory array  50  pairs of bit lines  53 A and  53 B intersect word lines  54  at SRAM memory cells as previously described. Parallel to the word lines  54  are upper power supply lines  56  which are connected to the SRAM memory cells; pairs of word lines  54  and power supply lines  56  are connected to a current source circuit  55 . Current control is placed along the word line so that selection is made along the word line or world lines being read. 
       FIG. 5B  shows the current source circuit  55  in greater detail. When address signals to a decoder represented by a NAND gate turn on the word line  54 , the same output signal from the NAND gate turns on the PMOS transistor that short-circuits the current source  58 . The power supply line  56  is pulled up to V DDA  for the connected memory cells  57  to be read. Otherwise, when the word line  54  is off and the row of memory cells is not selected, the PMOS transistor is off and the current source  57  at voltage V DDA  supplies the necessary current to maintain the memory cells  57  in their respective logic states. Given current technologies, the required current from the current source  57  for a single row of memory cells in standby mode is in the range of 1-10 pA per bit. 
       FIG. 5C  shows a standby current source arrangement for memory cells  57  with cross-coupled thyristors in a cross point array. In this embodiment the cathodes of the thyristors are connected to complementary bit line pairs  53 A,  53 B and the anodes of the thyristors are connected to the word line  54 . The current source  51  is connected to the word line  54  by a PMOS transistor  59 . In Standby mode, the PMOS transistor  59  is on and the current source  51  supplies a maintenance current to the memory cells  57  through the word line  54  and out through the bit lines  53 A,  53 B. When the address signals to a decoder represented by the NAND gate turn on the PMOS transistor  56  and turn off the PMOS transistor  59 , the current source  51  is disconnected from the word line  54  and the word line  54  is raised to a word line selection voltage, represented by V WL   _   SEL , for a Read or Write operation. 
       FIG. 5D  shows another standby current source arrangement for memory cells  57  with cross-coupled thyristors in a cross point array, similar to that of  FIG. 5C . In this embodiment, however, the anodes of the thyristors are connected to the complementary bit line pair  53 A,  53 B, and the cathodes of the thyristors are connected to the word line  54 . The current source, now a current sink  52 , is connected to the word line  54 . In Standby mode, the PMOS transistor  59  is on and the current sink  57  pulls a maintenance current through the memory cells  57  from the bit lines  53 A,  53 B through the cells  57  and out to the word line  54 . When the address signals to a decoder represented by the NAND gate turn on the PMOS transistor  56  and turn off the PMOS transistor  59 , the current sink  52  is disconnected from the word line  54  and the word line  54  is raised to the word line selection voltage for a Read or Write operation. 
     It should be noted that the arrangements shown in  FIGS. 5C and 5D  provide for a current source/sink for more than one word line in the cross point array. Rather, the current source/sink works for a block with many word lines. In a similar fashion,  FIG. 6  shows a current source circuit for blocks of memory cells which have fixed voltage supplies. See  FIG. 5B . In  FIG. 6 , the current source  68  supplies the current for the plurality of rows of memory cells in the block. When a particular row, i.e., word line  64 , is selected, a second MOS transistor  69  turns off to decouple the current source  68  from the power supply line  66 , while a first MOS transistor  66  turns on to pull the power supply line  66  to voltage V DDA . When the particular word line is not selected, the first MOS transistor  66  is off and the second MOS transistor  69  connects the power supply line  66  to the current supply  68  at voltage V DDA . The current supply  68  supplies the current to a block of several rows of unselected memory cells; the total current is about 1 μA at current technology levels. Such a current supply is relative easy to design compared to a current supply in the range of 1 nA. 
     g. Cross-Coupled Thyristor SRAM Cell with Write Assist Circuitry 
     To speed the operations of the thyristor-based memory cell, Write assist circuitry can be added to the memory cell. In the example of  FIG. 7A , the anodes of the cross-coupled thyristors of the memory cell  77  are respectively connected to a complementary bit line pair,  73 A and  73 B, and the cathodes of the thyristors are connected a word line  74 . Two PMOS transistors  76 A and  76 B are respectively connected to the emitter and collector regions of the PNP transistors  70 A and  70 B. That is, a source/drain region of the PMOS transistor  76 A ( 76 B) is connected to, or part of, the emitter region of the PNP transistor  70 A ( 70 B), i.e., the anodes of the cross-coupled thyristors, and the second source/drain region of the PMOS transistor  76 A ( 76 B) is connected to, or part of, the collector region of the PNP transistor  70 A ( 70 B). The gates of the PMOS transistors  76 A and  76 B are connected to a Write assist line  75 . Additionally, the two capacitors  78 A and  78 B each have a terminal connected to the Write assist line  75 ; the other terminal of each capacitor  878 A and  78 B are respectively connected to the base regions of the PNP transistor  70 A and  70 B/collector regions of the NPN transistor  71 A and  71 B. 
     When the signal on the Write assist line  75  is low, the PMOS transistor  76 A short-circuits the P-type emitter and collector regions of the transistor  70 A. Likewise the PMOS transistor  76 B short-circuits the P-type emitter and collector regions of the transistor  70 B. The N-type carriers are driven from the base regions of the transistors  70 A and  70 B into the capacitors  78 A and  78 B respectively. The Write assist transistors  76 A and  76 B provide a conduction path from the anode of the individual thyristor to its P-well region (the collector region of the PNP transistor  70 A,  70 B and the base region of the NPN transistor  71 A,  71 B). If the thyristor is “OFF”, then its shallow N-well, the base region of the PNP transistor  70 A,  70 B/collector region of NPN transistor  71 A,  71 B, is at a higher voltage—somewhere near but not necessarily equal to the voltage on the anode. It is high enough that the magnitude of V be , the base-emitter voltage, is not sufficient to cause the PNP transistor  70 A,  70 B to conduct more than background leakage current. Similarly the P-well region, the collector region of the PNP transistor  70 A,  70 B and the base region of the NPN transistor  71 A,  71 B, is at a lower voltage—somewhere near but not necessarily equal to the voltage on the cathode. It is low enough that the magnitude of V be  is not sufficient to cause the NPN transistor  71 A,  71 B to conduct more than background leakage current. In order to cause a thyristor to turn “ON,” either the voltage of the shallow N-well must be lowered to turn on the PNP transistor  70 A,  70 B, or the voltage of the P-well must be raised to turn on the NPN transistor  71 A,  71 B. One approach is to increase the voltage between the anode and the cathode to larger values. This increases the background leakage currents through the PNP and NPN transistors which eventually causes one of the thyristor component transistors to turn “ON” and then the feedback loop in the thyristor snaps the device into the fully “ON” state. However, this approach requires either a large anode-cathode voltage (higher than may be provided with the CMOS circuits) or long write times at the applied voltage (longer than desired for SRAM operation). 
