Abstract:
A sensing circuit for sensing the binary state of a memory cell includes a first transistor wherein the first spaced-apart region is connected to a first voltage source, and the gate is connected to a second voltage source, a second transistor wherein the first spaced-apart region of the second transistor is connected to the second spaced-apart region of the first transistor, and a third transistor wherein the first spaced-apart region is connected to the second spaced-apart region of the second transistor, the gate is electrically connected to the memory cell, and the second spaced-apart region is connect to a ground potential. The circuit also includes a circuit to generate a first current in response to one binary state of the memory cell and a second current in response to another binary state of the memory cell wherein a feedback circuitry is created between the circuitry to generate currents and the second transistor, and a circuit to produce output signals in response to the first current and the second current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains in general to a non-volatile memory device and, more particularly, to a sensing circuit of a non-volatile memory device. 
     2. Description of the Related Art 
     Sensing circuits, or sense amplifiers, are commonly used in non-volatile memory devices to ascertain the binary states of the devices. For example, U.S. Pat. No. 5,386,158 (&#34;the &#39;158 patent&#34;) discloses a sensing circuit for a floating-gate memory device. 
     FIG. 1 shows a portion of the circuitry illustrated in FIG. 2a of the &#39;158 patent. Referring to FIG. 1, the &#39;158 patent discloses a sensing circuit 10 comprised of primarily MOS transistors. Sensing circuit 10 is coupled to a memory array 12 that includes a plurality of non-volatile memory cells 12a, 12b . . . 12n, only one of which (12a) is shown. Memory array 12 is connected to a sensing line 80 through a multiplexer 16. Sensing line 80 connects to the gate of an n-type transistor 36. The source of transistor 36 is connected to a ground potential and the drain is connected to the source of an n-type transistor 34. The terms &#34;source&#34; and &#34;drain,&#34; as well known in the art, can be used interchangeably. The drain and gate of transistor 34 are connected together and also to the drain of a p-type transistor 32. The gate of transistor 32 is connected to a reference potential V REF , and the source is connected to a voltage potential V c . Transistors 32, 34, and 36 operate as a voltage amplifier. 
     The gate and drain of transistor 34 are also connected to the gate of an n-type transistor 44. The drain of transistor 44 is connected to the drain and the gate of a p-type transistor 40 and also the gate of a p-type transistor 42. The source of transistor 40 and the source of transistor 42 are connected to voltage potential V c . Transistors 40, 44, and 42 operate as a current mirror. The drain of transistor 42 is connected to the input of an inverter 28 and the drain of an n-type transistor 48. The source of transistor 48 is connected to a ground potential and the gate is connected to a reference voltage V SENREF . Inverter 28 provides an output S OUT . 
     Sensing line 80 is additionally connected to the drain and the gate of an n-type transistor 46. The drain and the gate of transistor 46 are also connected to the source of transistor 44. The source of transistor 46 is connected to a ground potential. 
     As described by the &#39;158 patent, transistors 34 and 44 will be on regardless of the conductive state of the memory cell being sensed, for example, memory cell 12a. When memory cell 12a is in a conductive state, transistor 34 generates a first voltage to maintain the voltage at sensing line 80. Current then flows from transistor 40, through transistor 44, to sensing line 80 to maintain the conductive state of memory cell 12a. When memory cell 12a is in a non-conductive state, there is no current flow in sensing line 80 initially. Because transistor 44 is on, the voltage on sensing line 80 will subsequently be charged up. When the voltage on sensing line 80 increases, the voltage at the gate of transistor 34 drops, generating a second voltage. Transistor 40 continues to supply current to sensing line 80 through transistor 44 until sensing line 80 reaches a saturation level, at which point transistor 46, also known as a &#34;leaker,&#34; conducts excess current to ground to prevent the voltage on sensing line 80 from cutting off transistor 44. 
     The sensing operation is conducted through transistors 40, 42, 48 and inverter 28. Depending upon the net current flow into or out from the input of inverter 28, S OUT  will either be high or low. Because the current flow through transistor 42 mirrors the current flow through transistor 40, the current flow to the input of inverter 28 also mirrors the current flow through transistor 40. The &#39;158 patent additionally describes how reference voltage V SENREF  is generated from which the net current flow into inverter 28 is determined. The &#39;158 patent is hereby incorporated by reference. 
