Abstract:
A voltage level shifter formed by single-typed transistors comprises two input terminals, two power supply terminals, a plurality of thin-film transistors, and an output terminal. Another voltage level shifter formed by single-typed transistors comprises two input terminals, an output terminal, two power supply terminals, two input units, a first thin-film transistor, a disable unit, a feedback unit, and a second thin-film transistor. The voltage level shifters are formed by single-typed TFTs. When integrating the voltage level shifters into a substrate of a TFT display, the manufacturing processes are simplified. Besides, power is saved.

Description:
This application claims priority to Taiwan Patent Application No. 095114010 filed on Apr. 19, 2006. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage level shifter, and more particularly, to a voltage level shifter formed by single-typed thin-film transistors. 
     2. Descriptions of the Related Art 
     Recently, thin-film transistor liquid crystal displays (TFT LCDs) are widely applied in personal computer monitors, televisions, cellular phones, digital cameras, and other electronic appliances. A TFT array is scanned according to a clock signal to activate pixels in turns. Since a high voltage level of the clock signal is required while the TFT array is scanned, the clock signal with a low voltage level has to be transferred to the high voltage level by a peripheral driving circuit, such as a voltage level shifter, and then provided to the TFT array. 
       FIG. 1  shows the circuit of one of conventional voltage level shifters, which comprises NMOS TFTs  101 ,  103 , and PMOS TFTs  105 ,  107 . Due to the coexistence of NMOS TFTs and PMOS TFTs, multiple doping MOS processes are generally necessary. This increases processing steps when integrating the voltage level shifter into a substrate of a TFT display, and manufacture cost increases. 
     One of the drawbacks of the conventional voltage level shifter is high manufacture cost. Therefore, it is desired in the industrial field that a voltage level shifter formed by single-typed TFTs to reduce manufacture cost. 
     SUMMARY OF THE INVENTION 
     The present invention, in one aspect, relates to a voltage level shifter formed by single-typed TFTs. In one embodiment, the voltage level shifter comprises a first input terminal, a second input terminal, a first power supply terminal, a second power supply terminal, a first TFT, a second TFT, a third TFT, a fourth TFT, a fifth TFT, a sixth TFT, and an output terminal. The first input terminal is configured to receive a first input signal. The second input terminal is configured to receive a second input signal. The first TFT, the second TFT, the third TFT, the fourth TFT, the fifth TFT, and the sixth TFT comprise a gate, a source, and a drain, respectively. The drain of the first TFT is electrically coupled to the first input terminal and the gate of the first TFT. The source of the second TFT is electrically coupled to the first power supply terminal. The gate of the second TFT is electrically coupled to the source of the first TFT. The source of the third TFT is electrically coupled to the drain of the second TFT. The drain of the third TFT is electrically coupled to the second power supply terminal. The source of the fourth TFT is electrically coupled to the gate of the second TFT. The drain of the fourth TFT is electrically coupled to the second power supply terminal. The gate of the fourth TFT is electrically coupled to the gate of the third TFT. The gate and the drain of the fifth TFT are electrically coupled to the second input terminal. The source of the fifth TFT is electrically coupled to the gate of the fourth TFT. The gate of the sixth TFT is electrically coupled to the first input terminal. The drain of the sixth TFT is electrically coupled to the second power supply terminal. The source of the sixth TFT is electrically coupled to the source of the fifth TFT. The output terminal is electrically coupled to the source of the third TFT. 
     In another aspect, the present invention relates to a voltage level shifter formed by single-typed TFTs. In one embodiment, the voltage level shifter comprises a first input terminal, a second input terminal, an output terminal, a first power supply terminal, a second power supply terminal, a first input unit, a second input unit, a first TFT, a disable unit, a feedback unit, and a second TFT. The first TFT and second TFT comprise a gate, a source, and a drain, respectively. The first input unit is configured to receive a first input signal via the first input terminal so as to output a first switching control signal. The second input unit is configured to receive a second input signal via the second input terminal so as to output a second switching control signal. The gate of the first TFT is electrically coupled to the first input unit