Patent Publication Number: US-8125840-B2

Title: Reference level generation with offset compensation for sense amplifier

Description:
BACKGROUND 
     The present invention relates generally to memory sense amplifiers, and more specifically to reference level generation with offset compensation for a sense amplifier. 
     Contemporary memory access circuits rely on differential sense amplifiers to read small signals generated by a selected memory cell from a memory device such as a dynamic random access memory (DRAM) device. These sense amplifiers typically use transistors configured in a cross-coupled fashion to amplify a small voltage differential created by the cell. This cross-coupled configuration of the transistors is used to tolerate global variation in device threshold by relying on device matching of the transistors as opposed to the absolute threshold voltages of the transistors. As semiconductor device technology continues to evolve towards providing smaller device sizes and more devices per integrated circuit (IC) (and thus smaller voltages utilized within the circuits within the IC), there is an increase in local device mismatch caused by random variation, including random dopant fluctuation and line edge roughness. Consequently, the increase in device mismatch requires more of a signal from the memory cell to reliably overcome the device mismatch, resulting in larger and slower memories. 
     SUMMARY 
     In one embodiment, there is a circuit that comprises a first inverter device having an input node and an output node and a second inverter device having an input node and an output node. A gated shunt device is configured to connect and disconnect the input node of the first inverter device to the output node of the first inverter device and the input node of the second inverter device to the output node of the second inverter device. A cross-coupling switch is configured to enable and disable a connection of the input node of the first inverter device to the output node of the second inverter device and the input node of the second inverter device to the output node of the first inverter device. The first inverter device, second inverter device, gated shunt device and cross-coupling switch are operatively coupled to perform one of a pre-charge time operation and sense time operation on a bit line having a bit line true and bit line complement connected to a storage cell from a memory device. 
     In a second embodiment, there is a sense amplifier that comprises a first inverter device having an input node and an output node and a second inverter device having an input node and an output node. A gated shunt device is configured to connect and disconnect the input node of the first inverter device to the output node of the first inverter device and the input node of the second inverter device to the output node of the second inverter device. A cross-coupling switch is configured to enable and disable a connection of the input node of the first inverter device to the output node of the second inverter device and the input node of the second inverter device to the output node of the first inverter device. The first inverter device, second inverter device, gated shunt device and cross-coupling switch are operatively coupled to perform one of a pre-charge time operation and sense time operation on a bit line having a bit line true and bit line complement connected to a storage cell from a dynamic random access memory (DRAM) array of storage cells. During the pre-charge time operation the gated shunt device is gated on and the cross-coupling switch is disabled, while the gated shunt device is gated off and the cross-coupling switch is enabled during the sense time operation. 
     In a third embodiment, there is a method for sensing voltage on a bit line having a bit line true and bit line complement connected to a storage cell from a memory device. In this embodiment, the method comprises pre-charging the bit line true and bit line complement; generating an arbitrary mid-level voltage reference to charge the bit line true and bit line complement while simultaneously generating an offset for the bit line true and bit line complement; activating a word line in the storage cell to connect to the bit line in response to generating the arbitrary mid-level voltage reference and the offset; developing a differential signal on the bit line true and bit line complement in response to the storage cell connecting to the bit line; and amplifying the differential signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional sense amplifier; 
         FIG. 2  shows a simulation of the operation of the conventional sense amplifier shown in  FIG. 1 ; 
         FIG. 3  shows a simulation of the operation of the conventional sense amplifier shown in  FIG. 1  when presented with device and voltage threshold mismatch; 
         FIG. 4  shows a circuit of a sense amplifier according to one embodiment of the present invention; 
         FIG. 5  shows a simulation of the operation of the sense amplifier shown in  FIG. 4  according to one embodiment of the present invention; and 
