Dynamic flip flop

Abstract

It is intended to provide a dynamic flip flop that prevents a floating signal from maintaining a voltage below a substrate voltage level in a P-type semiconductor substrate and that prevents a floating signal from maintaining a voltage exceeding the substrate voltage level in an N-type semiconductor substrate. In the dynamic flip flop, an N-type MOSFET (5)controlled by an output signal MX from an inverter (2) and an N-type MOSFET (6) controlled by an output signal Q from an inverter (4) are provided as switches for short-circuiting a signal M and a signal QX to be brought into a floating state to a substrate.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to a dynamic flip flop that, at points of variation in clocks, prevents a signal line to be brought into a floating state from maintaining a voltage below a substrate voltage level when using a P-type semiconductor substrate and prevents a signal line to be brought into a floating state from maintaining a voltage exceeding a substrate voltage level when using an N-type semiconductor substrate.

[0003] 2. Related Background Art

[0004] FIG. 7 shows an example of a circuit diagram illustrating a dynamic flip flop conventionally used. As shown in FIG. 7, a dynamic flip flop has been known that is composed of a transmission gate 101, an inverter 102, a transmission gate 103, and an inverter 104. The transmission gate 101, into which a signal D is input, outputs a signal M in synchronization with a clock C1 and a clock C1X. The inverter 102 inverts the signal M to output a signal MX. The transmission gate 103, into which the signal MX is input, outputs a signal QX in synchronization with a clock C2 and a clock C2X. The inverter 104 inverts the signal QX to output a signal Q. FIG. 8 shows a diagram illustrating an operation of the dynamic flip flop conventionally used.

[0005] In such a dynamic flip flop, however, when the conduction in the transmission gate 101 ceases according to timing of the clock C1, the signal M is brought into a floating state. When the transmission gate 101 is in a conducting state and the signal M is at a substrate voltage level, the signal M maintains a voltage below the substrate voltage level due to coupling caused by parasitic capacitance generated between the clock C1 and the signal M at the instant when the conduction in the transmission gate 101 ceases. Similarly, when the conduction in the transmission gate 103 ceases according to timing of the clock C2X, the signal M is brought into a floating state. When the transmission gate 103 is in a conducting state and the signal M is at a substrate voltage level, the signal QX maintains a voltage below the substrate voltage level due to coupling caused by parasitic capacitance generated between the clock C2X and the signal QX at the instant when the conduction in the transmission gate 103 ceases. When using a P-type substrate, a PN junction diode between the substrate and a drain with a voltage below the substrate voltage level is subjected to a forward bias and a substrate current flows. When the substrate current flows, electrons are generated. The electrons thus generated are minority carriers in the P-type substrate and become stray carriers. The stray carriers diffuse inside the semiconductor substrate. For example, in a semiconductor device having a built-in light-receiving element such as a photoelectric conversion device, there has been a problem of causing fixed pattern noise since electrons as stray carriers contaminate the light-receiving element and the degree of contamination is not uniform over the whole light-receiving element.

SUMMARY OF THE INVENTION

[0006] A dynamic flip flop of the present invention is characterized by being provided with switches for short-circuiting signal lines to be brought into a floating state to the substrate side according to timing of clocks.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the accompanying drawings:

[0008] FIG. 1 is a circuit diagram showing a dynamic flip flop according to Embodiment 1 of the present invention.

[0009] FIG. 2 is a timing chart as to the dynamic flip flop according to Embodiment 1 of the present invention.

[0010] FIG. 3 is a circuit diagram showing a dynamic flip flop according to Embodiment 2 of the present invention.

[0011] FIG. 4 is a timing chart as to the dynamic flip flop according to Embodiment 2 of the present invention.

[0012] FIG. 5 is a circuit diagram showing a dynamic flip flop according to Embodiment 3 of the present invention.

[0013] FIG. 6 is a circuit diagram showing a dynamic flip flop according to Embodiment 4 of the present invention.

[0014] FIG. 7 is a circuit diagram showing a conventional dynamic flip flop.

[0015] FIG. 8 is a timing chart as to the conventional dynamic flip flop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A shift register according to the present invention is provided with switches for short-circuiting signal lines to be brought into a floating state to the substrate side according to timing of clocks. This allows retention of a voltage below a substrate voltage level to be eliminated in a p-type semiconductor substrate. Similarly, retention of a voltage exceeding a substrate voltage level can be eliminated in an N-type semiconductor substrate.

