Abstract:
An SRAM cell with gate tunneling load devices. The SRAM cell uses PFET wordline transistors and NFET cross-coupled transistors. The PFET wordline transistors are fully conductive during read operations, thus a full voltage level is passed through the PFET to the high node of the cell from the bitline. Tunnel current load devices maintain the high node of the cell at full voltage level during standby state.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to the field of static storage elements; more specifically, it relates to a static random access memory (SRAM) cell using tunnel current loading n-channel field effect transistors (NFETs).  
       BACKGROUND OF THE INVENTION  
       [0002]     Static storage devices such as SRAM cells use a write operation to store data in the cell and a read operation to sense the data stored in the cell. To ensure no read data disturbs occur, the write operation needs to write full power supply voltage levels to the SRAM cell so when the data is read it is not corrupted. In current SRAM cell designs, large P-channel field effect transistors (PFETs) are required to supply retention and write-recovery currents to maintain the full power supply voltage level. As integrated circuits become smaller and denser and as power consumption specifications for battery powered integrated circuits decrease, along with power supply voltages, the present SRAM cell designs are increasingly inefficient in both silicon area used and power consumed.  
         [0003]     Therefore, there is a need for writing full power supply voltage levels to SRAM cells that have reduced area requirements and low power consumption.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention provides an SRAM cell that can be written with full power supply voltage levels and has reduced area requirements and low power consumption compared to conventional SRAM cells by utilizing the tunneling leakage current of load devices to maintain the nodes of the SRAM cell at full power supply voltages levels.  
         [0005]     A first aspect of the present invention is an integrated circuit, comprising: a node; a PFET connected between the node and a data signal source, a gate of the PFET coupled to a control signal source; a first NFET connected between the node and ground; and a second NFET, a gate of the second NFET connected to a power supply, a source and a drain of the second NFET both connected to the node.  
         [0006]     A second aspect of the present invention is an integrated circuit, comprising: a first node and a second node; a first PFET connected between the first node and a first data signal source, a gate of the first PFET coupled to a control signal source; a second PFET connected between the second node and a second data signal source, a gate of the second PFET coupled to the control signal source; a first NFET connected between the first node and ground, a gate of the first NFET connected to the second node; a second NFET, a gate of the second NFET connected to a power supply, a source and a drain of the second NFET both connected to the first node; a third NFET connected between ground and the second node, a gate of the third NFET connected to the first node; and a fourth NFET, a gate of the fourth NFET connected to the power supply, a source and a drain of the fourth NFET both connected to the second node.  
         [0007]     A third aspect of the present invention is a method, comprising: providing an SRAM cell, the SRAM cell comprising: a first node and a second node; a first PFET connected between the first node and a true bitline, a gate of the first PFET coupled to a wordline; a second PFET connected between the second node and a complimentary bitline, a gate of the second PFET coupled to the wordline; a first NFET connected between the first node and ground, a gate of the first NFET connected to the second node; a second NFET, a gate of the second NFET connected to a power supply, a source and a drain of the second NFET both connected to the first node; a third NFET connected between ground and the second node, a gate of the third NFET connected to the first node; and a fourth NFET, a gate of the fourth NFET connected to the power supply, a source and a drain of the fourth NFET both connected to the second node. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a schematic circuit diagram of an SRAM cell according to the present invention;  
         [0010]      FIG. 2  is a read-cycle simulation of an SRAM cell according to the present invention;  
         [0011]      FIG. 3  is a plot of NFET gate current versus gate dielectric thickness for various gate voltages;  
         [0012]      FIG. 4  is a plot of NFET gate tunneling current as a function of temperature;  
         [0013]      FIG. 5  is a plot of the slope AN 1  (for an NFET) as a function of gate voltage;  
         [0014]      FIG. 6  is a plot of the magnitude of the intercept AN 2  (for an NFET) as a function of gate voltage; and  
