U.S. patent application number 09/812573 was filed with the patent office on 2001-10-25 for booster circuit for raising voltage by sequentially transferring charges from input terminals of booster units to output terminals thereof in response to clock signals having different phases.
Invention is credited to Tanzawa, Toru.
Application Number | 20010033515 09/812573 |
Document ID | / |
Family ID | 18597515 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010033515 |
Kind Code |
A1 |
Tanzawa, Toru |
October 25, 2001 |
Booster circuit for raising voltage by sequentially transferring
charges from input terminals of booster units to output terminals
thereof in response to clock signals having different phases
Abstract
A booster circuit includes a plurality of booster units whose
output terminals are respectively connected to the input terminals
of the next-stage booster units, and an output circuit section.
Each of the booster units sequentially transfers charges from the
input terminal to the output terminal in response to clock signals
to raise a positive or negative voltage input to a first-stage
booster unit. Then, a node in which charges are left behind is
reset by use of a reset signal after the charge transfer. The
potential of the reset signal for turning ON the resetting
transistor is created from the positive or negative boosted
potential output from the output circuit section.
Inventors: |
Tanzawa, Toru; (Ebina-shi,
JP) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Family ID: |
18597515 |
Appl. No.: |
09/812573 |
Filed: |
March 21, 2001 |
Current U.S.
Class: |
365/189.11 |
Current CPC
Class: |
G11C 5/145 20130101;
G11C 16/08 20130101; G11C 16/30 20130101 |
Class at
Publication: |
365/189.11 |
International
Class: |
G11C 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2000 |
JP |
2000-080410 |
Claims
What is claimed is:
1. A booster circuit comprising: a booster unit including an input
terminal to which one of a power supply potential and ground
potential is applied, an output terminal for outputting one of
positive and negative boosted potentials, first and second clock
input terminals to which first and second clock signals of
different phases are input, and a reset signal input terminal to
which a reset signal is input, said booster unit transferring
charges from the input terminal to the output terminal in response
to the first and second clock signals and turning ON resetting
transistors in response to the reset signal after termination of
the charge transfer to reset gate nodes of transistors in which
charges are left behind; wherein the potential of the reset signal
for turning ON the resetting transistor is created based on one of
the positive and negative boosted potentials output from the output
terminal.
2. The booster circuit according to claim 1, wherein said booster
unit includes a first MOS transistor having a current path
connected at one end thereof to the input terminal and connected at
the other end thereof to the output terminal, a second MOS
transistor having a current path connected at one end thereof to
the input terminal and connected at the other end thereof to a gate
of the first MOS transistor and a gate connected to the output
terminal, a third resetting MOS transistor having a current path
connected at one end thereof to the gate of the first MOS
transistor and applied at the other end thereof with one of a power
supply potential and ground potential and a gate supplied with the
reset signal, a fourth resetting MOS transistor having a current
path connected at one end thereof to the gate of the second MOS
transistor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal, a first capacitor having one electrode
connected to the output terminal and the other electrode supplied
with the first clock signal, and a second capacitor having one
electrode connected to the gate of the first MOS transistor and the
other electrode supplied with the second clock signal.
3. The booster circuit according to claim 1, further comprising a
third capacitor having one electrode connected to the other
electrode of the first capacitor and the other electrode supplied
with the first clock signal; a fifth resetting MOS transistor
having a current path connected at one end thereof to the other
electrode of the first capacitor and applied at the other end
thereof with one of the power supply potential and ground potential
and a gate supplied with the reset signal; a fourth capacitor
having one electrode connected to the other electrode of the second
capacitor and the other electrode supplied with the second clock
signal; and a sixth resetting MOS transistor having a current path
connected at one end thereof to the other electrode of the second
capacitor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal.
4. A booster circuit comprising: a plurality of booster units which
include a first-stage booster unit having an input terminal applied
with one of a power supply potential and ground potential and whose
output terminals are respectively connected to the input terminals
of the next stage booster units; and an output circuit section
controlled by an output potential of a final stage of said booster
units, for transferring an output potential of one of said booster
units which lies in the preceding stage of the final-stage booster
unit; each of said booster units including first and second clock
input terminals to which first and second clock signals of
different phases are input and a reset signal input terminal to
which a reset signal is input, transferring charges from the input
terminal to the output terminal in response to the first and second
clock signals and turning ON resetting transistors in response to
the reset signal after termination of the charge transfer to reset
gate nodes of transistors in which charges are left behind; wherein
the potential of the reset signal for turning ON the resetting
transistors is created based on one of positive and negative
boosted potentials output from said output circuit section.
5. The booster circuit according to claim 4, wherein each of said
booster units includes a first MOS transistor having a current path
connected at one end thereof to the input terminal and connected at
the other end thereof to the output terminal, a second MOS
transistor having a current path connected at one end thereof to
the input terminal and connected at the other end thereof to a gate
of the first MOS transistor and a gate connected to the output
terminal, a third resetting MOS transistor having a current path
connected at one end thereof to the gate of the first MOS
transistor and applied at the other end thereof with one of a power
supply potential and ground potential and a gate supplied with the
reset signal, a fourth resetting MOS transistor having a current
path connected at one end thereof to the gate of the second MOS
transistor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal, a first capacitor having one electrode
connected to the output terminal and the other electrode supplied
with the first clock signal, and a second capacitor having one
electrode connected to the gate of the first MOS transistor and the
other electrode supplied with the second clock signal.
6. The booster circuit according to claim 5, further comprising a
third capacitor having one electrode connected to the other
electrode of the first capacitor and the other electrode supplied
with the first clock signal; a fifth resetting MOS transistor
having a current path connected at one end thereof to the other
electrode of the first capacitor and applied at the other end
thereof with one of the power supply potential and ground potential
and a gate supplied with the reset signal; a fourth capacitor
having one electrode connected to the other electrode of the second
capacitor and the other electrode supplied with the second clock
signal; and a sixth resetting MOS transistor having a current path
connected at one end thereof to the other electrode of the second
capacitor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal.
7. A booster circuit comprising: a plurality of booster units which
include a first-stage booster unit having an input terminal applied
with one of a power supply potential and ground potential and whose
output terminals are respectively connected to the input terminals
of the next stage booster units; and an output circuit section
controlled by an output potential of a final stage of said booster
units, for transferring an output potential of one of said booster
units which lies in the preceding stage of said final-stage booster
unit; a reset pulse generator for using one of positive and
negative boosted potential outputs from said output circuit section
as one of power supply potentials to create a reset signal having
an amplitude between one of the positive and negative boosted
potentials and a ground potential and supplying the reset signal to
each of said booster units to reset gate nodes of transistors of
said booster units in which charges are left behind; and a
discharge circuit for discharging a node of said reset pulse
generator to which one of the positive and negative boosted
potentials is applied after resetting the gate nodes of the
transistors in which charges are left behind.
8. The booster circuit according to claim 7, wherein each of said
booster units includes a first MOS transistor having a current path
connected at one end thereof to the input terminal and connected at
the other end thereof to the output terminal, a second MOS
transistor having a current path connected at one end thereof to
the input terminal and connected at the other end thereof to a gate
of the first MOS transistor and a gate connected to the output
terminal, a third resetting MOS transistor having a current path
connected at one end thereof to the gate of the first MOS
transistor and applied at the other end thereof with one of a power
supply potential and ground potential and a gate supplied with the
reset signal, a fourth resetting MOS transistor having a current
path connected at one end thereof to the gate of the second MOS
transistor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal, a first capacitor having one electrode
connected to the output terminal and the other electrode supplied
with the first clock signal, and a second capacitor having one
electrode connected to the gate of the first MOS transistor and the
other electrode supplied with the second clock signal.
9. The booster circuit according to claim 8, further comprising a
third capacitor having one electrode connected to the other
electrode of the first capacitor and the other electrode supplied
with the first clock signal; a fifth resetting MOS transistor
having a current path connected at one end thereof to the other
electrode of the first capacitor and applied at the other end
thereof with one of the power supply potential and ground potential
and a gate supplied with the reset signal; a fourth capacitor
having one electrode connected to the other electrode of the second
capacitor and the other electrode supplied with the second clock
signal; and a sixth resetting MOS transistor having a current path
connected at one end thereof to the other electrode of the second
capacitor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal.
10. The booster circuit according to claim 8, further comprising a
fifth capacitor having one electrode connected the node of said
reset signal generator applied with one of the positive and
negative boosted potentials and the other electrode applied with
one of the ground potential and power supply potential.
11. The booster circuit according to claim 8, wherein said reset
pulse generator includes a high-level shifter operated on voltage
between the positive voltage and the ground potential, for shifting
a high level of the reset signal to the positive potential, a
low-level shifter operated on voltage between the positive voltage
and a negative boosted potential output from said output circuit
section, for shifting a low level of an output from said high-level
shifter to the negative boosted potential, inverter circuits of odd
stages operated on voltage between the positive voltage and the
negative boosted potential output from said output circuit section,
and a logic circuit supplied with an output signal from an
even-stage inverter circuit and an output signal from an odd-stage
inverter circuit, for supplying the reset signal to each of said
booster units, and said discharge circuit includes a high-level
shifter operated on voltage between the positive voltage and the
ground potential, for shifting a high level of a discharge enable
signal to the positive potential, a low-level shifter operated on
voltage between the positive voltage and the negative boosted
potential output from said output circuit section, for shifting a
low level of an output from said high-level shifter to the negative
boosted potential, inverter circuits of even stages operated on a
voltage between the positive voltage and the negative boosted
potential output from said output circuit section, and a seventh
MOS transistor having a current path connected at one end thereof
to a power supply line of said reset pulse generator to which the
negative boosted potential is applied and connected at the other
end thereof to the ground node and a gate supplied with an output
signal from an even-stage inverter circuit.
