Title: IC device having a transistor switch for a power supply
Abstract: An IC device has a MOSFET serving as a power switch, a condenser connected between a first input terminal of the IC and the gate of the MOSFET, and a ferroelectric condenser connected between a second input terminal of the IC and the gate of the MOSFET. A prescribed voltage having a predetermined polarity is applied across the first and the second input terminals to generate a remanent polarization oriented in a specific direction in the ferroelectric condenser, thereby raising the threshold voltage of the MOSFET to a higher level than its original level. The power switching MOSFET is fabricated in the same manufacturing process as for other circuit blocks of the IC device such that it has substantially the same threshold voltage as that of the MOSFETs in other circuit blocks.
Patent Number: 7,012,460 Issued on 03/14/2006 to Fujimori
| Inventors:
|
Fujimori; Yoshikazu (Kyoto, JP)
|
| Assignee:
|
Rohm Co., Ltd. (Kyoto, JP)
|
| Appl. No.:
|
932643 |
| Filed:
|
September 1, 2004 |
Foreign Application Priority Data
| Oct 15, 2001[JP] | 2001-316700 |
| Current U.S. Class: |
327/530; 327/437; 307/130; 307/156 |
| Current Intern'l Class: |
G05F 3/02 (20060101) |
| Field of Search: |
327/365,419,427,437,530
307/126,130,131
|
References Cited [Referenced By]
U.S. Patent Documents
| 5155573 | Oct., 1992 | Abe et al.
| |
| 6327172 | Dec., 2001 | Ishiwara.
| |
| 2002/0043676 | Apr., 2002 | Ohtsuka et al.
| |
| 2003/0094640 | May., 2003 | Ohtsuka et al.
| |
| 2003/0142533 | Jul., 2003 | Ueda et al.
| |
Primary Examiner: Cunningham; Terry D.
Attorney, Agent or Firm: Hogan & Hartson, LLP
Parent Case Text
This is a continuation of application Ser. No. 10/269,277, filed Oct. 11, 2002,
now U.S. Pat. No. 6,806,758 which application is hereby incorporated by reference
in its entirety.
Claims
What we claim is:
1. A semiconductor integrated circuit device (IC device) comprising:
a circuit block having a transistor circuit supplied with a potential difference
between a power supply potential and a ground potential; and
a switch circuit for switching on and off the supplying of a power supply of
potential or ground of potential to said circuit block; said switch circuit including:
a power switching transistor which is an insulated gate FET (field effect transistor),
a nonferroelectric condenser having one end connected to the gate of said power
switching transistor, and
a ferroelectric condenser having one end connected to the gate of said power
switching transistor.
2. The IC device according to claim 1, wherein said power switching transistor
is a MISFET (metal insulator semiconductor field effect transistor), and the transistors
of said circuit block are MISFETs.
3. The IC device according to claim 1, wherein said power switching transistor
is a an MOSFET (metal-oxide-semiconductor field effect transistor), and the transistors
of said circuit block are MOSFETs.
4. The IC device according to claim 1, wherein said transistor circuit of said
circuit block includes CMOS (complementary MOS) FETs, each of said CMOSFETs formed
of a P-type MOSFET and an N-type MOSFET.
5. The IC device according to claim 1, wherein said power switching transistor
and the transistors of said circuit block are fabricated in the same manufacturing process.
6. The IC device according to claim 1, wherein said power switching transistor
and the transistors of said circuit block have substantially the same threshold voltage.
Description
FIELD OF THE INVENTION
The invention relates to a semiconductor integrated circuit device (IC device)
having a transistor switch for a power supply.
BACKGROUND OF THE INVENTION
Most IC devices generally include numerous transistors such as MOSFET. They
are required to have limited power consumption. This is true particularly when
they are used in portable electronic devices.
FIG. 1 shows schematically a structure of a typical IC device configured to
operate at a reduced power. Block 10 represents a transistor circuit block
involving many transistors like CMOSFETs. The IC device as shown in FIG. 1 further
includes a power switch consisting of a P-type MOSFET 11 and an N-type MOSFET
12 for turning on and off power to the circuit block 10. They are
connected between a power supply having a potential E1 and the ground of
potential Egnd. The supply voltage Vcc amounts to the potential difference between
the two potentials.