         FIG. 6  shows a simulation of the operation of the sense amplifier shown in  FIG. 4  as used to reliably read a high level from a storage cell in a dynamic random access memory (DRAM) array of storage cells according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is a schematic circuit diagram of a conventional sense amplifier  100  that is in a cross-coupled configuration. In  FIG. 1 , a P-type field effect transistor (PFET) T 1  and an N-type field effect transistor (NFET) T 4  form one gated inverter, while a PFET T 0  and an NFET (T 3 ) form another gated inverter (hereinafter referred to as “inverter(s)”). The inverters connect to a storage cell  110  from a dynamic random access memory (DRAM) array of storage cells. Typically, a DRAM array may contain hundreds or thousands of storage cells, where each cell includes a capacitor for storing a voltage value that represents a logical binary “0” or “1” value and a transistor that switches the voltage value of the storage cell onto a bit line signal line. As shown in  FIG. 1 , the inverters connect to the storage cell via a bit line lbl that includes a pair of bit lines that provide the “true” and “complement” states of the logical binary value stored in storage cell  110  that are selected for reading or writing. Bit line true and bit line complement are represented in  FIG. 1  by signal line bt and signal line bc. Note that in a typical DRAM array, there are an equal number of storage cells coupled to bit line bt and bit be to balance parasitic capacitances, however, for ease of illustration and explanation, only one cell is illustrated in  FIGS. 1 and 4 . As shown in  FIG. 1 , bit line bt is inputted into the inverter formed by PFET T 0  and NFET T 3 , and bit line be is inputted into the inverter formed by PFET T 1  and NFET T 4 . Both bit line bt and bit line be connect to ground via a capacitor, capacitor true CT and capacitor complement CC, respectively. Capacitor true CT and capacitor complement CC represent the parasitic capacitance of the wiring and cells coupled to the bit line bt and bit line be, respectively. 
     The inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4  are cross-coupled because the output of one inverter is coupled to the input of the other inverter, while the output of the other inverter is coupled to the input of the first inverter. In  FIG. 1 , the inverter formed by PFET T 0  and NFET T 3  has its output coupled to the input of the inverter formed by PFET T 1  and NFET T 4  along the be line, while the inverter formed by PFET T 1  and NFET T 4  has its output coupled to the input of the inverter formed by PFET T 0  and NFET T 3  along the bt line. 
     Before the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4 , can receive data from storage cell  110  along bit line lbl, the bit lines bt and bc are pre-charged or pre-conditioned to a “ready” state after initially being in a “stand-by” state waiting for a command. In  FIG. 1 , a pre-charger formed by NFET T 7 , NFET T 20  and NFET T 8  receives a voltage reference vrefx as controlled by an equalize phase signal eqp. In one embodiment, when the eqp signal is “high”, then NFET T 7 , NFET T 20  and NFET T 8  are set to voltage reference vrefx, which in turn is provided to bit lines bt and bc. After the bit lines bt and bc have been pre-charged to the voltage reference vrefx, then the bit lines will be released from the voltage reference when the eqp signal turns “low”. 
     When the bit lines bt and bc are released from the state of pre-charging to the voltage reference vrefx, then the inverters formed by PFET T 0  and NFET T 3  and PFET T 1 , and NFET T 4  are connected to storage cell  110  along bit line lbl and a signal is applied. In particular, a word line wl in storage cell  110  is activated and bit line lbl connects to the cell through one of the bt bit line or bc bit line. Once connected there is a charge-sharing that occurs with the storage cell  110  and one of the bt bit line or bc bit line via capacitors CT and CC, respectively. A result of this charge sharing is that the charge in the storage cell and the charge in one of the bt bit line or bc bit line will change, leaving a voltage differential with the other bit line not taking part in the charge sharing. 
     It is this voltage differential that sense amplifier  100  will amplify. In particular, a pair of set devices is used to amplify the voltage differential formed between bt bit line and bc bit line. As shown in  FIG. 1 , one set device is a psetn device formed by PFET T 2  and PFET T 6  and the second set device is an nsetp device formed by NFET T 21  and NFET T 22 . The psetn device formed by PFET T 2  and PFET T 6  which is coupled at their gates is coupled to power supply Vdd, while the nsetp device formed by NFET T 21  and NFET T 22  is also coupled at their gates and is coupled to ground. Enabling the psetn device and the nsetp device causes the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4  to amplify bt bit line and bc bit line. 