         [0015]      FIG. 7  is a cross-sectional view through an NFET.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     For the purposes of the present invention the term gate current, tunneling leakage current and gate tunneling leakage current should be considered as equivalent terms. It should be understood that the structure of PFETs and NFETs used in the present invention in their simplest form comprise a gate electrode over a gate dielectric over a channel region in a semiconductor substrate with a source and a drain formed in the substrate on opposite sides of the channel region. However, more structurally complex PFETs and NFETs as known in the art may be used as well.  
         [0017]      FIG. 1  is a schematic circuit diagram of an SRAM cell according to the present invention. In  FIG. 1 , an SRAM cell  100  includes PFETs T 1  and T 2  and NFETs T 3 , T 4 , T 5  and T 6 . The source of PFET T 1  is coupled to bitline BLC (bitline complement), the drain of PFET T 1  is coupled to a node NC and the gate of PFET T 1  is coupled to a wordline WL. The source of PFET T 2  is coupled to bitline BLT (bitline true), the drain of PFET T 2  is coupled to a node NT and the gate of PFET T 2  is coupled to wordline WL. The source of NFET T 3  is coupled to GND (ground), the drain of NFET T 3  is coupled to node NC and the gate of NFET T 3  is coupled to node NT. The source of NFET T 4  is coupled to GND, the drain of NFET T 4  is coupled to node NT and the gate of NFET T 4  is coupled to node NC. The source and drain of NFET T 5  are coupled to node NC and the gate of NFET T 5  is coupled to VDD 1 . The source and drain of NFET T 6  are coupled to node NT and the gate of NFET T 6  is coupled to VDD 1 . Wordline WL carries a control signal often referred to a wordline signal, thus wordline WL may be considered a control signal source. Bitlines BLC and BLT carry data bit signals and may be considered data signal sources. Wordline WL, and bitlines BLC and BLT are supplied from a power supply VDD 2 . In a first example VDD 1  is equal to VDD 2 . In a second example VDD 1  is greater than VDD 2 . The term full POWER SUPPLY voltage swing when applied to read and write operations of SRAM cell  100  refers to a swing between VDD 1  and GND of node NC or node NT and a swing between VDD 2  and GND of the signal on bitline BLC or bitline BLT.  
         [0018]     To write a logical 1 to SRAM cell  100  wordline WL is turned on (at GND), turning PFETs T 1  and T 2  on, so with bitline BLT at GND and bitline BLC at VDD 2 , node NC rises to VDD 2  and node NT falls to GND.  
         [0019]     To write a logical 0 to SRAM cell  100  wordline WL is turned on (at GND), turning on PFETs T 1  and T 2 , so with bitline BLC at GND and bitline BLT at VDD 2 , node NC falls to GND and node NT rises to VDD 2 .  
         [0020]     Because of current leakage through NFET T 3  and T 4  respective nodes NC or NT will discharge over time and the voltage level on nodes NC or NT will drop. It is the node (NC or NT) at VDD 2  that is of concern for leakage current. If the voltage drops to a predetermined level below VDD 2 , read stability and read performance specifications may be compromised and data errors on read operations may occur.  
         [0021]     NFETS T 5  and T 6  supply retention current to respective nodes NC or NT to compensate for the leakage through NFETs T 3  and T 4  by keeping the HIGH node at VDD 1  and the LOW node is at GND.  
         [0022]     There are two types of gate tunneling current leakage, inversion tunneling current leakage and accumulation tunneling current leakage. Inversion tunneling current leakage occurs when the gate of an NFET is at VDD 2 . Accumulation tunneling current leakage occurs when the gate of an NFET is at GND. NFETs T 5  and T 6  are load devices operated in inversion mode in SRAM cell  100 .  
         [0023]     The write recovery operation is also of concern because the node NT or NC must be pulled to VDD 2  very quickly and that requires a substantial amount of current. In conventional SRAM cells NFETs T 5  and T 6  are PFET devices that pull the internal nodes NC or NT to VDD 2 . Generally, without NFETs T 5  and T 6 , the node (NT or NC) would discharge from VDD 2  due to current leakage through NFETs T 3  or T 4 .  
         [0024]     In the example of SRAM cell  100  storing a logical 1 (NT at GND NC at VDD 2 ), the retention current (which is a gate tunneling current) supplied by NFET T 5  (I TUNT5 ) should be about equal to or greater than the sub-threshold voltage leakage current through NFET T 3  (I SUBVTT3 ) plus the gate tunneling current through NFET T 4  (I TUNT4 ) minus the sub-threshold voltage leakage current through PFET T 1  (I SUBVTT1 ). It should be understood that I TUNT5  and I TUNT4  are inversion gate tunneling currents through NFETs T 5  and T 4  respectively and that I SUBVTT1  and I SUBVTT3  are sub-threshold voltage leakage currents through PFET T 1  and NFET T 3  respectively.  
         [0025]     In the example of SRAM cell  100  storing a logical 0 (NT at VDD 2 , NC at GND), the retention current (which is a gate tunneling current) supplied by NFET T 6  (I TUNT6 ) should be greater than the sub-threshold voltage leakage current through NFET T 4  (I SUBVTT4 ) plus the gate tunneling current through NFET T 3  (I TUNT3 ) minus the sub-threshold voltage leakage current through PFET T 2  (I SUBVTT2 ). Again, it should be understood that I TUNT6  and I TUNT3  are inversion gate tunneling currents through NFETs T 6  and T 3 , respectively and that I SUBVTT2  and I SUBVTT4  are sub-threshold voltage leakage currents through PFET T 2  and NFET T 4 , respectively.  
         [0026]     The amount of gate tunneling inversion current through NFET T 6  (or NFET T 5 ) is controlled by the value of VDD 1 , the gate dielectric thickness and the dielectric constant of the gate dielectric. When comparing gate dielectric thicknesses, electrically equivalent gate dielectric thicknesses are compared. The electrically equivalent gate dielectric thickness takes into account the different permittivity of different dielectric materials, because it is possible for a thin layer of a dielectric material with a high permittivity to have a higher electrically equivalent gate dielectric thickness than a physically thicker layer of a dielectric material with a lower permittivity. Since thermal silicon oxide is a traditional, well characterized and common dielectric material, gate dielectric thickness is often described in terms of thermal silicon oxide equivalent (T OXEQ ) thickness which is the physical thickness of the gate dielectric multiplied by the ratio of the permittivity of thermal silicon oxide divided by the permittivity of the material of the gate dielectric.  
         [0027]     In one example, the area of gate over channel region of NFETs T 5  and T 6  may be greater than the area of gate over channel region of NFETs T 3  and T 4  to allow for more current drive to maintain respective nodes NC or NT at VDD 2 . In one example, the T OXEQ  of PFETs T 1  and T 2  may be about the same as the T OXEQ  of NFETs T 5  and T 6  to take advantage of the fact that sub-threshold leakage through PFETs T 1  or T 2  will also help to maintain nodes NC or NT respectively at VDD 2 .  
         [0028]      FIG. 2  is a read-cycle simulation of an SRAM cell according to the present invention. Reference to  FIG. 1  during the following discussion will be helpful. In  FIG. 2 , the SRAM cell is holding a logical 0 (node NT at VDD 2  and node NC at GND). As wordline WL is turned on (transitions from high voltage to low voltage) node NT and bitline BLT remain at the full power supply voltage, node NC charges to about 10% of the power supply voltage and bitline BLC discharges to about 90% of the power supply-voltage. As wordline WL is turned off (transitions from low voltage to high voltage) node NT and bitline BLT remain at a full power supply voltage level, node NC discharges to GND and bitline BLC is pre-charged to a full power supply voltage level. Thus, operation of an SRAM cell according to the present invention is highly reliable in terms of read stability.  
         [0029]      FIGS. 3 through 6  and the discussion infra describe determination of NFET gate current (tunneling leakage) in amperes/um 2  as a function of temperature, T OXEQ  and gate voltage (V G ) and are useful in designing SRAM cell  100  (see  FIG. 1 ).  
         [0030]      FIG. 3  is a plot of NFET gate current versus gate dielectric thickness for various gate voltages. All curves were plotted for NFETs at 25° C. with the NFETs in inversion mode. In  FIG. 3 , curve  105  is for a gate voltage of 0.2 volts, curve  110  is for a gate voltage of 0.4 volts curve  115  is for a gate voltage of 0.6 volts, curve  120  is for a gate voltage of 0.8 volts and curve  125  is for a gate voltage of 1.0 volts. The gate dielectric thickness (T OXEQ ) has been measured electrically in  FIG. 3 .  FIG. 3  illustrates that gate current on a natural logarithmic scale is a linear function of T OXEQ  where the slope and intercept of the straight-line function are functions of the gate voltage. (Note, because the gate current is a log scale, the slopes of curves  105 ,  110 ,  125 ,  120  and  125  are parallel, but the slopes increase from curve  105  through  125 .)  FIG. 3  may be used, in a first example, to select appropriate T OXEQ  values for NFETs T 3 , T 4 , T 5  and T 6  (see  FIG. 1 ) when operated at the same voltages to ensure more gate tunneling leakage current through NFETs T 5  and T 6  than through NFETs T 3  and T 4 .  
         [0031]      FIG. 4  is a plot of NFET gate tunneling current as a function of temperature. From  FIG. 4 , the activation energy ΔH may be calculated to be 0.017 eV.  
         [0032]     Returning to  FIG. 3 , the equation for curves  105  through  125  may be written in the form of equation (1): 
 
 ln ( I   G )=( AN 1× T   OXEQ )+ AN 2  (1) 
    where: I G  is the gate tunneling leakage current in amperes/um 2 ,     AN 1  is the slope, which is itself a function of gate voltage (see  FIG. 5  and equation 2 infra),     AN 2  is the T OXEQ  intercept (the gate dielectric thickness axis of  FIG. 3 ), hereafter intercept, which is itself a function of gate voltage (see  FIG. 6  and equation 3 infra) and     T OXEQ  is the gate dielectric thickness in nm.     Equation (1) is for 25° C. only. A more general equation for any temperature is given by equation (4) described infra.    
 
         [0038]      FIG. 5  is a plot of the slope AN 1  (for an NFET) as a function of gate voltage. The equation for  FIG. 5  is: 
   AN 1=(0.673 ln ( V   G ))−9.917  (2)  
 where: V G  is the gate voltage in volts. 
 
         [0039]      FIG. 6  is a plot of the magnitude of the intercept AN 2  (for an NFET) as a function of gate voltage. The equation for AN 2  is: 
   AN 2=−9.685 e   (−1.159×VG)   (3)  
 where: V G  is the gate voltage in volts. 
 
         [0040]     Equation (4) is a more general version of equation (1) for any temperature: 
 
 ln ( I   G )=( AN 1× T   OXEQ )+ AN 2+{ ΔH [(1/ T 1)−(1/ T 2)]/ K}   (4) 
 
 where: K is Bolztmann&#39;s constant, 
    T 1  is 298° K. (25° C.), and     T 2  is the operating temperature of the NFET is an SRAM cell in ° K.    
 
         [0043]     Equation (4) reduces to equation (1) when T 2 =25° C.  
         [0044]      FIG. 7  is a cross-sectional view through an NFET. In  FIG. 7 , an NFET  130  includes a gate dielectric  135  formed on a top surface of a silicon substrate  140 , a gate electrode  145  formed over a channel region  150  in a P-well  155  in substrate  140  and a source  160  and a drain  165  formed on opposites sides of channel region  150 . NFET  130  is surrounded by shallow trench isolation (STI)  170 . Spacers  175  are formed on opposite sides of gate electrode  145 . The physical thickness of gate dielectric  135  is D 1 . Equation (4) may be used to determine a T OXEQ  based on a value of I G  for NFETS T 5  and T 6  (also T 3  and T 4 ) (see  FIG. 1 ) required to meet stability and performance specifications for SRAM cell  100  (see  FIG. 1 ). A value for D 1  may then be determined from the calculated T OXEQ  and the dielectric constant of dielectric layer  135 .  
         [0045]     Thus, the present invention provides an SRAM cell capable of writing full power supply voltage levels and also provides reduced area requirements and low power consumption.  
         [0046]     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.