12. The booster circuit according to claim 7, wherein said reset
pulse generator includes a high-level shifter operated on voltage
between a positive boosted voltage output from said output circuit
section and the ground potential, for shifting a high level of the
reset signal to the boosted voltage, and inverter circuits of even
stages operated on a voltage between the positive boosted voltage
and the ground potential, for supplying the reset signal to each of
said booster units, and said discharge circuit includes a
high-level shifter operated on voltage between the boosted voltage
and the ground potential, for shifting a high level of a discharge
enable signal to the boosted voltage, inverter circuits of even
stages operated on voltage between the boosted voltage and the
ground potential, and a seventh MOS transistor having a current
path connected at one end thereof to a power supply line of said
reset pulse generator to which the positive boosted potential is
applied and connected at the other end thereof to a power supply
and a gate supplied with an output signal from an even-stage
inverter circuit.
13. A semiconductor memory device comprising: a booster circuit for
raising voltage by sequentially transferring charges in response to
clock signals having different phases; and a circuit operated based
on an output voltage of said booster circuit; said booster circuit
including a plurality of booster units which include a first-stage
booster unit having an input terminal applied with one of a power
supply potential and ground potential and whose output terminals
are respectively connected to the input terminals of the next-stage
booster units, and an output circuit section controlled by an
output potential of a final-stage booster unit, for transferring an
output potential of one of the booster units which lies in the
preceding stage of the final-stage booster unit, each of the
booster units having first and second clock input terminals to
which first and second clock signals of different phases are input
and a reset signal input terminal to which a reset signal is input,
for transferring charges from the input terminal to the output
terminal in response to the first and second clock signals and
turning ON resetting transistors in response to the reset signal
after termination of the charge transfer to reset gate nodes of
transistors in which charges are left behind, wherein the potential
of the reset signal for turning ON the resetting transistors is
created based on one of the positive and negative boosted
potentials output from said output circuit section.
14. The semiconductor memory device according to claim 13, wherein
each of the booster units includes a first MOS transistor having a
current path connected at one end thereof to the input terminal and
connected at the other end thereof to the output terminal, a second
MOS transistor having a current path connected at one end thereof
to the input terminal and connected at the other end thereof to a
gate of the first MOS transistor and a gate connected to the output
terminal, a third resetting MOS transistor having a current path
connected at one end thereof to the gate of the first MOS
transistor and applied at the other end thereof with one of a power
supply potential and ground potential and a gate supplied with the
reset signal, a fourth resetting MOS transistor having a current
path connected at one end thereof to the gate of the second MOS
transistor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal, a first capacitor having one electrode
connected to the output terminal and the other electrode supplied
with the first clock signal, and a second capacitor having one
electrode connected to the gate of the first MOS transistor and the
other electrode supplied with the second clock signal.
15. The semiconductor memory device according to claim 14, further
comprising a third capacitor having one electrode connected to the
other electrode of the first capacitor and the other electrode
supplied with the first clock signal; a fifth resetting MOS
transistor having a current path connected at one end thereof to
the other electrode of the first capacitor and applied at the other
end thereof with one of the power supply potential and ground
potential and a gate supplied with the reset signal; a fourth
capacitor having one electrode connected to the other electrode of
the second capacitor and the other electrode supplied with the
second clock signal; and a sixth resetting MOS transistor having a
current path connected at one end thereof to the other electrode of
the second capacitor and applied at the other end thereof with one
of the power supply potential and ground potential and a gate
supplied with the reset signal.
16. The semiconductor memory device according to claim 13, wherein
said circuit operated on the output voltage of said booster circuit
is a row decoder.
17. A semiconductor memory device comprising: a booster circuit for
raising voltage by sequentially transferring charges in response to
clock signals having different phases; and a circuit operated based
on an output voltage of said booster circuit; said booster circuit
including a plurality of booster units which include a first-stage
booster unit having an input terminal applied with one of a power
supply potential and ground potential and whose output terminals
are respectively connected to the input terminals of the next-stage
booster units, an output circuit section controlled by an output
potential of the final-stage booster unit, for transferring an
output potential of one of the booster units which lies in the
preceding stage of the final-stage booster unit, a reset pulse
generator for using one of positive and negative boosted potentials
output from the output circuit section as one of power supply
potentials to create a reset signal having an amplitude between one
of the positive and negative boosted potentials and a ground
potential and supplying the reset signal to the booster units to
reset gate nodes of transistors of the booster units in which
charges are left behind, and a discharge circuit for discharging a
node of the reset pulse generator to which one of the positive and
negative boosted potentials is applied after resetting the gate
nodes of the transistors in which charges are left behind.
18. The semiconductor memory device according to claim 17, wherein
each of the booster units includes a first MOS transistor having a
current path connected at one end thereof to the input terminal and
connected at the other end thereof to the output terminal, a second
MOS transistor having a current path connected at one end thereof
to the input terminal and connected at the other end thereof to a
gate of the first MOS transistor and a gate connected to the output
terminal, a third resetting MOS transistor having a current path
connected at one end thereof to the gate of the first MOS
transistor and applied at the other end thereof with one of a power
supply potential and ground potential and a gate supplied with the
reset signal, a fourth resetting MOS transistor having a current
path connected at one end thereof to the gate of the second MOS
transistor and applied at the other end thereof with one of the
power supply potential and ground potential and a gate supplied
with the reset signal, a first capacitor having one electrode
connected to the output terminal and the other electrode supplied
with the first clock signal, and a second capacitor having one
electrode connected to the gate of the first MOS transistor and the
other electrode supplied with the second clock signal.
19. The semiconductor memory device according to claim 18, further
comprising a third capacitor having one electrode connected to the
other electrode of the first capacitor and the other electrode
supplied with the first clock signal; a fifth resetting MOS
transistor having a current path connected at one end thereof to
the other electrode of the first capacitor and applied at the other
end thereof with one of the power supply potential and ground
potential and a gate supplied with the reset signal; a fourth
capacitor having one electrode connected to the other electrode of
the second capacitor and the other electrode supplied with the
second clock signal; and a sixth resetting MOS transistor having a
current path connected at one end thereof to the other electrode of
the second capacitor and applied at the other end thereof with one
of the power supply potential and ground potential and a gate
supplied with the reset signal.
20. The semiconductor memory device according to claim 17, further
comprising a fifth capacitor having one electrode connected to a
node of said reset pulse generator to which one of the positive and
negative boosted potentials is applied and the other electrode
applied with one of the ground potential and the power supply
voltage.
21. The semiconductor memory device according to claim 17, wherein
said reset pulse generator includes a high-level shifter operated
on voltage between the positive voltage and the ground potential,
for shifting a high level of the reset signal to the positive
potential, a low-level shifter operated on voltage between the
positive voltage and a negative boosted potential output from said
output circuit section, for shifting a low level of an output from
the high-level shifter to the negative boosted potential, inverter
circuits of odd stages operated on voltage between the positive
voltage and the negative boosted potential output from said output
circuit section, and a logic circuit supplied with an output signal
from an even-stage inverter circuit and an output signal from an
odd-stage inverter circuit, for supplying the reset signal to each
of said booster units, and said discharge circuit includes a
high-level shifter operated on voltage between the positive voltage
and the ground potential, for shifting a high level of a discharge
enable signal to the positive potential, a low-level shifter
operated on voltage between the positive voltage and the negative
boosted potential output from said output circuit section, for
shifting a low level of an output from the high-level shifter to
the negative boosted potential, inverter circuits of even stages
operated on voltage between the positive voltage and the negative
boosted potential output from said output circuit section, and a
seventh MOS transistor having a current path connected at one end
thereof to a power supply line of said reset pulse generator to
which the negative boosted potential is applied and connected at
the other end thereof to the ground node and a gate supplied with
an output signal from an even-stage inverter circuit.
22. The semiconductor memory device according to claim 17, wherein
said reset pulse generator includes a high-level shifter operated
on voltage between a positive boosted voltage output from said
output circuit section and the ground potential, for shifting a
high level of the reset signal to the boosted voltage, and inverter
circuits of even stages operated on voltage between the positive
boosted voltage and the ground potential, for supplying the reset
signal to each of said booster units, and said discharge circuit
includes a high-level shifter operated on voltage between the
boosted voltage and the ground potential, for shifting a high level
of a discharge enable signal to the boosted voltage, inverter
circuits of even stages operated on voltage between the boosted
voltage and the ground potential, and a seventh MOS transistor
having a current path connected at one end thereof to a power
supply line of said reset pulse generator to which the positive
boosted potential is applied and connected at the other end thereof
to a power supply and a gate supplied with an output signal from an
even-stage inverter circuit.
