Abstract:
Disclosed is an apparatus and method for decreasing the timing delay variation of output signals obtained from an SOI technology sense amplifier. The cross-coupled latch includes FETs where the body is connected to one of source and drain to minimize switching history effects while the input FETs have a higher than normal gate switching voltage to increase input signal sensitivity.

Description:
TECHNICAL FIELD 
     The invention relates to an SOI (silicon on insulator) sense amplifier design and, more particularly, to improving clocking, sense history and loading characteristics. 
     BACKGROUND 
     Typically, high density, high performance SRAM (Static Random Access Memory) signal state detection circuit designs rely on small signal sense amplifiers to detect the state of a stored value. Several approaches are available to the SRAM designer. When SRAM circuit design transitioned from bulk technologies to SOI technologies, sense amplifiers FETs (Field Effect Transistor) required the addition of body contacts to maintain matching between the sensing devices inside the sense amplifier. 
     One prior art traditional cross-coupled inverter sense amplifier approach used requires synchronization of the bitline precharge and the sense amplifier timing. 
     A prior art SOI sense amplifier circuit design approach had the advantage of only having gate loads on the small signal inputs. The approach also allowed for different timings between pre-charging of the small signal inputs and the output of the sense amplifier itself. Such an amplifier used cross-connected inverters operating in a latch node that is triggered during a different portion of the cycle than the pre-charging. As known to those skilled in the art, SOI FETs typically have the body floating (ungrounded), as opposed to bulk technology FETs. The floating body of the FET can cause a history problem in that such FETs receiving a plurality of logic ones become biased in a given direction and require a much longer time than typical to react to a logic zero input. Thus, timing becomes critical in high performance circuits. 
     One might expect to minimize this history problem by connecting the bodies of critical FETs to ground (body contact FETs). However, when the body of an SOI FET is grounded, the reaction time to a signal input is reduced. Further, in such a circuit, the history related voltage buildup on the latch portion of the sense amplifier affects the timing of the output (reaction time is voltage dependent). Additionally, a grounded body contact FET has a lower input signal sensitivity than does an ungrounded SOI FET. 
     It would thus be desirable to have a sense amplifier that does not need synchronized clocks, can be relatively insensitive to the variable delay related to history problems and has a higher sensitivity to input signals than prior art circuits. 
     SUMMARY OF THE INVENTION 
     The present invention comprises using grounded body FETs in combination with high threshold input voltage FETs to provide an improved sense amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and its advantages, reference will now be made in the following Detailed Description to the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a prior art sense amplifier; 
     FIG. 2 comprises a set of waveforms used in conjunction with an explanation of operation of the circuit figures; 
     FIG. 3 is a circuit diagram of an initial attempt at improving the sense amplifier of FIG. 1; and 
     FIG. 4 is a circuit diagram of a preferred embodiment of a sense amplifier in accordance with this invention. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, a plurality of P-type or P-channel FETs (field effect transistors)  10 ,  12 ,  14 ,  16 , and  18  are shown. Each of these designated FETs, as well as the remaining FETs in the figure, have designators “s” and “d” for source and drain terminals, respectively. FETs  10 ,  12 ,  14 , and  16  each have their source terminal connected to a positive voltage designated as  22 . The gates of each of FETs  10 ,  16  and  18  are connected to a lead  24  that provides a PC (pre-charge) signal. This signal may be identical to that shown as a PC (pre-charge) signal, a waveform representation of which is further illustrated in FIG.  2 . 
     An N-type FET  20  is shown having the SASET (Sense Amplifier SET) signal of FIG. 2 supplied to a gate thereof. Further N-type or N-channel FET transistors are labeled as  26 ,  28 ,  30 , and  32 . As known to those skilled in the art, P-type FETs act as closed switches or, in other words, turn ON to allow current flow from source to drain when the gate terminal is at a low or negative potential with respect to the source. When the gate potential is positive or the same as the source, the FET is OFF or, in other words, does not conduct electricity. On the other hand, N-type FETs act as closed or ON switches to allow current flow therethrough when the gate terminal is high or positive with respect to the source. An output lead  34 , further labeled as SAT (Sense Amplifier True) is connected to the drains of FETs  14  and  16 , to the drain of FET  28  and to the gates of FETs  12  and  26 . An output lead  36 , further labeled as SAC (Sense Amplifier Compliment), is connected to the drains of FETs  10  and  12 , to the drain of FET  26  and to the gates of FETs  14  and  28 . A lead  38  is connected to the source of FET  28  and to the drain of FET  32 . A lead  40  is connected to the source of FET  26  and to the drain of FET  30 . The source and drain leads of FET  18  are connected between leads  38  and  40 . A lead  42  interconnects the sources of FETs  30  and  32  and to the drain of FET  20 . The source of FET  20  is connected to ground or reference potential  44 . FETs  12  and  26 , as well as  14  and  28 , are physically interconnected to act in the same manner as commercially available CMOS (complementary metal oxide on silicon) FETs. The gate of FET  30  is labeled  46  and receives a BLT (Bit Line True) signal from one memory cell, of a set of memory cells, whose logic value is being sensed. The gate of FET  32  is labeled  48  and receives a BLC (Bit Line Compliment) signal from the same memory cell. 