In operation, transistor circuits inside the block 10 are switched at high
frequency. In order to reduce power consumption by the circuit block 10,
the supply voltage Vcc is set as low as 2 V say. In a transistor circuit, reduction
of the supply voltage Vcc is effective in reducing its power consumption, since
power consumption is proportional to the square of the supply voltage. However,
when the supply voltage is reduced, operational speed of the transistor circuits
in the block is also reduced, making an appreciable delay in the device.
One way to circumvent such delay is to reduce the threshold voltage of the transistors
(e.g. CMOSFETs) of the circuit block 10 in correspondence with the reduction
of the supply voltage Vcc. This reduces only the power consumption without reducing
the operational speed of the block 10.
However, with a reduced threshold voltage as mentioned above, leak current
through the MOSFETs, and hence the standby power consumption by the MOSFETs, increases
while they are turned off. It is obviously necessary to reduce the standby power
as much as possible.
To attain this goal, normally a power switch is used which has a P-type MOSFET
11 and an N-type MOSFET 12 both having a high threshold voltage (HIGH
Vth). With this switch, the transistor circuits in the block 10 are operated
at a high frequency at the low threshold voltage (LOW Vth), but when they are in
standby condition, the MOSFETs 11 and 12 are turned off at the high
threshold voltage (HIGH Vth), thereby suppressing the leak currents (i.e., the
standby power).
In this conventional IC device, the threshold voltage of the MOSFETs such as
CMOSs
of the circuit block 10 is set to a low level (LOW Vth) while the threshold
voltages of the P-type MOSFET 11 and of N-type MOSFET 12 serving
as a power switch must be set to a high level. In providing the MOSFETs 11,
12 with a threshold voltage higher than that of other MOSFETs, a special
mask is needed to perform extra ion implantation in the process of manufacturing
the IC device.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide MOSFETs serving as a
power
supplying switch in an IC device in such a way that the MOSFETs can be fabricated
in the same manufacturing process as other MOSFETs and their threshold voltages
can be raised to reduce leak current through them while they are turned off, thereby
reducing power consumption during standby.
In accordance with one aspect of the invention, there is provided an IC device
having at least one power supplying transistor, wherein
said power supplying transistor is a metal insulator semiconductor field effect
transistor (MISFET), said IC device further comprising:
a condenser having one end connected with the gate of said power supplying MISFET; and
a ferroelectric condenser having one end connected to the gate of said power
supplying MISFET.
In accordance with another aspect of the invention, there is provided an IC device, comprising:
a first power supplying transistor switch circuit that includes
- a power supplying P-type MISFET;
- a condenser having one end connected to the gate of said power supplying
P-type MISFET,
- a ferroelectric condenser having one end connected to the gate of said
power supplying P-type MISFET;
a transistor circuit block that includes MISFETs;
a second power supplying transistor switch circuit including
- a power supplying N-type MISFET,
- a condenser having one end connected to the gate of said power supplying
N-type MISFET, and
- a ferroelectric condenser having one end connected to the gate of said
power supplying N-type MISFET, wherein
said first power supplying transistor switch circuit, transistor circuit block,
and second power supplying transistor switch circuit are connected in series in
the order mentioned.
In accordance with further aspect of the invention, there is provided a method
of controlling a power supplying transistor switch circuit, wherein said transistor
switch circuit has
a power supplying MISFET,
a condenser having one end connected to the gate of said MISFET,
a ferroelectric condenser having one end connected to the gate of said MISFET,
said method comprising steps of:
temporarily applying, in a prescribed direction, a voltage across the
other end of said condenser and the other end of said ferroelectric condenser;
generating a remanent polarization in a prescribed direction in said ferroelectric
condenser, thereby impressing said MISFET with a threshold voltage higher than
the threshold voltage of said power supplying MISFET itself; and
applying a control voltage to the other end of said condenser to turn ON/OFF
said power supplying MISFET.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in detail by way of example with reference
to accompanying drawings, in which:
FIG. 1 is a circuit diagram of a conventional IC device;
FIG. 2 is a circuit diagram of an IC device according to the invention;
FIG. 3 is a circuit diagram illustrating a method of initializing the transistor
switch circuit shown in FIG. 2;
FIG. 4 is a diagram representation of gate voltage versus drain current characteristic
of the transistor switch circuit shown in FIG. 2;
FIG. 5 is a graphical representation of a first operational state of the IC
device shown in FIG. 2; and
FIG. 6 is a graphical representation of a second operational state of the IC
device shown in FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2, there is shown a circuit diagram of an IC device according
to the invention. In what follows the invention will be described with reference
to a preferred embodiment of an IC device which comprises MOSFETs as MISFETs.