23. The semiconductor memory device according to claim 17, wherein
said circuit operated on the output voltage of said booster circuit
is a row decoder.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2000-080410, filed Mar. 22, 2000, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a booster circuit having a
plurality of booster units (booster cells) whose output terminals
are respectively connected to input terminals of the next-stage
booster units, for raising voltage by sequentially transferring
charges from the input terminals of the booster units to the output
terminals thereof in response to clock signals of different phases
and more particularly to the technique for resetting the gate nodes
of transistors of the booster units in which charges are left
behind.
[0003] FIG. 1 is a circuit diagram showing an example of the
construction of a conventional booster circuit. The booster circuit
is described in IEEE Journal of Solid-State Circuits. Vol. 27, No.
11, November 1992, pp. 1540 to 1546, A. Umezawa et al. "A 5-V-Only
Operation 0.6-.mu.m Flash EEPROM with Row Decoder Scheme in Triple
Well Structure".
[0004] The booster circuit is constructed by booster units (booster
cells) 11a, 11b, booster unit 12 and output circuit section 13.
Each of the booster units 11a, 11b, 12 is constructed by two MOS
transistors (MOSFETs) and two capacitors and the output terminals
thereof are cascade-connected to the input terminals of the
next-stage booster units.
[0005] That is, the booster unit 11a includes N-channel MOS
transistors QN1, QN2 and capacitors C1, C2. One-side ends of the
current paths of the MOS transistors QN1, QN2 are connected to a
power supply terminal 14 to which a power supply voltage VCC is
applied. The other end of the current path of the MOS transistor
QN2 is connected to the gate of the MOS transistor QN1 and the gate
thereof is connected to the other end-of the current path of the
MOS transistor QN1. One electrode of the capacitor C1 is connected
to the other end of the current path of the MOS transistor QN1 and
the other electrode of the capacitor C1 is supplied with a clock
signal phi1. Further, one electrode of the capacitor C2 is
connected to the gate of the MOS transistor QN1 and the other
electrode of the capacitor C2 is supplied with a clock signal
phi3.
[0006] Likewise, the booster unit 11b includes N-channel MOS
transistors QN3, QN4 and capacitors C3, C4. One-side ends of the
current paths of the MOS transistors QN3, QN4 are connected to the
other end of the current path of the MOS transistor QN1. The other
end of the current path of the MOS transistor QN4 is connected to
the gate of the MOS transistor QN3 and the gate thereof is
connected to the other end of the current path of the MOS
transistor QN3. One electrode of the capacitor C3 is connected to
the other end of the current path of the MOS transistor QN3 and the
other electrode of the capacitor C3 is supplied with a clock signal
phi2. Further, one electrode of the capacitor C4 is connected to
the gate of the MOS transistor QN3 and the other electrode of the
capacitor C4 is supplied with a clock signal phi4.
[0007] The booster unit 12 includes N-channel MOS transistors QN5,
QN6 and capacitors C5, C6. One-side ends of the current paths of
the MOS transistors QN5, QN6 are connected to the other end of the
current path of the MOS transistor QN3. The other end of the
current path of the MOS transistor QN6 is connected to the gate of
the MOS transistor QN5 and the gate thereof is connected to the
other end of the current path of the MOS transistor QN5. One
electrode of the capacitor C5 is connected to the other end of the
current path of the MOS transistor QN5 and the other electrode of
the capacitor C5 is supplied with the clock signal phi1. Further,
one electrode of the capacitor C6 is connected to the gate of the
MOS transistor QN5 and the other electrode of the capacitor C6 is
supplied with a clock signal phi3.
[0008] The output circuit section 13 is constructed by an N-channel
MOS transistor QN7. One end of the current path of the MOS
transistor QN7 is connected to the other end of the current path of
the MOS transistor QN3 and the gate thereof is connected to the
gate of the MOS transistor QN5. Further, a positive voltage VPP
obtained by raising or boosting the power supply voltage VCC is
output from the other end of the current path of the MOS transistor
QN7.
[0009] With the above construction, when four-phase clock signals
phi1, phi2, phi3, phi4 of different phases are input as shown in
the timing chart of FIG. 2, the power supply voltage VCC is
sequentially raised by the booster circuits 11a, 11b for each cycle
of the clock signals and supplied to one end of the current path of
the MOS transistor QN7. Further, the boosted or raised voltage is
supplied to and further raised by the booster unit 12 to produce
voltage VG which is in turn supplied to the gate of the MOS
transistor QN7. Thus, the gate of the transfer MOS transistor QN7
of the final stage is overdriven by the booster unit 12 so as to
prevent a lowering in the output voltage VPP by the threshold
voltage of the MOS transistor QN7.
[0010] In the above four-phase booster circuit, if a high voltage
is left behind on the gate node of each MOS transistor in an
electrically floating state after termination of the boosting
operation, the transfer MOS transistors QN1, QN3, QN7 maintain the
ON state at the time of re-boosting operation and the boosting
operation cannot be effected.
[0011] Therefore, as shown in FIG. 3, the construction in which
resetting N-channel MOS transistors QN8 to QN13 are respectively
connected between the connection nodes of the gates of the MOS
transistors QN1 to QN6 and the capacitors C1 to C6 and a ground
node (GND) is proposed. A reset signal RST is supplied to the gates
of the MOS transistors QN8 to QN13.
[0012] With the above construction, the gate voltage VG of the
transfer MOS transistor QN7 of the final stage is raised for each
cycle of the clock signals phi1 to phi4 as shown in the timing
chart of FIG. 4 and the output voltage VPP is raised to a desired
voltage. Then, if the reset signal RST is set to the high level at
the time of termination of the boosting operation to turn ON the
MOS transistors QN8 to QN13 and ground and discharge the connection
nodes of the gates of the MOS transistors QN1 to QN6 and the
capacitors C1 to C6, then the transfer MOS transistors QN1, QN3,
QN7 can be forcedly turned OFF, thereby making it possible to
prevent occurrence of an erroneous operation in the re-boosting
operation.
[0013] Further, as shown in FIG. 5, the construction in which a
resetting N-channel MOS transistor QN21 is connected between the
output node of the power supply voltage VPP and the ground node GND
in the booster circuit shown in FIG. 3 is also known in the art.
After termination of the boosting operation, the output node is
grounded and reset by supplying the reset signal RST to the gate of
the MOS transistor QN21.
[0014] Thus, in the case of the booster circuit for generating the
positive voltage, each node can be relatively easily reset by using
the N-channel MOS transistor having a source grounded and a gate
supplied with the reset signal having an amplitude between the
ground potential and the power supply voltage VCC.
[0015] Further, in the above document, a booster circuit for
generating a negative voltage as shown in FIG. 6 is also disclosed.
The circuit includes P-channel MOS transistors QP1 to QP7 instead
of the N-channel MOS transistors QN1 to QN7 shown in FIG. 1. The
input terminal of the first-stage booster unit 11a, that is, one
end of the current path of each of the MOS transistors QP1, QP2 is
connected to the ground node GND instead of the power supply
terminal 14.
[0016] Likewise, a booster circuit for generating a negative
boosted voltage as shown in FIG. 7 and including P-channel MOS
transistors QP1 to QP13 instead of the N-channel MOS transistors
QN1 to QN13 shown in FIG. 3 can be provided.
[0017] If the booster circuit for generating a negative boosted
voltage as shown in FIG. 7 is constructed, it is necessary to
supply a reset signal RST' with an amplitude between the ground
potential and a preset negative boosted potential to the gates of
the resetting P-channel MOS transistors QP8 to QP13. However, since
the normal resetting signal RST is a signal whose amplitude lies
between the ground potential and the power supply potential, the
reset signal cannot be simply supplied to the gates of the
P-channel MOS transistors QP8 to QP13. In order to create the rest
signal RST', it is of course possible to provide a separate
negative voltage generating circuit, but there occurs a problem
that the circuit scale is made large and the control operation
becomes complicated.
[0018] As described above, in the conventional booster circuit, it
is required to create a reset signal having a large amplitude in
order to reset the gate nodes of the transistors in which charges
are left behind and a problem that the circuit scale is made large
and the control operation becomes complicated occurs if a separate
voltage generating circuit is used to create the reset signal.
BRIEF SUMMARY OF THE INVENTION
[0019] Accordingly, an object of this invention is to provide a
booster circuit capable of suppressing an increase in the circuit
scale and simplifying the control operation even if it is so
constructed as to reset the gate nodes of transistors in which
charges are left behind.
[0020] Another object of this invention is to provide a
semiconductor memory device having a booster circuit capable of
suppressing an increase in the circuit scale and simplifying the
control operation even if it is so constructed as to reset the gate
nodes of transistors in which charges are left behind.
[0021] The above object of this invention can be attained by a
booster circuit comprising a booster unit including an input
terminal to which a power supply potential or ground potential is
applied, an output terminal for outputting a positive or negative
boosted potential, first and second clock input terminals to which
first and second clock signals of different phases are input, and a
reset signal input terminal to which a reset signal is input, the
booster unit transferring charges from the input terminal to the
output terminal in response to the first and second clock signals
and turning ON resetting transistors in response to the reset
signal after termination of the charge transfer to reset the gate
nodes of transistors in which charges are left behind; wherein the
potential of the reset signal for turning ON the resetting
transistor is created based on the positive or negative boosted
potential output from the output terminal.