     Thus the PMOS transistors  76 A,  76 B are added to assist the Write operation to the “ON” state. When a normal operating voltage from the anode to the cathode (approx. 1.2-1.4V) is applied and the voltage on the gate of the MOS transistor low enough to turn on the PMOS transistor, a conduction path is provided between the anode voltage and the P-well, the collector region of the PNP transistor  70 A,  70 B/base region of the NPN transistor  71 A,  71 B. This conduction path pulls the voltage on the P-well upward towards the anode and when it reaches the normal PN-diode turn-on voltage, the NPN base-emitter junction (P-well to cathode) starts to conduct, turning on the NPN transistor  71 A,  71 B and then the feedback of the thyristor kicks it into the fully “ON” state. 
     If the thyristor is in the “ON” state, voltage of the shallow N-well, the base region of the PNP transistor  70 A,  70 B/collector region of NPN transistor  71 A,  71 B, is relatively low (sufficient to place the PNP transistor  70 A,  70 B in the “ON” state) and the P-well, the collector region of the PNP transistor  70 A,  70 B/the base region of the NPN transistor  71 A,  71 B, is relatively high (sufficient to place the NPN transistor  71 A,  71 B in the “ON” state.) 
     To switch the thyristor into the “OFF” state, the direct current flow through the thyristor must first be stopped by reducing the voltage between the anode and the cathode to sufficiently low voltage to eliminate the “turn-on” forward biasing of the base-emitter junctions of both the PNP (sNW to anode) transistors  70 A,  70 B and the NPN (PW to cathode)  71 A,  71 B transistors. However, this is not sufficient. Even though current flow through the thyristor is stopped by the reduced voltage between the anode and the cathode, there is a large amount of charge (minority carriers) stored in the N base region of the PNP transistor  70 A,  70 B and P base region of the PNP transistor  70 A,  70 B which internally holds them very near but slightly less than the V be  turn-on point of the PNP and NPN devices. If the anode-cathode voltage are increased again, these junctions quickly forward bias again and the thyristor turns back “ON”. Thus without some “assistance,” a thyristor must be held in this “OFF” state for a sufficiently long time for all the stored charge to dissipate through background leakage and recombination. 
     The Write assist PMOS transistor  76 A,  76 B help in removing charge from the thyristor P-well, the collector region of the PNP transistor  70 A,  70 B/the base region of the NPN transistor  71 A,  71 B. In the “Write-to-OFF” operation, the voltage on the cathode is driven up to near or perhaps even above the voltage on the anode. With the charge stored in the P-well, this pushes via capacitive coupling the voltage in the P-well to above the voltage on the anode. Thus when the gate of the Write assist PMOS transistor  76 A,  76 B is pulled low and the transistor is turned on, the charge stored in the P-well gets drained out through the PMOS transistor  76 A,  76 B to the anode which brings the voltage drop between the P-well and the anode to well below the NPN base-emitter turn-on voltage, V be . There is still the charge stored in the shallow N-well, the base region of the PNP transistor  70 A,  70 B/collector region of NPN transistor  37 A,  37 B, to contend with. But the P-well being discharged through the PMOS transistor  76 A,  76 B, the overall redistribution of voltage in the PNPN thyristor stack helps shorten the time needed for the shallow N-well to reach a voltage that keeps the thyristor “OFF”. 
     The memory cell  77  of  FIG. 7A  has two cross-coupled thyristors with shallow N-well of one thyristor connected to the P-well of the other. In this configuration one of the thyristors is always “ON” and the other thyristor is always “OFF”. The “lower” shallow N-well in the “ON” thyristor re-enforces the “lower” voltage of the P-well in the “OFF” thyristor and vise-versa. The Write operation to the memory cell actually involves doing two simultaneous thyristor writes—turning “ON” the “OFF” thyristor by putting the full access voltage across it, and at the same time turning “OFF” the “ON” thyristor by putting near 0V across it. In this example of a cross-coupled thyristor memory cell, the word line  74  is connected to the two cathodes and the BL/BL_N lines  73 A,  73 B to the respective anodes. The bit line  73 A,  73 B connected to “ON” thyristor anode is low and the bit line  73 B,  73 A connected to the “OFF” thyristor anode is high. This puts that maximum available turn-on voltage across the “OFF” thyristor and a low enough voltage drop (˜0.2-0.3V) across the “ON” thyristor to turn off its current flow. At the same time the voltage on the gates of the 2 PMOS Write assist transistor  76 A and  76 B is pulled low so that they can operate as described above. But with the addition of the cross-coupling of the two thyristors there are additional paths for the charge stored in the “OFF” thyristor to be removed—namely through the cross connections to the thyristor that is being turned on. The “OFF” thyristor is switched on almost the exact manner described above. The turn-on of the PMOS Write assist transistor  76 A,  76 B provides a path for the “low” P-well to be pulled up toward the anode voltage, turning on the NPN transistor  71 A,  71 B. This actually takes a little longer because this PMOS transistor now has to pull-up not only P-well in the “OFF” thyristor, but also the “low” shallow N-well in the “ON” thyristor through the cross connecting wire. But in doing this the charge stored in this shallow N-well is removed. Thus a small push-out in the time to turn on the “OFF” NPN transistor  71 A,  71 B is exchanged for a large improvement in the time to eliminate the stored charge in the base of the PNP transistor  70 B,  70 A of the “ON” thyristor. As the “OFF” NPN transistor  71 A,  71 B turns on, its collector current pulls down the base (shallow N-well) of the “OFF” PNP transistor  70 A,  70 B turning it on and then its collector current supplies the base of the NPN transistor  71 A,  71 B, re-enforcing the “ON” state (i.e.—the thyristor feedback loop). At the same time, the collector current of the newly turned on NPN transistor  71 A,  71 B also pulls down the “high” P-well of the “ON” thyristor, removing its stored charge and re-enforcing the effort to fully turn that thyristor “OFF”. So the cross-coupling of two thyristors solves the write-speed problem in the isolated single thyristor described above. 