[0017] Embodiment 1

[0018] FIG. 1 is a circuit diagram showing a dynamic flip flop according to Embodiment 1 of the present invention. FIG. 2 shows an example of timing in respective signal lines in Embodiment 1 of the present invention. When a P-type semiconductor substrate is used, a drain of an N-type MOSFET in a transmission gate 1 is connected to a drain of a P-type MOSFET in the transmission gate 1. A source of the N-type MOSFET in the transmission gate 1 is connected to a source of the P-type MOSFET in the transmission gate 1. Thus, the transmission gate 1 is configured. A clock C1 is input to a gate of the N-type MOSFET in the transmission gate 1. An inverted signal C1X of the clock C1 is input to a gate of the P-type MOSFET in the transmission gate 1. The transmission gate 1 outputs a signal M converted from a signal D input thereinto when the clock C1 reaches a VDD level and the clock C1X reaches a substrate voltage level. An inverter 2, a drain of an N-type MOSFET in a transmission gate 3, and a drain of a P-type MOSFET in the transmission gate 3 are connected to one another. The inverter 2, into which the signal M as an output signal from the transmission gate 1 is input, inverts the signal M to output a signal MX. A source of the N-type MOSFET in the transmission gate 3 is connected to a source of the P-type MOSFET in the transmission gate 3. Thus, the transmission gate 3 is configured. A clock C2 is input to a gate of the N-type MOSFET in the transmission gate 3 and an inverted signal C2X of the clock C2 is input to a gate of the P-type MOSFET in the transmission gate 3.

[0019] The transmission gate 3 outputs a signal QX converted from the signal MX when the clock C2 and the clock C2X reach the VDD level and the substrate voltage level, respectively. In order to eliminate retention of a voltage below the substrate voltage level, which is a characteristic of the present invention, and thus to suppress the generation of minority carriers to a minimum, an N-type MOSFET 5 and an N-type MOSFET 6 are added to a dynamic flip flop of a conventional technique including an inverter 4, into which the signal QX as an output signal from the transmission gate 3 is input, that inverts the signal QX to output a signal Q. In the N-type MOSFET 5, the signal MX as an output signal from the inverter 2 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal M. In the N-type MOSFET 6, the signal Q as an output signal from the inverter 4 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal QX. For convenience of the invention, the transmission gate 1 and the transmission gate 3 are described as transmission gates. Instead of the transmission gates, however, P-type MOSFETs or N-type MOSFETs alone may be included. Furthermore, it is possible to eliminate the retention of a voltage below the substrate voltage level, which is a characteristic of the present invention, even when the clock C1 and the clock C2X are subjected to common connection and the clock C1X and the clock C2 also are subjected to common connection.

[0020] FIG. 2 shows an example of timing in the respective signal lines. When the clock C2 falls and the clock C2X rises in timing 1 and timing 3, the conduction in the transmission gate 3 ceases. Until the conduction in the transmission gate 3 ceases, an output at the substrate voltage level from the inverter 2 is supplied to the signal QX. When the conduction in the transmission gate 3 ceases, the signal QX is not brought into a floating state but is maintained at the substrate voltage level by the N-type MOSFET 6 as a switch for short-circuiting the signal line to the substrate side, which is a characteristic of the present invention. Since the signal QX is maintained at the substrate voltage level, the signal QX is prevented from maintaining a voltage below the substrate voltage level through the coupling caused by parasitic capacitance generated between the clock C2 and the signal QX at the instant when the conduction in the transmission gate 3 ceases. Similarly, when the clock C1 falls and the clock C1X rises in timing 4, the conduction in the transmission gate 1 ceases. Until the conduction in the transmission gate 1 ceases, an input at the substrate voltage level of the signal D is supplied to the signal M. When the conduction in the transmission gate 1 ceases, the signal M is not brought into a floating state but is maintained at the substrate voltage level by the N-type MOSFET 5 as a switch for short-circuiting the signal line to the substrate side, which is a characteristic of the present invention.

[0021] Since the signal M is maintained at the substrate voltage level, the signal M is prevented from maintaining a voltage below the substrate voltage level through the coupling caused by parasitic capacitance generated between the clock C1 and the signal M at the instant when the conduction in the transmission gate 1 ceases.