[0022] Further, the above object of this invention can be attained
by a booster circuit comprising a plurality of booster units which
includes a first-stage booster unit having an input terminal
applied with a power supply potential or ground potential and whose
output terminals are respectively connected to the input terminals
of the next stage booster units; and an output circuit section
controlled by an output potential of the final-stage booster unit,
for transferring an output potential of one of the booster units
which lies in the preceding stage of the final-stage booster unit;
wherein each of the booster units includes first and second clock
input terminals to which first and second clock signals of
different phases are input and a reset signal input terminal to
which a reset signal is input, transfers charges from the input
terminal to the output terminal in response to the first and second
clock signals and turns ON resetting transistors in response to the
reset signal after termination of the charge transfer to reset the
gate nodes of transistors in which charges are left behind and the
potential of the reset signal for turning ON the resetting
transistor is created based on the positive or negative boosted
potential output from the output circuit section.
[0023] Further, the above object of this invention can be attained
by a booster circuit comprising a plurality of booster units
including a first-stage booster unit having an input terminal
applied with a power supply potential or ground potential and whose
output terminals are respectively connected to the input terminals
of the next stage booster units; an output circuit section
controlled by an output potential of the final-stage booster unit,
for transferring an output potential of one of the booster units
which lies in the preceding stage of the final-stage booster unit;
a reset pulse generator for using a positive or negative boosted
potential output from the output circuit section as one of power
supply potentials to create a reset signal having an amplitude
between the positive or negative boosted potential and a ground
potential and supplying the reset signal to each of the booster
units to reset the gate nodes of transistors of the booster units
in which charges are left behind; and a discharge circuit for
discharging the node of the reset pulse generator to which the
positive or negative boosted potential is applied after resetting
the gate nodes of the transistors in which charges are left
behind.
[0024] The above object of this invention can be attained by a
semiconductor memory device comprising a booster circuit for
raising voltage by sequentially transferring charges in response to
clock signals having different phases; and a circuit operated based
on an output voltage of the booster circuit; wherein the booster
circuit includes a plurality of booster units including a
first-stage booster unit having an input terminal applied with a
power supply potential or ground potential and whose output
terminals are respectively connected to the input terminals of the
next stage booster units, and an output circuit section controlled
by an output potential of the final-stage booster unit, for
transferring an output potential of one of the booster units which
lies in the preceding stage of the final-stage booster unit, each
of the booster units has first and second clock input terminals to
which first and second clock signals of different phases are input
and a reset signal input terminal to which a reset signal is input,
transfers charges from the input terminal to the output terminal in
response to the first and second clock signals and turns ON
resetting transistors in response to the reset signal after
termination of the charge transfer to reset the gate nodes of
transistors in which charges are left behind, and the potential of
the reset signal for turning ON the resetting transistor is created
based on the positive or negative boosted potential output from the
output circuit section.
[0025] Further, the above object of this invention can be attained
by a semiconductor memory device comprising a booster circuit for
raising voltage by sequentially transferring charges in response to
clock signals having different phases; and a circuit operated based
on an output voltage of the booster circuit; wherein the booster
circuit includes a plurality of booster units including a
first-stage booster unit having an input terminal applied with a
power supply potential or ground potential and whose output
terminals are respectively connected to the input terminals of the
next stage booster units, an output circuit section controlled by
an output potential of the final-stage booster unit, for
transferring an output potential of one of the booster units which
lies in the preceding stage of the final-stage booster unit, a
reset pulse generator for using a positive or negative boosted
potential output from the output circuit section as one of power
supply potentials to create a reset signal having an amplitude
between the positive or negative boosted potential and a ground
potential and supplying the reset signal to each of the booster
units to reset the gate nodes of transistors of the booster units
in which charges are left behind, and a discharge circuit for
discharging the node of the reset pulse generator to which the
positive or negative boosted potential is applied after resetting
the gate nodes of the transistors in which charges are left
behind.
[0026] With the above construction, since the output voltage of the
booster section is used for creating the reset signal, the booster
circuit can be constructed by use of a simple logic circuit, it
becomes unnecessary to use a separate negative voltage generating
circuit, and the gate nodes of the transistors of the booster unit
in which charges are left behind can be reset by a relatively
simple control operation. Therefore, a booster circuit capable of
suppressing an increase in the circuit scale and making the control
operation simple and a semiconductor memory device having the
booster circuit can be provided.
[0027] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0028] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0029] FIG. 1 is a circuit diagram showing an example of the
construction of a conventional booster circuit;
[0030] FIG. 2 is a timing chart for illustrating the operation of
the booster circuit shown in FIG. 1;
[0031] FIG. 3 is a circuit diagram showing another example of the
construction of the conventional booster circuit;
[0032] FIG. 4 is a timing chart for illustrating the operation of
the booster circuit shown in FIG. 3;
[0033] FIG. 5 is a circuit diagram showing still another example of
the construction of the conventional booster circuit;
[0034] FIG. 6 is a circuit diagram showing an example of the
construction of a conventional booster circuit for generating a
negative voltage;
[0035] FIG. 7 is a circuit diagram showing another example of the
construction of the conventional booster circuit for generating a
negative voltage;
[0036] FIG. 8 is a block diagram showing the schematic construction
of a flash memory system, for illustrating a booster circuit
according to a first embodiment of this invention and a
semiconductor memory device having the booster circuit;
[0037] FIG. 9 is a cross sectional view of a memory cell in a
memory cell array shown in FIG. 8;
[0038] FIG. 10 is a characteristic diagram showing the relation
between the control gate voltage and drain current of a memory cell
transistor shown in FIG. 9;
[0039] FIG. 11 is a circuit diagram showing an extracted portion of
the memory cell array of FIG. 8 having the memory cell transistors
shown in FIG. 9 arranged in an array form;
[0040] FIG. 12 is a block diagram specifically showing an extracted
portion of the memory cell array and row decoders of the flash
memory system shown in FIG. 8;
[0041] FIG. 13 is a circuit diagram showing one of the row decoders
of FIG. 12 as a representative in detail;
[0042] FIG. 14A is a circuit diagram showing an example of the
construction of a VCG1 pre-decoder circuit for supplying a
pre-decode signal to the row decoder in the circuit shown in FIG.
13;
[0043] FIG. 14B is a circuit diagram showing an example of the
construction of a GAm pre-decoder circuit for supplying a
pre-decode signal to the row decoder in the circuit shown in FIG.
13;
[0044] FIG. 14C is a circuit diagram showing an example of the
construction of a GBn pre-decoder circuit for supplying a
pre-decode signal to the row decoder in the circuit shown in FIG.
13;
[0045] FIG. 15 is a circuit diagram showing a four-phase type
negative voltage generating booster circuit, for illustrating the
booster circuit according to the first embodiment of this invention
in detail;
[0046] FIG. 16A is a circuit diagram showing an example of the
concrete construction of the circuit shown in FIG. 15 and an
example of the construction of a high level shifter;
[0047] FIG. 16B is a circuit diagram showing an example of the
concrete construction of the circuit shown in FIG. 15 and an
example of the construction of a low level shifter;
[0048] FIG. 17A is a circuit diagram showing an example of the
concrete construction of the circuit shown in FIG. 15 and an
example of the construction of an inverter circuit;
[0049] FIG. 17B is a circuit diagram showing an example of the
concrete construction of the circuit shown in FIG. 15 and another
example of the construction of the inverter circuit;
[0050] FIG. 18 is a circuit diagram for illustrating an example of
the construction of a booster section in the circuit shown in FIG.
15;
[0051] FIG. 19 is a timing chart for illustrating the operations of
the booster circuits shown in FIGS. 15 to 18;
[0052] FIG. 20 is a circuit diagram showing a four-phase type
positive voltage generating booster circuit, for illustrating a
booster circuit according to a second embodiment of this
invention;
[0053] FIG. 21 is a circuit diagram for illustrating an example of
the construction of a booster section in the circuit shown in FIG.
20;
[0054] FIG. 22 is a timing chart for illustrating the operations of
the circuits shown in FIGS. 20 and 21; and
[0055] FIG. 23 is a circuit diagram showing another example of the
construction of the booster section of the first embodiment shown
in FIG. 15, for illustrating a booster circuit according to a third
embodiment of this invention.
DETAILED DESCRIPTION OF THE INVENTION
[First Embodiment]
[0056] A booster circuit according to a first embodiment of this
invention and a semiconductor memory device having the booster
circuit are explained by taking a flash memory system as an
example.
[0057] FIG. 8 is a block diagram showing the schematic construction
of the flash memory system. The flash memory system includes a
memory cell array 101, row decoder 102, column decoder 103, column
gates 104, sense amplifier 105, program circuit 106, positive
charge pump 107, negative charge pump 108, regulator 109, I/O
buffer 110, controller 111, command register 112, address buffer
113 and the like.
