Abstract:
A description is given of a memory device having memory cells for storing data. The memory device described is distinguished by the fact that a current switch-off device is provided, which prevents an existing current flow through the memory cell to be read in response to the identification of the memory cell content, and/or that a discharge device is provided, which partly discharges again a node in the memory cell which is to be precharged before the memory cell is read.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE00/03682, filed Oct. 19, 2000, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a memory device having memory cells for storing data, a sense amplifier, by which the content of the relevant memory cell can be determined on the basis of the magnitude and/or the direction of a current flow established through a memory cell during the read-out thereof. 
     Memory devices of this type have been known for many years in innumerable embodiments. 
     In the development of memory devices, as also in the case of other electrical circuits, great efforts are made in an attempt to keep the energy consumption as low as possible, or to reduce it further. In this context, considerable success has already been achieved particularly in recent years. However, the energy consumption of memory devices is still too high for certain applications. 
     One of these applications is memory devices for use in contactless smart cards. 
     As is known, contactless smart cards are connected to other devices exclusively in a wire-free manner. Even the energy required for their operation is transmitted in a wire-free manner; it is extracted from electromagnetic oscillations received by the smart card and is buffer-stored in a capacitor. However, the energy that can be buffer-stored in smart cards is extremely low in particular owing to the highly restricted storage possibilities. Accordingly, components contained in wire-free smart cards must be able to be operated with a minimum of energy. Conventional memory devices do not yet satisfy this requirement. 
     Published, European Patent Application EP 0 390 404 A shows a memory device having nonvolatile memory cells, and also a sense amplifier and the driving thereof. The sense amplifier contains a sense amplifier having feedback inverters. A current branch that is driven by the bit line connected to the memory cell is connected in parallel with an input-side transistor of the sense amplifier. The output of the sense amplifier is fed back to an NMOS transistor in order to decouple the bit line from the sense amplifier. The NMOS transistor is connected in series with a PMOS transistor and is located on the side of the memory cell. 
     U.S. Pat. No. 5,258,669 shows a memory cell and also a read-out circuit in which the bit line capacitance is precharged from a current path connected to the supply voltage. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a memory device which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which operates as intended with a minimum of energy. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a memory device. The memory device contains a terminal for supplying a pole of a supply voltage, a memory cell for storing data in a memory transistor, and a sense amplifier for ascertaining a content of the memory cell on a basis of a magnitude and/or a direction of a current flow established through the memory cell during a read-out of the memory cell. The sense amplifier has an output and complementary transistors coupled in series with each other, and the output of the sense amplifier is fed back to the complementary transistors. A current mirror has a mirror transistor coupled in series with the memory transistor and a current branch connected in parallel with one of the complementary transistors of the sense amplifier. The mirror transistor has a remote terminal remote from the memory cell. A current switch-off device is provided for interrupting the current flow through the memory cell to be read while the memory cell content is ascertained. The current switch-off device has a transistor connected between the remote terminal of the mirror transistor and the terminal for supplying the pole of the supply voltage. 
     The invention has the positive effect that the time during which electric currents flow in the memory device and the sense amplifier can be reduced to a minimum. 
     The invention further has the positive effect that this considerably shortens the time required for reading from the memory cells; as a result, the memory device and/or a system containing it has to be activated for a shorter time than has previously been the case. 
     The energy consumption of the memory device decreases in both cases; such memory devices can be operated as intended with a minimum of energy. 
     In accordance with an added feature of the invention, the mirror transistor is a PMOS transistor, the current branch has a PMOS transistor, the complementary transistor of the sense amplifier which is connected in parallel with the current branch is a PMOS transistor, and the transistor contained in the current switch-off device is a PMOS transistor. 
     In accordance with another feature of the invention, the mirror transistor, the PMOS transistor contained in the current branch of the current mirror circuit, and the complementary transistor of the sense amplifier which is connected in parallel with the current branch are directly connected to the terminal for the pole of the supply voltage. 
