Abstract:
An electrostatic discharge protection circuit is provided. The electrostatic discharge protection circuit includes a first metal-oxide-semiconductor (MOS) transistor, a second MOS transistor, and a third MOS transistor. The first MOS transistor is coupled between a power terminal and a ground terminal. The first MOS transistor has a control electrode terminal coupled to a first node to receive a first signal. The second MOS transistor has a control electrode terminal and a first electrode terminal both coupled to the first node and a second electrode terminal coupled to a bulk of the first MOS transistor. The third MOS transistor has a control electrode terminal coupled to a second node to receive a second node, a first electrode terminal coupled to the first node, and a second electrode terminal coupled to the bulk of the first MOS transistor. The first signal is inverse to the second signal.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    The invention relates to an integrated circuit, and, more particularly, to an integrated circuit with an electrostatic discharge protection circuit. 
       Description of the Related Art 
       [0002]    In the development of the semiconductor manufacturing process, the dimensions of semiconductor elements have reached the sub-micron level, upgrading the performance and computational speed of integrated circuits. As dimensions shrink, the reliability and capability of electrostatic discharge (ESD) protection of integrated circuits decline significantly. When the dimensions are reduced with the developed manufacture process, the capability of ESD protection is seriously lowered, which causes the ESD tolerance of the elements to become degraded. Thus, an ESD protection circuit is provided to provide a discharge path for electrostatic charges. Particularly, how an ESD protection circuit can quickly provide a discharge path is an important issue. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    An exemplary embodiment of an electrostatic discharge protection circuit is provided. The electrostatic discharge protection circuit comprises a first metal-oxide-semiconductor (MOS) transistor, a second MOS transistor, and a third MOS transistor. The first MOS transistor is coupled between a power terminal and a ground terminal. The first MOS transistor has a control electrode terminal coupled to a first node to receive a first signal. The second MOS transistor has a control electrode terminal and a first electrode terminal both coupled to the first node and a second electrode terminal coupled to a bulk of the first MOS transistor. The third MOS transistor has a control electrode terminal coupled to a second node to receive a second node, a first electrode terminal coupled to the first node, and a second electrode terminal coupled to the bulk of the first MOS transistor. The first signal is inverse to the second signal. 
         [0004]    An exemplary embodiment of an integrated circuit comprises a core circuit and an electrostatic discharge protection circuit. The core circuit is coupled between a first pad and a second pad. The electrostatic discharge protection circuit is coupled to the first pad. When an electrostatic discharge event occurs at the first pad, the electrostatic discharge protection circuit provides a discharge path between the first pad and the second pad to protect the core circuit. The electrostatic discharge protection circuit comprises a first metal-oxide-semiconductor (MOS) transistor, a second MOS transistor, and a third MOS transistor. The first MOS transistor is coupled between the power terminal and the ground terminal. The first MOS transistor has a control electrode terminal coupled to a first node to receive a first signal. The second MOS transistor has a control electrode terminal and a first electrode terminal both coupled to the first node and a second electrode terminal coupled to a bulk of the first MOS transistor. The third MOS transistor has a control electrode terminal coupled to a second node to receive a second node, a first electrode terminal coupled to the first node, and a second electrode terminal coupled to the bulk of the first MOS transistor. The first signal is inverse to the second signal. 
         [0005]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0007]      FIG. 1  shows an exemplary embodiment of an integrated circuit; 
           [0008]      FIG. 2  shows one exemplary embodiment of an electrostatic discharge (ESD) protection circuit with an electrostatic discharge path by using N-type transistors; 
           [0009]      FIG. 3  shows another exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using N-type transistors; 
           [0010]      FIG. 4  shows another exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using N-type transistors; 
           [0011]      FIG. 5  shows another exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using N-type transistors; 
           [0012]      FIG. 6  shows one exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using P-type transistors; 
           [0013]      FIG. 7  shows another exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using P-type transistors; 
           [0014]      FIG. 8  shows another exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using P-type transistors; and 
           [0015]      FIG. 9  shows another exemplary embodiment of an ESD protection circuit with an electrostatic discharge path by using P-type transistors. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0017]      FIG. 1  shows an exemplary embodiment of an integrated circuit. Referring to  FIG. 1 , an integrated circuit  1  comprises a core circuit  10  and an electrostatic discharge (ESD) protection circuit  11 . The core circuit  10  is coupled to pads PAD 10  and PAD 11 . The PAD 11  is coupled to a ground GND. When the core circuit  10  operates in a normal operation mode, an operation voltage VDD is applied to the pad PAD 10 . When the core circuit  10  does not operate in the normal operation mode, the pad PAD 10  does not receive the operation voltage VDD. The ESD protection circuit  11  is coupled between the pads PAD 10  and PAD 11 . During the period when the core circuit  10  does not operate in the normal operation mode, once an ESD event occurs at the pad PAD 10 , the ESD protection circuit  11  provides a discharge path between the pads PAD 10  and PAD 11 , so that the electrostatic charges (ESD current) at the pad PAD 10  are conducted to the pad PAD 11  through the discharge path, which prevents the core circuit  10  from being damaged by the electrostatic charges. The various embodiments of the ESD protection circuit  11  will be described in the following. 
