Abstract:
A display device includes a pixel driver circuit. Each of level converter circuits in the pixel driver circuit has an input terminal supplied with a signal swinging between a first voltage and a second voltage lower than the first voltage; a first first-conductivity-type transistor having a gate electrode coupled to the input terminal, and a source region coupled to a reference voltage; a second second-conductivity-type transistor having a gate electrode coupled to a drain region of the first transistor, a source region-coupled to a power supply, and a drain region coupled to an output terminal; one circuit element among a diode, a resistor and a fourth second-conductivity-type transistor, coupled between the gate electrode of the second transistor and the power supply; a third first-conductivity-type transistor having a source region coupled to the input terminal, a drain region coupled to the output terminal, and a gate electrode supplied with a do voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation application of U.S. application Ser. No. 11/283,745 filed Nov. 22, 2005, which is a Continuation application of U.S. application Ser. No. 10/460,154 filed Jun. 13, 2003, which is a Continuation application of U.S. application Ser. No. 09/780,492 filed Feb. 12, 2001. Priority is claimed based on U.S. application Ser. No. 11/283,745 filed Nov. 22, 2005, which claims priority to U.S. application Ser. No. 10/460,154 filed Jun. 13, 2003, which claims priority to U.S. application Ser. No. 09/780,492 filed Feb. 12, 2001, which claims priority to Japanese Patent Application No. 2000-047164 filed on Feb. 24, 2000, all of which is incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a level converter circuit and a liquid crystal display device employing the level converter circuit, and in particular to a level converter circuit formed by polysilicon transistors. 
         [0003]    Liquid crystal display modules of the STN (Super Twisted Nematic) type or the TFT (Thin Film Transistor) type are widely used as a display device for a notebook personal computer and the like. Some driver circuits for driving such liquid crystal display panels need a level converter circuit external to the liquid crystal display panel. Such a level converter circuit is disclosed in Japanese Patent Application Laid-open No. Hei 6-204,850 (laid-open on Jul. 22, 1994), for example. 
         [0004]      FIG. 13  is a circuit diagram of an example of a prior art level converter circuit. The level converter circuit shown in  FIG. 13  is formed by MOS transistors using single crystal silicon for their semiconductor layers, and is of the same circuit configuration as that shown in FIG. 4 of Japanese Patent Application Laid-open No. Hei 6-204,850. 
         [0005]    The level converter circuit shown in  FIG. 13  has a CMOS inverter INV 1  to which a low-voltage input signal ø 1  is supplied and a CMOS inverter INV 2  to which an output signal ø 2  from the CMOS inverter INV 1  is supplied. 
         [0006]    The CMOS inverter INV 1  is formed by a p-channel MOS transistor (hereinafter referred to as a PMOS) M 5  and an n-channel MOS transistor (hereinafter referred to as an NMOS) M 6  which are connected in series between a low voltage VCC and a reference voltage (or ground potential) VSS. 
         [0007]    The CMOS inverter INV 2  is formed by a PMOS M 7  and an NMOS M 8  which are connected in series between the low voltage VCC and the reference voltage (or ground potential) VSS. 
         [0008]    Further, the level converter circuit includes a series combination of a PMOS M 9  and an NMOS M 11  and a series combination of a PMOS M 10  and an NMOS M 12 , which are connected between a high voltage VDD and the reference voltage VSS. 
         [0009]    An output signal ø 3  from the CMOS inverter INV 2  is supplied to a gate electrode of the NMOS M 11 , and an output signal ø 2  from the CMOS inverter INV 1  is supplied to a gate electrode of the NMOS M 12 . A gate electrode of the PMOS M 9  is connected to a drain electrode of the PMOS M 10 , and a gate electrode of the PMOS M 10  is connected to a drain electrode of the PMOS M 9 . 
         [0010]    The input signal ø 1  supplied via an input terminal VIN has an amplitude between the low voltage VCC and the reference voltage VSS, and is converted into the low voltage outputs ø 2  and ø 3  each having amplitudes between the low voltage VCC and the reference voltage VSS. 
         [0011]    The low voltage output signals ø 2  and ø 3  are supplied to gate electrodes of the NMOS M 11  and the NMOS M 12 , respectively, and outputs from output terminals VOUT 1  and VOUT 2  are two level-converted signals, that is, two complementary output signals ø 4  and ø 5  having amplitudes between the high supply voltage VDD and ground potential VSS, respectively. 
         [0012]    For example, suppose that the low voltage output signal ø 2  is at a high level (hereafter referred to merely as an H level) and the low voltage output signal ø 3  is at a low level (hereafter referred to merely as an L level). Then the NMOS M 12  is ON, PMOS M 9  is ON, NMOS M 11  is OFF, and PMOS M 10  is OFF, and therefore the output terminal VOUT 2  outputs the ground potential VSS and the output terminal VOUT 1  outputs the high voltage VDD. 
         [0013]    Next, suppose that the low voltage output signal ø 2  is at the L level and the low voltage output signal ø 3  is at the H level. Then the NMOS M 12  is OFF, the PMOS M 9  is OFF, the NMOS M 11  is ON, and the PMOS M 10  is ON, and therefore the output terminal VOUT 2  outputs the high supply voltage VDD and the output terminal VOUT 1  outputs the ground potential VSS. 
         [0014]      FIG. 14  is a circuit diagram of another example of a prior art level converter circuit. The level converter circuit shown in  FIG. 14  is also formed by MOS transistors using single crystal silicon for their semiconductor layers, and is of the same circuit configuration as that shown in FIG. 1 of Japanese Patent Application Laid-open No. Hei 6-204,850. 
         [0015]    The level converter circuit shown in  FIG. 14  differs from that shown in  FIG. 13 , in that the CMOS inverter INV 2  is omitted, the output signal ø 2  from the CMOS inverter INV 1  is supplied to the source electrode of the NMOS M 11 , and the gate of which is supplied with the low voltage VCC. 