[0058] The address buffer 113 is supplied with an address Add of a
memory cell subjected to the readout, program or erase process. The
I/O buffer 110 outputs memory cell data Dout sensed and amplified
by the sense amplifier 105 at the readout time and is supplied with
program data Din at the program time to apply a program voltage
from the program circuit 106 to the drain of the memory cell. The
command register 112 holds a command which is input for controlling
the program or erase process. The controller 111 generates various
control signals based on the command held in the command register
112 and supplies the control signals to the sense amplifier 105,
program circuit 106, positive charge pump 107 and regulator 109 to
control the whole operation of the memory system. Further, the row
decoder 102 selects one of word lines provided in the memory cell
array 101 and the column decoder 103 selects one of the column
gates 104 to connect one of bit lines provided in the memory cell
array 101 to the sense amplifier 105 or program circuit 106. The
positive charge pump 107 generates a positive voltage for the
readout, program or erase process. An output voltage of the
negative charge pump 108 is supplied to the row decoder 102 at the
erase time to bias the word line selected for the erase process to
a negative voltage. The regulator 109 generates a controlled
voltage Vreg and supplies the voltage to the selected word line via
the row decoder 102.
[0059] FIG. 9 is a cross sectional view of a memory cell in the
memory cell array 101 shown in FIG. 8. The memory cell (memory cell
transistor) is constructed by a MOSFET having a so-called stacked
gate structure in which a floating gate FG and control gate CG are
stacked with an insulating film disposed therebetween. That is, in
this example, an n-type well region 201 is formed on a p-type
semiconductor substrate 200 and a p-type well region 202 is formed
on the n-type well region 201. On the surface area of the p-type
well region 202, an n.sup.+-type impurity diffused region 203
acting as the drain region of the MOSFET, an n.sup.+-type impurity
diffused region 204 acting as the source region and an p.sup.+-type
impurity diffused region 205 are formed. On a portion of the
substrate 200 which lies between the impurity diffused regions 203
and 204, a gate insulating film 206, floating gate FG, insulating
film 207 and control gate CG are stacked. Further, on the surface
area of the n-type well region 201, an n.sup.+-type impurity
diffused region 208 is formed and the impurity diffused region 208
is connected to the impurity diffused regions 204 and 205. In
addition, a p.sup.+-type impurity diffused region 209 is formed on
the main surface of the substrate 200 and connected to the ground
node.
[0060] In the above memory cell transistor, the threshold voltage
as viewed from the control gate CG varies according to the number
of electrons stored in the floating gate FG and data "0" or "1" is
stored according to a variation in the threshold voltage.
[0061] FIG. 10 shows the relation between the control gate voltage
and the drain current of the memory cell transistor shown in FIG.
9. In this example, a state in which the number of electrons stored
in the floating gate FG is relatively large (that is, the threshold
voltage Vt of the memory cell transistor is high) is defined as "0"
data and a state in which the number of electrons is small is
defined as "1" data. The bias condition for data readout, erase and
program is shown in the following table 1.
1TABLE 1 OPERATION VOLTAGE OF CELL Read Program Erase Vg 5 V 9 V -7
V Vd 1 V 5 V("0") Floating 0 V("1") Vs 0 V 0 V 10 V
[0062] The data readout operation is effected by respectively
applying the voltage Vd (=1 V), voltage Vs (=0 V) and voltage Vg
(=5 V) to the drain, source and control gate of the memory cell
transistor and "1" or "0" of stored data is determined based on
whether a cell current flows or not.
[0063] Further, the erase operation is simultaneously effected for
a plurality of memory cells commonly having the source and p-type
well region 202. Electrons flow from the floating gate FG to the
substrate by an F-N tunnel phenomenon by setting the drain into an
electrically floating state, setting the source voltage Vs to 10 V
and setting the control gate voltage Vg to -7 V and memory cells to
be subjected to the erase operation are all set to have data
"1".
[0064] On the other hand, the program operation is effected for
each bit line. The bit line of a cell into which "0" is to be
programmed is biased to 5 V (drain voltage Vd=5 V) while the source
voltage Vs is set at 0 V and the control gate voltage Vg is set at
9 V so as to cause high energy electrons generated by a channel hot
electron phenomenon to be injected into the floating gate. At this
time, if the bit line of a cell which is desired to be kept at "1"
is set at 0 V (drain voltage Vd=0 V), no injection of electrons
occurs and the threshold voltage Vt is not changed.
[0065] Next, in order to verify the program or erase operation, the
program verify operation or erase verify operation is effected. The
program verify operation is to read out "0" by setting the control
gate voltage Vg to a higher voltage Vpv in comparison with the
voltage set at the readout time. Then, the program operations and
program verify operations are repeatedly and alternately effected
and when all of the cells to be programmed are set to have "0", the
program operation is terminated. Likewise, in the erase operation,
the erase verify operation for reading out "1" by applying a lower
voltage Vev in comparison with the voltage set at the readout time
to the control gate CG so as to attain a sufficiently large cell
current Icell.
[0066] FIG. 11 shows an extracted portion of the memory cell array
having the above memory cell transistors arranged in an array form.
The control gates of the memory cell transistors MC on the same row
are connected to a corresponding one of word lines WL0 to WLn. The
drains of the memory cell transistors on the same column are
connected to a corresponding one of bit lines BL0 to BLm and the
sources of the memory cell transistors are commonly connected to a
ground node Vss (source line).
[0067] FIG. 12 is a block diagram specifically showing an extracted
portion of the memory cell array 101 and row decoder 102 of the
flash memory system shown in FIG. 8. In this example, the
construction of cell array block MCA commonly using one source line
and a row decoder array associated therewith is shown.
[0068] The cell array block MCA of FIG. 12 has i.times.j memory
cells, i bit lines BL0 to BL(i-1), j word lines WL0 to WL(j-1) and
one source line SL. In this case, i=1024 and j=512, that is, the
cell array block MCA has a construction of 512 word lines.times.1k
bit lines and is constructed by eight sub-cell array blocks MCAB0
to MCAB7 each having a bit capacity of 8 kbytes. The sub-cell array
blocks MCAB0 to MCAB7 commonly use the 1024 bit lines BL0 to
BL(i-1) and one source line SL. A row decoder array RDA includes
eight row decoder blocks RDB0 to RDB7 respectively provided for the
eight sub-cell array blocks MCAB0 to MCAB7. Each of the row decoder
blocks RDB0 to RDB7 includes eight row decoders RD and the whole
portion of the row decoder array has 64 row decoders RD in total.
Each row decoder Rd decodes pre-decode signals GAm, GBn, VCGl (m=0
to 7, n=0 to 7, 1=0 to 7).
[0069] FIG. 13 shows one of the row decoders RD of FIG. 12 as a
representative in detail. The row decoder Rd includes a NAND gate
NA18 supplied with the pre-decode signals GAm, GBn, a level shifter
LS supplied with an output of the NAND gate NA18, and eight word
line drivers WLD0 to WLD7 each supplied with outputs of the level
shifter LS. The signal VCGl (l=0 to 7) is supplied to a
corresponding one of the word line drivers as a word line driver
voltage.
[0070] In the row decoder RD, a first-stage NMOS input type CMOS
differential circuit of the level shifter LS is connected between a
VSW node (for example, VSW=3 V) and the ground node. A next-stage
PMOS input type CMOS differential circuit is connected between the
VSW node and a VBB node (VBB is a negative voltage of -7.5 V). The
word line drivers WLD0 to WLD7 are connected between the respective
word line driver voltage sources (VCGl) and the ground node.
[0071] In the row decoder RD with the above construction, driving
signals for eight word lines WLmnl (l=0 to 7) are created based on
the pre-decode signals GAm, GBn, VCGl, and in the whole portion of
the row decoder array, driving signals for 512 word lines WLmnl can
be selectively created by a combination of m, n, l.
[0072] In this case, at the erase time, VBB is supplied as the word
line driver voltage source signal VCGl as will be described later
and the output potential of each row decoder RD of the selected row
decoder block selected by the pre-decode signals GAm, GBn is set to
VBB. On the other hand, the output potential of each row decoder of
the non-selected row decoder block which is not selected is set to
0 V.
[0073] FIG. 14A shows an example of the construction of a VCGl
pre-decoder circuit for creating a pre-decode signal VCGl in the
circuit shown in FIG. 13.
[0074] The VCGl pre-decoder circuit includes a NAND gate NA19,
level shifter LS and CMOS inverter (driver) IV23. Signals RA0 to
RA2, {overscore (RA0)} to {overscore (RA2)} which are part of
complementary internal row address signals RA0 to RA8, {overscore
(RA0)} to {overscore (RA8)} and an erase mode signal {overscore
(ERA)} are supplied to the NAND gate NA19. An output of the NAND
gate NA19 is input to the level shifter LS. An output of the level
shifter LS is input to the CMOS inverter (driver) IV23.
[0075] In the VCGl pre-decoder circuit, the level shifter LS has
substantially the same construction as the level shifter LS shown
in FIG. 13 and the CMOS inverter IV23 is connected between the VSW
node and the VBB node.
[0076] The VCGl pre-decoder circuit with the above construction
decodes the signals RA0 to {overscore (RA2)}, {overscore (RA0)} to
{overscore (RA2)} and outputs a pre-decode signal VCGl when the
erase mode signal {overscore (ERA)} is set in the non-activated
state ("H" level) (at the readout/program time). On the other hand,
if the erase mode signal {overscore (ERA)} is set in the activated
state ("L" level), it outputs VBB as the pre-decode signal
VCGl.
[0077] FIG. 14B shows an example of the construction of a GAm
pre-decoder circuit for creating a pre-decode signal GAm in the
circuit shown in FIG. 13. The GAm pre-decoder circuit includes NAND
gates NA20, NA21. Complementary internal row address signals RA3 to
RA5, {overscore (RA3)} to {overscore (RA5)} are input to the NAND
gate NA20. An output of the NAND gate NA20 and erase mode signal
{overscore (ERA)} are input to the NAND gate NA21.