         [0018]      FIG. 2  shows an exemplary embodiment of the ESD protection circuit. In order to illustrate the circuit structure of the ESD protection circuit  11 ,  FIG. 2  only shows the ESD protection circuit  11  and the pads PAD 10  and PAD 11 . Referring to  FIG. 2 , the ESD protection circuit  11  comprises an ESD detection circuit  20 , an inverter  21 , N-type metal-oxide-semiconductor (NMOS) transistors N 20 -N 22 , a power terminal T 20 , and a ground terminal T 21 . The power terminal T 20  is coupled to the pad PAD 10 , and the ground terminal T 21  is coupled to the pad PAD 11 . The ESD detection circuit  20  comprises a resistor R 20  and a capacitor C 20  which are coupled in series. The resistor R 20  is coupled between the power terminal T 20  and a common node ND 20 . The capacitor C 20  is coupled between the common node ND 20  and the ground T 21 . A signal S 20  is generated at the common node ND 20 . The inverter  21  is coupled to the common node ND 20  to receive the signal S 20 . The inverter  21  inverts the signal S 20  to generate a signal S 21  at the node ND 21 . The inverter  21  comprises a P-type metal-oxide-semiconductor (PMOS) transistor P 20  and an NMOS transistor N 23 . The gate (control electrode terminal) of the PMOS transistor P 20  is coupled to the common node ND 20 , the source (electrode terminal) thereof is coupled to the power terminal T 20 , and the drain (electrode terminal) thereof is coupled to the node ND 21 . The bulk and the source of the PMOS transistor P 20  are coupled together. The gate of the NMOS transistor N 23  is coupled to the common node ND 20 , the drain thereof is coupled to the node ND 21 , and the source thereof is coupled to the ground terminal T 21 . The bulk and the source of the NMOS transistor N 23  are coupled together. The gate of the NMOS transistor N 20  is coupled to the node ND 21  to receive the signal S 21 , the drain thereof is coupled to the power terminal T 20 , and the source thereof is coupled to the ground terminal T 21 . The gate and the drain of the NMOS transistor N 21  are coupled together at the node ND 21 , and the source thereof is coupled to the bulk of the NMOS transistor N 20 . The bulk of the NMOS transistor N 21  is coupled to the ground terminal T 21 . The gate of the NMOS transistor N 22  is coupled to the node ND 20  to receive the signal S 20 , the drain thereof is coupled to the node ND 21  to receive the signal S 21 , and the source thereof is coupled to the bulk of the NMOS transistor N 20 . The bulk of the NMOS transistor N 22  is coupled to the ground terminal T 21 . 
         [0019]    When the core circuit  10  operates in the normal operation mode, an operation voltage VDD is applied to the pad PAD 10 , and the pad PAD 11  is coupled to the ground (such as 0 volts (V)). At this time, the signal S 20  at the node ND 20  is at a high voltage level: that is, there is a high voltage at the node ND 20 . The inverter  21  inverts the signal S 20  with the high voltage level to generate the signal S 21  with a low voltage level. In detail, the high voltage at the node ND 20  turns off the PMOS transistor P 20  and turns on the NMOS transistor N 23 . Thus, the signal S 21  at the node ND 21  is at the low voltage level: that is, there is a low voltage (0V) at the node ND 21 , to turn off the NMOS transistors N 20  and N 21 . Moreover, the high voltage at the node ND 20  turns on the NMOS transistor N 22 . Through the turned-on NMOS transistor N 22 , the bulk of the NMOS transistor N 20  is pulled to 0V. Accordingly, both the gate and the bulk of the NMOS transistor N 20  are at 0V. Thus, during the period when the core circuit  10  operates normally, the NMOS transistor N 20  can be in a stable turned-off state, so that there is no discharge path between the pads PAD 10  and PAD 11  in the ESD protection circuit  11 , and the operation of the core circuit  10  cannot be affected by any unexpected discharge path in the ESD protection circuit  11 . 