and receives the first switching control signal. The drain of the first TFT is electrically coupled to the output terminal. The source of the first TFT is electrically coupled to the first power supply terminal. The disable unit is electrically coupled to the first input unit, the second input unit, the first TFT, and the second power supply terminal so as to control the first TFT disable. The feedback unit transmits a feedback signal to the first input unit and the disable unit in responding to an output signal of the output terminal. The gate of the second TFT is electrically coupled to the second input unit and receives the second switching control signal. The source of the second TFT is electrically coupled to the output terminal. The drain of the second TFT is electrically coupled to the second power supply terminal. 
     The present invention discloses voltage level shifters formed by single-typed TFTs. When integrating the voltage level shifters into a substrate of a TFT display, the manufacturing processes are simplified. Besides, power is saved. 
     These aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate one or more embodiments of the present invention and, together with the written description, serve to explain the principles of the present invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment, and wherein: 
         FIG. 1  illustrates a circuit of a conventional voltage level shifter; 
         FIG. 2A  illustrates a first embodiment of the present invention; 
         FIGS. 2B ,  2 C, and  2 D illustrate waveforms of an input terminal and an output terminal of the first embodiment of the present invention; 
         FIG. 3A  illustrates a second embodiment of the present invention; 
         FIGS. 3B ,  3 C, and  3 D illustrate waveforms of an input terminal and an output terminal of the second embodiment of the present invention; 
         FIG. 4A  illustrates a third embodiment of the present invention; 
         FIGS. 4B ,  4 C, and  4 D illustrate waveforms of an input terminal and an output terminal of the third embodiment of the present invention; 
         FIG. 5A  illustrates a fourth embodiment of the present invention; and 
         FIGS. 5B ,  5 C, and  5 D illustrate waveforms of an input terminal and an output terminal of the fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. Various embodiments of the present invention are now described in detail. 
       FIG. 2A  shows a first embodiment of the present invention which comprises a first input terminal Vin, a second terminal Vxin, a first power supply terminal V DD , a second power supply terminal V SS , a first TFT  201 , a second TFT  203 , a third TFT  205 , a fourth TFT  207 , a fifth TFT  209 , a sixth TFT  211 , and an output terminal Vout. The first input terminal Vin is configured to input a first input signal and the second input terminal Vxin is configured to receive a second input signal, wherein the first input signal and the second input signal are complementary. In other words, a device (not shown) is configured to generate the first input signal and the second input signal to the first input terminal Vin and the second terminal Vxin, respectively. The first input terminal Vin and the second input terminal Vxin are configured to receive the first input signal and the second input signal, and to transmit the first input signal and the second input signal. The output terminal Vout outputs an output signal. The first TFT  201 , second TFT  203 , third TFT  205 , fourth TFT  207 , fifth TFT  209 , and sixth TFT  211  are P-type in the first embodiment. Those skilled in the art can easily realize that N-type TFTs are also available. Moreover, the materials of the TFTs, such as amorphous silicon, poly-crystal silicon, micro-crystal silicon, single-crystal silicon, or combinations thereof, and the formations of the TFTs, such as top gate TFTs, bottom gate TFTs, or the like are not a limitation to the present invention. The connections among these elements are described below. 
     The drain  201   a  of the first TFT  201  is electrically coupled to the first input terminal Vin and the gate  201   c  thereof. The source  203   b  of the second TFT  203  is electrically coupled to the first power supply terminal V DD . The gate  203   c  of the second TFT  203  is electrically coupled to the source  201   b  of the first TFT  201 . The source  205   b  of the third TFT  205  is electrically coupled to the drain  203   a  of the second TFT  203 . The drain  205   a  of the third TFT  205  is electrically coupled to the second power supply terminal V SS . The source  207   b  of the fourth TFT  207  is electrically coupled to the gate  203   c  of the second TFT  203 . The drain  207   a  of the fourth TFT  207  is electrically coupled to the second power supply terminal V SS . The gate  207   c  of the fourth TFT  207  is electrically coupled to the gate  205   c  of the third TFT  205 . The gate  209   c  and the drain  209   a  of the fifth TFT  209  are electrically coupled to the second input terminal Vxin. The source  209   b  of the fifth TFT  209  is electrically coupled to the gate  207   c  of the fourth TFT  207 . The gate  211   c  of the sixth TFT  211  is electrically coupled to the first input terminal Vin. The drain  211   a  of the sixth TFT  211  is electrically coupled to the second power supply terminal V SS . The source  211   b  of the sixth TFT  211  is electrically coupled to the source  209   b  of the fifth TFT  209 . The output terminal Vout is electrically coupled to the source  205   b  of the third TFT  205 . 
       FIGS. 2B ,  2 C, and  2 D show simulation voltage versus time waveforms of the first input terminal Vin and the output terminal Vout under three different TFT threshold voltages, respectively.  FIG. 2B  shows the waveforms under a first threshold voltage, substantially −1V,  FIG. 2C  shows the waveforms under a second threshold voltage, substantially −2.5V, and  FIG. 2D  shows the waveforms under a third threshold voltage, substantially −4V. Meanwhile, the simulation conditions for deriving the waveforms in  FIGS. 2B ,  2 C, and  2 D are that: the first power supply terminal V DD  is substantially equal to −6V, the second power supply terminal V SS  is substantially equal to 9V, the first input terminal Vin swings from about 0V to about 5V, the electron mobility of the PMOS TFTs is about 60 cm 2 /Vsec, and an output load has about 20 pF capacitance. 
     As shown in  FIG. 2B , the low level of the output terminal Vout is far apart from the voltage level of the first power supply V DD , but the high level of the output terminal Vout is close to the voltage level of the second power supply V SS  when the threshold voltage is about −1V. As shown in  FIG. 2C , the low level and high level of the output terminal Vout are more acceptable when the threshold voltage of TFT is about −2.5V. As shown in  FIG. 2D , although the low level of the output terminal Vout can reach the voltage level of the first power supply V DD , it takes approximately 20 μs, and the rising time of the output signal is longer when the threshold voltage of TFT is about −4V. 
       FIG. 3A  shows a second embodiment of the present invention, which comprises a first input terminal Vin, a second input terminal Vxin, an output terminal Vout, a first power supply terminal V DD , a second power supply terminal V SS , a first input unit  31 , a second input unit  33 , a first TFT  301 , a disable unit  35 , a feedback unit  37 , and a second TFT  303 . The first input terminal Vin is configured to input a first input signal. The second input terminal Vxin is configured to input a second input signal. The output terminal Vout is configured to output an output signal. The first input signal and the second input signal are complementary, and the output signal of the output terminal Vout and the first input signal are substantially in phase. The connections among these elements are described below. 
     The first input unit  31  receives the first input signal via the first input terminal Vin, and outputs a first switching control signal  300 . The second input unit  33 , electrically coupled to the second power supply terminal V SS , receives the second input signal via the second input terminal Vxin, and outputs a second switching control signal  302 . The gate  301   c  of the first TFT  301 , electrically coupled to the first input unit  31 , receives the first switching control signal  300 . The drain  301   a  of the first TFT  301  is electrically coupled to the output terminal Vout. The source  301   b  of the first TFT  301  is electrically coupled to the first power supply terminal V DD . The disable unit  35 , electrically coupled to the first input unit  31 , the second input unit  33 , the first TFT  301 , and the second power supply terminal V SS , receives the second switching control signal  302  and disables the first TFT  301 . In other words, the disable unit  35  can control the first TFT  301  to disable (namely turned off). The feedback unit  37  respectively transmits feedback signals  304  and  306  to the first input unit  31  and the disable unit  35  in response to the output signal of the output terminal Vout. The gate  303   c  of the second TFT  303 , electrically coupled to the second input unit  33 , receives the second switching control signal  302 . The source  303   b  of the second TFT  303  is electrically coupled to the output terminal Vout. The drain  303   a  of the second TFT  303  is electrically coupled to the second power supply terminal V SS . In other words, the second TFT  303  receives the second switching control signal  302 . 
