Abstract:
A sense amplifier ( 40 ) uses a body shorting device ( 60 ) to selectively electrically short circuit the bodies of two transistors ( 44, 48 ) that function as a differential sensing pair. Equalization of charge injected into the bodies functions to minimize offset voltage between the two bodies. The body shorting device selectively shorts the bodies in response to a body control signal after a sense operation and after asserting a precharging signal to initiate precharging of the sense amplifier&#39;s outputs.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to semiconductor memories, and more specifically, to sense amplifiers used in semiconductor memories. 
     BACKGROUND OF THE INVENTION 
     Sense amplifiers are used in conjunction with memories such as, for example, a static random access memory array (SRAM), a dynamic random access memory (DRAM) or a read only memory (ROM). Sense amplifiers function to detect when bit lines in a memory array exhibit a voltage transition in response to column and row decoding and a sense enable signal. In such memories, there is a need to amplify and decode signals provided via columns of memory cells. 
     It is desirable to sense a data signal with an amplifier containing a cross-coupled differential pair of transistors. The timing of turn-on of the sense amplifier is critical. One measure of a sense amplifier&#39;s quality is the minimum differential signal that the sense amplifier is able to accurately sense. An objective in sense amplifier design is to provide the maximum differential signal to the difference in gate-to-source drive (delta V GS ) of the differential cross-coupled pair. Another critical design parameter associated with sense amplifiers is associated with the operation of a differential cross-coupled pair of transistors. The design parameter involves insuring that the difference in gate-to-source drives is greater than zero at the time the pair is clocked. If not, the output signal may not be accurate. In general, prior sense amplifiers have involved a trade-off between speed, size and power consumption. 
     One transistor implementation of memories is the use of silicon on insulator (SOI) processing in which a transistor is formed on an insulating material. A characteristic of a transistor formed with an SOI process is an isolated portion of the device that exists between the current conducting electrodes and below a control electrode. This portion of the transistor is commonly referred to as the “body” of the transistor and is the portion where current conduction occurs between the current conducting electrodes. The body is otherwise electrically isolated and is frequently not electrically contacted. However, simply allowing non-contacted bodies to electrically float makes the voltage associated with the body to be vulnerable to noise, leakage currents and other process variations. Voltage variations of the body cause numerous detrimental characteristics for a memory sense amplifier. For example, the body voltage variation affects the transistor threshold voltage. The amount of voltage variation is dependent upon the previous switching history of the transistor and is therefore data dependent. The reason this matters is that any offset in the sense amplifier is effectively subtracted from the bit line signal to be detected by the sense amplifier. In other words, the signal to noise ratio is reduced. For memory sense amplifiers where small differential voltages are being sensed and accurate threshold voltages are important, the body voltage variation is very problematic. One technique that has been used to minimize such disadvantages is to connect the bodies of transistors used in memory sense amplifiers to a ground potential. However, the resistance of the body contacts is large and the body often does not have time to be equalized to ground potential when very short memory cycle times are used. Because there is a relatively large voltage potential difference between the bodies of the differential pair in a sense amplifier, the voltages are not equalized at high frequencies. 
     Another known technique that has been used to minimize such disadvantages is to connect the bodies of a differential pair of sensing transistors in a sense amplifier together. However, when the bodies are connected together, a voltage gradient builds up in the bodies due to the lack of the ability to create a low resistance connection. Also, due to the resistance and capacitive coupling effects of the body, a relatively large time constant exists when trying to equalize the voltages of the bodies. 
     Yet another known technique that has been used to minimize such disadvantages is to connect the body to the source of each transistor of a differential pair of sensing transistors in a sense amplifier. The resistance of the body contacts is large and the body often does not have time to be equalized to the source when very short memory cycle times are used. Therefore, there is no ability for the electrical connection to fully discharge the body of either transistor of the differential pair of sensing transistors to ground. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. 
     FIG. 1 illustrates a cross section of an SOI transistor having a floating body; 
     FIG. 2 illustrates a top plan view of a body contacted SOI transistor illustrating the distributed resistive/capacitive characteristic of the body; 
     FIG. 3 illustrates in schematic form an equivalent circuit of the high resistance portion of the body of the transistor of FIG. 2; 
     FIG. 4 illustrates in schematic form a sense amplifier in accordance with the present invention; and 
     FIG. 5 illustrates in graphical form signal waveforms associated with the sense amplifier of FIG.  4 . 
    
    
     Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention. 
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a body isolated transistor  10  generally having an insulator  12 , a source  14 , a drain  16 , a body  18 , an insulator  20  and a gate  22 . The source  14 , body  18  and drain  16  adjoin insulator  12 , and body  18  physically separates source  14  from drain  16 . Insulator  20  separates gate  22  from body  18 . Electrical contacts to each of source  14 , drain  16  and gate  22  are implied but not expressly shown. In the illustrated form, body  18  is electrically isolated from external contact. Because source  14 , drain  16 , body  18  and gate  22  may be formed of silicon, one common term for the illustrated structure is ‘silicon on insulator’ or SOI. It should be well understood that underlying insulator  20  is a substrate (not shown) of other material. 