       FIG. 2  shows a simulation  200  of the operation of the conventional sense amplifier  100  shown in  FIG. 1 . In simulation  200 , line  210  represents the pre-charge time operation of bt bit line and bc bit line by voltage reference vrefx, line  220  represents the voltage that is stored in storage cell  110  and line  230  represents the word line wl in the cell. As shown in  FIG. 2 , once the word line wl is activated (goes “high” at around 160 mV) charge sharing takes place between bt bit line and bc bit line. In this simulation, there is a charge sharing between bt bit line  240  and the voltage of the storage cell as represented by line  220 . In particular, simulation  200  shows that the voltage of bt bit line decreases while the charge of the storage cell  220  increases. Because there is no charge sharing with bc bit line, its voltage level remains the same. As a result, there is a voltage differential between bt bit line and bc bit line. At around time 2.8 nanoseconds (ns) the set devices (nsetp and psetn) are turned on to amplify the voltage differential between bt bit line and bc bit line. Consequently, the voltages of bt bit line and bc bit line are moved in the direction that further illustrates the amplification of this voltage differential. 
     As mentioned above, local device mismatch caused by random variation, including random dopant fluctuation and line edge roughness has become an ever increasing problem as semiconductor device technology continues to evolve towards providing smaller device sizes and more devices per integrated circuit (IC).  FIG. 3  shows a simulation  300  of the operation of a conventional sense amplifier like the one shown in  FIG. 1  when presented with device and voltage threshold mismatch. Like simulation  200  shown in  FIG. 2 , simulation  300  uses like reference elements to represent like elements except in  FIG. 3  the like elements are preceded by a “3” as opposed to a “2”. As shown in  FIG. 3 , once the word line wl is activated charge sharing takes place between bt bit line and bc bit line. Like  FIG. 2 , there is a charge sharing between bt bit line  340  and the voltage of the storage cell as represented by line  320 . Because there is no charge sharing with bc bit line  350 , its voltage level remains the same. As a result, there is a voltage differential between bt bit line  340  and bc bit line  350 . The set devices (nsetp and psetn) are turned at around time 2.8 ns to amplify the voltage differential between bt bit line and bc bit line. But, because the device and voltage threshold mismatch is more prominent in this example, voltage lines for bt bit line  340  and bc bit line  350  move in opposite directions causing their paths to cross instead of diverge. The crossing of paths represent that the inverters have chosen the wrong states and this is an indication that the sense amplifier is not operating properly. 
     Embodiments of the present invention have addressed this problem associated with local device mismatch caused by random variation by establishing a reference level which adjusts for device and voltage threshold mismatch.  FIG. 4  shows a circuit  400  of a sense amplifier according to one embodiment of the present invention. In  FIG. 4 , PFET T 1  and NFET T 4  form one inverter, while PFET T 0  and NFET T 3  form another inverter. The inverters connect to a storage cell  410  from a DRAM array of storage cells. Although the description that follows is directed to using the sense amplifier of  FIG. 4  with a DRAM array, those skilled in the art will recognize that embodiments of the sense amplifier are also suited for use with other memory devices such as for example a static RAM (SRAM) device. 
     As shown in  FIG. 4 , the inverters connect to the storage cell via a bit line lbl that includes a pair of bit lines, bit line true bt and bit line complement bc. As shown in  FIG. 4 , bit line bt is inputted into the inverter formed by PFET T 0  and NFET T 3 , and bit line be is inputted into the inverter formed by PFET T 1  and NFET T 4 . Both bit line bt and bit line be connect to ground via a capacitor, capacitor true CT and capacitor complement CC, respectively. Capacitor true CT and capacitor complement CC represent the parasitic capacitance of the wiring and cells coupled to the bit line bt and bit line be, respectively. Note that these capacitances are placed on the inputs to the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4 , to hold the offset compensation values on the inputs of the inverters. If the capacitances were placed on the outputs from the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4 , then this configuration would render sense amplifier  400  more sensitive to offset than the circuit shown in the prior art of  FIG. 1 . 