     Capacitors, such as shown in  FIG. 7A , can also be used in Write assist operations. As illustrated by the capacitors  78 A and  78 B, capacitance is added between the Write assist line  75  and the shallow N-well region, the base of the PNP transistor  70 A,  70 B/collector of the NPN transistor  71 A,  71 B, of a thyristor. It is theoretically possible to utilize these capacitors by capacitively coupling a voltage shift into the shallow N-well from either the rising or falling transitions of the Write assist line  75 . A falling transition of the Write assist line  75  pushes the shallow N-well downward to a lower voltage by some percentage of the change in voltage on the Write assist line  75 . This percentage is determined by the ratio of the Write assist gate capacitance to the total capacitance seen by the shallow N-well. If the thyristor is in the “OFF” state, this downward coupling of the voltage on the shallow N-well increases the magnitude of the V be  of the PNP transistor  70 A,  70 B which can possibly help turn transistor on, resulting in the thyristor switching from the “OFF” to the “ON” state. 
     Conversely, if the thyristor is in the “ON” state, a rising transition on the Write assist line  75  couple a voltage upward into the shallow N-well by some percentage (as described above). In a Write operation where the voltage between the anode and the cathode of the “ON” thyristor has been reduced to near 0V or even to a negative voltage, and the current flow through the “ON” thyristor has ceased, the thyristor is not truly “OFF” because of the charge still stored in the shallow N-well region (base of the PNP transistor  70 A,  70 B) and the P-well region (base of the NPN transistor  71 A,  71 B). The effect of the upward coupling of the shallow N-well voltage after the DC current flow has been cut off helps remove some of the charge stored in the shallow N-well, thus helping truly turn “OFF” the thyristor. 
     Read and Write operations for the cross point array of  FIG. 7A  memory cells  77  are illustrated in  FIGS. 7B and 7C . The conditions for Standby are also implicitly illustrated. In these exemplary operations, a digital “0” and “1” is read from and written into respectively the first and third memory cells  77  in the first row of the array. Similar to the memory cell of  FIG. 7A ,  FIG. 7D  shows a cross-coupled thyristor memory cell for a cross point array. In this example, the anodes of the cross-coupled thyristors are connected to the word line  74  and the cathodes of the thyristors are respectively connected to the complementary bit lines  73 A,  73 B. Read and Write operations for the cross point array of  FIG. 7D  memory cells  77  are illustrated in  FIGS. 7E and 7F  with a digital “0” and “1” is read from and written into respectively the first and third memory cells  77  in the first row of the array. Again, it should be noted the voltage values shown represent a range of values and are used to best illustrate the described operations. Optimum values are dependent upon the particular characteristics of the devices of an integrated circuit. 
       FIG. 36  et seq. illustrate the structure and method of manufacture of a memory cell with Write assist MOS(FET)s. 
     h. SRAM Cell with Single Thyristor 
     A memory cell comprising a single thyristor can also be used.  FIGS. 8A and 8B  show some basic configurations of a single thyristor SRAM memory cell.  FIG. 8A  shows one embodiment of the present invention in which a memory cell having a thyristor represented by a PNP transistor  80  and NPN transistor  81  connected between an upper voltage supply line at V DD  and lower voltage supply line at V SS . A select NMOS(FET) transistor  82  connects the single bit line  83  to the base region of the PNP transistor  80 /collector region of the NPN transistor  81 . The gate of the NMOS transistor  82  is connected to the word line  84 .  FIG. 8B  shows another embodiment of the present invention. In this case the select transistor is eliminated so that the emitter region of the PNP transistor  80  is connected directly to the word line  84  and the emitter region of the NPN transistor  81  is connected to the bit line  83 . 
     i. Single Thyristor SRAM Cell with Write Assist Transistors 
     The single thyristor memory cell can be arranged in many configurations.  FIG. 9A  shows a single thyristor memory cell in which the thyristor anode is connected to the bit line and the cathode connected to the word line. To speed operations, two Write assist MOS transistors are connected to the thyristor. A PMOS Write assist transistor  96  has a source/drain connected to the anode of thyristor and a second source/drain connected to the collector region of the PNP transistor  90 /base region of the NPN transistor  91 . Its MOS transistor gate is connected to a Write assist line  95 B. An NMOS Write assist transistor  98  has a source/drain connected to the cathode and the collector region of the NPN transistor  91 /base region of the PNP transistor  90 . Its gate is connected to another Write assist line  95 A.  FIG. 9B  shows another single thyristor memory cell similar to that of  FIG. 9A . In this example the anode of the thyristor is connected to the word line and the cathode connected to the bit line. Two MOS transistors, a PMOS transistor and NMOS transistor, are connected to the thyristor to act as Write assist transistors as described for the  FIG. 9A  memory cell. 
     The memory cell of  FIG. 9D  shows a single thyristor with its anode connected to the bit line and the cathode connected to the word line. A single PMOS transistor  96  having a source/drain region connected to the anode and the second source/drain connected to the collector region of the PNP transistor  90 /base region of the NPN transistor  91  acts as a Write assist transistor. The gate of the PMOS transistor  96  is connected to a single Write assist line  95 .  FIG. 9H  shows another single thyristor memory cell which has its anode and cathode connections reversed compared to that of  FIG. 9D . 
     j. Reduced Power Standby Operation for Single Thyristor SRAM Cells 
     The operation of the single thyristor cell in a cross-point array is very similar to that of the earlier described cross-coupled thyristor memory cell. In Standby the single bit line  93 , BL, is maintained at its “pre-charge” level (0V if the BL connects to the cathode, the array “high” voltage (˜1.4V) if the BL connects to the anode of the thyristor.) The word line  94 , WL, is held an intermediate voltage which is sufficient to maintain the “ON” state of “ON” thyristors in the memory array but low enough to keep the current flow through the “ON” thyristors at a minimum level in the range of pico-amperes or 10&#39;s of pico-amperes per bit. This is tricky because the current drawn by an deselected row in deselect now depends on the number of “ON” cells contained in the row. With the cross-coupled thyristor cell, each cell always has one “ON” thyristor and one “OFF” thyristor, so the row Standby current is always the same which allows the necessary Standby current to be supplied with a pre-set constant current source as described earlier. 