     FIG. 2 illustrates a top view of a body isolated transistor such as transistor  10  of FIG.  1 . Common elements between FIG.  1  and FIG. 2 are similarly numbered. The source  14  is separated from drain  16  via the gate  22 . Gate  22  is represented by a cross hatching running from the bottom left of the drawing to upper right. Gate  22  overlies body  18  in areas that are represented in both directions. A body contact  21  is extended beyond the structure for making electrical contact to underlying body  18 . The length of body  18  is illustrated and has a high resistance. The resistance of body  18  is determined by the resistivity of the material, rho, multiplied by the ratio of the length divided by the width of body  18 . As a result of both the small width in relation to the long length and the high resistivity of the body, the resistance of the body  18  is high. This resistance is represented along five equal portions of underlying body  18  as Rb. Additionally, along each of the five portions of the underlying body  18  there is a drain-to-body capacitance, Cb. The effect of the cumulative Rb resistances and Cb capacitances is to create a significant delay in transferring charge within body  18 . 
     Illustrated in FIG. 3 is a schematic representative circuit  30  of the capacitive and resistive elements associated with body  18  of FIG.  2 . For purposes of comparison, common elements shared between FIGS. 1-3 are numbered the same. The equivalent circuit of FIG. 2 represents a single transistor that is divided into five separate portions to further illustrate the net ohmic nature of body  18 . Source  14  is illustrated as connected to a plurality of source electrodes of N-channel transistors  32 - 36 . A gate of each of N-channel transistors  32 - 36  is connected together to form gate  22 . A drain of each of N-channel transistors  32 - 36  is connected together to form drain  16 . The individual resistive elements Rb of body  18  are connected in series to body contract  21 . Similarly, the individual drain-to-body capacitances Cb of body  18  are distributed between the various portions of body  18  to the body contact  21 . 
     Illustrated in FIG. 4 is a hysteresis reduced sense amplifier  40  in accordance with one form of the present invention. A P-channel transistor  42  has a source connected to a power supply terminal for receiving a power supply voltage labeled Vdd. A drain of transistor  42  is connected to a first sense input/output (I/O)  43  and to a drain of an N-channel transistor  44 . A gate of transistor  42  is connected to a gate of transistor  44  at a second sense input/output (I/O)  45 . Because first sense input/output  43  and second sense input/output  45  are differential outputs, the two signals represent true and complement signals. As illustrated, second sense input/output  45  is a true input/output and first sense input/output  43  is a complementary input/output. A P-channel transistor  46  has a source connected to the Vdd power supply terminal, a gate connected to the first sense input/output  43  and a drain connected to the second sense input/output  45 . A drain of an N-channel transistor  48  is connected to the drain of transistor  46  at the second sense input/output  45 , and a source of transistor  48  is connected to a source of transistor  44  and to a drain of an N-channel transistor  50 . A gate of transistor  48  is connected to the first sense input/output  43 . A gate of transistor  50  is connected to a Sense Enable signal. A source of transistor  50  is connected to a ground terminal labeled Gnd. A P-channel transistor  52  has a source connected to the Vdd power supply terminal, a gate for receiving a Precharge signal, and a source connected to the first sense input/output  43 . A P-channel transistor  53  has a source connected to the Vdd power supply terminal, a gate for receiving the Precharge signal, and a source connected to the second input/output  45 . A P-channel transistor  56  has a source connected to a complement bit line, BL-bar, a gate connected to a Column Select signal, and a drain connected to the first sense input/output  43 . A P-channel transistor  57  has a source connected to a bit line, BL, a gate connected to the Column Select signal, and a drain connected to the second sense input/output  45 . A body shorting device  60  is implemented, in one form, as a P-channel transistor having a first current electrode (source or drain) connected to the body of transistor  44 , a second current electrode (drain or source) connected to the body of transistor  48 , and a gate connected to a control signal labeled Body Control. 
     The operation of sense amplifier  40  may be more readily understood in connection with reference to the waveforms of FIG.  5 . The Column Select signal is made active to couple a bit line and a bit line-bar to sense amplifier  40 . Initially, a Precharge signal is asserted during a precharge phase to make transistors  52  and  53  conductive and thereby place first sense input/output  43  and second sense input/output  45  at a predetermined voltage level, such as supply voltage Vdd. During the precharge phase, the isolated body voltage of transistor  44  reaches a first voltage level and the isolated body voltage of transistor  46  reaches a second voltage level different from the first voltage level. As illustrated, the Precharge signal is deasserted to make transistors  52  and  53  nonconductive and allow first and second sense inputs/outputs  43  and  45  to form a differential voltage. This operation initiates a memory access, such as a Read access. During a read access, transistors  44  and  48  function as a cross-coupled differential pair of sensing transistors. Asserting the Body Control signal at the same time, or substantially close in time, makes transistor  60  nonconductive. 