     The first input unit  31  comprises a third TFT  305  and a fourth TFT  307 . The second input unit  33  comprises a fifth TFT  309  and a sixth TFT  311 . The disable unit  35  comprises a seventh TFT  313  and an eighth TFT  315 . The feedback unit  37  comprises a ninth TFT  317  and a tenth TFT  319 . All the TFTs included in the second embodiment are P-type. Those skilled in the art can easily realize that N-type TFTs are also available. The materials of the TFTs, such as amorphous silicon, poly-crystal silicon, micro-crystal silicon, single-crystal silicon, or combinations thereof, and the formations of the TFTs, such as top gate TFTs, bottom gate TFTs, or the like are not a limitation to the present invention. The connections among these elements are described below. 
     The gate  305   c  of the third TFT  305  is electrically coupled to the first input terminal Vin and the drain  305   a  thereof. The gate  307   c  of the fourth TFT  307  is electrically coupled to the gate  305   c  of the third TFT  305 . The source  307   b  of the fourth TFT  307  is electrically coupled to the gate  301   c  of the first TFT  301 . The drain  307   a  of the fourth TFT  307 , electrically coupled to the source  305   b  of the third TFT  305 , receives the feedback signal  304 . 
     The gate  309   e  of the fifth TFT  309  is electrically coupled to the second input terminal Vxin and the drain  309   a  of the fifth TFT  309 . The source  309   b  of the fifth TFT  309 , electrically coupled to the gate  303   c  of the second TFT  303 , transmits the second switching control signal  302 . The gate  311   c  of the sixth TFT  311  is electrically coupled to the first input terminal Vin. The source  311   b  of the sixth TFT  311  is electrically coupled to the gate  303   c  of the second TFT  303  and the source  309   b  of the fifth TFT  309 . The drain  311   a  of the sixth TFT  311  is electrically coupled to the second power supply terminal V SS . 
     The source  313   b  of the seventh TFT  313  is electrically coupled to the gate  301   c  of the first TFT  301 . The source  315   b  of the eighth TFT  315 , electrically coupled to the drain  313   a  of the seventh TFT  313 , receives the feedback signal  306 . The gate  315   c  of the eighth TFT  315  and the gate  313   c  of the seventh TFT  313 , electrically coupled to the gate  303   c  of the second TFT  303 , receive the second switching control signal  302 . The drain  315   a  of the eighth TFT  315  is electrically coupled to the second power supply terminal V SS . In other words, the eighth TFT  315  receives the second switching control signal  302 . 
     The gate  317   c  of the ninth TFT  317  is electrically coupled to the output terminal Vout and the drain  317   a  of the ninth TFT  317 . The source  317   b  of the ninth TFT  317 , electrically coupled to the source  305   b  of the third TFT  305 , provides the feedback signal  304 . The source  319   b  of the tenth TFT  319 , electrically coupled to drain  313   a  of the seventh TFT  313  and the source  315   b  of the eighth TFT  315 , provides the feedback signal  306 . The gate  319   c  of the tenth TFT  319  is electrically coupled to the output terminal Vout and the drain  319   a  of the tenth TFT  319 . 
       FIGS. 3B ,  3 C, and  3 D show simulation voltage versus time waveforms of the first input terminal Vin and the output terminal Vout under three different TFT threshold voltages in accordance to the second embodiment, respectively.  FIG. 3B  shows the waveforms under a first threshold voltage, substantially −1V,  FIG. 3C  shows the waveforms under a second threshold voltage, substantially −2.5V, and  FIG. 3D  shows the waveforms under a third threshold voltage, substantially −4V. Meanwhile, the simulation conditions for deriving the waveforms in  FIGS. 3B ,  3 C, and  3 D are that: the first power supply terminal V DD  is substantially equal to −6V, the second power supply terminal V SS  is substantially equal to 9V, the first input terminal Vin swings from about 0V to about 5V, the electron mobility of the PMOS TFTs is about 60 cm 2 /Vsec, and an output load has about 20 pF capacitance. 
     As shown in  FIG. 3B , the low level of the output terminal Vout is close to the voltage level of the first power supply V DD  when the threshold voltage is about −1V. As shown in  FIG. 3C , the low level and high level of the output terminal Vout are more acceptable when the threshold voltage of TFT is about −2.5V. As shown in  FIG. 3D , the output signal of the output terminal Vout still requires long time to reach the low level and the high level when the threshold voltage of TFT is about −4V. 
       FIG. 4A  shows a third embodiment of the present invention. In contrast to the second embodiment, the first input unit  31  and the second input unit  33  of the third embodiment are different. As  FIG. 4A  shows, the first input unit  31  further comprises an eleventh TFT  401  and a twelfth TFT  403 , and the second input unit  33  further comprises a thirteenth TFT  405 . The connections among these elements are described below. 