[0078] The GAm pre-decoder circuit decodes the signals RA3 to RA5,
{overscore (RA3)} to {overscore (RA5)} and outputs a pre-decode
signal GAm when the erase mode signal {overscore (ERA)} is set in
the non-activated state ("H" level) (at the readout/program time).
On the other hand, if the erase mode signal {overscore (ERA)} is
set in the activated state ("L" level), it outputs an "H" level as
the pre-decode signal GAm.
[0079] FIG. 14C shows an example of the construction of a GBn
pre-decoder circuit for creating a pre-decode signal GBn in the
circuit shown in FIG. 13. The GBn pre-decoder circuit includes a
NAND gate NA22 and inverter IV24. Complementary internal row
address signals RA6 to RA8, {overscore (RA6)} to {overscore (RA8)}
are input to the NAND gate NA22 and an output of the NAND gate NA22
is input to the inverter IV24.
[0080] The GBn pre-decoder circuit decodes the signals RA6 to RA8,
{overscore (RA6)} to {overscore (RA8)} and outputs a pre-decode
signal GBn. In this case, at the erase time, a 3-bit binary signal
generated from a binary counter provided inside the chip is
supplied as the signals RA6 to RA8, {overscore (RA6)} to {overscore
(RA8)} as will be described later. Therefore, the pre-decode signal
GBn functions as a row decoder block selection signal for
alternatively selecting one of the eight row decoder blocks RDB0 to
RDB7.
[0081] FIG. 15 illustrates the booster circuit according to the
first embodiment of this invention in detail and shows a four-phase
type negative voltage generating booster circuit for creating a
negative voltage VBB which corresponds to the negative charge pump
108 of the flash memory system shown in FIG. 8. The circuit is a
so-called voltage lowering circuit, but it is explained as a
negative voltage generating booster circuit in the following
description.
[0082] The four-phase type negative voltage generating booster
circuit includes a reset signal generating section 21, booster
section 22 and transistor section 23. The transistor section 23 is
provided between the booster section 22 and another circuit, the
ON/OFF state thereof is controlled by a switching signal SWN and it
supplies or interrupts supply of a negative voltage VBB output from
the booster section 22 to the other circuit acting as a load, for
example, the row decoder RD.
[0083] The reset signal generating section 21 includes a reset
pulse generator 24, discharge circuit 25 and capacitor 26. The
reset pulse generator 24 includes a high-level shifter 27,
low-level shifter 28, inverter circuits 29 to 31, N-channel MOS
transistors QN50, QN51 and P-channel MOS transistor QP50. The
output terminals of the high-level shifter 27, low-level shifter 28
and inverter circuits 29 to 31 are respectively connected to the
input terminals of the next-stage circuits, that is, they are
cascade-connected. The high-level shifter 27 is operated on voltage
between the positive voltage VSW and the ground node GND and the
low-level shifter 28 and inverter circuits 29 to 31 are operated on
voltage between the positive voltage VSW and the negative voltage
VBB output from the booster section 22. The positive voltage VSW is
3 V, for example, but may be set at the power supply voltage VCC or
another voltage created based on the power supply voltage VCC. The
reset signal RST is supplied to the input terminal IN of the
high-level shifter 27 and signals output from the output terminals
OUT, OUTB of the high-level shifter 27 are supplied to the input
terminals IN, INB of the low-level shifter 28. A signal output from
the output terminal OUT of the low-level shifter 28 is supplied to
the input terminal of the inverter circuit 29, and an output signal
of the inverter circuit 29 is supplied to the input terminal of the
inverter circuit 30. An output signal of the inverter circuit 30 is
supplied to the input terminal of the inverter circuit 31 and the
gates of the MOS transistors QP50, QN51. An output signal of the
inverter circuit 31 is supplied to the gate of the MOS transistor
QN50.
[0084] One-side ends of the current paths of the MOS transistors
QN50, QP50 are connected to the ground node GND and the other ends
thereof are commonly connected to one end of the current path of
the MOS transistor QN51. The other end of the current path of the
MOS transistor QN51 is connected to a power supply line 32. The
power supply line 32 is supplied with the negative voltage VBB
generated from the booster section 22. An internal reset signal
RSTHB is generated from the connection node of the current paths of
the MOS transistors QN50, QN51, QP50 and supplied to the booster
section 22. The internal reset signal RSTHB is set at 0 V when the
reset signal RST is set at the low level and it is set at the VBB
level when the reset signal is set at the high level.
[0085] The discharge circuit 25 includes a high-level shifter 33,
low-level shifter 34, inverter circuits 35, 36 and N-channel MOS
transistor QN52. The high-level shifter 33, low-level shifter 34
and inverter circuits 35, 36 are cascade-connected. The high-level
shifter 33 is operated on voltage between the positive voltage VSW
and the ground node GND and the low-level shifter 34 and inverter
circuits 35, 36 are operated on voltage between the positive
voltage VSW and the negative voltage VBB output from the booster
section 22. A discharge signal GNDEN is supplied to the input
terminal IN of the high-level shifter 33 and signals output from
the output terminals OUT, OUTB of the high-level shifter 33 are
supplied to the input terminals IN, INB of the low-level shifter
34. A signal output from the output terminal OUT of the low-level
shifter 34 is supplied to the input terminal of the inverter
circuit 35, and an output signal of the inverter circuit 35 is
supplied to the input terminal of the inverter circuit 36. An
output signal of the inverter circuit 36 is supplied to the gate of
the MOS transistor QN52. One end of the current path of the MOS
transistor QN52 is connected to the power supply line 32 and the
other end thereof is connected to the ground node GND.
[0086] The capacitor 26 is connected between the power supply line
32 and the ground node GND. The capacitor 26 is a parasitic
capacitor or a capacitor having a capacitance larger than or
approximately equal to the whole capacitance associated with the
gates of the P-channel MOS transistors in the booster section 22 to
which the internal reset signal RSTHB is input. As the capacitor
26, a gate capacitor can be used and an electrostatic capacitor
between two polysilicon layers, between two metal layers or between
a polysilicon layer and a metal layer can be used.
[0087] A large parasitic capacitance such as a PN junction
capacitance in a portion in which the power supply line 32 is wired
or an overlap capacitance between the gate electrode of the MOS
transistor and the source region may be associated with the power
supply line 32 to which the negative voltage VBB is supplied in
some cases. In such a case, the externally attached capacitor 26
becomes unnecessary. The reason why a capacitance larger than or
approximately equal to the whole capacitance associated with the
gates of the P-channel MOS transistors to which the internal reset
signal RSTHB is input becomes necessary is that the ON-resistance
of the P-channel MOS transistor can be suppressed to a sufficiently
small value even when the negative voltage VBB on one electrode
side of the capacitor 26 is divided by the gate capacitance at the
reset time.
[0088] FIG. 16A is a circuit diagram showing an example of the
construction of the high-level shifters 27, 33 in the circuit shown
in FIG. 15. The high-level shifter includes P-channel MOS
transistors QP60, QP61, N-channel MOS transistors QN60, QN61 and
inverter 40. The positive voltage VSW is applied to the back-gates
and one-side ends of the current paths of the MOS transistors QP60,
QP61. The current paths of the MOS transistors QN60 and QN61 are
respectively connected between the other ends of the current paths
of the MOS transistors QP60, QP61 and the ground node GND. The gate
of the MOS transistor QP60 is connected to a connection node
(output terminal OUT) of the current paths of the MOS transistors
QP61 and QN61. The gate of the MOS transistor QP61 is connected to
a connection node (output terminal OUTB) of the current paths of
the MOS transistors QP60 and QN60. The gate of the MOS transistor
QN60 is connected to the input terminal IN. The input terminal of
the inverter 40 is connected to the input terminal IN and the
output terminal of the inverter 40 is connected to the gate of the
MOS transistor QN61.
[0089] The high level of the reset signal RST or discharge signal
GNDEN supplied to the input terminal IN is level-shifted to the
positive voltage VSW and is output from the output terminal OUT or
OUTB in a complementary fashion.
[0090] FIG. 16B is a circuit diagram showing an example of the
construction of the low-level shifters 28, 34 in the circuit shown
in FIG. 15. The low-level shifter includes P-channel MOS
transistors QP62, QP63 and N-channel MOS transistors QN62, QN63.
The positive voltage VSW is applied to the back-gates and one-side
ends of the current paths of the MOS transistors QP62, QP63. The
current paths of the MOS transistors QN62 and QN63 are respectively
connected between the other ends of the current paths of the MOS
transistors QP62, QP63 and the power supply line 32 to which the
negative voltage VBB is applied. The input terminal IN is connected
to the gate of the MOS transistor QP62 and the input terminal INB
is connected to the gate of the MOS transistor QP63. The output
terminals OUT, OUTB of the high-level shifter 27 or 33 are
connected to the input terminals IN, INB. The gate of the MOS
transistor QN62 is connected to a connection node (output terminal
OUT) of the current paths of the MOS transistors QP63 and QN63.
Further, the gate of the MOS transistor QN63 is connected to a
connection node (output terminal OUTB) of the current paths of the
MOS transistors QP62 and QN62.