     A gated shunt device  420  formed from NFET T 17  and NFET T 18  is configured to connect the input node of the inverter device formed from PFET T 0  and NFET T 3  to its output and similarly to connect the input node of the inverter device formed from PFET T 1  and NFET T 4  to its output. In particular,  FIG. 4  shows that NFET T 17  connects the input node bt of inverter PFET T 0  and NFET T 3  to its output node sc, and that NFET T 18  connects the input node be of inverter PFET T 1  and NFET T 4  to its output node st. 
       FIG. 4  shows that the sense amplifier  400  further includes a cross-coupling switch  430  that is configured to enable and disable a connection of the input node of the inverter device formed by PFET T 0  and NFET T 3  to the output node of the inverter device formed by PFET T 1  and NFET T 4  and the input node of the inverter device formed by PFET T 1  and NFET T 4  to the output node of the inverter device formed by PFET T 0  and NFET T 3 . In one embodiment as shown in  FIG. 4 , the cross-coupling switch  430  comprises a first NFET/PFET transmission gate formed from PFET T 19  and NFET T 11  and a second NFET/PFET transmission gate formed from PFET T 10  and NFET T 12 . In this embodiment, the NFET/PFET transmission gate formed from PFET T 19  and NFET T 11  and the NFET/PFET transmission gate formed from PFET T 10  and NFET T 12  each has complementary enable and disable signals to enable and disable the cross-coupling of the inverters. As shown in  FIG. 4 , the transmission gate formed from PFET T 19  and NFET T 11  couples the output st from the inverter formed by PFET T 1  and NFET T 4  to the input bt of the inverter formed by PFET T 0  and NFET T 3 . Similarly, the transmission gate formed from PFET T 10  and NFET T 12  couples the output sc from the inverter formed by PFET T 0  and NFET T 3  to the input bc of the inverter formed by PFET T 1  and NFET T 4 . 
     Note that the cross-coupling switch  430  formed from PFET T 19  and NFET T 11  and PFET T 10  and NFET T 12  are configured to ensure that the parasitic capacitances represented by capacitor true CT and capacitor complement CC are placed on the inputs to the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4 , to hold the offset compensation values on the inputs of the inverters. If the capacitances were placed on the outputs from the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4 , then enabling the cross-coupling switch  430  would result in the offset compensation values being placed on the wrong input and lead to a negative compensation. 
       FIG. 4  further shows that the sense amplifier  400  includes a pair of set devices that as explained below are used during various phases of operation of the amplifier. One set device is a psetn device formed by PFET T 2  and PFET T 6  and the second set device is an nset device formed by NFET T 21  and NFET T 22 . The psetn device formed by PFET T 2  and PFET T 6 , which is coupled at their gates, is coupled to power supply Vdd; while the nset device formed by NFET T 21  and NFET T 22  is also coupled at their gates and is coupled to ground gnd. 
     The sense amplifier  400  of  FIG. 4  further includes a pre-charger that as explained below is used during stand-by and pre-charge phases of operation of the amplifier. The pre-charger in  FIG. 4  is formed from NFET T 7  and NFET T 8  and is controlled by an equalize phase signal eqp. One node of NFET T 7  is coupled to bit line be, while its other node goes to ground. Similarly, NFET T 8  is coupled to bit line bt, while its other node goes to ground. 
     In operation, sense amplifier  400  is configured to perform four phases of operation. The four phases of operation are referred to herein as the stand-by time operation, pre-charge time operation, signal time operation and amplify time operation. As used herein, a sense time operation collectively describes the signal time operation and the amplify time operation. 