     In accordance with an additional feature of the invention, the sense amplifier has an inverter with an input and a feedback output forming the output of the sense amplifier. The transistor contained in the current switch-off device has a control input connected to the input of the inverter. 
     In accordance with a further feature of the invention, the sense amplifier outputs a signal which signals the content of the memory cell to be read. The signal is held in a previous state after the current flow through the memory cell to be read has been prevented by the current switch-off device. 
     In accordance with a further added feature of the invention, the process of preventing the current flow through the memory cell to be read, which is performed by the current switch-off device, is ended with an ending of a read-out operation of the memory cell. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a memory device. The memory device contains a memory cell for storing data and sense amplifier for ascertaining a content of the memory cell on a basis of a magnitude and/or a direction of a current flow established through the memory cell during a read-out of the memory cell. The sense amplifier is coupled to the memory cell. A node is connected to the memory cell, the node being precharged before the memory cell is read, and a discharge device is connected to the node for partly discharging again the node. 
     In accordance with an added feature of the invention, a bit line is provided, and the node is part of the bit line assigned to the memory cell to be read. 
     In accordance with an additional feature of the invention, the memory cell has a memory transistor with a terminal connected to the node. 
     In accordance with a further feature of the invention, the discharge device discharges the node to a potential that is established there when a current that is to be detected by the sense amplifier flows through the memory cell. 
     In accordance with another feature of the invention, the node is discharged before a beginning of the read-out of the memory cell by the sense amplifier. 
     In accordance with a concomitant feature of the invention, the node is discharged with an adjustable current. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a memory device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The single FIGURE of the drawing is a circuit diagram of a detail of a memory device according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the single FIGURE of the drawing in detail, there is shown a memory device embodied as an EEPROM. However, there is no restriction thereto. The special features of the EEPROM which are described below can also be used beneficially in any other nonvolatile or volatile (semiconductor) memory devices such as, for example flash memories, RAMS, etc. 
     For the sake of clarity, the FIGURE illustrates only a single memory cell of the many memory cells that an EEPROM usually has. 
     The memory cell shown contains a memory transistor designated by the reference symbol NVM 1  and a selection transistor designated by the reference symbol M 1 , which is connected in series with the memory transistor. The selection transistor M 1  is connected to ground via a so-called source line SL, and the memory transistor NVM 1  is connected to a supply voltage potential V DD  via a so-called bit line BL and transistors M 2 , M 3  and M 10 . 
     The selection transistor M 1  is driven by a read signal READ via a so-called word line WL. The transistors NVM 1  and M 2  are driven by signals VCONST 1  (transistor M 2 ) and VCONST 2  (transistor NVM 1 ). The aforementioned signals are constant—at least during the precharge and/or read-out described in more detail below—and, in the example considered, have values of 1.2 V (VCONST 1 ) and 2 V (VCONST 2 ). 
     The transistor M 2  is a precharge transistor and serves for charging a node KBL located between it and the transistor NVM 1  (the bit line BL running between it and the transistor NVM 1 ) to a predetermined potential. 
     The transistor M 3  serves for mirroring a current flowing via the bit line BL into a branch Z 2 , which will be described in more detail below. 
     The transistor M 10  serves to interrupt, as required, the connection of the branch which contains it (the branch Z 1  defined below) to the supply voltage potential V DD  (to prevent a current flowing through the branch Z 1 ). 
     The circuit section described above, i.e. the circuit branch containing the transistors M 10 , M 3 , M 2 , NVM 1  and M 1 , forms the (first) branch Z 1  already mentioned. 
     The second branch Z 2 , likewise already mentioned, is located in parallel with the first branch Z 1 . The second branch Z 2  contains a series circuit of transistors M 4 , M 5 , and M 9 , the transistor M 4  being connected to the supply voltage potential V DD , and the transistor M 9  being connected to ground. 
     The transistors M 4  and M 5  are driven in such a way that quite specific currents flow through each of them, to be precise a current corresponding to the current flowing through the first branch Z 1  flows through the transistor M 4  (the transistors M 3  and M 4  form a current mirror which causes a current corresponding to the current flowing through the transistor M 3  to flow through the transistor M 4 ), and a reference current InCur which is caused to flow through a transistor M 6  flows through the transistor M 5  (the transistors M 5  and M 6  form a current mirror which causes a current corresponding to the current flowing through the transistor M 6  to flow through the transistor M 5 ). 
     The transistor M 9  serves “only” to interrupt the second branch Z 2  as required (to prevent a current flowing through the second branch Z 2 ). 
     The currents flowing through the transistors M 4  and M 5  meet one another at a node K 1  located between the transistors M 4  and M 5 . A potential that is dependent on the relative magnitude of the currents is established at the node K 1 . The level of the potential represents the content of the memory cell to be read, but is not yet the output signal of the configuration representing the content of the memory cell. 
     The potential of the node K 1  is inverted by a first inverter I 1 . An output signal of the first inverter I 1  is fed to a NOR element NOR, where it is NORed with the already mentioned READ signal, which is inverted by a second inverter I 2 . The output signal of the NOR element NOR is used for driving the transistor M 10  and is furthermore fed to a third inverter I 3 . The output signal of the third inverter I 3  is used as an output signal KOUT of the configuration shown. 
     In the example considered, KOUT has the value 0, if the signal READ has the level 1, that is to say the memory cell is read, and the transistor NVM 1  is programmed in such a way that it has a low threshold voltage, that is to say turns on during the read-out operation. KOUT has the value 1, if the signal READ has the level 1, that is to say the memory cell is read, and the transistor NVM 1  is programmed in such a way that it has a high threshold voltage, that is to say turns off during the read-out operation, or if and as long as the signal READ has the level 0, that is to say the memory cell is not read. 
     The signal KOUT is also used to drive the transistor M 9  located in the second branch and a transistor M 8  disposed in parallel with the transistor M 4 . 
     The memory cell is read in two stages, namely a precharge stage and a subsequent detection stage. 
     The precharge stage is a preparation for the read operation that takes place (in the detection stage); the (read) signal READ still has the level 0 in the precharge stage. The aim of the precharge stage is to precharge the node KBL to a specific potential. In the precharge stage, the transistors M 10 , M 3 , M 2 , NVM 1  and M 1  contained in the first branch Z 1  are driven in such a way that the transistors M 10 , M 3  and M 2  turn on, and that the transistor M 1  turns off. As a result, and because the node KBL is connected to ground via a parasitic capacitance C, the node KBL is charged asymptotically to VCONST 1  (first by a so-called strong inversion current and then by a so-called weak inversion current). 
     Since the transistor M 1  turns off, no current, or at least no appreciable current, can flow via the first branch Z 1  in the precharge stage. 
     As a result, the transistor M 4  turns off, so that no current, or at least no appreciable current, flows in the second branch Z 2  either. 
     On account of the level 0 of the signal READ, the output signal of the second inverter I 2  has the level 1, the output signal of the NOR element NOR has the level 0, and the output signal of the third inverter I 3  (and thus also the output signal KOUT of the arrangement) has the level 1. 
     Through the level 0 of the output signal of the NOR element NOR, the (PMOS) transistor M 10  driven thereby is or remains turned on; through the level 1 of KOUT, the (NMOS) transistor M 9  driven thereby is or remains turned on and the (PMOS) transistor M 8  likewise driven thereby is or remains turned off. 
     The node K 1  of the second branch Z 2  can be pulled to ground potential via the transistor M 9 . 
     The precharge stage is followed by the already mentioned detection stage, in which the memory cell is actually read; the read signal READ now has the level 1. In the detection stage, the transistors contained in the first branch Z 1  are driven in such a way that they turn on given the presence of suitable voltages at the source or drain terminals. The fact of whether actually all the transistors in the first branch Z 1  turn on depends on the state of the transistor NVM 1 . If the transistor NVM 1  is programmed in such a way that it has a high threshold voltage, then the transistor NVM 1  turns off; if the transistor NVM 1  is programmed in such a way that it has a low threshold voltage, then the transistor NVM 1  and the other transistors provided in the first branch Z 1  turn on. These operations are known and require no further explanation. 
     If the transistor NVM 1  is programmed in such a way that it turns off in the detection stage, no current can flow via the first branch Z 1 . 
     As a result, the transistor M 4  in the second branch maintains its state unchanged. In other words, like in the precharge stage, it remains turned off, so that now, too, no current can flow through the second branch Z 2 . 
     The potential of the node K 1  in the second branch Z 2  also remains unchanged. In other words, like in the precharge stage, the node K 1  remains at ground potential. 
     Thus, the output signal of the first inverter I 1  remains at the level 1, the output signal of the NOR element NOR remains at the level 0, and the output signal of the third inverter I 3  (the output signal KOUT of the entire configuration) remains at the level 1, which in turn has the consequence that the transistors M 9  and M 10  remain turned on and the transistor M 8  remains turned off. 
     If the transistor NVM 1  is programmed in such a way that it turns on in the detection stage, a current can flow via the first branch Z 1 . In the example considered, the transistors (in particular the transistor NVM 1 ) provided in the first branch Z 1  are dimensioned and driven in such a way that the current flowing via the first branch Z 1  is 11 μA. 
     By the transistors M 3  and M 4  acting as a current mirror, the current is also caused to flow in that part of the second branch Z 2  that lies above the node K 1 . The reference current InCur already mentioned flows in that part of the second branch Z 2  that lies below the node K 1 . The flowing of the current in the second branch Z 2  is based on current mirroring by the transistors M 6  and M 5  acting as a current mirror. The reference current InCur is caused to flow to ground through the transistor M 6  and from there is mirrored into the second branch Z 2  using the transistors M 6  and MS. The (reference) current flowing in that part of the second branch Z 2  which lies below the node K 1  is in the opposite direction to the current flowing in that part of the second branch Z 2  which lies above the node K 1 . However, the (reference) current flowing in that part of the second branch Z 2  which lies below the node K 1  is in this case significantly smaller than the current flowing in that part of the second branch Z 2  which lies above the node K 1 ; it is only half as large, that is to say has a value of only 5.5 μA, in the example considered. 
     Since the current flowing in that part of the second branch Z 2  which lies above the node K 1  is greater than the current flowing in that part of the second branch Z 2  which lies below the node K 1 , the node K 1  is pulled toward the supply voltage potential V DD . 
     Thus, the output signal of the first inverter I 1  goes to the level 0, the output signal of the NOR element NOR to the level 1 (the output signal of the second inverter I 2  likewise has the level 0 owing to READ=1), and the output signal of the third inverter I 3  (the output signal KOUT of the entire configuration) to the level 0. 
     Through the level 1 of the output signal of the NOR element NOR, the transistor M 10  driven thereby is put into a non-conducting state, as a result of which the current flow in the first branch Z 1  is interrupted. 
     At the same time, through the level 0 of the output signal KOUT, the transistor M 8  disposed in parallel with the transistor M 4 , which is now turned off, is put into the on state, as a result of which the node K 1  is held at high potential. At the same time, through the level 0 of the output signal KOUT, the transistor M 9  is put into the off state, as a result of which current can no longer flow through the second branch Z 2  either, and as a result of which the node K 1  is pulled completely to V DD . 
     The current flow to be detected by the configuration, i.e. the current flow ensuing via the transistor NVM 1  and the bit line BL, and current flows required for detecting the current flow (in particular the current flow which ensues through the second branch Z 2 ) are thus maintained only for as long as is required to detect the current to be detected. The above-mentioned current flows are prevented immediately after detection, more precisely in response to the detection of the current flow to be detected. 
     Normally, i.e. without the above-described prevention of the current flows, the latter would last until the detection stage representing the actual read operation is ended by a change in the READ signal from the level 1 to the level 0. By virtue of the fact that the current flows are already prevented within the detection stage, the relevant currents flow for a considerably shorter time than has previously been the case. As a result, the power consumption and the energy consumption of memory devices can be considerably reduced. 
     A positive effect is had on the power consumption and the energy consumption of memory devices if, in addition or as an alternative, care is taken to ensure that the node KBL is precharged only approximately to the potential required to turn on the transistor NVM 1  which is programmed for a low threshold voltage. 
     In the example considered, this is achieved by virtue of the fact that the node KBL is discharged via a transistor M 21  with a very small current (in the nA range). In the example considered, a current (signal MU) sent through a transistor M 20  can be used to define whether such a discharge process is carried out and when such a discharge process is to begin and end. The transistors M 20  and M 21  are connected up to form a current mirror, the current flowing through the transistor  20  being mirrored into the transistor M 21 . Given suitable dimensioning and driving of the transistors M 20  and M 21 , what can be achieved is that the node KBL is discharged to a greater or lesser extent precisely until a predetermined desired potential is established at the node KBL. The desired potential is preferably approximately the potential established on the bit line when all the transistors in the first branch Z 1  turn on (the current to be detected flows through the first branch Z 1 ). 
     A process of discharging the node KBL which is effected in the manner described or differently takes account of the circumstance that in practice it is not possible, or at any rate not readily possible, to limit the precharging of the node in such a way that the abovementioned desired potential is established there. 
     This can be illustrated using the example of the (precharge) transistor M 2 . In order to ensure intended functioning of the transistor in the detection stage, the following must hold true for the voltage VCONST 1  applied to the gate terminal of the transistor M 2 : 
     
       
           VCONST   1 = V   th   +V   KBLdesired    
       
     
     where V th  is the threshold voltage of the transistor M 2 , and where V KBLdesired  is the abovementioned desired potential to which the node KBL must be brought in order that a transistor NVM 1  which is programmed for a low threshold voltage, but not a transistor NVM 1  which is programmed for a high threshold voltage, is reliably turned on and kept turned on. 
     In the example considered, V KBLdesired  has a value of approximately 0.6 V. This potential suffices to turn on and keep turned on a transistor NVM 1  that is programmed for a low threshold voltage. 
     As has already been mentioned above, however, the node KBL is charged to VCONST 1 , that is to say to approximately 1.2 V, in the precharge stage. Therefore, after the beginning of the detection stage (after the change in the READ signal from the level 0 to the level 1) it takes a relatively long time until the current to be detected is established in the branch Z 1 . This is because if and for as long as the potential at the node KBL is greater than VCONST 1 −V th , the transistor M 2  turns off and thus prevents the flowing of the current to be detected. In this case, it takes a relatively long time until the node KBL is discharged from VCONST 1  to VCONST 1 −V th , because in this stage only a static current which gradually reverses the charge of the bit line capacitance C flows through the transistor NVM 1 . 
     If a process of discharging the node KBL that is effected in the manner described brings the potential established there in the precharge stage to approximately VCONST 1 −V th , then the current to be detected is established immediately at the beginning of the detection stage (directly after the change in the READ signal from the level 0 to the level 1). 
     The potential established at the node KBL need not necessarily be brought to approximately VCONST 1 −V th  through the discharge process. It already proves to be advantageous if it is not all that much greater than VCONST 1 −V th . As a result, the time between the beginning of the precharge stage and the beginning of the current flow to be detected can be reduced to a greater or lesser extent. 
     An early occurrence of the current to be detected within the detection stages makes it possible to shorten the detection time and thus also the length of the detection stages. This proves to be an enormous advantage because considerably shorter memory access times can thereby be obtained in an amazingly simple manner. Shorter memory access times enable the realization of memory devices which operate faster and can be utilized under certain circumstances (for example in the case of use in wire-free smart cards) to the effect that the memory device or the system containing the memory device is activated for a shorter time or is put into an energy-saving standby operating mode earlier. 
     The memory device described can thus be operated with a minimum of energy.