Abstract:
A read and write assist and restore circuit for a memory device includes a first device, which is responsive to a potential on a bit line such that the potential on the bit line activates the first device. A second device is driven by the first device such that when the first device is activated, a change in the bit line potential is reinforced with positive feedback by the second device during a wordline active period to enable write-back of data lost as a result of threshold voltage fluctuations in memory cell transistors coupled to the bit line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to random access memory technology and architecture, and more particularly to tolerating large threshold voltage fluctuation in memory cells. 
     2. Description of the Related Art 
     Large threshold voltage (Vt) fluctuations due to random placement of dopant atoms in small geometry static random access memory (SRAM) cell metal oxide semiconductor field effect transistors (MOSFETs) significantly degrades SRAM array yields in functionality and performance. Presently, there are no known circuit solutions to address this limitation other than brute force reverse scaling of SRAM cell transistor geometries and cell size. One drawback of the reverse scaling approach is that it increases the production cost of the device by increasing die size significantly. Thus, making high performance processors more expensive. 
     Therefore, a need exists for a structure and method, which counters the negative effects threshold voltage fluctuation in memory cells. 
     SUMMARY OF THE INVENTION 
     A read and write assist and restore circuit for a memory device includes a first device, which is responsive to a potential on a bit line such that the potential on the bit line activates the first device. A second device is driven by the first device such that when the first device is activated, a change in the bit line potential is reinforced with positive feedback by the second device during a wordline active period to enable write-back of data lost as a result of threshold voltage fluctuations in memory cell transistors coupled to the bit line. 
     Another read and write assist and restore circuit for a memory device includes sensing device responsive to a potential on a bit line such that the potential on the bit line activates the sensing device, and a driver device driven by the sensing device such that when the sensing device is activated, a change in the bit line potential is reinforced with positive feedback by the second device during a wordline active period to enable write-back of data lost as a result of threshold voltage fluctuations in memory cell transistors coupled to the bit line. A virtual ground provides a higher potential than a global ground to the memory cell transistors to assist in write operations and during standby. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein: 
     FIG. 1 is a schematic diagram conceptually showing operation of a memory cell, during a read operation; 
     FIG. 2 is a schematic diagram showing a read and write assist and restore circuit in accordance with an illustrative embodiment of the present invention; 
     FIG. 3 is a timing diagram for a simulation of a prior art SRAM cell showing data loss; 
     FIGS. 4 and 5 are timing diagrams for a simulation of a circuit employing a read and write assist and restore circuit of the present invention; and 
     FIG. 6 is a schematic diagram showing a virtual ground enabling circuit in accordance with an illustrative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention captures data being read by a bit line (BL) from a memory cell, before the cell asymmetries due to Vt fluctuations upset the cell. The captured data is restored to the cell while the word line (WL) is still selected, enabling the cell to retain data that would be otherwise lost. 
     It should be understood that the present invention will be illustratively described in terms of an SRAM cell; however, the present invention is much broader and can be used with other memory storage devices, such as register files and DRAM or even other storage or capacitive elements that are affected by threshold voltage fluctuations. 
     Referring now to the drawings in which like numerals represent the same or similar elements and initially to FIG. 1, a half (½) cell  10  is illustratively shown to demonstrate the operation of the present invention. Cell  10  may be any type memory cell, e.g., SRAM, DRAM or other memory cell. In FIG. 1, cell  10  includes an SRAM cell having a plurality of MOSFETs. Cell  10  is preferably included in an SRAM memory circuit  12  in a memory array region. Although, memory cell  10  may be include as a single device, for example, on an application specific integrated circuit. In an array region, circuit  12  includes a plurality of cells  10 . 
     Half cell  10  shows three transistors, however a full cell may include six transistors. PFETs and NFETs may be switched as would be known to one skilled in the art. 
     The present invention uses devices having a threshold voltage Vt. A word line (WL) is included in the gate of device  16  and is activated with a supply voltage, Vdd, by the word line (WL). When the WL selects a row of cells, each cell begins pulling down (one of 2) precharged BLs with its cell read current Iread. 
     In a conventional SRAM, the cell storage node at ‘0’ rises to a voltage determined by a voltage divider between the precharged BL and the cell ground GND across cell transistors  16  and  18 . If the Vt of the cell access transistor,  16  is smaller due to a Vt fluctuation, and/or if the pull-down transistor  18  has a larger Vt due to random Vt variations, then the node at ‘0’ rises high enough to potentially trip an inverter (e.g., device  18  and  20  form an inverter on the half of the cell shown, a mirror image is included on the other half) formed by the transistors (not shown) on the other half of the cell. The positive feedback between the two cell inverters then ensures that the cell flips state and the data originally stored is lost. 
     The present invention uses the fact that the cell with severe Vt fluctuation takes a finite time to flip. This time will be referred to as Tdata. Tdata is usually long enough for the BL pair driven by the unstable cell to capture the data from the cell that is sinking read current (Iread) and developing a differential voltage signal across the BL pair. A more unstable cell is also faster (smaller Vt in access devices degrades cell stability and increases read current). Vt increases in the cell transistor  18  do not affect the node voltage at ‘0’ as much as transistor  16  does. In addition, Vt increases in transistor  18  do not occur as often as Vt decreases do, since the skew in the Vt distribution favors smaller Vt from short channel effects. 