[0091] The low level of each of the output signals of the
high-level shifter supplied to the input terminals IN, INB is
level-shifted to the negative voltage VBB and output from the
output terminal OUT or OUTB in a complementary fashion. Therefore,
the reset signal RST or discharge signal GNDEN which has passed
through the high-level shifter and low-level shifter becomes a
signal having an amplitude between the positive voltage VSW and the
negative voltage VBB.
[0092] The low-level shifter shown in FIG. 16B has the output
terminals OUT, OUTB, but in the case of the circuit construction
shown in FIG. 15, only the output terminal OUT is used.
[0093] FIG. 17A is a circuit diagram showing an example of the
construction of the inverter circuits 29 to 31, 35, 36 in the
circuit shown in FIG. 15. The inverter circuit has a CMOS inverter
construction operated on the positive voltage VSW and negative
voltage VBB used as the power supply voltages. That is, the gates
of a P-channel MOS transistor QP70 and N-channel MOS transistor
QN70 are connected to an input terminal IN. The positive voltage
VSW is applied to one end of the current path and the back-gate of
the P-channel MOS transistor QP70. The negative voltage VBB is
applied to one end of the current path of the N-channel MOS
transistor QN70. The other ends of the current paths of the MOS
transistors QP70, QN70 are commonly connected to an output terminal
OUT.
[0094] With the above construction, a signal supplied to the input
terminal IN is inverted and the inverted signal is output from the
output terminal OUT with the high level of the inverted signal set
to the positive voltage VSW and the low level thereof set to the
negative voltage VBB.
[0095] FIG. 17B is a circuit diagram showing another example of the
construction of the inverter circuits 29 to 31, 35, 36 in the
circuit shown in FIG. 15. The inverter circuit has a P-channel MOS
transistor QP71 and N-channel MOS transistors QN71, QN72. The
current paths of the MOS transistors QP71, QN71, QN72 are serially
connected. The positive voltage VSW is applied to one end of the
current path and the back-gate of the P-channel MOS transistor
QP71. The negative voltage VBB is applied to one end of the current
path of the N-channel MOS transistor QN72. The gates of the MOS
transistors QP71, QN72 are connected to a first input terminal IN1
and the gate of the MOS transistor QN71 is connected to a second
input terminal IN2. A connection node of the current paths of the
MOS transistors QP71, QN71 is connected to an output terminal
OUT.
[0096] The inverter circuit with the above construction is used
with the first and second input terminals IN1, IN2 short-circuited.
With this construction, since the MOS transistor QN71 is connected
between the MOS transistors QP71 and QN72, a higher withstanding
voltage can be attained. The circuit is similar to the circuit
shown in FIG. 17A in that a signal supplied to the short-circuited
input terminals IN1, IN2 is inverted and the inverted signal is
output from the output terminal OUT with the high level of the
inverted signal set to the positive voltage VSW and the low level
thereof set to the negative voltage VBB.
[0097] FIG. 18 is a circuit diagram for illustrating an example of
the construction of the booster section 22 in the circuit shown in
FIG. 15. The booster section 22 includes booster units (booster
cells) 51a, 51b, booster unit 52 and output circuit section 53.
Each of the booster units 51a, 51b, 52 includes four MOS
transistors (MOSFETs) and two capacitors and the output terminals
of the booster units are respectively connected to the input
terminals of the next-stage booster units.
[0098] That is, the booster unit 51a includes P-channel MOS
transistors QP31, QP32, QP41, QP42 and capacitors C31, C32.
One-side ends of the current paths of the MOS transistors QP31,
QP32 are connected to the ground node GND. The other end of the
current path of the MOS transistor QP32 is connected to the gate of
the MOS transistor QP31 and the gate thereof is connected to the
other end of the current path of the MOS transistor QP31. One
electrode of the capacitor C31 is connected to the other end of the
current path of the MOS transistor QP31 and the other electrode of
the capacitor C31 is supplied with a clock signal phi1. One
electrode of the capacitor C32 is connected to the gate of the MOS
transistor QP31 and the other electrode of the capacitor C32 is
supplied with a clock signal phi3. The current path of the MOS
transistor QP41 is connected between the gate of the MOS transistor
QP32 and the ground node GND and the gate of the MOS transistor
QP41 is supplied with an internal reset signal RSTHB output from
the reset signal generating section 21. Further, the current path
of the MOS transistor QP42 is connected between the gate of the MOS
transistor QP31 and the ground node GND and the gate of the MOS
transistor QP42 is supplied with the internal reset signal
RSTHB.
[0099] Likewise, the booster unit 51b includes P-channel MOS
transistors QP33, QP34, QP43, QP44 and capacitors C33, C34.
One-side ends of the current paths of the MOS transistors QP33,
QP34 are connected to the other end of the current path of the MOS
transistor QP31. The other end of the current path of the MOS
transistor QP34 is connected to the gate of the MOS transistor QP33
and the gate thereof is connected to the other end of the current
path of the MOS transistor QP33. One electrode of the capacitor C33
is connected to the other end of the current path of the MOS
transistor QP33 and the other electrode of the capacitor C33 is
supplied with a clock signal phi2. One electrode of the capacitor
C34 is connected to the gate of the MOS transistor QP33 and the
other electrode of the capacitor C34 is supplied with a clock
signal phi4. The current path of the MOS transistor QP43 is
connected between the gate of the MOS transistor QP34 and the
ground node GND and the gate of the MOS transistor QP43 is supplied
with the internal reset signal RSTHB. Further, the current path of
the MOS transistor QP44 is connected between the gate of the MOS
transistor QP33 and the ground node GND and the gate of the MOS
transistor QP44 is supplied with the internal reset signal
RSTHB.
[0100] The booster unit 52 includes P-channel MOS transistors QP35,
QP36, QP45, QP46 and capacitors C35, C36. One-side ends of the
current paths of the MOS transistors QP35, QP36 are connected to
the other end of the current path of the MOS transistor QP33. The
other end of the current path of the MOS transistor QP36 is
connected to the gate of the MOS transistor QP35 and the gate
thereof is connected to the other end of the current path of the
MOS transistor QP35. One electrode of the capacitor C35 is
connected to the other end of the current path of the MOS
transistor QP35 and the other electrode of the capacitor C35 is
supplied with the clock signal phi1. One electrode of the capacitor
C36 is connected to the gate of the MOS transistor QP35 and the
other electrode of the capacitor C36 is supplied with the clock
signal phi3. The current path of the MOS transistor QP45 is
connected between the gate of the MOS transistor QP35 and the
ground node GND and the gate of the MOS transistor QP45 is supplied
with the internal reset signal RSTHB. Further, the current path of
the MOS transistor QP46 is connected between the gate of the MOS
transistor QP36 and the ground node GND and the gate of the MOS
transistor QP46 is supplied with the internal reset signal
RSTHB.
[0101] The output circuit section 53 includes a P-channel MOS
transistor QP37. One end of the current path of the MOS transistor
QP37 is connected to the other end of the current path of the MOS
transistor QP33 and the gate of the MOS transistor QP37 is
connected to the gate of the MOS transistor QP35. The boosted
negative voltage VBB is output from the other end of the current
path of the MOS transistor QP37.
[0102] As described above, the circuit construction of the booster
section 22 is basically the same as that of the circuit shown in
FIG. 7, but in this circuit, an output signal of the reset signal
generating section 21 is used as the internal reset signal RSTHB
supplied to the gates of the resetting MOS transistors QP41 to
QP46. The reset signal generating section 21 generates the internal
reset signal RSTHB by using the negative voltage VBB output from
the booster section 22.
[0103] FIG. 19 is a timing chart for illustrating the operation of
the booster circuit shown in FIGS. 15 to 18. As shown in FIG. 19,
when the clock signals phi1, phi2, phi3, phi4 are input as in the
conventional booster circuit, the gate voltage VG of the MOS
transistor QP37 and the output voltage VBB are gradually lowered
for each cycle of the clock signals. If the output voltage VBB is
lowered to a desired boosted level Vneg and a reset signal RST is
input, then an internal reset signal RSTHB is created from the
reset signal RST to drive the resetting MOS transistors QP41 to
QP46 so as to set the gate potentials of the MOS transistors QP31
to QP37 to the ground potential (0 V). At this time, a switching
signal SWN is set to the negative boosted level Vneg to interrupt
the transistor section 23. At this time point, the output voltage
VBB keeps the negative boosted level Vneg. After this, when a
discharge signal GNDEN is set to the high level, the output voltage
VBB and switching signal SWN are set to the ground potential (0
V).
[0104] In this case, if an attempt is made to activate the
transistor section 23 irrespective of the timing of the reset
signal RST, it is fixed at the positive voltage VSW as shown by
SWN'.
[0105] With the above construction, since the output voltage VBB of
the booster section 22 is used to generate the reset signal, it
becomes easy to generate a reset signal having an amplitude between
the ground potential and a preset negative boosted potential only
by use of the electrostatic capacitance and some logic circuits.
Therefore, it becomes unnecessary to use a separate negative
voltage generating circuit and an increase in the circuit scale can
be suppressed and the control operation can be simplified.