As shown in FIG. 2, in order to minimize power consumption, a transistor circuit
block
10 is formed of a multiplicity of CMOSFETs, operable at a low supply
voltage Vcc of, for example, 1-2 Volts, given by the difference in potential of
a first power supply of potential E
1 and the ground of potential Egnd.
In order to circumvent operational delay of the transistor circuit on account
of the supply voltage Vcc being low, the threshold voltage of the internal MOSFETs
of the circuit block
10 is set to a low level (referred to as LOW Vth).
Thus, the operating speed of the circuit block
10 can be fast even though
the supply voltage Vcc is low.
A power supplying transistor switch circuit
20 is connected between the
power supply of potential E
1 and the circuit block
10. Another power
supplying transistor switch circuit
30 is connected between the circuit
block
10 and the ground of potential Egnd. Power is supplied to the circuit
block
10 and cut off by turning ON and OFF the transistor switch circuits
20 and
30.
The power supplying transistor switch circuit
20 has a P-type MOSFET
21
whose gate G
1 is coupled to first and second input terminals
22 and
23, respectively, via a condenser C
1 and a ferroelectric condenser
Cf
1, respectively. Similarly, the power supplying transistor switch circuit
30 has an N-type MOSFET
31 whose gate G
2 is connected to first
and second input terminals
32 and
33 via a condenser C
2 and
a ferroelectric condenser Cf
2, respectively. Here, condensers (C
1
and C
2) other than ferroelectric condensers (Cf
1 and Cf
2)
are simply referred to condensers.
In the example shown above, condensers C
1 and C
2 and ferroelectric
condensers Cf
1 and Cf
2 are connected to the respective first and
second input terminals. It should be understood, however, that the input terminals
are shown for convenience in that the condensers may be connected directly to other
elements in the IC device without the terminals.
The P-type MOSFET
21 and the N-type MOSFET
31 are fabricated together
with other MOSFETs in the circuit block
10, so that their threshold voltages
are set to the low level (LOW Vth).
In using the IC device shown in FIG. 2 for the first time, the power supplying
transistor switch circuits
20 and
30 are first initialized.
The initialization is carried out by applying to the second input terminal
23
a high potential (referred to as HIGH level potential), e.g. the potential E
1,
while a low potential (referred to as LOW level potential), e.g. the ground potential
Egnd, is applied to the first input terminal
22, as shown in FIG. 3. Because
the ferroelectric condenser Cf
1 has a polarization hysteresis, it will have
a remanent polarization oriented in a specific direction after the removal of the
voltage applied thereto.
In the example shown, the second input terminal
23 is applied with HIGH
level voltage and the first input terminal
22 with LOW level voltage. Consequently,
polarization will occur in the condenser Cf
1 in the direction as indicated
by an arrow shown in FIG. 3, creating a positive charge on the gate G
1 of
the P-type MOSFET
21 connected to the node of the condensers C
1 and
Cf
1. Due to the remanent polarization in the ferroelectric condenser, a
certain amount of the positive charge will remain on the gate G
1 even after
the removal of the voltages from the first and the second input terminals
22
and
23, respectively.
In the ferroelectric condenser Cf
2, polarization will occur in the direction
indicated by an arrow as shown under the HIGH level potential applied to the first
input terminal
32 and the LOW level potential to the second input terminal
33, creating a negative charge on the gate G
2 of the N-type MOSFET
31 connected to the node of the condensers C
2 and Cf
2. The
negative charge will partly remain on the gate G
2 as a result of remanent
polarization in the condenser Cf
2 even after the removal of applied potentials
from the respective input terminals
32 and
33.
Consequently, after the initialization, the remanent polarizations
in the ferroelectric condensers Cf
1 and Cf
2 are conserved, the second
input terminals
23 and
33 of the respective power supplying transistor
switch circuit
20 and
30 may be left open.