     Referring to FIG. 2, a self-timed read and write assist and restore circuit  100  is shown in accordance with one illustrative embodiment of the present invention. Circuit  100  may be placed within an array of cells  110  and is coupled to a bit line BL. During operation, transistors  112  and  120  are enabled by a subarray select bar signal and a subarray select signal, respectively. Once activated, a faster cell (unstable cell with a large Vt fluctuation) in subarray  111  sinks BL charge, dropping the BL voltage. At the same time, this charge drop activates a transistor  114 . Transistor  114  has a gate connected to BL, which turns on the PFET  114  when the BL voltage drops below its threshold. PFET  114  in turn drives a device (NFET)  118 , which, on turning on begins to ‘assist’ a selected SRAM cell in developing signal on the BL. Transistors  114  and  118  are preferably low threshold voltage device (LVt) or may have their threshold voltages adjusted during fabrication to handle predetermined condition on BL. 
     Advantageously, feedback action is provided in the assist circuitry  100 , which is self-timed in that there is no external signal that is needed to initiate this feedback mechanism. The feedback is initiated by the voltage of the BL itself. The sensitivity of the feedback to the BL voltage is controlled by body bias (BB) on the well of the PFET  114  or the well of the PFET  114  and the NFET  118 . 
     In the embodiment shown, the read assist mechanism is gated by the sub-array select and subarray-select-bar signals also shown below so that the feedback can be invoked only when the subarray is selected. Note also that the feedback does not degrade the write operation. Instead, the feedback ‘assists’ the write in providing the write buffers with more drive, potentially relaxing their drive strength requirements. 
     Referring to FIG. 3, a conventional SRAM cell simulation is shown without read assist or the present invention. The circuit simulation demonstrates the read upset due to Vt mismatch in a conventional SRAM cell. According to FIG. 3, data is lost on the left bit line (left BL) as indicated in a region marker  202 . A CLK signal shows the read (R) and write (W) cycles. WL shows the word line signal. Left cell and right cell show the data signals for storage in a given half of a cell. Tdata is the time needed for a cell to flip. In the flip region, static noise margin (SNM)=0, and the voltage at node ‘0’ (see e.g., FIG. 1) is greater than Vcritical (Vcrit). Vcrit is the minimum node voltage needed to trip the cell state. As a result of the reduced bit line voltage a false read cycle was experienced as indicated by label  204 . 
     Referring now to FIG. 4, a simulation of an identical SRAM cell with read assist in accordance with the present invention is shown. The simulation of FIG. 4 demonstrates that the cell is able to hold data even in the presence of large Vt variations (e.g., despite Vnode exceeding Vcrit). As shown in FIG. 5, the symmetrical matched SRAM cell simulation with the addition of read assist circuitry shows that performance is maintained using read assist circuitry of the present invention. FIG. 5 shows bit line timing for right and left BL signals. 
     Write Assist using SRB 
     Referring again to FIG. 1, the noise margin for a write operation is smaller when Vt of the cell access transistor increases. This is because, the cell node at ‘0’ is being held low by the pull-down NFET  18 , which fights the pull-up device  20  of the write driver that is trying to write a ‘1’ to that node. If the cell access transistor Vt is large enough, then there will be enough of a voltage drop across  16  to inhibit the write driver  20  from raising the voltage of the node at ‘0’ making it harder to write data into the cell. 
     However, on keeping the cell ground voltage(VGND) higher than ground (GND) during a write operation, the cell pull-down NFET  18  is much weaker because its gate-source voltage is smaller, making it easier for the write driver  20  to flip the cell, large Vt variation in the cell access transistors notwithstanding. 
     The write assist operation may be implemented using a technique disclosed with reference to FIG. 6 by not activating a SRB READ signal for write operations and during standby. This ensures that the VGND stays at Vt above ground during writes and during standby. 
     Referring to FIG. 6, a schematic diagram provides an illustrative model circuit to explain VGND for write operations in accordance with the present invention. 
     In a logic portion  304  of circuit  300 , a virtual ground line VGND connects to VGND of array  302 . VGND is coupled to global ground GND via one or more transistors  334  and  336 . Transistor  334  may include an NFET which has its gate controlled by an SRB (self-reverse bias) signal. Transistor  336  may include a PFET which has its gate controlled by an SLP_mode (“sleep” mode) signal. 
     During idle mode, nodes  322  (and  323 ) store a one or a zero. In conventional systems, leakage through NFETs  314  and  316  to GND caused charge (ones) from storage nodes  322  (and  323 ) to discharge. By employing VGND at a value greater than GND, leakage is reduced. Transistor  336  acts as a diode in pinning the VGND at a voltage (e.g., Vt above GND). Hence, charge that builds up in VGND during idle mode increases the potential of VGND. When the VGND potential exceeds a threshold value, charge leaks through to GND, through device  336  thereby pinning the voltage, which can build up on VGND. Cell  110  includes transistors  310 - 320  for operations of the SRAM cell as described above (see e.g., FIG. 1) with the assist circuitry of FIG. 2 in accordance with the present invention. Note BLB is bit line bar. 
     Having described preferred embodiments of self-timed read and write assist and restore circuit (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.