         [0020]    When the core circuit  10  does not operate in the normal operation mode, the operation voltage VDD is not applied to the pad PAD 10 . When an ESD event occurs at the pad PAD 10 , the voltage at the power terminal T 20  rises immediately. At this time, based on the element characteristics of the capacitor C 20 , the signal S 20  at the node ND 20  is at a lower voltage level (that is, there is a low voltage at the node ND 20 ) to turn off the NMOS transistor N 22 . The inverter  21  inverts the signal S 20  with the low voltage level to generate the signal S 21  with a high voltage level. In detail, the low voltage at the node ND 20  turns on the PMOS transistor P 20  and turns off the NMOS transistor N 23 . Thus, the signal S 21  at the node ND 21  is at the high voltage level: that is, there is a high voltage at the node ND 21 , to turn on the NMOS transistors N 20  and N 21 . Due to the turned-on NMOS transistor N 21 , there is a voltage difference between the gate and source of the NMOS transistor N 21  (the voltage difference is V TH , V TH  is the threshold voltage of the NMOS transistor N 21 ). As described above, the gates of the NMOS transistors N 20  and N 21  are coupled together through the node ND 21 , and the source of the NMOS transistor N 21  is coupled to the bulk of the NMOS transistor N 20 . In other words, the NMOS transistor N 21  is coupled between the gate and bulk of the NMOS transistor N 20 . Thus, there is a voltage difference between the gate and bulk of the NMOS transistor N 20 , so the gate-bulk voltage V GB  is not equal to zero, which ensures that the NMOS transistor N 20  is turned on. Due to the turned-on NMOS transistor N 20 , a discharge path is formed between the power terminal T 20  and the ground terminal T 21  (that is, between the pads PAD 10  and PAD 11 ). Accordingly, the electrostatic charges at the pad PAD 10  can be conducted to the pad PAD 11  through the discharge path, thereby protecting the elements in the core circuit  10  from being damaged by the electrostatic charges. 
         [0021]    In an embodiment, the speed of turning on the NMOS transistor N 20  can be increased by raising the gate-bulk voltage of the NMOS transistor N 20 . Thus, the ESD protection circuit  11  may further comprise at least one NMOS transistor which is coupled to the NMOS transistor N 21  in series. Referring to  FIG. 3 , the ESD protection circuit  11  further comprises an NMOS transistor N 30 . The gate and drain of the NMOS transistor N 30  are coupled to the source of the NMOS transistor N 21 , and the source thereof is coupled to the bulk of the NMOS transistor N 20 . The bulk of the NMOS transistor N 30  is coupled to the ground terminal T 21 . In the structure of  FIG. 3 , the source of the NMOS transistor N 21  is coupled to the bulk of the NMOS transistor N 20  through the NMOS transistor N 30 . In  FIGS. 2 and 3 , the elements with the same reference signs perform the same operation, thus, the related operations are omitted. In the embodiment, when the core circuit  10  does not operate in the normal operation mode and an ESD event occurs at the pad PAD 10 , both the NMOS transistors N 21  and N 30  are turned on. At this time, the voltage difference between the gate of the NMOS transistor N 21  and the source of the NMOS transistor N 30  is two times the value of V TH . As described above, the gates of the NMOS transistors N 20  and N 21  are coupled together through the node ND 21 , and the source of the NMOS transistor N 30  is coupled to the bulk of the NMOS transistor N 20 . In other words, there are two NMOS transistors N 21  and N 30  coupled between the gate and bulk of the NMOS transistor N 20 . Thus, the gate-bulk voltage V GB (=2V TH ) of the NMOS transistor N 20  in  FIG. 3  is larger than the gate-bulk voltage V GB (=V TH ) of the NMOS transistor N 20  in  FIG. 2 . Compared with the embodiment of  FIG. 2 , the NMOS transistor N 20  in  FIG. 3  can be turned on more quickly when an ESD event occurs at the pad PAD 10 : that is, the NMOS transistor N 20  in  FIG. 3  can provide a discharge path in a short time. 
         [0022]    In the embodiment of  FIG. 3 , one NMOS transistor which is coupled to the NMOS transistor N 21  in series is given as an example for illustration. However, in other embodiments, the number of NMOS transistors coupled to the NMOS transistor N 21  in series can be determined according to the system requirements. The higher the number of NMOS transistors coupled to the NMOS transistor N 21  in series is, the more the gate-bulk voltage V GB  of the NMOS transistor N 20  is, so that the NMOS transistor N 20  can be turned on more quickly when an ESD event occurs at the pad PAD 10 . 
         [0023]      FIG. 4  shows another exemplary embodiment of the ESD protection circuit. In order to illustrate the circuit structure of the ESD protection circuit  11 ,  FIG. 4  only shows the ESD protection circuit  11  and the pads PAD 10  and PAD 11 . Referring to  FIG. 4 , the ESD protection circuit  11  comprises an ESD detection circuit  40 , an inverter  41 , NMOS transistors N 40 -N 42 , a power terminal T 40 , and a ground terminal T 41 . The power terminal T 40  is coupled to the pad PAD 10 , and the ground terminal T 41  is coupled to the pad PAD 11 . The ESD detection circuit  40  comprises a resistor R 40  and a capacitor C 40  which are coupled in series. The capacitor C 40  is coupled between the power terminal T 40  and a common node ND 40 . The resistor R 40  is coupled between the common node ND 40  and the ground T 41 . A signal S 40  is generated at the common node ND 40 . The inverter  41  is coupled to the common node ND 40  to receive the signal S 40 . The inverter  41  inverts the signal S 40  to generate a signal S 41  at the node ND 41 . The inverter  41  comprises a PMOS transistor P 40  and an NMOS transistor N 43 . The gate (control electrode terminal) of the PMOS transistor P 40  is coupled to the common node ND 40 , the source (electrode terminal) thereof is coupled to the power terminal T 40 , and the drain (electrode terminal) thereof is coupled to the node ND 41 . The bulk and the source of the PMOS transistor P 40  are coupled together. The gate of the NMOS transistor N 43  is coupled to the common node ND 40 , the drain thereof is coupled to the node ND 41 , and the source thereof is coupled to the ground terminal T 41 . The bulk and the source of the NMOS transistor N 43  are coupled together. The gate of the NMOS transistor N 40  is coupled to the node ND 40  to receive the signal S 40 , the drain thereof is coupled to the power terminal T 40 , and the source thereof is coupled to the ground terminal T 41 . The gate and the drain of the NMOS transistor N 41  are coupled together at the node ND 40 , and the source thereof is coupled to the bulk of the NMOS transistor N 40 . The bulk of the NMOS transistor N 41  is coupled to the ground terminal T 41 . The gate of the NMOS transistor N 42  is coupled to the node ND 41  to receive the signal S 41 , the drain thereof is coupled to the node ND 40  to receive the signal S 40 , and the source thereof is coupled to the bulk of the NMOS transistor N 40 . The bulk of the NMOS transistor N 42  is coupled to the ground terminal T 41 . 
         [0024]    When the core circuit  10  operates in the normal operation mode, an operation voltage VDD is applied to the pad PAD 10 , and the pad PAD 11  is coupled to the ground (such as 0V). At this time, the signal S 40  at the node ND 40  is at a low voltage level: that is, there is a low voltage at the node ND 40 , to turn off the NMOS transistors N 40  and N 41 . The inverter  41  inverts the signal S 40  with the low voltage level to generate the signal S 41  with a high voltage level. In detail, the low voltage at the node N 40  turns off the NMOS transistor N 43  and turns on the PMOS transistor P 40 . Thus, the signal S 41  at the node ND 41  is at the high voltage level: that is, there is a high voltage at the node ND 41 , to turn on the NMOS transistor N 42 . Through the turned-on NMOS transistor N 42 , the bulk of the NMOS transistor N 40  is pulled to the low level voltage. Accordingly, both the gate and the bulk of the NMOS transistor N 40  are at 0V. Thus, during the period when the core circuit  10  operates normally, the NMOS transistor N 40  can be in a stable turned-off state, so that there is no leakage current path between the pads PAD 10  and PAD 11  in the ESD protection circuit  11 , and the operation of the core circuit  10  cannot be affected by any unexpected discharge path in the ESD protection circuit  11 . 
         [0025]    When the core circuit  10  does not operate in the normal operation mode, the operation voltage VDD is not applied to the pad PAD 10 . When an ESD event occurs at the pad PAD 10 , the voltage at the power terminal T 40  rises immediately. At this time, based on the element characteristics of the capacitor C 40 , the signal S 40  at the node ND 40  is at a high voltage level (that is, there is a high voltage at the node ND 40 ) to turn on the NMOS transistors N 40  and N 41 . The inverter  41  inverts the signal S 40  with the high voltage level to generate the signal S 41  with a low voltage level. In detail, the high voltage at the node ND 40  turns off the PMOS transistor P 40  and turns on the NMOS transistor N 43 . Thus, the signal S 41  at the node ND 41  is at the low voltage level: that is, there is a low voltage at the node ND 41 , to turn off the NMOS transistor N 42 . Due to the turned-on NMOS transistor N 41 , there is a voltage difference between the gate and source of the NMOS transistor N 41  (the voltage difference is V TH , V TH  is the threshold voltage of the NMOS transistor N 41 ). As described above, the gates of the NMOS transistors N 40  and N 41  are coupled together through the node ND 40 , and the source of the NMOS transistor N 41  is coupled to the bulk of the NMOS transistor N 40 . In other words, the NMOS transistor N 41  is coupled between the gate and bulk of the NMOS transistor N 40 . Thus, there is a voltage difference between the gate and bulk of the NMOS transistor N 40 , so the gate-bulk voltage V GB  is not equal to zero, which ensures that the NMOS transistor N 40  is turned on. Due to the turned-on NMOS transistor N 40 , a discharge path is formed between the power terminal T 40  and the ground terminal T 41  (that is, between the pads PAD 10  and PAD 11 ). Accordingly, the electrostatic charges at the pad PAD 10  can be conducted to the pad PAD 11  through the discharge path, thereby protecting the elements in the core circuit  10  from being damaged by the electrostatic charges. 
         [0026]    In an embodiment, the speed of turning on the NMOS transistor N 40  can be increased by raising the gate-bulk voltage of the NMOS transistor N 40 . Thus, the ESD protection circuit  11  may further comprise at least one NMOS transistor which is coupled to the NMOS transistor N 41  in series. Referring to  FIG. 5 , the ESD protection circuit  11  further comprises an NMOS transistor N 50 . The gate and drain of the NMOS transistor N 50  are coupled to the source of the NMOS transistor N 41 , and the source thereof is coupled to the bulk of the NMOS transistor N 40 . The bulk of the NMOS transistor N 50  is coupled to the ground terminal T 41 . In the structure of  FIG. 5 , the source of the NMOS transistor N 41  is coupled to the bulk of the NMOS transistor N 40  through the NMOS transistor N 50 . In  FIGS. 4 and 5 , the elements with the same reference signs perform the same operation, thus, the related operations are omitted. In the embodiment, when the core circuit  10  does not operate in the normal operation mode and an ESD event occurs at the pad PAD 10 , both the NMOS transistors N 41  and N 50  are turned on. At this time, the gate-bulk voltage V GB  of the NMOS transistor N 40  in  FIG. 5  is two times the value of V TH , which is larger than the gate-bulk voltage V GB (=V TH ) of the NMOS transistor N 40  in  FIG. 4 . Compared with the embodiment of  FIG. 4 , the NMOS transistor N 40  in  FIG. 5  can be turned on more quickly when an ESD event occurs at the pad PAD 10 : that is, the NMOS transistor N 40  in  FIG. 5  can provide a discharge path in a short time. 
         [0027]    In the embodiment of  FIG. 5 , one NMOS transistor which is coupled to the NMOS transistor N 41  in series is given as an example for illustration. However, in other embodiments, the number of NMOS transistors coupled to the NMOS transistor N 41  in series can be determined according to the system requirements. The higher the number of NMOS transistors coupled to the NMOS transistor N 41  in series is, the more the gate-bulk voltage V GB  of the NMOS transistor N 40  is, so that the NMOS transistor N 40  can be turned on more quickly when an ESD event occurs at the pad PAD 10 . 
         [0028]    In the above embodiments, the transistors which provide the discharge paths are implemented by NMOS transistors. In other embodiments, PMOS transistors can be used to provide discharge paths.  FIG. 6  shows another exemplary embodiment of the ESD protection circuit. Referring to  FIG. 6 , the ESD protection circuit  11  comprises an ESD detection circuit  60 , an inverter  61 , PMOS transistors P 60 -P 62 , a power terminal T 60 , and a ground terminal T 61 . The power terminal T 60  is coupled to the pad PAD 10 , and the ground terminal T 61  is coupled to the pad PAD 11 . The ESD detection circuit  60  comprises a resistor R 60  and a capacitor C 60  which are coupled in series. The capacitor C 60  is coupled between the power terminal T 60  and a common node ND 60 . The resistor R 60  is coupled between the common node ND 60  and the ground T 61 . A signal S 60  is generated at the common node ND 60 . The inverter  61  is coupled to the common node ND 60  to receive the signal S 60 . The inverter  61  inverts the signal S 60  to generate a signal S 61  at the node ND 61 . The inverter  61  comprises a PMOS transistor P 63  and an NMOS transistor N 60 . The gate (control electrode terminal) of the PMOS transistor P 63  is coupled to the common node ND 60 , the source electrode terminal thereof is coupled to the power terminal T 60 , and the drain (electrode terminal) thereof is coupled to the node ND 61 . The bulk and the source of the PMOS transistor P 63  are coupled together. The gate of the NMOS transistor N 60  is coupled to the common node ND 60 , the drain thereof is coupled to the node ND 61 , and the source thereof is coupled to the ground terminal T 61 . The bulk and the source of the NMOS transistor N 60  are coupled together. The gate of the PMOS transistor P 60  is coupled to the node ND 61  to receive the signal S 61 , the source thereof is coupled to the power terminal T 60 , and the drain thereof is coupled to the ground terminal T 61 . The gate and the source of the PMOS transistor P 61  are coupled together at the node ND 61 , and the drain thereof is coupled to the bulk of the PMOS transistor P 60 . The bulk of the PMOS transistor P 61  is coupled to the power terminal T 60 . The gate of the PMOS transistor P 62  is coupled to the node ND 60  to receive the signal S 60 , the source thereof is coupled to the node ND 61  to receive the signal S 61 , and the drain thereof is coupled to the bulk of the PMOS transistor P 60 . The bulk of the PMOS transistor P 62  is coupled to the power terminal T 60 . 
         [0029]    When the core circuit  10  operates in the normal operation mode, an operation voltage VDD is applied to the pad PAD 10 , and the pad PAD 11  is coupled to the ground (such as 0V). At this time, the signal S 60  at the node ND 60  is at a low voltage level: that is, there is a low voltage at the node ND 60 . The inverter  61  inverts the signal S 60  with the low voltage level to generate the signal S 61  with a high voltage level. In detail, the low voltage at the node ND 60  turns off the NMOS transistor N 60  and turns on the PMOS transistor P 63 . Thus, the signal S 61  at the node ND 61  is at the high voltage level: that is, there is a high voltage at the node ND 61 , to turn off the PMOS transistors P 60  and P 61 . Moreover, the low voltage at the node ND 60  turns on the PMOS transistor P 62 . Through the turned-on PMOS transistor P 62 , the bulk of the PMOS transistor P 60  is pulled to the high level of the node ND 61 . Accordingly, both the gate and the bulk of the PMOS transistor P 60  are at the same high voltage level. Thus, during the period when the core circuit  10  operates normally, the PMOS transistor P 60  can be in a stable turned-off state, so that there is no discharge path between the pads PAD 10  and PAD 11  in the ESD protection circuit  11 , and the operation of the core circuit  10  cannot be affected by any unexpected discharge path in the ESD protection circuit  11 . 
         [0030]    When the core circuit  10  does not operate in the normal operation mode, the operation voltage VDD is not applied to the pad PAD 10 . When an ESD event occurs at the pad PAD 10 , the voltage at the power terminal T 60  rises immediately. At this time, based on the element characteristics of the capacitor C 60 , the signal S 60  at the node ND 60  is at a high voltage level (that is, there is a high voltage at the node ND 60 ) to turn off the PMOS transistor P 62 . The inverter  61  inverts the signal S 60  with the high voltage level to generate the signal S 61  with a low voltage level. In detail, the high voltage at the node ND 60  turns off the PMOS transistor P 63  and turns on the NMOS transistor N 60 . Thus, the signal S 61  at the node ND 61  is at the low voltage level: that is, there is a low voltage (0V) at the node ND 61 , to turn on the PMOS transistors P 60  and P 61 . Due to the turned-on PMOS transistor P 61 , there is a voltage difference between the gate and drain of the PMOS transistor P 61  (the voltage difference is V TH , V TH  is the threshold voltage of the PMOS transistor P 61 ). As described above, the gates of the PMOS transistors P 60  and P 61  are coupled together through the node ND 61 , and the drain of the PMOS transistor P 61  is coupled to the bulk of the PMOS transistor P 60 . In other words, the PMOS transistor P 61  is coupled between the gate and bulk of the PMOS transistor P 60 . Thus, there is a voltage difference between the gate and bulk of the PMOS transistor P 60 , so the gate-bulk voltage V GB  is not equal to zero, which ensures that the PMOS transistor P 60  is turned on. Due to the turned-on PMOS transistor P 60 , a discharge path is formed between the power terminal T 60  and the ground terminal T 61  (that is, between the pads PAD 10  and PAD 11 ). Accordingly, the electrostatic charges at the pad PAD 10  can be conducted to the pad PAD 11  through the discharge path, thereby protecting the elements in the core circuit  10  from being damaged by the electrostatic charges. 