     When sensing circuit 10 is first enabled, transistors 40 and 42 are operating in the cut-off region. If V SENREF  is provided to transistor 48, which turns transistor 48 on, while transistors 40 and 42 are off, output signal S OUT  will be pulled to zero, creating a transition output noise. Damping noise may also be created during the same period between transistors 40 and 42 as transistor 42 generates a current that &#34;mirrors&#34; the charging current through transistor 40. In addition, because only one current path, namely the one through transistors 40 and 44, is provided to sensing line 80, insufficient charging current is provided to bring sensing circuit 10 from the cut-off region into its linear operating region within a short enough period of time to avoid transition noises or to allow sensing circuit 10 to provide high sensing speed. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a high speed sensing circuit for a non-volatile memory device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structures and methods particularly pointed out in the written description and claims thereof, as well as the appended drawings. 
     To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and broadly described, there is provided a sensing circuit for sensing the binary state of a memory cell in a non-volatile memory device that includes a first transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to a first voltage source, and the gate is connected to a second voltage source, and a second transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region of the second transistor is connected to the second spaced-apart region of the first transistor. The sensing circuit also includes a third transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the second spaced-apart region of the second transistor, the gate is electrically connected to the memory cell, and the second spaced-apart region is connect to a ground potential. The sensing circuit additionally includes means for generating a first current in response to one binary state of the memory cell and a second current in response to another binary state of the memory cell wherein the means for generating currents is connected to the first spaced-apart region and the gate of the second transistor such that a feedback circuit is created between the means for generating currents and the second transistor, and means, connected to the means for generating currents, for producing a first output signal in response to the first current and a second output signal in response to the second current. 
     In one aspect of the invention, the means for generating currents includes a fourth transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the first voltage source and the gate is connected to the second spaced-apart region, and a fifth transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region of the fifth transistor is connected to the gate and the second spaced-apart region of the fourth transistor. In addition, the gate of the fifth transistor is connected to the second spaced-apart region of the first transistor and to the first spaced-apart region of the second transistor, and the second spaced-apart region of the fifth transistor is connected to the gate of the second transistor and to the gate of the third transistor such that the feedback circuit is created between the second transistor and the fifth transistor. The means for generating current also includes a sixth transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the first voltage source, the gate of the sixth transistor is connected to the gate and the second-spaced apart region of the fourth transistor, and the second spaced-apart region is connected to the means for producing output signals. 
     In another aspect, the means for generating currents further includes a seventh transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the second spaced-apart region of the fifth transistor and the gate of the second transistor, the second spaced-apart region of the seventh transistor is connected to the second spaced-apart region of the sixth transistor and the means for producing output signals, and the gate of the seventh transistor is provided with a first clock signal. 
     In yet another aspect, the means for producing output signals includes a feedback loop. 
     In still another aspect, the feedback loop comprises an eighth transistor having a gate provided with a second clock signal. 
     In another aspect of the invention, the first clock signal has a pulse of a first duration, and the second clock signal has a pulse of a second duration. 
     In yet another aspect of the invention, the second duration is longer than the first duration. 
     Also in accordance with the invention, there is provided a sensing circuit for sensing the binary state of a memory cell in a non-volatile memory device that includes a first transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to a first voltage source, and the gate is connected to a second voltage source, and a second transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region of the second transistor is connected to the second spaced-apart region of the first transistor. The sensing circuit also includes a third transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the second spaced-apart region of the second transistor, the gate is electrically connected to the memory cell, and the second spaced-apart region is connected to a ground potential, and means for generating a first current in response to one binary state of the memory cell and a second current in response to another binary state of the memory cell wherein the means for generating currents is connected to the first spaced-apart region and the gate of the second transistor such that a feedback circuit is created between the means for generating currents and the second transistor. The sensing circuit additionally includes means, connected to the means for generating currents, for producing a first output signal in response to the first current and a second output signal in response to the second current, and an eighth transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to an input of the means for providing output signals and the second spaced-apart region is connected to an output of the means for providing output signals, and the gate is provided with a second clock signal. 
     Additionally in accordance with the invention, there is provided a sensing circuit for sensing the binary state of a memory cell in a non-volatile memory device that includes an amplifier electrically connected to the memory cell, a first transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to a first voltage source and the gate is connected to the second spaced-apart region, and a second transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region of the second transistor is connected to the gate and the second spaced-apart region of the first transistor, and the gate and the second spaced-apart region of the second transistor is connected to the amplifier such that a feedback circuit is created between the second transistor and the amplifier. The sensing circuit also includes a third transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the first voltage source, and the gate of the third transistor is connected to the gate and the second-spaced apart region of the first transistor such that a first current is generated in response to one binary state of the memory cell and a second current is generated in response to another binary state of the memory cell. The sensing circuit additionally includes means, connected to the second spaced-apart region of the third transistor, for producing a first output signal in response to the first current and a second output signal in response to the second current. 