     With a single thyristor cell, a memory cell with digital bit D=1 has a single “OFF” thyristor and a memory cell with digital bit D=0 has a single “ON” thyristor, so the total current drawn in Standby now depends on the data pattern stored in the row, so the constant current source described elsewhere doesn&#39;t work correctly. If a constant current source is used to maintain the row in Standby, the supplied constant current must be sufficiently large to supply the current required if ALL the cells in the row are “ON”, otherwise the current draw of all the cells would pull the voltage on the word line too far towards the bit line voltage and at least some cells on the row would be starved of current and eventually flip to the “OFF” state. But if the current source feeding the word line in Standby sends the current required when all the cells are “ON,” a problem occurs when many of the cells are actually off. Then the current supplied is larger than the current being pulled out by the “ON” cells and this causes the WL voltage to move away from the BL voltage, increasing the voltage drop across the cells in the row. This would not cause a Standby current draw problem because the total current would still be equal to the worst-case current when all cells are “ON”. The problem occurs when only a very few or perhaps none of the cells on the row are in the “ON” state. In this case the word line voltage is pulled all the way to the supply voltage that connects to the other side of the current source (presumably either the array “high” voltage of ˜1.4 v if the bit lines are connected to the memory cell cathodes, or to 0V if the bit lines are connect to the memory cell anodes). This can be tolerated for some period of time (perhaps 100&#39;s of micro-seconds, but very prolonged exposure to this full voltage drop across the cells eventually causes some of the “OFF” cells to flip to the “ON” state, disturbing the data in the memory array. 
     Hence with the single thyristor cell, if a current source is to control the Standby condition (i.e.—the word line WL voltage), then some sort of “clamp” must be provided on the wire driven by the current source that prevents the Standby voltage on the word lines from increasing (if the word line WL is connected to the cell&#39;s thyristor anode) or decreasing (if the word line WL is connected to the cell&#39;s thyristor cathode) beyond the minimum disturb voltage when there are few or no “ON” cells in the group of cells being held in Standby. This “clamp” consists of some cascode-type device (FET or BJT) that turns on when the Standby voltage crosses a designated threshold and then shunts away the excess current being provided by the Standby constant current source. For example,  FIG. 9C  shows a Standby current source arrangement for memory cells with single thyristors in a cross point array. In this embodiment the cathode of the cell thyristor is connected to the bit line  93  and the anode of the thyristor is connected to the word line  94 . The current source  100  is connected to the word line  94  through a PMOS transistor  99 . In another embodiment (not shown) the anode and cathode connections to the bit and word lines are reversed. Another more brute force approach uses an analog voltage regulator, such as an op-amp comparing the stand-by voltage to a reference voltage, and activating a shunt transistor (FET or BJT) when the reference voltage is exceeded. 
     k. Read and Write Operations in a Single Thyristor Cell 
     In Read operations the single thyristor cell operates identically to the cross-coupled thyristor cell. Before the word line WL is selected, the bit line BL is pre-charged (low if the word line is connected to the memory cell thyristor anode (e.g., see  FIGS. 9B and 9H ), high if the word line is connected to the thyristor cathode (e.g., see  FIGS. 9A and 9D )) and then left floating. Then the word line WL is selected. If the selected cell is “OFF”, the bit line BL is left floating at the pre-charge voltage. If the selected cell is “ON,” then the bit line BL is pulled toward the WL voltage thru the “ON” thyristor of the selected cell. See  FIGS. 9E and 9I  for some representative voltages for Read operations for memory arrays for  FIGS. 9D and 9H  cells respectively. 
     In Write operations, the Write-to-ON operation is identical to the bit line BL connected to the “OFF” thyristor in the cross-coupled thyristor memory cell. The bit line BL is driven to the opposite voltage from the word line WL (low if the word line is connected to the thyristor anode (e.g., see  FIGS. 9B and 9H ), high if the word line is connected to the thyristor cathode (e.g., see  FIGS. 9A and 9D )) and a PMOS Write assist transistor  96  (see  FIG. 9D ) is turned on by dropping the voltage on the Write-assist line. The cell is flipped to the ON state just as described above. 
     For the Write-to-OFF operation in a single thyristor cell, the useful cross-coupling effect to help remove charge of a cross-coupled thyristor cell is not present. Instead, the Write-to-ON and the Write-to-OFF operations are segregated into two separate voltage bias schemes. In the Write-to-OFF operation, the selected bit line BL is driven toward the normal word line WL select voltage (˜1.0 v if the word line is connected to the cell anodes, ˜0.2V if the word line is connected to the cell cathodes), just as for the cross-coupled thyristor cell arrays. The difference is that the selected word line WL is also driven toward an intermediate voltage near or at the Standby level. This applies a negative anode-cathode voltage across the cell to maximize the leakage currents trying to dissipate the charge stored in the shallow N-well, the base region of the PNP transistor  90 /collector region of the NPN transistor  91 , of the thyristor. The PMOS Write assist transistor  96  still works to remove charge from the P-well, the collector region of the PNP transistor  90 /base region of the NPN transistor  91 , of the thyristor as described above. 
     Therefore, a Write operation for a data pattern into a row is performed in two stages, one in which the word line WL is driven to the select voltage and the selected bit lines BLs are driven pre-charge levels to flip the “OFF” cells to “ON&#39;s”. Then the word line WL is switched to the fully OFF voltage (not just the Standby level) and the “other” selected bit lines BLs are driven to the opposite voltage from the pre-charge level to flip “ON” cells to “OFF&#39;s”. When writing the “ON&#39;s” on the selected bit lines, the non-selected bit lines (including the BLs to be subsequently written to “OFF”) need to be driven to near the WL level to prevent non-selected “OFF” cells on the row from being flipped to “ON”. Then when writing the “OFF&#39;s” on the “other” selected bit lines, the non-selected bit lines BLs (including the bit lines just written to “ON”) need to be driven to the pre-charge level to prevent the non-selected “ON” cells on the row from being flipped to the “OFF” state. In this second case, the word line WL voltage is set at some intermediate level near the Stand-by voltage so that there is enough current in the “ON” cell(s) to keep the cells alive when the Write assist gate is turned on. This is the cross-point memory “half-select” condition. 