     In the stand-by state of time operation, bit lines bt and be are waiting for a command to initiate the pre-charge time operation that puts the bit lines in the ready state. In particular, equalize phase signal eqp is high during the stand-by state, which causes NFET T 7  and NFET T 8  to turn on and hold bit lines bt and be at ground. When phase signal eqp goes low, NFET T 7  and NFET T 8  turn off which causes bit lines bt and be to float. 
     The pre-charge time phase of operation that puts the bit lines in the ready state is initiated when the psetn device formed from PFET T 2  and PFET T 6  and the nset device formed from NFET T 21  and NFET T 22  are turned on. The psetn device is turned on when the psetn signal is low and nset device is turned on when the nset signal is high. In addition to turning on the psetn device and the nset device during the pre-charge time operation, the gated shunt device  420  formed from NFET T 17  and NFET T 18  is enabled by the pre signal and the cross-coupling switch  430  formed from PFET T 19  and NFET T 11  and PFET T 10  and NFET T 12  is disabled by both the pre signal and pre_b signal. 
     This causes the connection of the input node of the inverter device formed from PFET T 0  and NFET T 3  to its output and similarly the connection of the input node of the inverter device formed from PFET T 1  and NFET T 4  to its output. Essentially, enabling the gated shunt device  420  puts each inverter at its trip point, resulting in the generation of an offset voltage applied to the gates of the inverters. This results in the offset voltage being applied to the bit lines bt and be. The offset voltage is basically an arbitrary reference level that provides offset for bit lines bt and be that compensates for any mismatch. The offset voltage compensates for any device mismatch and any voltage threshold mismatch that exists with the inverters of the sense amplifier  400 . In one embodiment, the reference level voltage is an arbitrary mid-level voltage reference that has a value that is between Vdd and ground. After the bit lines bt and be have been pre-charged to the arbitrary reference level provided by the offset voltage, then the bit lines are released from the offset by disabling the psetn device and the nset device, as well as disabling the gated shunt device  420 . 
     When the bit lines bt and be are released from the state of pre-charging to the offset voltage, then the sense amplifier  400  is ready to perform the sense time operation which includes first the signal time phase of operation and then the amplify time phase of operation. During the sense time operation, the gated shunt device  420  remains gated off and the cross-coupling switch  430  formed from PFET T 19  and NFET T 11  and PFET T 10  and NFET T 12  is enabled by both the pre signal and pre_b signal. 
     During the signal time phase of operation, the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4  are connected to storage cell  410  by activating bit line lbl via cross-coupling switch  430 . In particular, a word line wl in storage cell  410  is activated and bit line lbl connects to the cell through one of the bt bit line or bc bit line. Once connected there is a charge-sharing that occurs with the storage cell  410  and one of the bt bit line or bc bit line via capacitors CT and CC, respectively. A result of this charge sharing is that the charge in the storage cell and the charge in one of the bt bit line or bc bit line will change, leaving a voltage differential with the other bit line not taking part in the charge sharing. 
     It is this voltage differential that sense amplifier  400  will amplify during the amplify time operation. In particular, during the amplify time operation, the cross-coupling switch  430  remains enabled and the psetn device formed by PFET T 2  and PFET T 6  and the nset device formed by NFET T 21  and NFET T 22  are turned on by a low psetn signal and a high nset signal to the psetn device and the nset device, respectively. Enabling the psetn device and the nsetp device causes the inverters formed by PFET T 0  and NFET T 3 , and PFET T 1  and NFET T 4 , to amplify bt bit line and bc bit line. 