     In FIG. 2, a plurality of waveforms, already mentioned above, are shown with initial designators of H and L to indicate High and Low potential values. Two vertical dash lines are also shown to illustrate identical times in associated waveform signals. 
     In FIGS. 3 and 4, the same designators are used for instances where components may be identical to those of FIG.  1 . As shown, an FET  70  receives the signal BLT. The symbol for FET  70  represents a configuration where the body is connected to ground and is often referred to as a grounded body contact FET. A similar body contact FET  72  receives the BLC signal. The FETs  26  and  28  of FIG. 1 are replaced in FIG. 3 by grounded body contact FETs  74  and  76 . 
     In FIG. 4, a further improvement over FIG. 3, replaces FETs  70  and  72  with Hvt (High threshold voltage) FETs  80  and  82 . FETs  80  and  82 , therefore, having higher threshold voltage than all other FETs in the circuit. N-type FETs with a high threshold voltage begin to turn ON (or OFF) at a larger source to gate voltage than is the case with typical N-type FETs. Further, the FETs  74  and  76  of FIG. 3 have been replaced in FIG. 4 with FETs  84  and  86 . Each of FETs  84  and  86  have the body connected to the source lead of the FET. This type of contact allows the body of the FET to float while equalizing the history effects between these FETs via the pre-charge P-type FET  18  interconnecting them. 
     In operation, a sense amplifier circuit, of the type shown in each of the figures, reacts to a differential voltage provided between inputs BLT and BLC as obtained from a memory cell logic value. Prior to the time when PC goes high, the FETs  26 ,  28 ,  30 , and  32  (and those FETs corresponding thereto in FIGS. 3 and 4) are pre-charged to a condition where they will quickly be turned ON. When SASET (and PC) goes high, current begins to flow through the FETs connected to receive BLT and BLC. As shown, BLT has a lower input voltage than BLC at the time SASET goes high. Thus, FET  30  will conduct less current than FET  32 . As a result, FET  26  will not pull down on lead  36  (SAC) as hard as FET  28  pulls down on lead  34 . 
     As shown in FIG. 2, initially, both SAT and SAC will fall, but SAT will fall faster. As the voltage SAT falls, it will begin to shut off FET  26  causing more of the current being sourced for the FET  20  to pull down the SAT lead even harder. When FET  26  is OFF, the FET  12  will turn ON and cause lead  36  to pull back to a high level. The two P-type FETs  10  and  16  do not participate in the signal amplification, but are used to pre-charge leads  34  and  36  to insure a good “up” level at the beginning of the next access. The present type of sense amplifier, shown in the various figures, depends upon the current regulation abilities of input signal FETs  30  and  32 . As previously mentioned, however, the floating body of the input FETs can cause a history problem in that when the input FETs receive a consecutive plurality of logic ones, the body of the FET typically becomes biased in a given direction. This body voltage bias results in a much longer time, than typical, for the input FETs to react to a logic zero input. Such a variation in delay between input and output signals is detrimental to reliable operation downstream circuitry utilizing these output signals. 
     By grounding the body of each of the FETs  70 ,  72 ,  74 , and  76 , as shown in FIG. 3, the delay variation is reduced because the body cannot develop a history voltage. However, the grounding of the body of the FETs as shown in FIG. 3 reduces the gain of the circuit to only slightly more than it would be when using bulk technology. 
     As may be noted in FIG. 4, two improvements have been made over the implementation of FIG.  3 . The first is that FETs  84  and  86  have the body connected to the source rather than to ground. This allows the body to float at the same potential as the source terminal to return the gain to that of FIG.  1 . However, due to the action of the equalizing FET  18 , the history problem is eliminated, thereby minimizing the delay variation in the switching of the output relative the input. The second improvement is in using high threshold voltage FETs for receiving the input signals BLT and BLC. Thus, even though the body is grounded, the effect is to raise the switch point of the FETs closer to the initial pre-charge voltage, since the switching action takes place sooner, thereby effectively raising the gain of the circuit. As may be known, the high threshold voltage reduces the percentage difference overdrive needed between these two FETs, thus providing a greater regulation of the current. Further, since the higher threshold voltage will reduce the maximum current capabilities of these two FETs, the devices should desirably be appropriately increased in physical size. Analysis has shown that, for a given signal, less small signal is required to overcome a mismatch with the circuit of FIG. 4 than the circuit of FIG.  3 . 
     It should noted that while the invention has been explained using N-type FETs to receive the input signal, the invention could also be practiced for using opposite types for each of FETs involved. In other words, substituting N-type FETs for P-type FETs, and visa versa. Such action might well be called for when attempting to sense ground sensed storage elements. 
     Further, although the invention has been described in connection with SOI technology, the improvements described should be equally applicable to other floating body FET configurations. Although the invention has been described with reference to a specific embodiment, the description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the claims will cover any such modifications or embodiments that fall within the true scope and spirit of the invention.