       FIGS. 9F and 9G  show some representative voltages for the described two-stage Write operations for  FIG. 9D  memory cell arrays.  FIGS. 9J and 9K  show some representative voltages for the described two-stage Write operations for  FIG. 9H  memory cell arrays. 
     l. Reduced Power Write Operations for Single Thyristor SRAM Cell 
     The Write operation stage in which the “OFF” single thyristor memory cells are turned “ON” can benefit from the previously described reduced power Write operation for cross-coupled thyristor SRAM cells. In the Write operation description above the bit line connected to the “OFF” thyristor memory cell which is to be written to an “ON” state is held high to guarantee that the thyristor receives the maximum Write voltage during the duration of the Write operation pulse to turn the thyristor “ON”. Once the thyristor turns “ON”, the newly “ON” thyristor conducts the maximum “ON” current while the thyristor&#39;s bit line is held high. This increases the power consumption during the Write operation. More importantly, the word line must carry away the current from all the cells on the row so that this current can be very large. This can cause a significant voltage drop along the word line due to the electrical resistance of the line and may result in instability in the memory array. 
     This situation can be ameliorated by pre-charging the bit line to the “OFF” cell high prior to the Write pulse in a fashion similar to a Read operation described above as illustrated in  FIG. 9L .  FIG. 9M  is a representation of the voltages at various locations of the  FIG. 9L  memory cell. After the pre-charge, the pre-charge is turned off to allow the bit line to “float” until the memory cell flips state. At that point the current through the newly “ON” thyristor pulls the bit line low (similar to a Read operation). As the bit line voltage falls, the voltage drop across the thyristor decreases and the current falls toward the minimum holding current. Again, there are three benefits. First, the peak current is reduced because the bit line voltage starts to drop before the internal voltages in the memory cell turn the thyristor fully “ON”. Secondly, the now narrow current pulse (shown by the shape of I ThyL  in  FIG. 9M ) reduces the power dissipated in the Write operation. Thirdly, since from a statistical standpoint the various cells in the row being written flip at different times, the peak current in the word line is reduced and spread over a larger time interval, further minimizing the undesirable voltage drop along the word line. 
       FIG. 9N  illustrates the low power Write operation for a single thyristor cell in which the connections to the word and bit lines are reversed compared to that of  FIG. 9L . That is, the  FIG. 9N  memory cell has its thyristor anode connected to the word line and its cathode to the bit line.  FIG. 9O  is a representation of the voltages at various locations of the  FIG. 9N  memory cell. In this example, the bit line to the “OFF” cell is pre-charged low prior to the Write pulse (similar to a Read operation). Then the pre-charge path is turned off to allow the bit line to “float” until the memory cell flips state. At that point current through the newly “ON” thyristor pulls the bit line high (similar to a Read operation). As the bit line rises, the voltage drop across the thyristor decreases and the current drops toward the minimum holding current. The same benefits as described in the previous paragraph are achieved. First, the peak current is reduced because the bit line voltage starts to rise before the internal voltages in the memory cell turn the thyristor fully “ON”. Secondly, the now narrow current pulse (shown by the shape of I ThyL  in  FIG. 9O ) reduces the power dissipated in the Write operation. Thirdly, since from a statistical standpoint the various cells in the row being written flip at different times, the peak current in the word line is reduced and spread over a larger time interval, further minimizing the undesirable voltage drop along the word line. 
     II. Description of Integrated Circuit Structures and Manufacture 
     The memory cells described above can be manufactured using existing manufacturing technology. New semiconductor fabrication operations are not required, eliminating the expense and complication associated with new process development steps. As a point of departure,  FIG. 10  illustrates the basic steps in a CMOS process commonly used today.  FIG. 10  illustrates a well-known prior art complementary metal oxide semiconductor (CMOS) process employing a P-conductivity type substrate. In the discussion that follows, the process illustrated in  FIG. 10  is referred to herein as a “standard CMOS process.” The process begins with formation of an annular region of insulating material (not shown), typically silicon dioxide, to isolate regions of the semiconductor from each other prior to formation of the transistors in the substrate. (This process is discussed below in conjunction with  FIG. 11 .)  FIG. 10  illustrates one such isolated region. Then as shown in step  1 , a layer of silicon dioxide is grown on the P conductivity type silicon substrate providing a field oxide. Next, in step  2 , using conventional semiconductor manufacturing techniques, a layer of photoresist (not shown) is defined over the field oxide to enable removal of the field oxide in a location where an N-conductivity type well is to be formed. Then, using ion implantation, or other well-known technology, N-conductivity type impurity is implanted into the substrate to define the N-well. 
     As next shown by step  4 , another masking operation is performed to expose the substrate and the location where the NMOS transistor is to be formed. Gate oxide is then formed over this portion of the substrate as well as over the N-well. The appearance of the structure at this stage of the process is shown by step  5 . As shown in step  6 , a layer of polycrystalline silicon is deposited across the surface of the semiconductor. This layer will ultimately become the gates of the complementary MOS transistors. 
     Using another masking operation, the polysilicon layer and the gate oxide layer are etched to remove them in the locations where the sources and drains are to be formed for the complementary MOS transistors. The appearance of the structure is shown at step  7 . Next, in separate operations the N-conductivity type and P-conductivity type impurity used to form the sources and drains of the transistors are implanted into the substrate. This is shown at step  8 . 
     A dielectric layer such as silicon dioxide or silicon nitride is then deposited across the surface of the structure, and etched to expose the locations for ohmic contacts to the sources and drains. These operations are shown in steps  9  and  10 . As shown by step  11 , a metal layer, for example aluminum, or other electrically conductive material, is then deposited across the surface of the structure to provide electrical connections to the sources and drains, as well as the gates. Finally, as shown by step  12 , the metal is etched to provide the metal interconnections to the CMOS transistors. 
