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.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a semiconductor integrated circuit device (IC device) having a transistor switch for a power supply.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    [0003]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 E 1  and the ground of potential Egnd. The supply voltage Vcc amounts to the potential difference between the two potentials.  
           [0004]    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.  
           [0005]    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 .  
           [0006]    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.  
           [0007]    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).  
           [0008]    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  
         [0009]    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.  
           [0010]    In accordance with one aspect of the invention, there is provided an IC device having at least one power supplying transistor, wherein  
           [0011]    said power supplying transistor is a metal insulator semiconductor field effect transistor (MISFET), said IC device further comprising:  
           [0012]    a condenser having one end connected with the gate of said power supplying MISFET; and  
           [0013]    a ferroelectric condenser having one end connected to the gate of said power supplying MISFET.  
           [0014]    In accordance with another aspect of the invention, there is provided an IC device, comprising:  
           [0015]    a first power supplying transistor switch circuit that includes  
           [0016]    a power supplying P-type MISFET;  
           [0017]    a condenser having one end connected to the gate of said power supplying P-type MISFET,  
           [0018]    a ferroelectric condenser having one end connected to the gate of said power supplying P-type MISFET;  
           [0019]    a transistor circuit block that includes MISFETs;  
           [0020]    a second power supplying transistor switch circuit including  
           [0021]    a power supplying N-type MISFET,  
           [0022]    a condenser having one end connected to the gate of said power supplying N-type MISFET, and  
           [0023]    a ferroelectric condenser having one end connected to the gate of said power supplying N-type MISFET, wherein  
           [0024]    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.  
           [0025]    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  
           [0026]    a power supplying MISFET,  
           [0027]    a condenser having one end connected to the gate of said MISFET,  
           [0028]    a ferroelectric condenser having one end connected to the gate of said MISFET, said method comprising steps of:  
           [0029]    temporarily applying, in a prescribed direction, a voltage across the other end of said condenser and the other end of said ferroelectric condenser;  
           [0030]    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  
           [0031]    applying a control voltage to the other end of said condenser to turn ON/OFF said power supplying MISFET. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    The invention will now be described in detail by way of example with reference to accompanying drawings, in which:  
         [0033]    [0033]FIG. 1 is a circuit diagram of a conventional IC device;  
         [0034]    [0034]FIG. 2 is a circuit diagram of an IC device according to the invention;  
         [0035]    [0035]FIG. 3 is a circuit diagram illustrating a method of initializing the transistor switch circuit shown in FIG. 2;  
         [0036]    [0036]FIG. 4 is a diagram representation of gate voltage versus drain current characteristic of the transistor switch circuit shown in FIG. 2;  
         [0037]    [0037]FIG. 5 is a graphical representation of a first operational state of the IC device shown in FIG. 2; and  
         [0038]    [0038]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  
       [0039]    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.  
         [0040]    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.  
         [0041]    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.  
         [0042]    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 .  
         [0043]    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.  
         [0044]    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.  
         [0045]    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).  
         [0046]    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.  
         [0047]    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.  
         [0048]    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.  
         [0049]    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 .  
         [0050]    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.  
         [0051]    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.  
         [0052]    [0052]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.  
         [0053]    [0053]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.  
         [0054]    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.  
         [0055]    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.  
         [0056]    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 .  
         [0057]    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.  
         [0058]    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.  
         [0059]    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.  
         [0060]    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.  
         [0061]    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.  
         [0062]    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).  
         [0063]    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 .  
         [0064]    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.