     Shortly thereafter, a sense amplifier wordline signal, WL, transitions from an inactive (low in the illustrated form) state to an active state. The wordline signal functions in a conventional manner to select a plurality of bit lines from an array of bit lines. Because the wordline selection used herein is conventional, details of the wordline operation are not shown in FIG.  4 . In response to the wordline signal being active, the Bit line, BL, and complement Bit line, BL-bar, signals begin to transition in a conventional manner. For purposes of explanation only, the BL-bar is selected as transitioning low while the BL signal remains near Vdd. During this time, assertion of a Sense Enable signal is being delayed a sufficient amount of time to permit enough differential signal to be created between sense input/output  43  and sense input/output  45  to permit sense amplifier  40  to properly resolve when enabled. The Column Select signal is made inactive at or close in time when the Sense Enable signal is asserted. In other words, the Column Select signal is turned off when a sense operation occurs. When the Sense Enable is asserted, the Sense inputs/outputs  43  and  45  are separated while transistors  56  and  57  are nonconductive, thereby isolating sense amplifier  40  from the bit line and bit line-bar. 
     The sense Enable signal latches the sense inputs/outputs  43  and  45  to the full supply rails, Vdd and ground. This latching results in differential capacitive coupling to the isolated bodies of transistors  44  and  48 . After sensing is complete, the sense inputs/outputs  43  and  45  are precharged back to Vdd as a result of asserting the Precharge signal. The precharge action of the Sense outputs couples charge into the body of transistors  44  and  48 . This charge is nearly the same as the charge that was coupled into the body of transistors  44  and  48  during latching, but is now removed from the isolated bodies. During the circuit operation description herein, the Body Control signal has to this point been negated. After waiting for the outputs to fully precharge, the Body Control signal is asserted, thereby making transistor  60  conductive. Transistor  60  functions to equalize any errors in the charging and discharging of the isolated bodies. Transistor  60  substantially equalizes a first body voltage of transistor  44  and a second body voltage of transistor  48 . Transistor  60  thereby removes any differential voltage existing between the isolated bodies of transistors  44  and  48 . Otherwise, a differential voltage in the isolated bodies will build up and have a hysteresis effect for any following accesses of the sense amplifier  40 . The use of transistor  60  to electrically short circuit the isolated bodies has minimized hysteresis and removed dependency from the previous switching history of sense amplifier  40 . Transistor  60  distributes the mismatched charges in the isolated bodies of transistors  44  and  48  within a predetermined amount of time. The required amount of time is small because the error to be corrected is small. Equalization in a conventional sense amplifier occurs primarily by normal operation coupling action. However, a short timed pulse completes the equalization process such as for example during a write mode and non-active states. In one form, the predetermined amount of time occurs between read accesses to the sense amplifier  40 . The cross-coupled isolated body transistors  44  and  48  provide an amplified output signal (true and complement versions as a result of the differential sensing) at the sense input/output  43  and  45 , respectively. 
     It should be noted that transistor  60  was not made conductive until completion of a full cycle in which charge is transferred into and out of the bodies of transistors  44  and  48 . Transistor  60  is only necessary to equalize any errors in the charge cycled. 
     By now it should be appreciated that there has been provided a sense amplifier that fulfills a need for equalized bodies in differential pair sense amplifiers. The equalizing is accomplished just prior to sensing and does not interfere with a natural coupling of charge into and out of the floating bodies of the sensing transistors. By allowing a full cycle of sense and precharge, the charge in the bodies of the differential pair transistors is restored very close to its initial state before sensing, thereby minimizing hysteresis effects created by floating bodies. Any errors due to process mismatches are equalized with the use of transistor  60  and the Body control signal. The equalization may be performed quickly because of the error voltages are small due to the fact that the charge was restored. 
     Because the apparatus implementing the present invention is, for the most part, composed of electronic components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, multiple sense amplifiers may be used in connection with the same shorting device transistor  60  in which the current electrodes of shorting device transistor  60  would be connected to other differential sensing pair(s) of transistors (not shown). Sense amplifier circuitry may be modified to use in conjunction with the present invention. As a further example, the present invention applies to various memories, such as MRAMs. Although memory bit line potentials may vary, the present invention is equally applicable for all voltage applications. Any semiconductor with a floating body may use the present invention; therefore semiconductors of materials other than silicon may be used. Additionally, MOS implementations using either P-channel transistors or N-channel transistors may be used, Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.