     In one aspect of the invention, the sensing circuit includes a fourth transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to the second spaced-apart region of the second transistor and the amplifier and the gate of the fourth transistor is provided with a first clock signal. 
     In another aspect of the invention, the sensing circuit includes a fifth transistor having first and second spaced-apart regions and a gate wherein the first spaced-apart region is connected to an input of the means for producing output signals, the second spaced-apart region is connected to an output of the means for producing output signals, and the gate is provided with a second clock signal. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the objects, advantages, and principles of the invention. 
     In the drawings: 
     FIG. 1 is a schematic diagram of a known sensing circuit; and 
     FIG. 2 is a schematic diagram of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, there is provided a sensing circuit for a non-volatile memory device. FIG. 2 is a diagram of an embodiment of the present invention. Referring to FIG. 2, a sensing circuit 50 includes a plurality of MOS transistors. Sensing circuit 50 is connected to a memory array 52 that includes a plurality of non-volatile memory cells 52a, 52b . . . 52n, only one of which (52a) is shown. Memory array 52 is connected to a sensing line 54 through a multiplexer 56. Sensing line 54 connects to the gate of an n-type transistor 58. The source of transistor 58 is connected to a ground potential and the drain is connected to the source of an n-type transistor 60. The drain of transistor 60 is connected to the drain of a p-type transistor 62. The gate of transistor 62 is connected to a reference potential V REF1 , and the source is connected to a voltage potential V c . Transistors 62, 60, and 58 operate as a voltage amplifier. 
     The gate of transistor 60 is connected to the source of an n-type transistor 64. The drain of transistor 64 is connected to the drain and the gate of a p-type transistor 66, and also the gate of a p-type transistor 68. The source of transistor 66 and the source of transistor 68 are connected to voltage potential V c . Transistors 66, 64, and 68 operate as a current mirror 80. The current mirror generates through transistor 68 a first current in response to one binary state of the memory cell being sensed, for example, memory cell 52a, and a second current in response to another binary state of the sensed memory cell. 
     The current mirror is also connected to the drain and the gate of transistor 60 to create a feedback circuit between the current mirror and transistor 60. Specifically, the drain of transistor 60 is connected to the gate of transistor 64, and the gate of transistor 60 is connected to the source of transistor 64, creating a feedback circuit between transistors 60 and 64. The drain of transistor 68 is connected to an input of an inverter 70 that provides an output S OUT .sbsb.--. A feedback loop is formed with inverter 70 through an n-type transistor 76. The drain and source of transistor 76 are connected to the input and output of inverter 70, respectively. The gate of transistor 76 is provided with a clock signal EQSA to trigger a sensing operation of sensing circuit 50. 
     Sensing line 54 additionally connects to the drain and the gate of an n-type transistor 78. The drain and the gate of transistor 78 are also connected to the source of transistor 64 and to the gate of transistor 60, in addition to the gate of transistor 58. The source of transistor 78 is connected to a ground potential. Sensing circuit 50 also includes an n-type transistor 74. The drain of transistor 74 is connected to the source of transistor 64, the gate of transistor 60, the gate of transistor 58, and the drain and the gate of transistor 78. The source of transistor 74 is connected to the drain of transistor 68, the drain of an n-type transistor 72, the input of inverter 70, and the drain of transistor 76. The gate of transistor 74 is provided with a clock signal EQSA2 to trigger a sensing operation of sensing circuit 50. The gate of transistor 72 is connected to reference voltage V SENREF  , and the source of transistor 72 is connected to a ground potential. V SENREF  may be generated and is used like the V SENREF  in the circuitry of the &#39;158 patent. 
     Clock signal EQSA includes a pulse having a peak-to-peak amplitude of V c , and clock signal EQSA2 includes a pulse having the same peak-to-peak amplitude as that of clock signal EQSA. The duration of the EQSA pulse is longer than that of the EQSA2. The duration of the EQSA2 pulse is approximately 3 to 5 nS. Each clock signal is triggered at the beginning of the sensing operations of sensing circuit 50, and each clock signal is turned off after sensing circuit 50 reaches the linear operating region. 
     In operation, when the memory cell being sensed, in this example cell 52a, is in a conductive state, current flows in sensing line 54, and transistor 60 generates a voltage to maintain the voltage at sensing line 54. The feedback circuit between transistor 60 and the current mirror, comprised of transistors 66, 64, and 68, stabilizes the voltage generated by transistor 60. Specifically, because the feedback circuit is created between transistors 60 and 64, as the gate-to-source voltage, V gs , of transistor 60 increases, the V gs  of transistor 64 decreases. Conversely, when the V gs  of transistor 60 decreases, the V gs  of transistor 64 increases. Therefore, during the transient period, defined as from the cut-off region when sensing circuit 50 is first enabled until sensing circuit 50 reaches the linear operating region, the voltage generated by transistor 60 remains stable and does not generate any transition output noise. 