[0022] As described above, when using a P-type substrate, it was kept to a minimum that a PN junction diode between the substrate and a drain with a voltage below the substrate voltage level was subjected to a forward bias, and thus the generation of electrons was suppressed. According to the results of simulation conducted by the present inventors and others, in the dynamic flip flop of the present invention, the number of generated electrons as minority carriers was reduced to 1/100000 as compared to that in a conventional dynamic flip flop. For instance, in a semiconductor device having a built-in light-receiving element such as a photoelectric conversion device, less electrons as stray carriers contaminated the light-receiving element and the degree of contamination was low. Hence, fixed pattern noise was prevented from being produced.

[0023] Embodiment 2

[0024] FIG. 3 is a circuit diagram showing a dynamic flip flop according to Embodiment 2 of the present invention. FIG. 4 shows an example of timing in respective signal lines in Embodiment 2 of the present invention.

[0025] When using an N-type semiconductor substrate, a drain of an N-type MOSFET in a transmission gate 11 is connected to a drain of a P-type MOSFET in the transmission gate 11. A source of the N-type MOSFET in the transmission gate 11 is connected to a source of the P-type MOSFET in the transmission gate 11. Thus, the transmission gate 11 is configured. A clock C1 is input to a gate of the N-type MOSFET in the transmission gate 11. An inverted signal C1X of the clock C1 is input to a gate of the P-type MOSFET in the transmission gate 11. The transmission gate 11 outputs a signal M converted from a signal D input thereinto when the clock C1 reaches a substrate voltage level and the clock C1X reaches a VSS level. An inverter 12, a drain of an N-type MOSFET in a transmission gate 13, and a drain of a P-type MOSFET in the transmission gate 13 are connected to one another. The inverter 12, into which the signal M as an output signal from the transmission gate 11 is input, inverts the signal M to output a signal MX. A source of the N-type MOSFET in the transmission gate 13 and a source of the P-type MOSFET in the transmission gate 13 are connected to each other. Thus, the transmission gate 13 is configured.

[0026] A clock C2 is input to a gate of the N-type MOSFET in the transmission gate 13 and an inverted signal C2X of the clock C2 is input to a gate of the P-type MOSFET in the transmission gate 13. The transmission gate 13 outputs a signal QX converted from the signal MX when the clock C2 and the clock C2X reach the substrate voltage level and the VSS level, respectively. In order to eliminate retention of a voltage exceeding the substrate voltage level, which is a characteristic of the present invention, and thus to suppress the generation of minority carriers to a minimum, a P-type MOSFET 15 and a P-type MOSFET 16 are added to a dynamic flip flop of a conventional technique including an inverter 14, into which the signal QX as an output signal from the transmission gate 13 is input, that inverts the signal QX to output a signal Q. In the P-type MOSFET 15, the signal MX as an output signal from the inverter 12 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal M. In the P-type MOSFET 16, the signal Q as an output signal from the inverter 14 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal QX. For convenience of the invention, the transmission gate 11 and the transmission gate 13 are described as transmission gates. Instead of the transmission gates, however, P-type MOSFETs or N-type MOSFETs alone may be included.

[0027] Furthermore, it is possible to eliminate the retention of a voltage exceeding the substrate voltage level, which is a characteristic of the present invention, even when the clock Cl and the clock C2X are subjected to common connection and the clock C1X and the clock C2 also are subjected to common connection.

[0028] FIG. 4 shows an example of timing in the respective signal lines. When the clock C2 falls and the clock C2X rises in timing 5, the conduction in the transmission gate 13 ceases. Until the conduction in the transmission gate 13 ceases, an output at the substrate voltage level from the inverter 12 is supplied to the signal QX. When the conduction in the transmission gate 13 ceases, the signal QX is not brought into a floating state but is maintained at the substrate voltage level by the P-type MOSFET 16 as a switch for short-circuiting the signal line to the substrate side, which is a characteristic of the present invention.

[0029] Since the signal QX is maintained at the substrate voltage level, the signal QX is prevented from maintaining a voltage exceeding the substrate voltage level through the coupling caused by parasitic capacitance generated between the clock C2X and the signal QX at the instant when the conduction in the transmission gate 13 ceases. Similarly, when the clock C1 falls and the clock C1X rises in timing 2, the conduction in the transmission gate 11 ceases. Until the conduction in the transmission gate 11 ceases, an input at the substrate voltage level of the signal D is supplied to the signal M. When the conduction in the transmission gate 11 ceases, the signal M is not brought into a floating state but is maintained at the substrate voltage level by the P-type MOSFET 15 as a switch for short-circuiting the signal line to the substrate side, which is a characteristic of the present invention.