     a. Shallow Trench Isolation 
       FIG. 11  illustrates a well-known process for forming shallow trench oxide isolation regions in integrated circuits. These regions are shown in many of the figures here, for example, region  111  in  FIG. 13 . The process shown in  FIG. 11 , as well as similar variants, is commonly referred to as shallow trench isolation, and is preferred over the previous technique, commonly referred to as local oxidation of silicon (LOCOS). In the shallow trench process a thin layer of silicon dioxide formed on the upper surface of the silicon substrate and then a layer of silicon nitride over that. Photoresist is then used to define areas where the shallow trenches are to be formed. The combined layers of silicon oxide, silicon nitride and photoresist are illustrated as a single layer  170  in  FIG. 11A . At locations desired for isolation regions, trenches  171  are etched into the substrate as shown in  FIG. 11B . The trench is then oxidized to fill it with silicon dioxide before chemical-mechanical planarization. For SRAM cells in this invention, a slight modification of the conventional trench isolation process can be optionally used for better isolation between the buried n-wells serving as thyristor cathodes. Here a conformal oxide or nitride is deposited and anisotropically etched to form a spacer of silicon dioxide  172  on the sidewalls of the trenches. If desired, an optional P-conductivity type impurity  173  is implanted into the structure to provide buried P regions for improved isolation of the “tubs” formed between the trenches. This procedure is then followed by the conventional trench fill and planarization. 
     b. Bipolar Transistor 
     A collateral development from the thyristor-based SRAM cells described above is described next. The SRAM cells described below preferably employ bipolar transistors, usually embodied as thyristors formed by merged PNP and NPN bipolar transistors. For this reason we first describe a preferred implementation for a bipolar transistor and a process for making it.  FIG. 12  illustrates a semiconductor structure in cross-section for a bipolar transistor/thyristor manufactured using a CMOS process. The use of a CMOS process for manufacture of bipolar transistors is particularly advantageous to the preferred embodiments here. That approach enables a CMOS logic circuit to be formed on a common substrate with high speed bipolar SRAM, for example, providing a cache memory for such a logic circuit, without need for more complicated and expensive BiCMOS process technology. 
     The structure depicted in  FIG. 12  includes a P-type substrate  110  having shallow trench isolation regions  111 . The annular shallow trench isolation shown in cross-section in  FIG. 12  encircles a region  121  of the substrate within which the bipolar transistor is disposed. Not shown are other isolation regions at cross-sections in front of, and behind, the one illustrated in  FIG. 12  that isolate this region of the semiconductor from other portions of the chip. Isolation  121  defines the sides of a “tub” in the substrate, with the bottom of the tub being defined by a buried N-conductivity type well  112 . The buried N-type well  112  is preferably implanted into the substrate through both the silicon and the trench isolation  111 . The tub  121  electrically isolates active or passive devices formed therein. 
     Note that in the depicted embodiment an electrical contact  114  to the buried n-well  112  is made by extending the n-type layer beneath a portion of the isolation region  111  to join more highly doped N-type region  114  that provides electrical contact to well  112  from the surface of the semiconductor. In an alternate embodiment the contact  114  is placed inside the annular trench isolation region  111 . This contact to the buried N-well  112  could also be made within the isolation region  121 . 
     A strongly doped N-type region  115  provides the emitter of the vertical bipolar transistor, with region  113  providing the base and region  112  the collector. Metal contacts  119 ,  118 , and  120  provide electrical connections to the emitter, base, and collector, respectively. The regions  117  are unused (dummy) CMOS gate regions used to define separation spacing among the various components of the bipolar transistor shown in  FIG. 12 . The purpose of these dummy gate regions is discussed further in conjunction with the figures below. 
       FIG. 13  is a flowchart illustrating the steps in a process for fabricating the semiconductor structure illustrated in  FIG. 11 . The process begins with formation of trench isolation regions in step  125  as described above in conjunction with  FIG. 11 . Next, in step  126 , operations  1  and  2  as described with respect to  FIG. 10  are performed. Then the buried N-well  112  is masked and implanted as shown by step  127 . This step is shown in more detail in  FIG. 14A  illustrating the photoresist mask  114  and the N-well implant that will ultimately provide a word line (or other functionality). Then step  128  is performed to implant the P-type well  113 , as also illustrated in  FIG. 14B . 
     Next, as shown by step  129 , conventional CMOS processing to form a field effect transistor gate is performed as per steps  5 ,  6 , and  7  in  FIG. 10 . In the illustration of  FIG. 14C , the gate thus formed includes gate oxide  136 , a conductive electrode  137  (typically polycrystalline silicon) and nitride spacer  138 . The gate region includes the residual silicon nitride  138  that remains on the sidewalls of the gate electrode  137  after the nitride layer is masked and etched away in a well-known manner. 
     Importantly in our process, however, the FET gate and gate oxide formed by this process are not used later as a gate electrode. Instead the gate is used to provide a “hard mask” which is later used to enable a self-aligned doping of the bipolar transistor emitter and the transistor base contact. As shown in  FIG. 14C  a photoresist mask  134  is used to protect one side of the gate region while an emitter implant is introduced on the other side of the gate  137 . This N-conductivity type bipolar emitter  115  is implanted in the same operation as the N-type sources and drains for MOS transistors being formed on the same integrated circuit. Note that the hard mask formed by the gate  137  provides the left edge of the photoresist  134  to have wide tolerance in its positioning, i.e. this left edge can fall anywhere over the gate, yet still protect the base contact region  116  from receiving the N-type dopant of the emitter  115 . In the various embodiments described below a “dummy” gate electrode is used to define the spacing between various regions. It will be appreciated, however, that it is not necessary to actually form a gate electrode. Instead a silicon dioxide/silicon nitride sandwich structure can be used, or other layers. One advantage of using a gate electrode is that this electrode is being formed in the same process operations as gates for CMOS logic are being formed elsewhere on the integrated circuit. 
     Referring to  FIG. 14D , at step  131  of  FIG. 13 , the P-type base pick-up implant  116  is introduced. As with the emitter doping, the use of the dummy gate enables a large tolerance for the location of the right edge of photoresist  139 . The bipolar base connection is shown in  FIG. 14D  by base contact region  116 . Conventional CMOS processes are then used to provide metal electrical connections as needed among the various regions. Examples of these metal connections are illustrated below. 
     c. Lateral Bipolar Transistor 
     Instead of a vertical bipolar transistor,  FIG. 15  illustrates an embodiment of the invention in which a lateral NPN bipolar transistor is provided. This is achieved by using the same N-type implant by which the emitter  115  was formed in the process described immediately above, to also form the collector region  140  in  FIG. 15 . An electrical contact to the P-well base region is formed either outside the annular trench isolation as depicted, or internally to that region at a cross-section other than the one depicted in  FIG. 15 . Because they require more substrate surface area, lateral bipolar transistors have increased base width, and thus usually have poorer performance than vertical bipolar transistors. In some applications where the number of masking steps is important, however, lateral bipolar transistors can be advantageous. 