     The drain  305   a  of the third TFT  305  is electrically coupled to the first input terminal Vin, the source  307   b  of the fourth TFT  307  is electrically coupled to the gate  301   c  of the first TFT  301  and the disable unit  35 . The gate  307   c  of the fourth TFT  307  is electrically coupled to the gate  305   c  of the third TFT  305 . The gate  307   a  of the fourth TFT  307  is electrically coupled to the source  305   b  of the third TFT  305 . The gate  401   c  of the eleventh TFT  401  is electrically coupled to the first input terminal Vin and the second input unit  33 . The drain  401   a  of the eleventh TFT  401  is electrically coupled to the first input terminal Vin. The gate  401   b  of the eleventh TFT  401  is electrically coupled to the gate  307   c  of the fourth TFT  307 . The gate  403   c  of the twelfth TFT  403  is electrically coupled to the gate  305   c  of the third TFT  305 . The source  403   b  of the twelfth TFT  403  is electrically coupled to the first input terminal Vin. The drain  403   a  of the twelfth TFT  403  is electrically coupled to the gate  305   c  of the third TFT  305 . 
     The source  309   b  of the fifth TFT  309  is electrically coupled to the gate  303   c  of the second TFT  303 . The drain  309   a  of the fifth TFT  309  is electrically coupled to the second input terminal Vxin. The gate  311   c  of the sixth TFT  311  is electrically coupled to the first input terminal Vin. The drain  311   a  of the sixth TFT  311  is electrically coupled to the second power supply terminal V SS . The source  311   b  of the sixth TFT  311  is electrically coupled to the gate  303   c  of the second TFT  303 . The gate  405   c  of the thirteenth TFT  405  is electrically coupled to the second input terminal Vxin. The source  405   b  of the thirteenth TFT  405  is electrically coupled to the gate  309   c  of the fifth TFT  309 . The drain  405   a  of the thirteenth TFT  405  is electrically coupled to the second input terminal Vxin. 
     The rest connections of the elements in the third embodiment are similar to those in the second embodiment so they are not repeated herein. 
     The eleventh TFT  401  and the twelfth TFT  403  cause a Bootstrap effect. They, as well as the thirteenth TFT  405  of the second input unit  33 , are capable of improving the performance of the whole circuit.  FIGS. 4B ,  4 C, and  4 D show simulation voltage versus time waveforms of the first input terminal Vin and the output terminal Vout under three different TFT threshold voltages in accordance to the third embodiment, respectively.  FIG. 4B  shows the waveforms under a first threshold voltage, substantially −1V,  FIG. 4C  shows the waveforms under a second threshold voltage, substantially −2.5V, and  FIG. 4D  shows the waveforms under a third threshold voltage, substantially −4V. Meanwhile, the simulation conditions for deriving the waveforms in  FIGS. 4B ,  4 C, and  4 D are that: the first power supply terminal V DD  is substantially equal to −6V, the second power supply terminal V SS  is substantially equal to 9V, the first input terminal Vin swings from about 0V to about 5V, the electron mobility of the PMOS TFTs is about 60 cm 2 /Vsec, and an output load has about 20 pF capacitance. One can observe that the waveforms of the output terminal Vout are excellent no matter the threshold voltage is low or high. 
       FIG. 5A  shows a fourth embodiment of the present invention. In contrast to the third embodiment, the second input unit  33  of the fourth embodiment is modified. The second input unit  33  further comprises a fourteenth TFT  501 , a fifteenth TFT  503 , a sixteenth TFT  505 , a seventeenth TFT  507 , an eighteenth TFT  509 , a nineteenth TFT  511 , a twentieth TFT  513 , and a twenty-first TFT  515 . All of the TFTs are P-type. The connections among those elements in the second input unit  33  are described below. 
     The drain  309   a  of the fifth TFT  309  is electrically coupled to the first input terminal Vin. The gate  311   c  of the sixth TFT  311  is electrically coupled to the second input terminal Vxin. The source  311   b  of the sixth TFT  311  is electrically coupled to the source  309   b  of the fifth TFT  309 . The drain  311   a  of the sixth TFT  311  is electrically coupled to the second power supply terminal V SS . The gate  405   c  of the thirteenth TFT  405  is electrically coupled to the first input terminal Vin. The source  405   b  of the thirteenth TFT  405  is electrically coupled to the gate  309   c  of the fifth TFT  309 . The drain  405   a  of the thirteenth TFT  405  is electrically coupled to the first input terminal Vin. 