[0030] Since the signal M is maintained at the substrate voltage level, the signal M is prevented from maintaining a voltage exceeding the substrate voltage level through the coupling caused by parasitic capacitance generated between the clock C1X and the signal M at the instant when the conduction in the transmission gate 11 ceases.

[0031] As described above, when using an N-type substrate, it was kept to a minimum that a PN junction diode between the substrate and a drain with a voltage exceeding the substrate voltage level was subjected to a forward bias, and thus the production of holes was suppressed. For instance, in a semiconductor device having a built-in light-receiving element such as a photoelectric conversion device, less holes as stray carriers contaminated the light-receiving element and the degree of contamination was low. Hence, fixed pattern noise was prevented from being produced.

[0032] Embodiment 3

[0033] FIG. 5 is a circuit diagram showing a dynamic flip flop according to Embodiment 3 of the present invention.

[0034] When using a P-type semiconductor substrate, an N-type MOSFET 23 and an N-type MOSFET 24 are added to a clocked inverter 21 and a clocked inverter 22 in order to eliminate retention of a voltage below a substrate voltage level, which is a characteristic of the present invention, and thus to suppress the generation of minority carriers to a minimum. The clocked inverter 21 is controlled by a clock C1, functions as an inverter when the clock C1 reaches a VDD level, and inverts a signal D input thereinto to output a signal MX. The clocked inverter 22 is controlled by a clock C2, functions as an inverter when the clock C2 reaches the VDD level, and inverts the signal MX input thereinto to output a signal Q. In the N-type MOSFET 23, the signal MX as an output signal from the clocked inverter 21 is input into its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal D. In the N-type MOSFET 24, the signal Q as an output signal from the clocked inverter 22 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal MX. With such a circuit configuration, it is possible to eliminate the retention of a voltage below the substrate voltage level with no influence exerted by the clock C1 and the clock C2 as in Embodiment 1.

[0035] Embodiment 4

[0036] FIG. 6 is a circuit diagram showing a dynamic flip flop according to Embodiment 4 of the present invention.

[0037] When using an N-type semiconductor substrate, a P-type MOSFET 33 and a P-type MOSFET 34 are added to a clocked inverter 31 and a clocked inverter 32 in order to eliminate retention of a voltage exceeding a substrate voltage level, which is a characteristic of the present invention, and thus to suppress the generation of minority carriers to a minimum. The clocked inverter 31 is controlled by a clock C1, functions as an inverter when the clock C1 reaches a VSS level, and inverts a signal D input thereinto to output a signal MX. The clocked inverter 32 is controlled by a clock C2, functions as an inverter when the clock C2 reaches the VSS level, and inverts the signal MX input thereinto to output a signal Q. In the P-type MOSFET 33, the signal MX as an output signal from the clocked inverter 31 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal D. In the P-type MOSFET 34, the signal Q as an output signal from the clocked inverter 32 is input to its gate, its source is connected to the substrate voltage level, and its drain is connected to the signal MX. With such a circuit configuration, it is possible to eliminate the retention of a voltage exceeding the substrate voltage level with no influence exerted by the clock C1 and the clock C2 as in Embodiment 2.

[0038] The present invention can be implemented in the forms described above and provides an excellent effect as described below. Switches are provided that short-circuit signal lines to be brought into a floating state to the substrate side according to timing of clocks, so that the retention of a voltage below the substrate voltage level can be eliminated in a P-type semiconductor substrate and the retention of a voltage exceeding the substrate voltage level can be eliminated in an N-type semiconductor substrate. Thus, there is provided an effect that the generation of minority carriers can be reduced to a minimum.

[0039] The invention maybe embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

1. A dynamic flip flop, comprising switches for short-circuiting signal lines to be brought into a floating state to a substrate side according to timing of clocks.

2. A dynamic shift register, comprising a dynamic flip flop according to claim 1 used in a shift register.

3. A semiconductor device, comprising a dynamic flip flop according to claim 1 used in a photoelectric conversion device.

4. A semiconductor device, comprising a dynamic shift register according to claim 2 used in a photoelectric conversion device.