     In this embodiment, the use of the gated shunt device  420 , the cross-coupling switch  430 , psetn device and nset device, pre-charger with the inverters formed by PFET T 1  and NFET T 4  and PFET T 0  and NFET T 3  enable the sense amplifier  400  to create a reference level for differential sensing in a way that compensates for random threshold mismatch. In essence, the offset compensation is achieved through a creative use of switches (i.e., gated shunt device  420  and the cross-coupling switch  430 ) to configure a cross-coupling sense amplifier that perform four phases of operation; the stand-by time phase of operation, pre-charge time phase of operation, signal time phase of operation and amplify time phase of operation. During the pre-charge time phase of operation, an offset compensation is provided by disabling the cross-coupling switch  430  and enabling the gated shunt device  420  along with the psetn device and nset device. This allows the inverters formed by PFET T 1  and NFET T 4 , and PFET T 0  and NFET T 3 , to have outputs shorted to inputs. Thus, when the psetn device and nset device apply a short pulse to the inverter pairs, each inverter will independently reach a trip point that is dependent on the inverter&#39;s specific device thresholds. With the proper connection of the bit line load, such that the offset voltage is maintained on the gate of the inputs of the FETs that created the offset (not the drain), the bit line lbl is also pre-charged to the same potential. This trip point not only provides offset compensation but also provides the mid-level reference required to enable differential sensing of the storage cell  410  while performing the sense time operation which collectively includes signal time phase of operation and the amplify time of operation. With both sides of the inverters that form the sense amplifier pre-charged to their individual trip points, a small shift in bit line potential created by reading the cell can be reliably amplified, even in the presence of significant device mismatch. During both the signal time phase of operation and amplify time phase of operation, the gated shunt device  420  is disabled and the cross-coupling switch  430  is enabled. This allows the sense amplifier  400  to generate a voltage differential signal on the bt bit line and bc bit line during the signal time phase of operation. Then, the psetn device and nset device are enabled which results in the amplification of the voltage differential signal during the amplify time operation. 
       FIG. 5  shows a simulation  500  of the operation of the sense amplifier  400  shown in  FIG. 4  according to one embodiment of the present invention. In simulation  500 , lines  510  and  520  represent the pre-charge time operation of bt bit line and bc bit line by the arbitrary reference level that simultaneously provides an offset for the bit lines that is used to compensate for any mismatch. Line  530  represents the voltage that is stored in storage cell  410  and line  540  represents the word line wl in the cell. As shown in  FIG. 5 , once the word line wl is activated charge sharing takes place between bt bit line and bc bit line and the storage cell. In this simulation, there is a charge sharing between bt bit line  520  and the voltage of the storage cell as represented by line  530 . In particular, simulation  500  shows that the voltage of bt bit line decreases while the charge of the storage cell  410  increases. Because there is no charge sharing with bc bit line, its voltage level remains the same. At around time 2.8 ns the set devices (nsetp and psetn) are turned on to amplify the voltage differential between bt bit line and bc bit line. The voltages of bt bit line and bc bit line are moved in the direction that further illustrates the amplification of this voltage differential. 
       FIG. 6  shows a simulation  600  of the operation of the sense amplifier shown in  FIG. 4  that indicates its capability to reliably read a high level from a storage cell in a DRAM according to one embodiment of the present invention. Like simulation  500  shown in  FIG. 5 , simulation  600  uses like reference elements to represent like elements, except in  FIG. 6  the like elements of the simulation are preceded by a “6” as opposed to a “5”. In this example, the storage cell  630  has a voltage level above 600 mV. Once the word line wl is activated charge sharing takes place between bt bit line and bc bit line which are offset by an amount proportional to the mismatch. There is a charge sharing between bc bit line  610  and the voltage of the storage cell  630 . Because there is no charge sharing with bc bit line its voltage level remains the same. The set devices (nset and psetn) are turned at around time 2.8 ns to amplify the voltage differential between bt bit line and bc bit line. The simulation shows the sense amplifier  400  is able to reliably read the cell voltage because there is no crossing of the bt bit line with the bc bit line and thus is able to perform suitable amplification. 
     It is apparent that there has been provided with this disclosure a reference level generation with offset compensation for a sense amplifier. While the disclosure has been particularly shown and described in conjunction with a preferred embodiment thereof, it will be appreciated that variations and modifications will occur to those skilled in the art. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.