     The drain  501   a  of the fourteenth TFT  501  is electrically coupled to the second input terminal Vxin. The source  501   b  of the fourteenth TFT  501  is coupled to the gate  303   c  of the second TFT  303 . The source of  503   b  the fifteenth TFT  503  is electrically coupled to the gate  303   c  of the second TFT  303 . The drain  503   a  of the fifteenth TFT  503  is electrically coupled to the second power supply terminal V SS . The gate  503   c  of the fifteenth TFT  503  is electrically coupled to the source  309   b  of the fifth TFT  309 . The source  505   b  of the sixteenth TFT  505  is electrically coupled to the gate  501   c  of the fourteenth TFT  501 . The gate  505   c  of the sixteenth TFT  505  is electrically coupled to the source  309   b  of the fifth TFT  309 . The gate  507   c  of the seventeenth TFT  507  is electrically coupled to the gate  505   c  of the sixteenth TFT  505 . The drain  507   a  of the seventeenth TFT  507  is electrically coupled to the second power supply terminal V SS . The source  507   b  of the seventeenth TFT  507  is electrically coupled to the drain  505   a  of the sixteenth TFT  505 . The gate  509   e  of the eighteenth TFT  509  is electrically coupled to the source  501   b  of the fourteenth TFT  501  and the drain  509   a  of the eighteenth TFT  509 . The source  509   b  of the eighteenth TFT  509  is electrically coupled to the drain  505   a  of the sixteenth TFT  505 . The source  511   b  of the nineteenth TFT  511  is electrically coupled to the source  505   b  of the sixteenth TFT  505 . The gate  513   c  of the twentieth TFT  513  is electrically coupled to the gate  511   c  of the nineteenth TFT  511  and the drain  513   a  of the twentieth TFT  513 . The source  513   b  of the twentieth TFT  513  is electrically coupled to the drain  511   a  of the nineteenth TFT  511  and the second input terminal Vxin. The gate  515   c  and the drain  515   a  of the twenty-first TFT  515  are electrically coupled to the second input terminal Vxin. The source  515   b  of the twenty-first TFT  515  is electrically coupled to the drain  513   a  of the twentieth TFT  513 . 
     The rest connections of the elements in the fourth embodiment are identical to those of the third embodiment so they are not repeated herein. 
       FIGS. 5B ,  5 C, and  5 D show simulation voltage versus time waveforms of the first input terminal Vin and the output terminal Vout under three different TFT threshold voltages in accordance to the fourth embodiment, respectively.  FIG. 5B  shows the waveforms under a first threshold voltage, substantially −1V,  FIG. 5C  shows the waveforms under a second threshold voltage, substantially −2.5V, and  FIG. 5D  shows the waveforms under a third threshold voltage, substantially −5V. Meanwhile, the simulation conditions for deriving the waveforms in  FIGS. 5B ,  5 C, and  5 D are that: the first power supply terminal V DD  is substantially equal to −6V, the second power supply terminal V SS  is substantially equal to 9V, the first input terminal Vin swings from about 0V to about 5V, the electron mobility of the PMOS TFTs is about 60 cm 2 /Vsec, and an output load has about 20 pF capacitance. One can observe that the waveforms of the output terminal Vout are excellent no matter the threshold voltage is low or high. 
     Table 1 shows the currents flowing through the first power supply terminal V DD  of third embodiment and fourth embodiment under the different threshold voltages. One can observe that the current flowing through V DD  of the fourth embodiment is apparently smaller than that of the third embodiment. Therefore, the fourth embodiment saves more power. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Current flowing through 
                 Current flowing through 
               
               
                   
                 the first power 
                 the first power 
               
               
                 Threshold voltage of 
                 supply terminal of third 
                 supply terminal of fourth 
               
               
                 TFT (V) 
                 embodiment (μA) 
                 embodiment (μA) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 −1 
                 58.0 
                 13.5 
               
               
                 −2 
                 8.5 
                 5.2 
               
               
                 −3 
                 3.3 
                 1.8 
               
               
                 −4 
                 1.3 
                 0.5 
               
               
                   
               
             
          
         
       
     
     The present invention discloses voltage level shifters formed by single-typed TFTs. When integrating the voltage level shifters into a substrate of a TFT display, the manufacturing processes are simplified. Besides, power is saved. 
     The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.