5. A dynamic flip flop according to claim 1, further comprising: a first transmission gate, including a first N-type MOSFET with a drain, a source, and a gate, and a first P-type MOSFET with a drain, a source, and a gate, the drain of the first N-type MOSFET being connected to the drain of the first P-type MOSFET, the source of the first N-type MOSFET being connected to the source of the first P-type MOSFET, a first clock C1 being input to the gate of the first N-type MOSFET, the first transmission gate, into which a signal D is input when the first clock C1 is at a VDD level, outputting a signal M, an inverted signal C1X of the first clock C1 being input to the gate of the first P-type MOSFET, and the first transmission gate outputting the signal M converted from the signal D when the inverted signal C1X is at a substrate voltage level; a first inverter, into which the signal M is input, inverting the signal M to output a signal MX; a second transmission gate, including a second N-type MOSFET with a drain, a source, and a gate, and a second P-type MOSFET with a drain, a source, and a gate, the drain of the second N-type MOSFET being connected to the drain of the second P-type MOSFET, the source of the second N-type MOSFET being connected to the source of the second P-type MOSFET, a second clock C2 being input to the gate of the second N-type MOSFET, the second transmission gate, into which the signal MX as an output from the first inverter is input when the second clock C2 is at the VDD level, outputting a signal QX, an inverted signal C2X of the second clock C2 being input to the gate of the second P-type MOSFET, and the second transmission gate outputting the signal QX converted from the signal MX when the inverted signal C2X is at the substrate voltage level; a second inverter, into which the signal QX is input, inverting the signal QX to output a signal Q; a third N-type MOSFET with a gate, a source, and a drain, the signal MX output from the first inverter being input to the gate of the third N-type MOSFET, the source of the third N-type MOSFET being connected to the substrate voltage level, and the drain of the third N-type MOSFET being connected to the signal M; and a fourth N-type MOSFET with a gate, a source, and a drain, the signal Q output from the second inverter being input to the gate of the fourth N-type MOSFET, the source of the fourth N-type MOSFET being connected to the substrate voltage level, and the drain of the fourth N-type MOSFET being connected to the signal QX.

6. A dynamic shift register, comprising a dynamic flip flop according to claim 5 used in a shift register.

7. A semiconductor device, comprising a dynamic flip flop according to claim 5 used in a photoelectric conversion device.

8. A semiconductor device, comprising a dynamic shift register according to claim 6 used in a photoelectric conversion device.

9. A dynamic flip flop according to claim 1, further comprising: a first transmission gate, including a first N-type MOSFET with a drain, a source, and a gate, and a first P-type MOSFET with a drain, a source, and a gate, the drain of the first N-type MOSFET being connected to the drain of the first P-type MOSFET, the source of the first N-type MOSFET being connected to the source of the first P-type MOSFET, a first clock C1 being input to the gate of the first N-type MOSFET, the first transmission gate, into which a signal D is input when the first clock C1 is at a substrate voltage level, outputting a signal M, an inverted signal C1X of the first clock C1 being input to the gate of the first P-type MOSFET, and the first transmission gate outputting the signal M converted from the signal D when the inverted signal C1X is at a VSS level; a first inverter, into which the signal M is input, inverting the signal M to output a signal MX; a second transmission gate, including a second N-type MOSFET with a drain, a source, and a gate, and a second P-type MOSFET with a drain, a source, and a gate, the drain of the second N-type MOSFET being connected to the drain of the second P-type MOSFET, the source of the second N-type MOSFET being connected to the source of the second P-type MOSFET, a second clock C2 being input to the gate of the second N-type MOSFET, the second transmission gate, into which the signal MX as an output from the first inverter is input when the second clock C2 is at the substrate voltage level, outputting a signal QX, an inverted signal C2X of the second clock C2 being input to the gate of the second P-type MOSFET, and the second transmission gate outputting the signal QX converted from the signal MX when the inverted signal C2X is at the VSS level; a second inverter, into which the signal QX is input, inverting the signal QX to output a signal Q; a third P-type MOSFET with a gate, a source, and a drain, the signal MX output from the first inverter being input to the gate of the third P-type MOSFET, the source of the third P-type MOSFET being connected to the substrate voltage level, and the drain of the third P-type MOSFET being connected to the signal M; and a fourth P-type MOSFET with a gate, a source, and a drain, the signal Q output from the second inverter being input to the gate of the fourth P-type MOSFET, the source of the fourth P-type MOSFET being connected to the substrate voltage level, and the drain of the fourth P-type MOSFET being connected to the signal QX.

10. A dynamic shift register, comprising a dynamic flip flop according to claim 9 used in a shift register.

11. A semiconductor device, comprising a dynamic flip flop according to claim 9 used in a photoelectric conversion device.

12. A semiconductor device, comprising a dynamic shift register according to claim 10 used in a photoelectric conversion device.