     During the conductive state, current also flows from transistor 66, through transistor 64, to sensing line 54 to maintain the conductive state of memory cell 52a. Because transistors 66, 64 and 68 constitute a current mirror, a current of the same magnitude also flows through transistor 68 to inverter 70. During the transient period, transistor 74 serves as an additional current mirror, or current loop, for transistor 68 to prevent damping noise from being generated between transistors 66 and 68. Also, transistor 74 provides an additional current path from transistor 68 to sensing line 54. Such an additional current path provides for a faster charging of sensing line 54, and thus shortens the transient period for sensing circuit 50. 
     Additionally, transistor 74 moves the operating point of sensing circuit 50 to a mid-point of sensing voltage between the voltage representing a logical zero &#34;0&#34; and the voltage representing a logical one &#34;1&#34;. For example, sensing logical &#34;1&#34; voltage is 1.9 V, and sensing logical &#34;0&#34; voltage is 1.8 V, than the mid-point of sensing voltage is 1.85 V. Without transistor 74, the operating point during the transient period tends to be closer to the zero state. By moving the operating point, the transient period of sensing circuit 50 is shortened as the time required to read a &#34;010&#34; state and a &#34;101&#34; state will be substantially equal. 
     Clock signal EQSA2 triggers transistor 74 at the beginning of a sensing operation so that the aforementioned additional current loop and current path are established during the transient period. After sensing circuit 50 reaches the linear operating region, EQSA2 is turned off, thereby turning off transistor 74. EQSA2 is a pulse signal having a peak-to-peak amplitude of V c , and the duration of the pulse is approximately 3 to 5 nS, but the duration may be adjusted according to the loading on sensing line 54. 
     In response to the current generated by transistors 66, 64, and 68, inverter 70 generates a first output signal in response to the first current, which corresponds to a conductive state of memory cell 52a and a second output signal in response to the second current, which corresponds to a non-conductive state of memory cell 52a. Transistor 76 creates a feedback loop for inverter 70 to stabilize output S OUT .sbsb.--  by moving the operating point of inverter 70 to approximately the mid-point of sensing voltage. In addition, the current path provided by transistor 76, from inverter 70 through transistor 74, provides yet another current path for which sensing line 54 may be charged. This current path additionally shortens the transient period of sensing circuit 50. 
     The current path created by transistor 76 also stabilizes the current path from transistor 68 to transistor 72. A large current flows through transistor 68 to transistor 72 and inverter 70 during sensing operations. Thus, during the transient period, a transient noise may be generated as inverter 70 abruptly receives a large current from transistors 68 and 72. Therefore, the current path created by transistor 76 moves the operating point along transistor 68 and 72 and inverter 70 to the mid-point of sensing voltage to stabilize the current flow and eliminate transient noises. 
     Similar to clock signal EQSA2, clock signal EQSA triggers transistor 76 at the beginning of a sensing operation. After sensing circuit 50 reaches the liner operating region, EQSA is turned off, thereby turning off transistor 76. EQSA is also a pulse signal having a peak-to-peak amplitude of V c . Clock signals EQSA and EQSA2 are triggered at the same time, but the EQSA pulse has a longer duration than the EQSA2 pulse because the EQSA pulse must account for an additional stage of circuit operations, namely transistors 68 and 72. 
     When memory cell 52a is in a non-conductive state, there is no current flow in sensing line 54. Because transistor 64 is on, the voltage on sensing line 54 will subsequently be charged up. When the voltage on sensing line 54 goes up, the voltage at the gate of transistor 60 drops, generating a second voltage. Transistor 66 continues to supply current to sensing line 54 through transistor 64 until sensing line 54 reaches a saturation level, at which point transistor 78 conducts excess current to ground to prevent the voltage on sensing line 54 from cutting off transistor 64. 
     The transient period for a non-conductive state of memory cell 52a may be longer than the transient period during a conductive state because there is initially no current flow in sensing line 54. The sensing circuit 50 of the present invention shortens the transient period during a non-conductive period by providing two additional current paths to sensing line 54. Specifically, the current paths created by transistors 74 and 76 provide current paths in addition to the path established by transistors 66, 64, and 78, to charge sensing line 54 and move sensing circuit 50 into the linear operating region in a considerably shorter period of time. 
     Although the foregoing discussion is limited to components of specific conductivity-types, the present invention should not be limited to the foregoing components. It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed process and product without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.