         [0016]    In the level converter circuit shown in  FIG. 13 , when the level-converted output signals ø 4 , ø 5  from the output terminals VOUT 1 , VOUT 2  change from the H level to the L level, or from the L level to the H level, all of the PMOS M 5 , the NMOS M 11 , the PMOS M 10  and the NMOS M 12  are turned ON simultaneously, and consequently, currents flow through a series combination of the PMOS M 9  and the NMOS M 11  and a series combination of the PMOS M 10  and the NMOS M 12 , respectively. The level converter circuit shown in  FIG. 14  is configured so as to prevent such currents from flowing through the series combination of the PMOS M 9  and the NMOS M 11  and the series combination of the PMOS M 10  and the NMOS M 12 . 
         [0017]    The level converter circuit shown in  FIG. 13  needs a total of eight MOS transistors comprising four MOS transistors M 5  to M 8  in the low-voltage circuit and four MOS transistors M 9  to M 12  in the high-voltage circuit, the level converter circuit shown in  FIG. 14  needs six MOS transistors, and therefore the prior art level converter circuits had the problem in that many MOS transistors are needed. 
         [0018]    It is known that mobility in MOS transistors using as their semiconductor layers, single crystal silicon, polysilicon and amorphous silicon are 1,000 to 2,000 cm.sup.2/(Vs), 10 to 100 cm.sup.2/(V6), and 0.1 to 10 cm.sup.2/(Vs), respectively. MOS transistors using as their semiconductor layers, polysilicon and amorphous silicon are capable of being fabricated on a transparent insulating substrate made of quartz glass or glass having a softening temperature not higher than 800.degree. C., and therefore electronic circuits can be fabricated directly on a display device such as a liquid crystal display device. 
         [0019]      FIG. 15  is a graph showing an example of switching characteristics of an n-channel MOS transistor having a semiconductor made of single crystal silicon, and  FIG. 16  is a graph showing an example of switching characteristics of an n-channel MOS transistor having a semiconductor layer made of polysilicon. 
         [0020]    In  FIGS. 15 and 16 , curves A represent characteristics for a standard threshold VTH, curves B represent characteristics for a threshold voltage VTH shifted by −1 V from the standard threshold voltage, and curves C represent characteristics for a threshold voltage VTH shifted by +1 V from the standard threshold voltage. 
         [0021]    As is understood from  FIGS. 15 and 16 , in the case of the polysilicon MOS transistor (a polysilicon thin film transistor, for example) using as a semiconductor layer a polysilicon obtained by a solid phase epitaxy method crystallizing at a temperature of 500.degree. C. to 1,100.degree. C., or a polysilicon obtained by crystallizing by laser-annealing amorphous silicon produced by a CVD method, when a gate-source voltage VGS is small (5 V or less), drain currents ID of the polysilicon MOS transistor is smaller than those of the MOS transistor having the semiconductor layer of single crystal silicon, and drain currents ID of the polysilicon MOS transistor vary greatly with variations of the threshold voltages VTH. 
         [0022]    As a result, when the level converter circuits shown in  FIGS. 13 and 14  are formed by MOS transistors having semiconductor layers made of single crystal silicon, satisfactory operation can be guaranteed, but when the level converter circuits shown in  FIGS. 13 and 14  are formed by polysilicon MOS transistors having semiconductor layers made of polysilicon, there was a disadvantage that sufficient driving capability could not be obtained in a case where the supply voltage is the low voltage VCC. 
         [0023]      FIG. 17  is a graph showing DC transfer characteristics of a CMOS inverter. 
         [0024]    In general, in a CMOS inverter, threshold voltages VTH are determined in the p-channel MOS transistors and the N-channel MOS transistors forming the CMOS inverter such that, when an input signal exceeds the middle between the H level and the L level of the input signals, the p-channel and N-channel MOS transistors forming the CMOS inverter change from ON to OFF, or from OFF to ON. Curve A in  FIG. 17  represent the DC transfer characteristic in this state. 
         [0025]    Curve B in  FIG. 17  represents a DC transfer characteristic in a case where the threshold voltages VTH of the p-channel and N-channel MOS transistors forming the CMOS inverter is shifted to the left of the curve A, and curve C in  FIG. 17  represents a DC transfer characteristic in a case where the threshold voltages VTH of the p-channel and N-channel MOS transistors forming the CMOS inverter is shifted to the right of the curve A. 
         [0026]      FIGS. 18A to 18D  are schematic illustrations for explaining input and output waveforms of the CMOS inverter. 
         [0027]      FIG. 18A  represents a waveform of an input signal to the CMOS inverter,  FIGS. 18B to 18D  represent waveforms of output signals from the CMOS inverters having DC transfer characteristics corresponding to the curves A to C of  FIG. 17 , respectively. 
         [0028]    If the DC transfer characteristic of the CMOS inverter is represented by the curve A of  FIG. 17 , the output signal starts to fall delayed by a time tDA from the time the input signal starts to rise, but a duration LHA of the H level and a duration LLA of the L level of the output signal are the same as durations of the H and L levels of the input signal, respectively, as shown in  FIG. 18B . 
         [0029]    But, if the DC transfer characteristic of the CMOS inverter is represented by the curve B of  FIG. 17 , the output signal starts to fall delayed by a time tDB which is shorter than the time tDA, from the time the input signal starts to rise, a duration LHB of the H level of the output signal is shorter than the duration of the H level of the input signal and a duration LLB of the L level of the output signal is longer than the duration of the L level of the input signal, as shown in  FIG. 18C . 