Under this condition, the gate G
1 of the P-type MOSFET
21 is
positively charged, while the gate G
2 of the N-type MOSFET
31 is
negatively charged, so that the threshold voltages of the respective MOSFETs are
held at the HIGH level.
FIG. 4 is a graph showing the experimental gate voltage-drain current characteristic
of an N-type power supplying transistor switch circuit according to the invention.
This is the characteristic of the N-type MOSFET switch circuit
30 shown
in FIGS. 2 and 3.
FIG. 4 compares the characteristic before initialization, that is, that of the
N-type MOSFET itself, with the characteristic after the initialization, that is
that of N-type MOSFET with its gate charged with a negative charge. From this comparison,
it is seen clearly in the Figure that the characteristic curve is shifted in the
direction of increasing gate voltage by the initialization. In the example shown
herein, the shift amounts to about 350 mV.
Thus, that the gate voltage-drain current characteristic of a transistor switch
circuit is shifted implies that the threshold voltage Vth is raised by about 350 mV.
The same is true in the P-type MOSFET. As a result, by connecting the power supplying
transistor switch circuit
20 and power supplying transistor switch circuit
30 in series, the combined threshold voltage of the switch circuits
20
and
30 is further raised. Therefore, the leakage current is suppressed efficiently.
After the power supplying transistor switch circuit
20 and power supplying
transistor switch circuit
30 are initialized, to place the IC device in
operation, the first input terminal
22 is impressed with the LOW level potential,
while the first input terminal
32 with HIGH level potential, as shown in
FIG. 5. While the input terminal is impressed with LOW level potential, the charge
on the gate G
1 is absorbed by the condenser C
1, lowering the gate
G
1 to a sufficiently LOW potential, thereby fully turning ON the P-type
MOSFET
21. While the input terminal is impressed with the HIGH level potential,
the charge on the gate G
2 is absorbed by the condenser C
2, raising
the gate G
2 to a sufficiently HIGH potential, thereby fully turning ON the
N-type MOSFET
31.
The ON-voltage, i.e. voltage drop across the conducive P-type MOSFET
21
and N-type MOSFET
31, is very small, irrespective of the magnitude of the
threshold voltage. Hence, the circuit block
10 is supplied with a voltage
which is substantially the supply voltage Vcc, allowing the circuit block
10
to operate without any hindrance even under the LOW level voltage.
On the other hand, when the IC device is on standby, the first input terminal
22 is supplied with the HIGH potential and the first input terminal
32
with the LOW potential.
In this case, the gate G
1 restores from the condenser C
1 the positive
charge, bringing the threshold voltage of the P-type MOSFET
21 back to HIGH
Vth again. Similarly, the gate G
2 restores the negative charge from the
condenser C
2, bringing the threshold voltage of the N-type MOSFET
31
back to HIGH Vth again.
As a result of the HIGH potential applied to the first input terminal
22
and LOW potential to the first input terminal
32, the P-type MOSFET
21
and the N-type MOSFET
31 are turned OFF. Since the threshold voltages of
these MOSFETs are high, only a suppressed leak current results.
After the initialization, the positive charge on the gate G
1 of the
P-type MOSFET
21 and the negative charge on the gate G
2 of the N-type
MOSFET
31 are conserved, so that they can reproduce the same conductive
state and standby state in repetition.
Thus, the leak current through the P-type MOSFET
21 and the N-type MOSFET
31, and hence the standby power consumption by the IC device, is suppressed
to a very low level if the internal transistors of the circuit block
10
have low threshold voltage (LOW Vth).
Although the invention has been described with particular reference to a
preferred embodiment in which two transistor switch circuits
20 and
30
are used in the above embodiment, the supply voltage may be implemented to the
circuit block
10 using only one of them. It will be understood that the
power supplying transistor switch circuits
20 and
30 may be provided
in pairs to each of such transistor circuit block
10.
The invention requires provision of condensers C
1 and C
2 and ferroelectric
condensers Cf
1 and Cf
2, which can be implemented by an ordinary technique
well known in the art. Ferroelectric condensers may be substituted for by ferroelectric
elements for use in ferroelectric memory, or may be independently formed.
*