       FIG. 16  illustrates a process for forming the structure shown in  FIG. 15 . In the case of  FIG. 16 , and all subsequent figures here, the step of forming the trench isolation is not described, but is carried out before the process illustrated by the flowchart in the figure. In addition, the conventional steps of the CMOS process such as forming metal contacts, e.g. as illustrated by  FIG. 10 , are not described again here. 
     d. NMOS FET Merged Bipolar Transistor 
       FIGS. 17A-17D  illustrate a process for forming an NMOS transistor merged with a vertical NPN bipolar transistor. Such a structure can be used in many applications beyond SRAMs, for example, voltage controlled oscillators, phase lock loop circuits, amplifiers, etc. The merged PMOS and/or NMOS structures recognizes that when we use the standard CMOS NFET/PFET self-aligned source/drain processing sequence to define the surface connections to the bipolar junction transistors (P+ anode, N+ shallow N-well connection, P+ P-well connection), the resulting “un-intended FET” formed between the BJT terminals can be used advantageously to enhance the operation of the BJT circuit—in the situations described herein, in assisting the write operation of the thyristor as described above. 
     In the same manner as described above,  FIG. 17A  illustrates the implantation of a buried N-well which will become the NPN collector, and  FIG. 17B  the implantation of a shallow P-type well providing the NPN base. In  FIG. 17C , FET gate regions have been formed which function as masks for the implantation of the N-type impurity for the NMOS source and drain. As shown in  FIG. 17D , this N-type dopant also forms the emitter  163  and collector contact  161  for the vertical bipolar transistor. The dummy MOS gate  162  establishes the separation between the collector contact and the emitter. Regions  163  and  165  are the NMOS transistor source and drain, with a (functional) gate  164 . Implant  166  provides a connection to the P-type region forming the base of the vertical NPN transistor. The NPN collector  167  is provided by the buried N-well. 
     e. SRAM Cell with FET Select Transistors 
       FIGS. 18A and 18B  illustrate in cross-section the SRAM cell whose circuit schematic is shown in  FIG. 3B . The SRAM cell consists of two pairs of cross-coupled bipolar transistors with MOS selection transistors coupled to the word line. The structure shown in  FIG. 18A  is formed in one tub isolated by shallow trench isolation regions, while the structure shown in  FIG. 18B  is formed in an adjacent tub. Connections between the two tubs are provided by metal interconnections shown at the top of each of  FIGS. 18A and 18B  and indicated as commonly connected by the double headed arrows between them. In  FIG. 18A , a first layer of metal  182  provides a connection between the voltage supply V DD  and the PNP emitter regions  186 . Other separate regions of the first metal  182  provide a connection  188  between the transistor bases. The buried N-well region  184  provides a connection to the emitters of the NPN transistors. An electrical connection to this buried N-well is provided by a separate contact to that region  185  to the left of the figure where the buried well extends under the trench isolation region. 
     A second layer of metal  181  provides an electrical connection to the shallow N-well that provides the base of the NPN transistor  186  and one node of the NMOS selection transistor  187 . The second layer of metal  181  also provides another connection (in a cross section not depicted in  FIG. 18A or 18B ) between the base of the PNP transistors  188  and the other node of the NMOS selection transistor  189 . A third layer of metal  180  provides the bit line connections to the SRAM cell, with the bit line shown in  FIG. 18A  being coupled to one of the cross-coupled bipolar pairs, and the bit line shown in  FIG. 18B  being coupled to the other of the cross coupled bipolar pairs.  FIG. 19  is a top view of the SRAM cell illustrating its layout on an integrated circuit. 
       FIGS. 20A-20D  illustrate in more detail portions of the process for forming the SRAM cell shown in  FIGS. 18 and 19  above. As shown in  FIG. 20A , after formation of the buried N-well to provide a V SS  connection, the P-well is implanted through an opening in photoresist mask PR. Then, using another mask as shown by  FIG. 20B , an N-type implant is performed to create the shallow N-well. As shown in  FIG. 20C , after formation of dummy gate regions for the bipolar (left) side of the structure and functional gate regions for the FET (right) side of the structure, the NPN bipolar emitters, and the N-type sources and drains for the FETs are implanted in one operation with N-conductivity type impurity. Following that step, as shown in  FIG. 20D , appropriate masking is performed to enable implantation of the P-type dopants to form the other electrodes of the bipolar (thyristor) devices. This step also forms PFET sources and drains elsewhere on the integrated circuit. The result is a compact fast SRAM cell that employs FETs as select transistors. 
     f. SRAM Cell with Bipolar Select Transistors 
       FIGS. 21A and 21B  illustrate cross-sections of an SRAM cell in which bipolar transistors are used as select transistors. ( FIG. 3A  is a circuit schematic for this cell.) As with the figures above, the devices illustrated in the cross-section of  FIG. 21A  are formed in one tub and the devices illustrated in the cross-section of  FIG. 21B  are formed in an adjacent tub, with metal interconnections between the two. The left-hand portion of  FIGS. 21A and 21B  correspond to the bipolar structure described in  FIGS. 18A and 18B  (in which MOS transistors are used as select transistors). The right-hand portion of  FIGS. 21A and 21B  illustrate the use of bipolar selection transistors instead of the FETs illustrated in  FIGS. 18A and 18B . These bipolar select transistors correspond to those depicted in the circuit schematic of  FIG. 3A . In  FIGS. 21A and 21B , the select transistors are vertical NPN bipolar transistors. Emitter  200 , base  201 , and collector  202  form one of the NPN select transistors. A strongly doped N-conductivity type region is used to provide a connection to the buried N-well  202 , which also provides the bit line. Of course, vertical PNP bipolar transistors or lateral bipolar transistors could also be used as select transistors. 
     A top view of the SRAM cell in  FIGS. 21A and 21B  is shown in  FIG. 22 . The SRAM cell can be manufactured using the process flow operations described with respect to  FIG. 18  et seq. Particular masking and implantation steps are shown in more detail in  FIGS. 23A-23E . As shown in  FIG. 23A , after formation of shallow trench isolation regions, the deep buried N-wells are implanted. Then, as shown in  FIG. 23B , following appropriate masking operations, a shallow P-well implant is performed. Next, as shown in  FIG. 23C , the shallow N-well is implanted into only a portion of one of the shallow P-well regions, thus leaving room for a later contact to the P-well. As shown by  FIG. 23D , after formation of dummy FET gates and functional FET gates elsewhere on the integrated circuit, appropriate masking steps are performed and an N-type implant is used to dope the emitters of the vertical NPN bipolar transistors, as well as the sources and drains of the NMOS transistors elsewhere on the substrate. Then, as shown in  FIG. 23E , after another masking step, P-type dopant is implanted to form the P-type emitters the PNP bipolar transistors, as well as the sources and drains of the PMOS transistors situated elsewhere on the substrate. 
     e. SRAM Cell in Deep Well 
       FIG. 24  illustrates another embodiment of the invention in which an SRAM cell structure is formed using a deep N-well. To function more efficiently, a bipolar SRAM cell prefers a voltage higher than standard logic core voltage, as described earlier. One way to achieve this is to use a higher voltage as the SRAM power supply, for example, V DDIO  or a voltage derived from V DDIO , i.e. about 1.5-2.5 volts with present technology of 28 nanometer design rules. Another approach is illustrated in  FIG. 24 . In this approach the SRAM cell is formed in a deep P-well  222  in a deep N-well  221  in the substrate  220 . This allows the P-well  222  to be biased negatively. With bipolar select transistors, this approach allows the word line driver circuits to use core logic levels, e.g., voltage swings from 0 volts to 1.0 volts. A deep strongly doped region  223  is used to provide electrical contact to the deep N-well  221 . First introducing the deep N-well implant  221 , and then doping the P-well  222  form the structure illustrated in  FIG. 31 . Semiconductor processing as described above can then form the SRAM cell. In an appropriate stage of the process a heavily doped implant is used to provide the connection  223  to the deep N-well  221 . 
     f. Three Transistor SRAM Cell 
       FIG. 25  is a cross-section of a 3-transistor SRAM cell whose circuit schematic is shown in  FIG. 8A . This cell has cross-coupled NPN and PNP bipolar transistors which are illustrated as being in the left-hand portion of the figure. An MOS select transistor is shown on the right hand side of the illustration. The power supply V DD    230  is coupled to the emitter  231  of the PNP transistor, while the ground connection is coupled through a buried N-well  232  to the collector of the NPN transistor. The word line  233  is coupled to the gate of the MOS transistor, while the bit line  234  is coupled to an electrode of the MOS transistor. A connection  235  in the first layer metal couples the N-type base of the PNP transistor to the other electrode of the MOS transistor. A word line  236  for the SRAM cell next to the illustrated SRAM cell is shown on the right hand side of the figure. 
       FIG. 26  is a top view of the layout of the 3-transistor SRAM cell illustrated in  FIG. 25 . Regions illustrated in  FIG. 26  that correspond to regions in  FIG. 25  are given the same reference numerals as those in  FIG. 25 . 
     g. Four Transistor SRAM Cell 
       FIGS. 27A and 27B  are cross-sectional views at two locations of a 4-transistor SRAM cell. The structure is almost identical to that described with respect to  FIG. 18 . In contrast with that structure, however, the structure of  FIGS. 27A and 27B  use the buried N-well  240  to provide the word line, and requires only two layers of metal, instead of three. A circuit schematic for the 4-transistor cell is shown in  FIG. 4A . The bit line ( FIG. 27B ), and complementary bit line ( FIG. 27A ), are coupled to the emitters  242  of the PNP bipolar transistors. The arrows labeled “Ml” illustrate the use of the first layer metal to provide cross coupling between the base of the PNP transistor and the collector of the NPN transistor, and between the base of the NPN transistors and the collector of the PNP transistors.  FIG. 28  is a top view of the structure shown in  FIGS. 27A and 27B . 
     h. Four Transistor SRAM Cell with Write Assist FETs 
       FIGS. 29A and 29B  illustrate cross-sectional views of a 4-transistor SRAM cell with the additional write assist FETs  250  earlier described in conjunction with the circuit schematic shown in  FIG. 7 . The write assist FETs  250  (PMOS transistors  46 A and  46 B in  FIG. 7 ), when turned on, short the collector to the base of the lateral PNP bipolar transistors. The write assist capacitors  251  (capacitors  47 A and  47 B in  FIG. 7 ) described above are also illustrated in the figure.  FIG. 30  is a top view of the structure shown in  FIGS. 29A and 29B . 
     g. Two Transistor SRAM Cell 
       FIG. 31  is cross-sectional views of a two bipolar transistor memory cell with write assist FETs provided to short the bipolar transistors, as needed, for writing data. The cell circuit was discussed above with regard to  FIG. 9B . The upper PNP transistor in  FIG. 31  includes emitter  281 , base  282  and collector  283  with the gate (PFET)  284  able to short the emitter and collector. Similarly the lower NPN transistor includes collector  284 , base  285  and emitter  286 . The gate (NFET) can short the emitter and collector. Note that the shallow N-well cross-couples the PNP base  282  to the NPN collector  284 , while the P-well cross-couples the NPN base  285  to the PNP emitter  281 . The buried N-well  288  and its coupling  289  to the second layer of metal provides the bit line, while the word line is coupled to the PNP emitter. In an alternative embodiment with the word line on the bottom, the word line is coupled to the NPN collector. The structure shown in  FIG. 31  can be manufactured using the process described with regard to  FIG. 18 . 
     i. Manufacturing Process Details 
       FIG. 32  illustrates a detail of the manufacturing process used throughout the various embodiments described above. In particular, using CMOS technology, the gates for the MOS transistors can be used in the process in which bipolar transistors are formed. Using CMOS technology to define gate regions, instead of being used as gates, those regions can be used to define the spacing of the various bipolar transistor regions. In  FIG. 32  a first “dummy” gate  260  is used to set the spacing between N doped emitter  261  and a nearby P-doped region  262 . In a similar manner, another “dummy” gate  263  is used to define the spacing between a P doped contact region  264  for a well and the P-doped region  262 . This technique allows the use of CMOS process technology to form bipolar transistors, yet have those bipolar regions self-aligned to various other desired regions in the integrated circuit. 
       FIG. 33  is a graph illustrating typical dopant concentrations for the vertical bipolar transistors described with regard to the figures above. The graph shows the net doping of the various regions used to form the bipolar transistors in preferred embodiments of the invention. 
     This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.