         [0030]    And, if the DC transfer characteristic of the CMOS inverter is represented by the curve C of  FIG. 17 , the output signal starts to fall delayed by a time tDC which is longer than the time tDA, from the time the input signal starts to rise, and a duration LHC of the H level of the output signal is longer than the duration of the H level of the input signal and a duration LLC of the L level of the output signal is shorter than the duration of the L level of the input signal, as shown in  FIG. 18D . 
         [0031]    In general, threshold voltages VTH of polysilicon MOS transistors vary more greatly than those of MOS transistors having single crystal silicon layer, and as is apparent from  FIG. 16 , drain currents ID vary greatly with variations of threshold voltages VTH of the polysilicon MOS transistors. 
         [0032]    As a result, if the prior art level converter circuit is formed by polysilicon MOS transistors, the DC transfer characteristics of the CMOS inverters INV 1 , INV 2  (see  FIG. 13 ) vary greatly mainly due to the variations of the threshold voltages VTH of the polysilicon MOS transistors of the CMOS inverters INV 1 , INV 2 , and consequently, there was a problem in that a delay time (or a phase difference) of the output signal with respect to the input signal and a variation of a duration of the H or L level of the output signal increase. 
         [0033]    For example,  FIG. 19  shows waveforms of input and output signals of the level converter circuit of  FIG. 13  formed by n-channel MOS transistors using polysilicon having mobility of about 80 cm.sup.2/(Vs) and p-channel MOS transistors using polysilicon having mobility of about 60 cm.sup.2/(Vs). 
         [0034]    In  FIG. 19 , curve ø 5  represents an output of the level converter circuit having standard threshold voltages VTH, curve ø 5 - 1  represents an output of the level converter circuit in a case where threshold voltages VTH of the NMOS and PMOS transistors shift by −1 V, and curve ø 5 - 2  represents an output of the level converter circuit in a case where threshold voltages VTH of the NMOS and PMOS transistors shift by +1 V. 
         [0035]    As is apparent from  FIG. 19 , the delay time of the output signal with respect to the input signal and a variation of a duration of the H level of the output signal vary greatly with the variations of the threshold voltages VTH of the MOS transistors. 
         [0036]    In a liquid crystal display module of the analog-sampling active-matrix type using polysilicon MOS transistors, such variations of the delay time of the output signal from the level converter circuit and the duration of the H level of the output signal cause a degradation in picture quality such as a picture defect in the form of a vertical line, when a half tone picture is displayed. 
         [0037]      FIG. 20  is an illustration for explaining a principle of displaying by the liquid crystal display module of the active matrix type using polysilicon MOS transistors. 
         [0038]    In the liquid crystal display module of the active matrix type using polysilicon MOS transistors, during one horizontal scanning period, a gate electrode line G 1 , for example, is selected by a scanning circuit and during this period analog video signals øsig are sampled and supplied to, . . . an (n−1)st drain electrode line, an nth drain electrode line, an (n+1)st drain electrode line, . . . , sequentially by shift scanning of shift registers SR of a horizontal scanning circuit, and this horizontal scanning is repeated the number of times equal to the number of the gate electrode lines to form a picture. 
         [0039]    The operation of sampling the analog video signals øsig for the (n−1)st, nth and (n+1)st drain electrode lines will be explained by referring to time charts in  FIG. 21 . 
         [0040]    First, voltage levels of complementary clock input signals øPL and øNL are level-converted by level converter circuits LV 1  and LV 2 , respectively, to produce level-converted mutually complementary signals øNH and øPH. 
         [0041]    The signal øPH and an output from one shift register SR are supplied to a NAND circuit NA 1  to produce a sampling pulse øN, and the signal øNH and an output from another shift register SR are supplied to a NAND circuit NA 2  to produce a sampling pulse øN+1. 
         [0042]    The inverted pulses /øN and /øN+1 (A slant “/” is used instead of the bar “    ” to indicate an inverted signal.) of the sampling pulses øN and øN+1 drive sample-and-hold circuits SH 1  and SH 2  to sample time-varying analog video signals øsig sequentially and supply video signal voltages øm−1, øm and øm+1 to the (n−1)st, nth and (n+1)st drain electrode lines. 
         [0043]    As a result, if the threshold voltages VTH of the MOS transistors of the level converter circuits LV 1  and LV 2  vary, the phases and the durations of the H level of the complementary signals øNH and øPH level-converted by the level converter circuits LV 1  and LV 2  vary, and consequently, the phases and the durations of the H level of the sampling pulses øN and øN+1 vary. 
         [0044]    The variations of the phases and the durations of the H level of the sampling pulses øN and øN+1 cause shortening of the sampling time, sampling of a portion of the analog video signal øsig different from a portion of the analog video signal øsig intended to be sampled, or overlapping of the sampling times of the two sampling pulses øN and øN+1. These produce a ghost in an image displayed on a liquid crystal display panel, and therefore deteriorate display quality of the displayed image extremely. 
         [0045]    In a digital-signal-input type liquid crystal display module of the active matrix type using polysilicon MOS transistors, if such level converter circuits are employed before a digital-analog converter (a D/A converter), variations of delay times occur in level converter circuits corresponding to respective data bits and consequently, a false picture is produced because some data bits are digital-to-analog converted in a state where they are inverted. 
       SUMMARY OF THE INVENTION 
       [0046]    The present invention is made so as to solve the above problems with the prior art, it is an object of the present invention to provide a technique capable of operating a level converter circuit at a high speed and stably irrespective of variations of threshold voltages of transistors. 
         [0047]    It is another object of the present invention to provide a technique capable of improving the quality of displayed images by a liquid crystal display device by using the above level converter circuit. 
         [0048]    The above-mentioned and other objects and novel features of the present invention will be made apparent by the following description and accompanying drawings. 
         [0049]    The following explains the representative ones of the present inventions briefly. 
         [0050]    In accordance with an embodiment of the present invention, there is provided a level converter circuit comprising: an input terminal adapted to be supplied with a signal swinging from a first voltage to a second voltage lower than the first voltage; a first transistor having a gate electrode connected to the input terminal, and a source electrode connected to ground potential; a second transistor having a gate electrode connected to a drain electrode of the first transistor, a source electrode connected to a supply voltage, and a drain electrode connected to an output terminal; a load circuit connected between the gate electrode of the second transistor and the supply voltage; a third transistor having a source electrode connected to the input terminal, a drain electrode connected to the output terminal, and a gate electrode supplied with a DC voltage higher than the second voltage and lower than the first voltage, wherein the level converter circuit outputs a third voltage higher than the second voltage when the input terminal is supplied with the first voltage, and the level converter circuit outputs the second voltage when the input terminal is supplied with the second voltage. 
         [0051]    In accordance with another embodiment of the present invention, there is provided a level converter circuit comprising: an input terminal adapted to be supplied with a digital signal swinging from a first voltage to a second voltage lower than the first voltage; a first transistor having a gate electrode connected to the input terminal, and a source electrode connected to ground potential; a second transistor having a gate electrode connected to a drain electrode of the first transistor, a source electrode connected to a supply voltage, and a drain electrode connected to an output terminal; a load circuit connected between the gate electrode of the second transistor and the supply voltage; a third transistor having a source electrode connected to the input terminal, a drain electrode connected to the output terminal, and a gate electrode supplied with a DC voltage higher than the second voltage and lower than the first voltage, wherein (a) when the input terminal is supplied with the first voltage, the first transistor and the second transistor are ON, and the level converter circuit outputs a third voltage higher than the first voltage; and (b) when the input terminal is supplied with the second voltage, the first transistor and the second transistor are OFF and the level converter circuit outputs the second voltage via the third transistor. 
         [0052]    In accordance with still another embodiment of the present invention, there is provided a liquid crystal display device including a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, a plurality of pixels formed between the pair of substrates and a driver circuit for driving the plurality of pixels, the driver circuit being provided with a level converter circuit, the level converter circuit comprising: an input terminal adapted to be supplied with a digital signal swinging from a first voltage to a second voltage lower than the first voltage; a first transistor of an n-channel type having a gate electrode connected to the input terminal, and a source electrode connected to ground potential; a second transistor of a p-channel type having a gate electrode connected to a drain electrode of the first transistor, a source electrode connected to a supply voltage, and a drain electrode connected to an output terminal; a load circuit connected between the gate electrode of the second transistor and the supply voltage; a third transistor having a source electrode connected to the input terminal, a drain electrode connected to the output terminal, and a gate electrode supplied with a DC voltage, the DC voltage being such that, (a) when the source electrode of the third transistor is supplied with the second voltage, the third transistor is ON, and (b) when the source electrode of the third transistor is supplied with the first voltage, the third transistor is OFF, wherein (i) when the input terminal is supplied with the first voltage, the first transistor and the second transistor are ON, and the level converter circuit outputs a third voltage higher than the first voltage; and (ii) when the input terminal is supplied with the second voltage, the first transistor and the second transistor are OFF and the level converter circuit outputs the second voltage via the third transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    In the accompanying drawings, in which like reference numerals designate similar components throughout the figures, and in which: 
           [0054]      FIG. 1  is a circuit diagram of a level converter circuit of Embodiment 1 of the present invention; 
           [0055]      FIG. 2  is an illustration of examples of input and output signal waveforms of the level converter circuit of Embodiment 1 of the present invention; 
           [0056]      FIG. 3  is a circuit diagram of a modification of the level converter circuit of Embodiment 1 of the present invention; 
           [0057]      FIG. 4  is a circuit diagram of another modification of the level converter circuit of Embodiment 1 of the present invention; 
           [0058]      FIG. 5  is a circuit diagram of still another modification of the level converter circuit of Embodiment 1 of the present invention; 
           [0059]      FIG. 6  is a circuit diagram of still another modification of the level converter circuit of Embodiment 1 of the present invention; 
           [0060]      FIG. 7  is a circuit diagram of still another modification of the level converter circuit of Embodiment 1 of the present invention; 
           [0061]      FIG. 8  is a circuit diagram of a level converter circuit of Embodiment 2 of the present invention; 
           [0062]      FIG. 9  is a circuit diagram of a level converter circuit of Embodiment 3 of the present invention; 
           [0063]      FIG. 10  is a circuit diagram of a level converter circuit of Embodiment 4 of the present invention; 
           [0064]      FIG. 11  is a block diagram representing a configuration of a display panel of an active-matrix type liquid crystal display module of the analog-sampling type using polysilicon MOS transistors in accordance with Embodiment 5 of the present invention; 
           [0065]      FIG. 12  is a block diagram representing a configuration of a display panel of a liquid crystal display module of the digital-signal-input active-matrix type using polysilicon MOS transistors in accordance with Embodiment 5 of the present invention; 
           [0066]      FIG. 13  is a circuit diagram of an example of a prior art level converter circuit; 
           [0067]      FIG. 14  is a circuit diagram of another example of a prior art level converter circuit; 
           [0068]      FIG. 15  is a graph showing an example of switching characteristics of a n-channel MOS transistor having a semiconductor made of single crystal silicon; 
           [0069]      FIG. 16  is a graph showing an example of switching characteristics of a MOS transistor having a semiconductor layer made of polysilicon; 
           [0070]      FIG. 17  is a graph showing DC transfer characteristics of a CMOS inverter; 
           [0071]      FIG. 18A  is an illustration of a waveform of an input signal to a CMOS inverter, and  FIGS. 18B to 18D  are illustrations of waveforms of output signals from the CMOS inverter; 
           [0072]      FIG. 19  is an illustration of an example of waveforms of input and output signals of the level converter circuit of  FIG. 13 . formed by polysil icon n-channel MOS transistors and polysil icon p-channel MOS transistors; 
           [0073]      FIG. 20  is an illustration for explaining a principle of displaying by a liquid crystal display module of the active matrix type using polysilicon MOS transistors; 
           [0074]      FIG. 21  is timing charts for explaining the operation of sampling analog video signals øsig to be supplied to a drain electrode line in  FIG. 20 ; and 
           [0075]      FIG. 22  is a circuit diagram of a prior art buffer circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0076]    The embodiments of the present invention will be explained in detail by reference to the drawings. All the drawings for the embodiments use the same reference numerals to identify parts performing the same functions, which are not repeatedly described in the specification. 
       Embodiment 1 
       [0077]      FIG. 1  is a circuit diagram representing a level converter circuit of Embodiment 1 of the present invention. 
         [0078]    As shown in  FIG. 1 , the level converter circuit of this embodiment is formed by a total of transistors including two enhancement-mode p-channel polysilicon MOS transistors and two enhancement-mode n-channel polysilicon MOS transistors, and the level converter circuit has a first stage formed by PMOS M 1  and NMOS M 3  and a second stage formed by PMOS M 2  and NMOS M 4 . 
         [0079]    A source electrode of NMOS M 3  of the first stage is connected to the reference voltage VSS (ground potential) and a gate electrode of NMOS M 3  is supplied with an input signal ø 6  from a input terminal VIN. 
         [0080]    The input signal ø 6  has an amplitude of VCC, or varies from a voltage higher than VCC to ground potential VSS. 
         [0081]    A drain electrode of PMOS M 1  is connected to a drain electrode of NMOS M 3 , and a source electrode and a gate electrode of PMOS M 1  are connected to the high voltage VDD and its drain electrode, respectively. 
         [0082]    A source electrode of NMOS M 4  of the second stage is supplied with the input signal ø 6  and a gate electrode of NMOS M 4  is connected to a low voltage VCC. 
         [0083]    A drain electrode of PMOS M 2  is connected to a drain electrode of NMOS M 4 , and a source electrode and a gate electrode of PMOS M 2  are connected to the high voltage VDD and the drain electrode of PMOS M 1 , respectively. Namely, PMOS M 1  forms an active load. 
         [0084]    A level-converted output signal ø 8  is output from the drain electrode of PMOS M 2  of the second stage. 
         [0085]    In the level converter circuit of this embodiment, among electrodes of NMOS M 3  and M 4  of the first and second stages, all the electrodes (i.e., the source and gate electrodes of NMOS M 3  and the source and gate electrodes of NMOS M 4 ) except for electrodes connected to an output terminal or a next stage are supplied with the input signal ø 6  or a direct-current voltage (the low voltage VCC or ground potential VSS). 
         [0086]    Next, the operation of the level converter circuit of this embodiment will be explained. 
         [0087]    When the input signal ø 6  from the input terminal VIN is at the H level, NMOS M 3  is ON, PMOS M 1  is ON, NMOS M 4  is OFF, PMOS M 2  is ON, and therefore the output terminal VOUT outputs the high voltage VDD. When the input signal ø 6  is at the L level, NMOS M 3  is OFF, PMOS M 1  is OFF, NMOS M 4  is ON, PMOS M 2  is OFF, and therefore the output terminal VOUT outputs the input signal ø 6  which is at the L level. 
         [0088]      FIG. 2  is illustrations of examples of waveforms of the input and output signals of the level converter circuit of this embodiment. 
         [0089]      FIG. 2  illustrates the waveforms of the input and output signals in a case where polysilicon n-channel MOS transistors having mobility of about 80 cm.sup.2/(Vs) are used as NMOS M 3  and M 4 , and polysilicon p-channel MOS transistors having mobility of about 60 cm.sup.2e(Vs) are used as PMOS M 1  and M 2 . 
         [0090]    In  FIG. 2 , curve ø 8  represents a waveform of an output in a case where NMOS M 3 , M 4  and PMOS M 1 , M 2  have standard threshold voltages VTH, curve ø 8 - 1  represents a waveform of an output in a case where NMOS M 3 , M 4  and PMOS M 1 , M 2  have threshold voltages changed by −1 V, and curve ø 8 - 2  represents a waveform of an output in a case where NMOS M 3 , M 4  and PMOS M 1 , M 2  have threshold voltages changed by +1 V. 
         [0091]    As is apparent from  FIG. 2 , the level converter circuit of this embodiment provides comparatively stable input and output characteristics irrespective of the variations of the threshold voltages VTH of NMOS M 3 , M 4  and PMOS M 1 , M 2 , compared with the waveforms of the input and output characteristics shown in  FIG. 19 . 
         [0092]    As described above, the threshold voltages VTH of the polysilicon MOS transistors vary greatly, and as shown in  FIG. 16 , when the supply voltage is low, the drain currents ID vary greatly with the variations of the threshold voltages VTH of the MOS transistors. 
         [0093]    However, in the level converter circuit of this embodiment, the external signal ø 6  is applied to the gate electrode of NMOS M 3  and the source electrode of NMOS M 4  directly from the input terminal VIN, and as a result, even if the threshold voltages VTH of the polysilicon MOS transistors vary, the drain currents ID do not vary much. 
         [0094]    Consequently, the level converter circuit of this embodiment can prevent the delay time of the output signal and the duration of the H level of the output signal from varying greatly with the variations of the threshold voltages VTH of the transistors NMOS M 3 , M 4  and PMOS M 1 , M 2  forming the level converter circuit. 
         [0095]    Incidentally, the advantages of this embodiment are obtained in a level converter circuit using transistors having single-crystal semiconductor layers. However, as shown in  FIG. 15 , the threshold voltages VTH of the MOS transistors having a single-crystal semiconductor layer do not vary much, and a large amount of the drain currents can be obtained, and consequently, it is common sense to use a conventional circuit shown in  FIG. 13  for the purpose of low power consumption. Therefore no one has thought of the level converter circuit of this embodiment shown in  FIG. 1 , because there is a disadvantage of increase of power consumption. 
         [0096]      FIGS. 3 to 7  are circuit diagrams for illustrating modifications of the level converter circuit of the embodiment of the present invention. 
         [0097]    A level converter circuit shown in  FIG. 3  uses a resistor element as a load of its first stage. In the level converter circuit of  FIG. 3 , the same polysilicon film and wiring electrodes as those of the thin film transistors (TFTS) can be used for the resistor element, and as a result, the level converter circuit can be fabricated simply and manufactured easily. 
         [0098]    A level converter circuit shown in  FIG. 4  uses as a load of its first stage a polysilicon PMOS M 1  a gate electrode of which is supplied with a specified bias supply voltage Vbb. In the level converter circuit of  FIG. 4 , a current flowing through NMOS M 3  is limited by PMOS M 1 , and consequently, its power consumption is suppressed. The limit of the current is determined by the bias supply voltage Vbb. 
         [0099]    A level converter circuit shown in  FIG. 5  uses as a load of its first stage an active load formed by a polysilicon NMOS M 20 . In the level converter circuit of  FIG. 5 , an input stage is formed only by MOS transistors of NMOS M 3  and M 20 , and the NMOS transistors have higher mobility than PMOS transistors and therefore the level converter circuit operates with greater speed. 
         [0100]    A level converter circuit shown in  FIG. 6  uses as a load of its first stage an active load formed by a depletion-mode polysilicon NMOS M 21 . In the level converter circuit of  FIG. 6 , NMOS M 21  is a depletion-mode MOS transistor, and it can flow a current therethrough at all times and therefore the level converter circuit operates with greater speed, but the power consumption is increased accordingly. 
         [0101]    A level converter circuit shown in  FIG. 7  uses a diode D as a load of its first stage. The diode D is fabricated by doping the same polysilicon film as that of the thin film transistors (TFT) with impurities for forming a p-type region and an n-type region, respectively, and therefore the level converter circuit of  FIG. 7  facilitates its manufacturing process. 
         [0102]    The level converter circuits shown in  FIGS. 3 to 7  are capable of providing the advantages similar to those provided by the level converter circuit of  FIG. 1 . 
       Embodiment 2 
       [0103]      FIG. 8  is a circuit diagram of a level converter circuit of Embodiment 2 of the present invention. 
         [0104]    As shown in  FIG. 8 , the level converter circuit of this embodiment also uses a total of four enhancement-mode transistors including two p-channel polysilicon MOS transistors and two n-channel polysilicon MOS transistors, and has the first stage formed by PMOS M 1  and NMOS M 3  and the second stage formed by PMOS M 2  and NMOS M 4 . 
         [0105]    The level converter circuit of this embodiment differs from that of Embodiment 1, in that a source electrode of NMOS M 3  of the first stage is supplied with the input signal ø 6 , a gate electrode of NMOS M 3  is connected to the low voltage VCC, a source electrode of NMOS M 4  of the second stage is connected to the reference voltage VSS and a gate electrode of NMOS M 4  is supplied with the input signal ø 6  from the input terminal VIN. 
         [0106]    In the level converter circuit of this embodiment, when the input signal ø 6  from the input terminal VIN is at the H level, NMOS M 3  is OFF, PMOS M 1  is OFF, NMOS M 4  is ON, PMOS M 2  is OFF, and therefore the output terminal VOUT outputs ground potential VSS. 
         [0107]    Next, when the input signal ø 6  is at the L level, NMOS M 3  is ON, PMOS M 1  is ON, NMOS M 4  is OFF, PMOS M 2  is ON, and therefore the output terminal VOUT outputs the high voltage VDD. 
         [0108]    While, in the level converter circuit of Embodiment 1, the level-converted output signal ø 8  is in the same phase with the input signal ø 6 , the level-converted output signal ø 8  of the level converter circuit of this embodiment is in the phase opposite from the input signal ø 6 . 
         [0109]    The level converter circuit of this embodiment also provides the advantages similar to those provided by the level converter circuit of Embodiment 1, and the level converter circuit of Embodiment 2 may use one of the loads represented in  FIGS. 3 to 7 , as the load of the first stage which is formed by PMOS M 1 . 
         [0110]    A buffer circuit similar to the level converter circuit of Embodiment 2 is disclosed in Japanese Patent Application Laid-open No. Hei 7-7414 (laid-open on Jan. 10, 1995).  FIG. 22  is a circuit diagram of the buffer circuit disclosed in Japanese Patent Application Laid-open No. Hei 7-7414. 
         [0111]    Only the voltage VDD and the reference voltage VSS are supplied to the buffer circuit of  FIG. 22  including PMOS Q 1  and NMOS Q 2  so as to perform a function of the buffer circuit. NMOS Q 2  is supplied with a signal having an amplitude varying between the voltage VDD and ground potential VSS, and consequentially, a depletion-mode n-channel MOS transistor is used as NMOS Q 2 . 
         [0112]    In the first place, the buffer circuit of  FIG. 22  is not a level converter circuit for shifting a voltage level of an input signal, and it differs from the level converter circuit of Embodiment 2 in that the depletion-mode n-channel MOS transistor, NMOS Q 2 , is used. 
         [0113]    Further, Japanese Patent Application Laid-open No. Hei 7-7414 does not disclose a technique for preventing the delay time of the output signal and the duration of the H level of the output signal from varying greatly with variations of the threshold voltages VTH of the transistors NMOS M 3 , M 4  and PMOS M 1 , M 2  of the level converter circuit of Embodiment 2 shown in  FIG. 8 . 
       Embodiment 3 
       [0114]      FIG. 9  is a circuit diagram of a level converter circuit of Embodiment 3 of the present invention. 
         [0115]    As shown in  FIG. 9 , the level converter circuit of this embodiment also uses a total of four enhancement-mode transistors including two p-channel polysilicon-MOS transistors and two n-channel polysilicon MOS transistors, and has the first stage formed by PMOS M 1  and NMOS M 3  and the second stage formed by PMOS M 2  and NMOS M 4 . 
         [0116]    The level converter circuit of this embodiment differs from that of Embodiment 1, in that a gate electrode of PMOS M 1  of the first stage is connected to a drain electrode (i.e., the output terminal VOUT) of PMOS N 2  of the second stage. 
         [0117]    In the level converter circuit of this embodiment, when the input signal ø 6  from the input terminal VIN is at the H level, NMOS M 3  is ON, PMOS M 1  is OFF, NMOS M 4  is OFF, PMOS M 2  is ON, and therefore the output terminal VOUT outputs the high voltage VDD. 
         [0118]    Next, when the input signal ø 6  is at the L level, NMOS M 3  is OFF, PMOS M 1  is ON, NMOS M 4  is ON, PMOS M 2  is OFF, and therefore the output terminal VOUT outputs the input signal ø 6  which is the low voltage. 
         [0119]    In this way, in the level converter circuit of this embodiment, the level-converted output signal ø 8  is in the same phase with the input signal ø 6  as in the case of Embodiment 1. 
         [0120]    The level converter circuit of this embodiment also provides the advantages similar to those provided by the level converter circuit of Embodiment 1. 
         [0121]    In the level converter circuit of this embodiment, as shown in  FIG. 9 , both NMOS M 3  and PMOS M 1  are not ON at the same time, both NMOS M 4  and PMOS M 2  are not ON at the same time, and consequently any currents do not flow except for switching times in the first and second stages and power consumption is reduced. 
         [0122]    However, the level converter circuit of Embodiment 1 shown in  FIG. 1  has an advantage of higher speed operation than this embodiment. 
         [0123]    The level converter circuit of this embodiment differs from the level converter circuit of  FIG. 14 , in that the external signal ø 6  from the external terminal VIN is applied directly to the gate electrode of NMOS M 3  and the source electrode of NMOS M 4 . 
         [0124]    As described above, threshold voltages VTH of polysilicon MOS transistors vary greatly, and if the supply voltage is low, drain currents ID vary greatly with the variations of the threshold voltages VTH of the MOS transistors. Therefore, if the level converter circuit of  FIG. 14  is formed by polysilicon MOS transistors, there has been a problem in that the variations of a delay time (or a phase difference) of the output signal with respect to the input signal and a duration of the H level (or a duration of the L level) become great mainly due to the threshold voltages VTH of the polysilicon. MOS transistors forming the CMOS inverter INV 1 . 
         [0125]    On the other hand, in the level converter circuit of this embodiment, the gate electrode of NMOS M 3  and the source electrode of NMOS M 4  have the external signal ø 6  applied directly from the external terminal VIN, and consequently, a delay time of the output signal and a duration of the H level of the output signal are prevented from varying greatly with the variations of the threshold voltages VTH of the transistors, NMOS M 3 , M 4  and PMOS M 1 , M 2 , forming the level converter circuit. 
       Embodiment 4 
       [0126]      FIG. 10  is a circuit diagram of a level converter circuit of Embodiment 4 of the present invention. 
         [0127]    As shown in  FIG. 10 , the level converter circuit of this embodiment also uses a total of four enhancement-mode transistors including two p-channel polysilicon MOS transistors and two n-channel polysilicon MOS transistors, and has the first stage formed by PMOS M 1  and NMOS M 3  and the second stage formed by PMOS M 2  and NMOS M 4 . 
         [0128]    The level converter circuit of this embodiment differs from that of Embodiment 2, in that a gate electrode of NMOS M 1  of the first stage is connected to a drain electrode (i.e., the output terminal VOUT) of PMOS M 2  of the second stage. 
         [0129]    In the level converter circuit of this embodiment, when the input signal ø 6  from the input terminal VIN is at the H level, NMOS M 3  is OFF, PMOS M 1  is ON, NMOS M 4  is ON, PMOS M 2  is OFF, and therefore the output terminal VOUT outputs ground potential VSS. 
         [0130]    Next, when the input signal ø 6  is at the L level, NMOS M 3  is ON, PMOS M 1  is OFF, NMOS M 4  is OFF, PMOS M 2  is ON, and therefore the output terminal VOUT outputs the high voltage VDD. 
         [0131]    In this way, in the level converter circuit of this embodiment, the level-converted output signal ø 8  is in the phase opposite from the input signal ø 6 , as in the case of Embodiment 2. 
         [0132]    As in the case of the level converter circuit of Embodiment 3, in the level converter circuit of this embodiment also, currents flow in the circuits of the first and second stages only during switching times, and power consumption is reduced. 
         [0133]    However, the level converter circuit of Embodiment 1 shown in  FIG. 1  has an advantage of higher speed operation than this embodiment. 
       Embodiment 5 
       [0134]      FIG. 11  is a block diagram representing a configuration of a display panel of an active-matrix type liquid crystal display module of the analog sampling type using polysilicon MOS transistors in accordance with Embodiment 5 of the present invention. 
         [0135]    In  FIG. 11 , reference character SUB 1  denotes a transparent insulating substrate made of glass having a softening temperature not higher than 800.degree. C. or quartz glass, reference numeral  3  denotes a display area having a plurality of pixels arranged in a matrix fashion and each pixel is provided with a polysilicon thin film transistor (TFT). 
         [0136]    Each pixel is disposed in an area surrounded by two adjacent drain electrode lines D and two adjacent gate electrode lines G. 
         [0137]    Each pixel has a thin film transistor TFT, a source electrode of which is connected to a pixel electrode (not shown). A liquid crystal layer is disposed between each pixel electrode and a common electrode (not shown) opposing all the pixel electrodes, and therefore a capacitor CLC formed by the liquid crystal layer is connected between the source electrode of the thin film transistor TFT and the common electrode in an electrical equivalent circuit. 
         [0138]    An additional capacitance CADD is connected between the source electrode of the thin film transistor TFT and an immediately preceding gate electrode line G. 
         [0139]    All the gate electrodes of thin film transistors TFT in the same row among the thin film transistors TFT arranged in a matrix fashion are connected to one of the gate electrode lines G, and each of the gate electrode lines G is connected to vertical scanning circuits  5  disposed on opposite sides of the display area  3 . 
         [0140]    All the drain electrodes of thin film transistors TFT in the same column among the thin film transistors TET arranged in the matrix fashion are connected to one of the drain electrode lines D, and each of the drain electrode lines D is connected to a horizontal scanning circuit  4  disposed below the display area  3 . Each of the drain electrode lines D is also connected to a precharge circuit  6  disposed above the display area  3 . 
         [0141]    Voltage levels of control signals input via control signal input terminals  9 ,  10  are level-shifted by level converter circuits  7  in accordance with one of the above embodiments, and are supplied to the horizontal scanning circuit  4 , the vertical scanning circuit  5  and the precharge circuit  6 . In this embodiment, the polysilicon MOS transistors forming the level converter circuits  7  are fabricated on the transparent insulating substrate SUB 1  simultaneously with the thin film transistors TFT of the pixels. 
         [0142]    In this embodiment, the liquid crystal display panel has incorporated therein the level converter circuits for converting signals (generally 0 to 5 V, 0 to 3.5 V or 0 to 3 V) input from an external circuit into signals of amplitudes (generally high voltages) sufficient to drive the liquid crystal display panel and the circuits formed by polysilicon MOS transistors. Therefore, the present embodiment makes it possible to drive the liquid crystal display panel with output signals from standard logic ICs. 
         [0143]    In the liquid crystal display module using polysilicon MOS transistors, of this Embodiment also, the first gate electrode line G 1 , for example, is selected by the vertical scanning circuit  5  during one horizontal scanning period, and during this period the horizontal scanning circuit  4  outputs sampling pulses to drive a sample-and-hold circuit SH (not shown) such that analog video signals supplied from video signal input terminals  8  are supplied to each of the drain electrode lines D. 
         [0144]    In this embodiment, the analog video signals whose frequencies are divided by 12 are supplied from the video signal input terminals  8 , and therefore with one sampling pulse, analog video signals are supplied to twelve drain electrode lines D, respectively. 
         [0145]    Further, within a retrace time of one horizontal scanning period, the precharge circuit  6  supplies a precharge voltage input from a precharge voltage input terminal  11  to each of the drain electrode lines D. 
         [0146]    In this embodiment, one of the level converter circuits of the embodiments of the present invention is used as the level converter circuit  7 , and therefore this circuit reduces variations of phases of the sampling pulses and durations of the H level supplied from the horizontal scanning circuit  4 , even if variations occur in the threshold voltages VTH of the polysilicon MOS transistors forming the level converter circuit. 
         [0147]    Consequently, this embodiment prevent occurrence of a ghost in an image displayed on the liquid crystal display panel, and improves the quality of the displayed image compared with that obtained by the prior art. 
         [0148]    The present invention is not limited to the liquid crystal display module of the analog-sampling active-matrix type using polysilicon MOS transistors, but is also applicable to a liquid crystal display module of the digital-signal-input active-matrix type using polysilicon MOS transistors shown in  FIG. 12 . 
         [0149]    The liquid crystal display module of the digital-signal-input active-matrix type using polysilicon MOS transistors shown in  FIG. 12  is the same as the liquid crystal display module of the analog-sampling active-matrix type using polysilicon MOS transistors shown in  FIG. 11 , except that the liquid crystal display module of the digital-signal-input active-matrix type is provided with a D/A converter DAC connected to the video signal input terminals  8 . 
         [0150]    The D/A converter DAC of the liquid crystal display module of  FIG. 12  is also comprised of polysilicon thin film transistors fabricated simultaneously with the thin film transistors TFT forming pixels, and therefore digital video signals can be input directly into the liquid crystal display panel. 
         [0151]    Further, level converter circuits  7  in accordance with one of the above-described embodiments are provided between the D/A converter DAC and the video signal input terminals  8 , and therefore output signals from standard logic ICs can be input directly to the video signal input terminals  8 . 
         [0152]    In the level converter circuit  7  formed by polysilicon thin film transistors in accordance with one of the above-described embodiments, delay times vary little with the variations of threshold voltages VTH of the polysilicon MOS transistors, and a portion of data is not inverted in the D/A converter DAC and therefore defective displays do not occur. 
         [0153]    The inventions made by the present inventors have been explained concretely based upon the above embodiments, but the present inventions are not limited to the above embodiments and it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the present inventions. For example, the active-matrix display panel shown in  FIG. 11  can be used for an electroluminescent (EL) display device. 
         [0154]    The following explains briefly advantages obtained by representative ones of the inventions disclosed in this specification. 
         [0155]    (1) The level converter circuits of the present invention can reduce the total number of transistors required for the level converter circuit. 
         [0156]    (2) The level converter circuits of the present invention can reduce influences due to variations of threshold voltages of transistors forming the level converter circuit. 
         [0157]    (3) The liquid crystal display device of the present invention can improve the quality of images displayed on its display panel.