[0106] In the booster section 22 shown in FIG. 18, it is of course
possible to connect a resetting P-channel MOS transistor between
the output node of the negative voltage VBB and the ground node GND
and supply an internal reset signal RSTHB to the gate of the above
MOS transistor after termination of the boosting operation, thereby
grounding and resetting the output node.
[0107] Further, in the first embodiment, the booster circuit used
in the flash memory system is explained as an example, but this
invention is not limited to a semiconductor memory device and can
be generally applied to a semiconductor integrated circuit
device.
[Second Embodiment]
[0108] FIGS. 20 to 22 are diagrams for illustrating a booster
circuit according to a second embodiment of this invention and show
an example in which this invention is applied to a four-phase type
positive voltage generating booster circuit. In order to reset the
gate node of an internal transistor (in which charges are left
behind) of the positive voltage generating circuit, it is
sufficient to use a signal having an amplitude of the power supply
voltage VCC, but in this case, the efficiency is low since it is
necessary to raise the potential of the gate node of the internal
transistor from 0 V. Therefore, the gate nodes of the internal
transistors are reset to the VCC level by applying the output
voltage VPP of the booster circuit to the gates of the resetting
MOS transistors. Thus, since the gate node of the internal
transistor is boosted from the VCC level, an amount of charges
required for the boosting operation can be reduced by VCC in
comparison with a case wherein the gate node of the internal
transistor is reset to 0 V.
[0109] The four-phase type positive voltage generating booster
circuit shown in FIG. 20 includes a reset signal generating section
41, booster section 42 and transistor section 43. The transistor
section 43 is connected between the output terminal of the booster
section 42 and another circuit and ON/OFF-controlled by a switching
signal SWP to supply or interrupt supply of the positive voltage
VPP output from the booster section 42 to the other circuit used as
a load.
[0110] The reset signal generating section 41 includes a reset
pulse generator 44, discharge circuit 45 and capacitor 46. The
reset pulse generator 44 includes a high-level shifter 47 and
inverter circuits 48, 49. The output terminals of the high-level
shifter 47 and inverter circuits 48, 49 are respectively connected
to the input terminals of the next-stage circuits and they are
operated on voltage between the positive boosted voltage VPP output
from the booster circuit 42 and the ground node GND. The reset
signal RST is supplied to the input terminal IN of the high-level
shifter 47, a signal output from the output terminal OUT of the
high-level shifter 47 is supplied to the input terminal of the
inverter circuit 48, and a signal output from the inverter circuit
48 is supplied to the input terminal of the inverter circuit 49. An
output signal of the inverter circuit 49 is output as the internal
reset signal RSTH. The internal reset signal RSTH is set at 0 V
when the reset signal RST is set at the low level and it is set at
the VPP level when the reset signal is set at the high level.
[0111] The discharge circuit 45 includes a high-level shifter 50,
inverter circuits 51, 52 and N-channel MOS transistor QN80. The
output terminals of the high-level shifter 50, inverter circuits
51, 52 are respectively connected to the output terminals of the
next-stage circuits and they are operated on voltage between the
positive boosted voltage VPP output from the booster circuit 42 and
the ground node GND. The discharge signal GNDEN is supplied to the
input terminal IN of the high-level shifter 50, a signal output
from the output terminal OUT of the high-level shifter 50 is
supplied to the input terminal of the inverter circuit 51, and a
signal output from the inverter circuit 51 is supplied to the input
terminal of the inverter circuit 52. An output signal of the
inverter circuit 52 is supplied to the gate of the MOS transistor
QN80. One end of the current path of the MOS transistor QN80 is
connected to a power supply line 53 to which the output voltage VPP
of the booster section 42 is supplied and the other end thereof is
connected to the power supply VCC.
[0112] The capacitor 46 is connected between the power supply line
53 and the power supply VCC. Like the first embodiment, the
capacitor 46 is a parasitic capacitor or a capacitor having a
capacitance larger than or approximately equal to the whole gate
capacitance associated with the gates of the N-channel MOS
transistors in the booster section 42 to which the internal reset
signal RSTH is input. As the capacitor 46, a gate capacitor can be
used or an electrostatic capacitor between two polysilicon layers,
between two metal layers or between a polysilicon layer and a metal
layer can be used.
[0113] A large parasitic capacitance such as a PN junction
capacitance in a portion in which the power supply line 53 is wired
or an overlap capacitance between the gate electrode of the MOS
transistor and the source region may be associated with the power
supply line 53 to which the boosted voltage VPP is supplied in some
cases. In such a case, the externally attached capacitor 46 becomes
unnecessary. The reason why a capacitance larger than or
approximately equal to the whole gate capacitance associated with
the gates of the N-channel MOS transistors to which the internal
reset signal RSTH is input is necessary is that the ON-resistance
of the N-channel MOS transistor can be suppressed to a sufficiently
small value even when the voltage VPP on one electrode side of the
capacitor 46 is divided by the gate capacitance at the reset
time.
[0114] FIG. 21 is a circuit diagram for illustrating an example of
the construction of the booster section 42 in the circuit shown in
FIG. 20. The circuit includes N-channel MOS transistors QN31 to
QN37, QN41 to QN46 instead of the P-channel MOS transistors QP31 to
QP37, QP41 to QP46 in the circuit shown in FIG. 18. The internal
reset signal RSTH output from the reset signal generating section
41 is supplied to the gates of the MOS transistors QN41 to QN46.
Further, one-side ends of the current paths of the resetting MOS
transistors QN41 to QN46 are connected to the power supply VCC
instead of the ground node.
[0115] With this construction, as shown in the timing chart of FIG.
22, when the clock signals phi1, phi2, phi3, phi4 are input, the
gate voltage VG of the MOS transistor QN37 and the output voltage
VPP are gradually raised for each cycle of the clock signals. If a
reset signal RST is input, an internal reset signal RSTH is created
from the reset signal RST to drive the resetting MOS transistors
QN41 to QN46 so as to set the gate potentials of the MOS
transistors QN31 to QN37 to the boosted voltage VPP. At this time,
a switching signal SWP is set to a positive boosted level Vpos to
interrupt the transistor section 43. At this time point, the output
voltage VPP keeps the positive boosted level Vpos. After this, when
a discharge signal GNDEN is set to the high level, the output
voltage VPP and switching signal SWP are set to the VCC level.
[0116] In this case, if an attempt is made to activate the
transistor section 43 irrespective of the timing of the reset
signal RST, it is fixed at 0 V as shown by SWP'.
[0117] Thus, in this embodiment, the gate nodes of the internal
transistors are reset to the VCC level by applying the output
voltage VPP of the booster section 42 to the gates of the resetting
MOS transistors QN41 to QN46. Therefore, since the gate node of the
internal transistor is started to be boosted from the VCC level, an
amount of charges required for the boosting operation can be
reduced by VCC in comparison with a case wherein the gate node of
the internal transistor is reset to 0 V.
[Third Embodiment]
[0118] FIG. 23 illustrates a booster circuit according to a third
embodiment of this invention and concretely shows another example
of the construction of the booster section 22 of the first
embodiment shown in FIG. 15. That is, the capacitors C31 to C36 in
the circuit shown in FIG. 18 are respectively constructed by two
series-connected capacitors C31-1, C31-2 to C36-1, C36-2 and
resetting MOS transistors QP100 to QP105 are respectively connected
between connection nodes of the capacitors C31-1, C31-2 to C36-1,
C36-2 and the ground node GND.
[0119] The third embodiment is made by taking the following point
into consideration. That is, an internal voltage of the booster
circuit becomes higher than an output voltage thereof. Therefore,
there occurs a possibility that the maximum input voltage may
exceed the breakdown voltage of an insulating film of the capacitor
when the thickness of the insulating film is small. Of course, it
is possible to use a capacitor having an insulating film whose
breakdown voltage is sufficiently high, but in this case, it cannot
be formed in the same process as that for forming a capacitor
having a thin insulating film as in a semiconductor memory device,
for example, and the manufacturing process becomes complicated.
Therefore, in this embodiment, the capacitors C31-1, C31-2 to
C36-1, C36-1 with thin insulating films which are serially
connected are used. If MOS capacitors are used as the capacitors
C31-1, C31-2 to C36-1, C36-1, a PN junction is formed in each
connection node of the two capacitors. It is necessary to reset the
connection node without fail, but it cannot be reset if a reset
signal having an amplitude of the power supply voltage VCC is used
as in the prior art. However, the connection nodes of the
capacitors C31-1, C31-2 to C36-1, C36-1 can be reset without fail
by using a reset signal RSTHB having an amplitude between a boosted
negative voltage VBB and a ground potential.
[0120] It is of course possible to construct each of the capacitors
C31-1 to C36-1 in the circuit of FIG. 21 by use of two
series-connected capacitors and connect a resetting MOS transistor
between the connection node of the capacitors and the power supply
VCC.
[0121] As described above, since it becomes unnecessary to use a
separate voltage generating circuit by creating the reset signal of
the four-phase type positive voltage generating booster circuit by
use of the output voltage of the booster section and the circuit
can be constructed by use of a simple logic circuit, the gate node
of the internal transistor can be relatively easily reset.
[0122] As explained above, according to this invention, it is
possible to provide a booster circuit which has a construction for
resetting the gate node of the transistor having charges left
behind therein and can suppress an increase in the circuit scale
and in which the control operation can be simplified.
[0123] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *