Abstract:
A boosting circuit comprises a first boosting cell row and a second boosting cell row. The boosting circuit further comprises an analog comparison circuit for comparing the potential of boosting cells on the same stage, and selecting and outputting the lower or higher of the potentials. The potential of an N well is controlled using the output potential of the analog comparison circuit. Thereby, the amplitude of an N well potential can be suppressed, and a single N well region can be shared.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a booster circuit employing a switching element having a triple-well structure. 
     2. Description of the Related Art 
     In recent years, flash memories, which are a type of non-volatile semiconductor memory devices, require data read and data write using a single power supply voltage or low power supply voltages, for which, therefore, a booster circuit for supplying a positive or negative boosted voltage is required on a chip when each operation is performed. Also, during CMOS processes, a power supply voltage generated by the booster circuit is used to improve characteristics of an analog circuit. 
     Conventionally, there is a known booster circuit employing a triple-well structure switching element (U.S. Pat. Nos. 6,100,557, 6,121,821, and 7,102,422). 
       FIG. 25  shows an exemplary conventional booster circuit. In  FIG. 25 ,  901  indicates a booster circuit which receives two-phase clock signals CLK 1  and CLK 2  and generates an output terminal voltage (boosted voltage) Vpump by a boosting operation.  902 ,  903 , and  904  are boosting cells which constitute an exemplary three-stage configuration, where CLK 1  is input to the odd-numbered-stage cells and CLK 2  is input to the even-numbered-stage cell.  905  indicates a backflow preventing circuit which prevents backflow of the boosted voltage Vpump.  906  indicates a charge transfer transistor which functions as a switching element.  907  indicates a P well (PW) of the charge transfer transistor  906 .  908  indicates a deep N well (NT) including the P well  907 .  909  indicates a parasitic diode between the P well  907  and the N well  908 .  910  indicates boosting capacitors which boost output terminals of the boosting cells  902 ,  903 , and  904 .  911 ,  912 ,  913 , and  914  indicate I/O terminals of the boosting cells. As shown in  FIG. 25 , the P well  907  and the N well  908  of each charge transfer transistor  906  of the boosting cells  902  to  904  is connected to the source of the charge transfer transistor  906  so that they have the same potential. 
       FIG. 26  is a waveform diagram showing the two-phase clock signals CLK 1  and CLK 2  in the booster circuit  901  of  FIG. 25 . An operation of the booster circuit  901  of  FIG. 25  will be briefly described with reference to  FIG. 26 . 
     Initially, at time T 1 , CLK 1  goes to “H” (power supply voltage Vdd) and CLK 2  goes to “L” (ground voltage Vss), so that the potentials of the I/O terminals  912  and  914  are boosted. At the same time, changes are transferred from the I/O terminal  912  to the I/O terminal  913  and from the I/O terminal  914  to the output terminal of the booster circuit  901 , via the charge transfer transistors  906  of the boosting cell  903  and the backflow preventing circuit  905 , respectively, so that the output terminal voltages of the I/O terminal  913  and the booster circuit  901  are increased. In this case, since the P well  907  of each of the boosting cell  903  and the backflow preventing circuit  905  has the same potential as that of the source terminal of the charge transfer transistor  906 , the substrate biasing effect of the charge transfer transistor  906  is suppressed, thereby making it possible to suppress a decrease in charge transfer efficiency. 
     At time T 2  when a charge transfer period Ttrans has been passed since time T 1 , CLK 2  goes to “H” and CLK 1  goes to “L”, so that the potential of the I/O terminal  913  is boosted. At the same time, charges are transferred from the I/O terminal  913  to the I/O terminal  914  via the charge transfer transistor  906  of the boosting cell  904 . In this case, since the P well  907  of the boosting cell  904  has the same potential as that of the source terminal of the charge transfer transistor  906 , the substrate biasing effect of the charge transfer transistor  906  is suppressed, thereby making it possible to suppress a decrease in charge transfer efficiency. 
     At time T 3 , an operation similar to that at time T 1  is performed. 
     Thus, according to the booster circuit  901  of  FIG. 25 , the substrate biasing effect is suppressed, thereby making it possible to suppress a decrease in charge transfer efficiency during the boosting operation. 
     However, in the conventional booster circuit  901 , the source and the N well  908  of the charge transfer transistor  906  are connected to each other, so that a parasitic capacitance formed by the N well  908  is charged and discharged by voltage transition widths of the clock signals CLK 1  and CLK 2  in response to voltage transitions of the clock signals CLK 1  and CLK 2 . 
     Also, charges supplied by the clock signals CLK 1  and CLK 2  are used to charge and discharge the N well  908 , disadvantageously resulting in a decrease in boost efficiency. 
     Also, since the source and the N well  908  of the charge transfer transistor  906  are connected to each other, it is necessary to separate the N wells  908  of the charge transfer transistors  906  from each other, disadvantageously resulting in an increase in layout area. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a booster circuit in which the current consumption and the layout area can be suppressed while the substrate biasing effect of a switching element used in each boosting cell can be suppressed. 
     To achieve the object, a booster circuit according to the present invention is provided in which the potential of an N well of each boosting cell is fixed to the input or output potential of the boosting cell stage to reduce the amount of charges which are charged and discharged between the N well and the substrate, thereby making it possible to improve the boost efficiency. 
     Specifically, in a first aspect, a boosting circuit comprises boosting cells each having a first-conductivity type first well region on a substrate, a second-conductivity type second well region in the first well region, and at least one switching element in the first well region or the second well region, in which the at least one switching element switches ON/OFF a connection between a first terminal and a second terminal so as to transfer charges from the first terminal to the second terminal, a first boosting cell row including N stages (N≧1) of the boosting cells, a second boosting cell row including M stages (M≧1) of the boosting cells, and at least one analog comparison circuit for outputting the higher or lower of an output potential of the boosting cell on the i-th stage (1≦i≦N) of the first boosting cell row and an output potential of the boosting cell on the i-th stage (1≦i≦M) of the second boosting cell row. The output potential of the at least one analog comparison circuit is applied to the first well region of the at least one switching element included in the boosting cell on the (i+1)-th stage, the boosting cell on the i-th stage, or at least one of the boosting cells on less than i-th stages of the first and second boosting cell rows. 
     In a second aspect, a booster circuit comprises boosting cells each having a first-conductivity type first well region on a substrate, a second-conductivity type second well region in the first well region, and at least one switching element in either or both of the first well region and the second well region, in which the at least one switching element switches ON/OFF a connection between a first terminal and a second terminal so as to transfer charges from the first terminal to the second terminal, a first boosting cell row including N stages (N≧1) of the boosting cells, a second boosting cell row including M stages (M≧1) of the boosting cells, and at least one analog comparison circuit for outputting the higher or lower of an input potential of the boosting cell on the i-th stage (1≦i≦N) of the first boosting cell row and an input potential of the boosting cell on the i-th stage (1≦i≦M) of the second boosting cell row. The output potential of the at least one analog comparison circuit is applied to the first well region of the at least one switching element included in the boosting cell on the (i+1)-th stage, the boosting cell on the i-th stage, or at least one of the boosting cells on less than i-th stages of the first and second boosting cell rows. 
     In a third aspect, a booster circuit comprises boosting cells and backflow preventing circuits each having a first-conductivity type first well region on a substrate, a second-conductivity type second well region in the first well region, and at least one switching element in the first well region or the second well region, wherein the at least one switching element switches ON/OFF a connection between a first terminal and a second terminal so as to transfer charges from the first terminal to the second terminal, a first boosting cell row including N stages (N≧1) of the boosting cells and the backflow preventing circuit, a second boosting cell row including M stages (M≧1) of the boosting cells and the backflow preventing circuit, and at least one analog comparison circuit for outputting the higher or lower of an output potential of the boosting cell on the i-th stage (1≦i≦N) of the first boosting cell row and an output potential of the boosting cell on the i-th stage (1≦i≦M) of the second boosting cell row. The output potential of the at least one analog comparison circuit is applied to the first well region of the at least one switching element included in the backflow preventing circuit, the boosting cell on the (i+1)-th stage, the boosting cell on the i-th stage, or at least one of the boosting cells on less than i-th stages of the first and second boosting cell rows. 
     In a fourth aspect, a booster circuit comprises boosting cells and backflow preventing circuits each having a first-conductivity type first well region on a substrate, a second-conductivity type second well region in the first well region, and at least one switching element in either or both of the first well region and the second well region, in which the at least one switching element switches ON/OFF a connection between a first terminal and a second terminal so as to transfer charges from the first terminal to the second terminal, a first boosting cell row including N stages (N≧1) of the boosting cells and the backflow preventing circuit, a second boosting cell row including M stages (M≧1) of the boosting cells and the backflow preventing circuit, and at least one analog comparison circuit for outputting the higher or lower of an intermediate potential of the backflow preventing circuit of the first boosting cell row and an intermediate potential of the backflow preventing circuit of the second boosting cell row. The output potential of the at least one analog comparison circuit is applied to the first well region of the at least one switching element included in the backflow preventing circuit of the first and second boosting cell rows or at least one of the boosting cells of the first and second boosting cell rows. 
     In a fifth aspect, a booster circuit comprises boosting cells and backflow preventing circuits each having a first-conductivity type first well region on a substrate, a second-conductivity type second well region in the first well region, and at least one switching element in either or both of the first well region and the second well region, in which the at least one switching element switches ON/OFF a connection between a first terminal and a second terminal so as to transfer charges from the first terminal to the second terminal, a first boosting cell row including N stages (N≧1) of the boosting cells and the backflow preventing circuit, a second boosting cell row including M stages (M≧1) of the boosting cells and the backflow preventing circuit, and at least one analog comparison circuit for comparing input potentials of the boosting cells on the i-th stages (1≦i≦N) of the first and second boosting cell rows or the backflow preventing circuits and outputting the higher or lower of the input potentials. The output potential of the at least one analog comparison circuit is applied to the first well region of the at least one switching element included in the backflow preventing circuit, the boosting cell on the (i+1)-th stage, the boosting cell on the i-th stage, or at least one of the boosting cells on less than i-th stages of the first and second boosting cell rows. 
     In a sixth aspect, in the booster circuit of any one of the first to fifth aspects, the second well region and the first terminal are connected to each other so that the second well region and the first terminal have the same potential. 
     In a seventh aspect, in the booster circuit of any one of the first to fifth aspects, the second well region and the first well region are connected to each other so that the second well region and the first well region have the same potential. 
     In an eighth aspect, in the booster circuit of any one of the first to fifth aspects, the at least one analog comparison circuit has a first-conductivity type first well region on the substrate, a second-conductivity type second well region in the first well region, and at least one switching element in the first well region or the second well region, and the at least one analog comparison circuit is provided one for each boosting cell stage. 
     In a ninth aspect, in the booster circuit of any one of the first to fifth aspects, the at least one analog comparison circuit has a first-conductivity type first well region on the substrate, a second-conductivity type second well region in the first well region, and at least one switching element in the first well region or the second well region, and the at least one analog comparison circuit is provided one every arbitrary number of boosting cell stages. 
     In a tenth aspect, in the booster circuit of any one of the first to fifth aspects, a diode element is provided between the first terminal and the first well region of the boosting cell. 
     In an eleventh aspect, in the booster circuit of any one of the first to fifth aspects, the at least one switching elements having the same potential of the first well region of the first and second boosting cell rows share a common first well region. 
     In a twelfth aspect, in the booster circuit of any one of the first to fifth aspects, the at least one switching elements having the same potential of the first well region of the first and second boosting cell rows and the at least analog comparison circuit share a common first well region. 
     In a thirteenth aspect, in the booster circuit of any one of the first to fifth aspects, the at least one switching circuit of the boosting cell on the i-th stage of the first boosting cell row and a first element of the at least analog comparison circuit share a common first well region, and the at least one switching circuit of the boosting cell on the i-th stage of the second boosting cell row and a second element of the at least analog comparison circuit share a common first well region. 
     According to the first aspect, the amount of charges which are charged and discharged in the first-conductivity type first well region can be caused to be smaller than the voltage swing of a clock signal, so that the apparent parasitic capacitance between the first well region and the substrate can be reduced. Thereby, it is possible to suppress current consumption during a boosting operation. Also, since the apparent parasitic capacitance between the first well region and the substrate can be reduced, boost efficiency can be improved. Also, by controlling the potentials of the first well regions of a plurality of boosting cells using a single analog comparison circuit, it is possible to suppress an increase in layout area due to the analog comparison circuit. 
     According to the second aspect, a well control can be performed using the input potential of a boosting cell on the first stage, so that a load capacitance connected to a boosting capacitor on each stage can be caused to be uniform. Thereby, a more stable boosting operation can be achieved. 
     According to the third aspect, the potential of the first well region of the backflow preventing circuit can be controlled, thereby making it possible to suppress a decrease in charge transfer efficiency of the backflow preventing circuit. 
     According to the fourth aspect, a well control can be performed using an intermediate potential of the backflow preventing circuit, so that a load capacitance connected to a boosting capacitor on each stage can be caused to be uniform. Thereby, a more stable boosting operation can be achieved. 
     According to the fifth aspect, a well control can be performed using the input potential of a boosting cell on the first stage, so that a load capacitance connected to a boosting capacitor on each stage can be caused to be uniform. Thereby, a more stable boosting operation can be achieved. 
     According to the sixth aspect, the input terminal and the second well region of a charge transfer transistor (switching element) in the boosting cell can be connected to each other, so that the current drive performance of a switching element can be suppressed from being decreased due to the substrate biasing effect. 
     According to the seventh aspect, the first well region and the second well region are caused to have the same potential, so that a single well region can be shared by an N-channel transistor and a P-channel transistor, resulting in a decrease in layout area. 
     According to the eighth aspect, an analog comparison circuit is provided for each of all stages, so that the boosting cells can be caused to be of uniform parasitic capacitance, thereby making it possible to improve easiness of design. 
     According to the ninth aspect, an analog comparison circuit is provided every arbitrary number of stages, so that an increase in circuit area can be suppressed while securing the margins of the breakdown voltages of the second well region and the first well region. 
     According to the tenth aspect, by supplying the input potential of a boosting cell to the first well region via a diode element, the potential increase of the first well region is caused to follow each boosting cell potential during start up of a booster circuit, thereby making it possible to prevent the occurrence of latch-up. 
     According to the eleventh and twelfth aspects, a common first well region is used, thereby making it possible to reduce the layout area. 
     According to the thirteenth aspect, noise interference between the first boosting cell row and the second boosting cell row which is pumped with clocks having different phases can be reduced while reducing the amount of charges which are charged and discharged of the first well region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing an exemplary configuration of a booster circuit according to the present invention. 
         FIG. 2  is a circuit diagram showing another exemplary configuration of a booster circuit according to the present invention. 
         FIG. 3  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 4  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 5  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 6  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 7  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 8  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 9  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 10  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 11  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 12  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 13  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 14  is a circuit diagram showing still another exemplary configuration of the booster circuit of the present invention. 
         FIG. 15  is a plan view showing an exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 16  is a plan view showing another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 17  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 18  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 19  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 20  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 21  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 22  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 23  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 24  is a plan view showing still another exemplary layout configuration of the booster circuit of the present invention. 
         FIG. 25  is a circuit diagram showing a conventional booster circuit. 
         FIG. 26  is a waveform diagram showing a two-phase clock signal in the booster circuit of  FIG. 25 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a booster circuit according to the present invention will be described, by way of examples, with reference to the accompanying drawings. 
       FIG. 1  shows an exemplary configuration of the booster circuit of the present invention. In  FIG. 1 ,  101  indicates a two-parallel booster circuit which receives two-phase clock signals CLK 1  and CLK 2  and generates an output terminal voltage (boosted voltage) Vpump by a boosting operation.  102 ,  103 ,  104 ,  105 ,  106 , and  107  indicate boosting cells which are arranged in a first line and a second line, where CLK 1  is input to the odd-numbered stages on the first line and the even-numbered stages on the second line, and CLK 2  is input to the even-numbered stages on the first line and the odd-numbered stages on the second line.  108  and  109  indicate backflow preventing circuits which prevent backflow of the boosted voltage Vpump.  110 ,  111 ,  112 ,  113 ,  114 ,  115 , and  116  indicate I/O terminals of the boosting cells  102  to  107 .  117 ,  118 , and  119  indicate exemplary low-voltage output analog comparison circuits which output the lower of the voltages of the I/O terminals of the boosting cells on the same stage of the first line and the second line.  120  and  121  indicate Nch (N-channel) transistors included in the low-voltage output analog comparison circuits  117 ,  118 , and  119 .  122 ,  123 , and  124  indicate output terminals of the low-voltage output analog comparison circuits  117  to  119  connected to N wells of the corresponding boosting cells.  125  indicates a high-voltage output analog comparison circuit which outputs the higher of the voltages of the I/O terminal  113  of the third-stage boosting cell  104  on the first line and the I/O terminal  116  of the third-stage boosting cell  107  on the second line.  126  and  127  indicate Pch (P-channel) transistors included in the high-voltage output analog comparison circuit  125 .  128  indicates an output terminal of the high-voltage output analog comparison circuit connected to N wells of the backflow preventing circuits  108  and  109 . Note that the same elements as those of the above-described conventional example are indicated by the same reference numerals. Also, the number of boosting cells connected in series in the booster circuit  101  shown in  FIG. 1  is only for illustrative purposes. 
     The two-phase clock signals CLK 1  and CLK 2  of the booster circuit  101  of  FIG. 1  have waveforms similar to those of  FIG. 26 . An operation of the booster circuit  101  of  FIG. 1  will be described with reference to  FIG. 26 . 
     At time T 1 , CLK 1  goes from “L” to “H” and CLK 2  goes from “H” to “L”, so that the potentials of the I/O terminals  111 ,  113 , and  115  of the boosting cells  102 ,  104 , and  106  are boosted, and the boosted charges are transferred via the charge transfer transistors  906  of the boosting cell  103 , the backflow preventing circuit  108 , and the boosting cell  107  to the I/O terminal  112 , the output terminal of the booster circuit  101 , and the I/O terminal  116 , respectively. In this case, in the low-voltage output analog comparison circuit  117 , the Nch transistor  120  is switched OFF and the Nch transistor  121  is switched ON due to a relationship in potential between the boosted I/O terminal  111  and the non-boosted I/O terminal  114 , so that the potential of the I/O terminal  114  is output from the output terminal  122  of the low-voltage output analog comparison circuit  117  and is supplied to the N wells of the boosting cell  102  and the boosting cell  105 . Similarly, the potential of the I/O terminal  112  is output from the output terminal  123  of the low-voltage output analog comparison circuit  118  and is supplied to the N wells of the boosting cell  103  and the boosting cell  106 . The potential of the I/O terminal  116  is output from the output terminal  124  of the low-voltage output analog comparison circuit  119  and is supplied to the N wells of the boosting cells  104  and the boosting cell  107 . Also, in the high-voltage output analog comparison circuit  125 , the Pch transistor  126  is switched ON and the Pch transistor  127  is switched OFF due to a relationship in potential between the boosted I/O terminal  113  and the non-boosted I/O terminal  116 , so that the potential of the I/O terminal  113  is output from the output terminal  128  of the high-voltage output analog comparison circuit  125  and is supplied to the N wells of the backflow preventing circuit  108  and the backflow preventing circuit  109 . 
     At time T 2 , if CLK 1  goes from “H” to “L” and CLK 2  goes from “L” to “H”, the potentials of the I/O terminals  112 ,  114 , and  116  of the boosting cells  103 ,  105 , and  107  are boosted, and boosted charges are transferred via the charge transfer transistors  906  of the boosting cells  104  and  106  and the backflow preventing circuit  109  to the output terminals of the I/O terminals  113  and  115  and the booster circuit  101 , respectively. In this case, in the low-voltage output analog comparison circuit  117 , the Nch transistor  120  is switched ON and the Nch transistor  121  is switched OFF due to a relationship in potential between the boosted I/O terminal  114  and the non-boosted I/O terminal  111 , so that the potential of the I/O terminal  111  is output from the output terminal  122  of the low-voltage output analog comparison circuit  117  and is supplied to the N wells of the boosting cell  102  and the boosting cell  105 . Similarly, the potential of the I/O terminal  115  is output from the output terminal  123  of the low-voltage output analog comparison circuit  118  and is supplied to the N wells of the boosting cell  103  and the boosting cell  106 . The potential of the I/O terminal  113  is output from the output terminal  124  of the low-voltage output analog comparison circuit  119  and is supplied to the N wells of the boosting cell  104  and the boosting cell  107 . Also, in the high-voltage output analog comparison circuit  125 , the Pch transistor  126  is switched OFF and the Pch transistor  127  is switched ON due to a relationship in potential between the boosted I/O terminal  116  and the non-boosted I/O terminal  113 , so that the potential of the I/O terminal  116  is output from the output terminal  128  of the high-voltage output analog comparison circuit  125  and is supplied to the N wells of the backflow preventing circuit  108  and the backflow preventing circuit  109 . 
     Thus, according to the booster circuit  101  of  FIG. 1 , the potentials of the N wells of the boosting cells  102  to  107  and the backflow preventing circuits  108  and  109  can be fixed to the input potentials or the output potentials of the respective boosting cell stage, so that the amount of charges which are charged and discharged between the N well and the substrate can be reduced, i.e., current consumption can be reduced. Also, by reducing the amount of charges which are charged and discharged between the N well and the substrate, the amount of charges transferred to the next stage can be increased, so that an improvement in boost efficiency can be expected. 
     Note that, as shown in  FIGS. 2 and 3 , the low-voltage output analog comparison circuits  117  to  119  and the high-voltage output analog comparison circuit  125  can be provided every arbitrary number of stages of boosting cells, taking into consideration the margin of the breakdown voltage between the P well and the N well and the circuit area, so that an effect similar to that of the above-described configuration can be obtained with a reduced number of elements. 
       FIG. 4  shows another exemplary configuration of the booster circuit of the present invention. In  FIG. 4 ,  701  indicates a two-parallel booster circuit which receives two-phase clock signals CLK 1  and CLK 2 , and generates a boosted voltage Vpump by a boosting operation.  702 ,  703 ,  704 ,  705 ,  706 , and  707  indicate the boosting cells  102  to  107  of  FIG. 1 , respectively, in each of which a transistor  710  diode-connected between the I/O terminal of the boosting cell and the output terminal  128  of the high-voltage output analog comparison circuit  125  is added.  708  and  709  indicate backflow preventing circuits. Note that elements similar to those of  FIG. 1  are indicated by the same reference numerals. The number of boosting cells connected in series in the booster circuit  701  is only for illustrative purposes. 
     The configuration of  FIG. 4  is different from that of  FIG. 1  in that the low- (or high-) voltage output analog comparison circuits  117  to  119  and  125  are replaced with a single common element, thereby reducing the number of elements. Thereby, during startup of the booster circuit  701 , when the potentials of the I/O terminals  111  to  116  of the boosting cells  702  to  707  increase, the N well potential is supplied as a forward current of the parasitic diode  909  from the P well  907  of each of the boosting cells  702  to  707 . To suppress the forward current of the parasitic diode  909 , the transistor  710  having a diode function is provided, thereby making it possible to provide a stable boosting operation even during startup of the booster circuit  701 . 
       FIG. 5  shows still another exemplary configuration of the booster circuit of the present invention. In  FIG. 5 ,  621  indicates a two-parallel booster circuit which receives two-phase clock signals CLK 1  and CLK 2 , and generates a boosted voltage Vpump by a boosting operation.  858  and  859  indicate backflow preventing circuits in which two-phase clock signals CLK 1  and CLK 2  are input and transistors  861  and  862  therein are controlled to cause a charge transfer transistor  860  to be in a conductive or non-conductive state. Thereby, a decrease in transfer efficiency occurring in the backflow preventing circuits  108  and  109  of  FIG. 1  is suppressed. Note that elements similar to those of  FIG. 1  are indicated by the same reference numerals. The number of boosting cells connected in series in the booster circuit  621  is only for illustrative purposes. 
     The configuration of  FIG. 5  is different from that of  FIG. 5  in that the two-phase clock signals CLK 1  and CLK 2  are input to control the gate potential of the charge transfer transistor  860 , and a low-voltage output analog comparison circuit  501  is used for the backflow preventing circuits  858  and  859  for improving charge transfer efficiency and boost efficiency. 
     According to  FIG. 5 , the low-voltage output analog comparison circuits  117 ,  118 ,  119 , and  501  having a similar structure are provided on the respective stages in the booster circuit  621 , so that a difference between the loads of the boosting capacitors  910  of the boosting cells  104  and  107  on the final stage of  FIG. 1  and the loads of the boosting capacitors  910  of the other boosting cells  102 ,  103 ,  105 , and  106  can be suppressed, thereby making it possible to cause the boosting capacitors  910  on the stages to be of substantially uniform parasitic capacitance. Therefore, the boosting cells on the stages are caused to be of uniform charge transfer amount, resulting in a stable boosting operation. 
     Note that a booster circuit  622  of  FIG. 6  is an example in which high-voltage output analog comparison circuits  511 ,  512 ,  513 , and  125  are used for the boosting cells  102  to  107  and the backflow preventing circuits  108  and  109  of  FIG. 1 , and only the gates of the transistors  126  and  127  of the high-voltage output analog comparison circuit  511  which controls the N wells of the boosting cells  102  and  105  on the first stage are fixed to VSS. Thereby, a stable boosting operation can be achieved as in  FIG. 5 . 
     Thus, the booster circuits employing the two-phase clock signals CLK 1  and CLK 2  have been described as exemplary booster circuit configurations. Alternatively, as shown in  FIG. 7 , a booster circuit  801  may employ four-phase clock signals CLK 1 , CLK 2 , CLK 3 , and CLK 4 . Alternatively, as shown in  FIGS. 8 ,  9 ,  10 , and  11 , booster circuit  851 ,  881 ,  601 , and  611  may employ two-phase clock signals CLK 1  and CLK 2 , and Nch transistors having triple wells in boosting cells, where the low-voltage output analog comparison circuits  117  to  119 , and  501  or the high-voltage output analog comparison circuits  511  to  513  and  125  are used, thereby making it possible to a similar effect irrespective of the configuration of the boosting cell. Also, as shown in  FIG. 11 , the P and N wells of an Nch transistor  612  of each of boosting cells  602  to  607  can be commonly connected, and further, the P and N wells of an Nch transistor  612  and the N well of the Pch transistor  611  can be commonly connected, thereby making it possible to reduce the layout area. 
     Note that, in  FIG. 7 ,  802 ,  803 ,  804 ,  805 ,  806 , and  807  indicate boosting cells,  808  and  809  indicate backflow preventing circuits,  810  indicates a charge transfer transistor (Nch transistor),  811  and  813  indicates Nch transistors, and  812  indicates a boosting capacitor. In  FIG. 8 ,  852 ,  853 ,  854 ,  855 ,  856 , and  857  indicate boosting cells,  858  and  859  indicate backflow preventing circuits,  860  indicates a charge transfer transistor (Nch transistor),  861  and  863  indicate Nch transistors, and  862  indicates a Pch transistor. In  FIGS. 10 and 11 ,  602 ,  603 ,  604 ,  605 ,  606 , and  607  indicate boosting cells,  608  and  609  indicate backflow preventing circuits,  610  and  612  indicate charge transfer transistors (Nch transistors),  611  and  614  indicate charge transfer transistors (Pch transistors), and  613  indicates a connection node. 
       FIGS. 12 ,  13 , and  14  show exemplary configurations including high-voltage (low-voltage) output analog comparison circuits, where either or both high- and low-voltage output analog comparison circuits can be used for all boosting cells and a backflow preventing circuit. If a boosting operation can be operated irrespective of the number of transistors (charge transfer transistors in the N wells of boosting cells, etc.) or the presence or absence of a Pch transistor and even when the P well is not necessarily connected directly to the source (e.g., the P well of the Nch transistor in a charge transfer transistor or the like is connected to the N well, the potential of the P well is supplied by switching the potentials of the drain and the source, etc.), a similar effect can be achieved. 
     The configuration of the low-voltage output analog comparison circuits  117  to  119  and  501  and the high-voltage output analog comparison circuits  511 ,  512 ,  513 , and  125  in the figures is only for illustrative purposes, and any other configurations that provide similar functions may be provided. 
       FIG. 15  is a plan view showing an exemplary layout configuration of booster circuits according to the present invention, indicating the charge transfer transistors  906  of the boosting cells  102  to  107  and the low-voltage output analog comparison circuits  117  to  119  of  FIG. 1 . 
     In  FIG. 15 , the output terminal  122  (or  123 ,  124 ) of the low-voltage output analog comparison circuit  117  (or  118 ,  119 ) is connected to a single N well (NT) which is shared by the charge transfer transistors  906  of the boosting cells  102  and  105  (or  103  and  106 ,  104  and  107 ). 
     According to  FIG. 15 , a single N well can be shared by the triple-well structure switching elements  906  of the two or more boosting cells  102  and  105  controlled by the output voltage of the low-voltage output analog comparison circuit  117 , thereby making it possible to reduce the layout area. 
     Note that the layout configuration of  FIG. 15  is only for illustrative purposes. Alternatively, as shown in  FIGS. 16 and 17 , the N well of the switching element  906  which is controlled by the output voltage of the low-voltage output analog comparison circuit  118  can be separated or shared irrespective of the number of stages of boosting cells. 
     Further, as shown in  FIGS. 18 and 19 , a single N well can be shared by the low-voltage output analog comparison circuits  117  to  119   u  and the switching elements  906  of the boosting cells  102  to  107 , respectively. 
     As shown in  FIGS. 20 and 21 , a single N well is shared by the transistors  120  of the single low-voltage output analog comparison circuits  117 ,  118 ,  119 , and  501 , and the boosting cells  102 ,  103 , and  104  and the backflow preventing circuit  108 , and another single N well is shared by the transistors  121  of the low-voltage output analog comparison circuits  117 ,  118 ,  119 , and  501 , and the boosting cells  105 ,  106 , and  107  and the backflow preventing circuit  109 . Thereby, the influence of noise in the boosting capacitor can be reduced while decreasing the amount of charges which are charged and discharged of the N well, thereby making it possible to achieve a stable boosting operation. 
     A similar layout can be applied to the high-voltage output analog comparison circuits  511 ,  512 ,  513 , and  125  as shown in  FIGS. 22 ,  23 , and  24 . 
     The above-described layout is only for illustrative purposes. A plurality of transistors having the same potential can share a single N or P well irrespective of the boosting cell row. 
     Further, in each of the above aspects, the output voltage generated by the analog comparison circuit in the boosting cell on the i-th stage can be applied to the N well of any of the boosting cells on the (i+1)-th stage, i-th stage, and stages anterior the i-th stage in the booster circuit in the N-th row other than the booster circuits in the first and second rows. This achieves not only reduction in area of the analog comparison circuit but also layout sharing to thus reduce the layout area. 
     As described above, in the booster circuit of the present invention, the substrate biasing effect can be suppressed in the triple-well structure element included in each boosting cell, so that the current consumption, the circuit area, and the layout area can be reduced. Therefore, the booster circuit of the present invention is useful as a power supply generating circuit or the like for improving analog circuit characteristics in a non-volatile semiconductor memory device and a CMOS process. 
     Also, the booster circuit of the present invention is applicable to power supply circuits for a volatile semiconductor memory device (DRAM, etc.), a liquid crystal device, a mobile device, and the like.