         [0031]    In an embodiment, the speed of turning on the PMOS transistor P 60  can be increased by raising the gate-bulk voltage of the PMOS transistor P 60 . Thus, the ESD protection circuit  11  may further comprise at least one PMOS transistor which is coupled to the PMOS transistor P 61  in series. Referring to  FIG. 7 , the ESD protection circuit  11  further comprises a PMOS transistor P 70 . The gate and source of the PMOS transistor P 70  are coupled to the drain of the PMOS transistor P 61 , and the drain thereof is coupled to the bulk of the PMOS transistor P 60 . The bulk of the PMOS transistor P 70  is coupled to the power terminal T 60 . In the structure of  FIG. 7 , the source of the PMOS transistor P 61  is coupled to the bulk of the PMOS transistor P 60  through the PMOS transistor P 70 . In  FIGS. 6 and 7 , the elements with the same reference signs perform the same operation, thus, the related operations are omitted. In the embodiment, when the core circuit  10  does not operate in the normal operation mode and an ESD event occurs at the pad PAD 10 , both the PMOS transistors P 61  and P 70  are turned on. At this time, the voltage difference between the gate of the PMOS transistor P 61  and the drain of the PMOS transistor P 70  is two times the value of V TH . As described above, the gates of the PMOS transistors P 60  and P 61  are coupled together through the node ND 61 , and the drain of the PMOS transistor P 70  is coupled to the bulk of the PMOS transistor P 60 . In other words, there are two PMOS transistors P 61  and P 70  coupled between the gate and bulk of the PMOS transistor P 60 . Thus, the gate-bulk voltage V GB (=2V TH ) of the PMOS transistor P 60  in  FIG. 7  is larger than the gate-bulk voltage V GB (=V TH ) of the PMOS transistor P 60  in  FIG. 6 . Compared with the embodiment of  FIG. 6 , the PMOS transistor P 60  in  FIG. 7  can be turned on more quickly when an ESD event occurs at the pad PAD 10 : that is, the PMOS transistor P 60  in  FIG. 7  can provide a discharge path in a short time. 
         [0032]    In the embodiment of  FIG. 7 , one PMOS transistor which is coupled to the PMOS transistor P 61  in series is given as an example for illustration. However, in other embodiments, the number of PMOS transistors coupled to the PMOS transistor P 61  in series can be determined according to the system requirements. The higher the number of PMOS transistors coupled to the PMOS transistor P 61  in series is, the more the gate-bulk voltage V GB  of the PMOS transistor P 60  is, so that the PMOS transistor P 60  can be turned on more quickly when an ESD event occurs at the pad PAD 10 . 
         [0033]      FIG. 8  shows another exemplary embodiment of the ESD protection circuit. In order to illustrate the circuit structure of the ESD protection circuit  11 ,  FIG. 8  only shows the ESD protection circuit  11  and the pads PAD 10  and PAD 11 . Referring to  FIG. 8 , the ESD protection circuit  11  comprises an ESD detection circuit  80 , an inverter  81 , PMOS transistors P 80 -P 82 , a power terminal T 80 , and a ground terminal T 81 . The power terminal T 80  is coupled to the pad PAD 10 , and the ground terminal T 81  is coupled to the pad PAD 11 . The ESD detection circuit  80  comprises a resistor R 80  and a capacitor C 80  which are coupled in series. The resistor R 80  is coupled between the power terminal T 80  and a common node ND 80 . The capacitor C 80  is coupled between the common node ND 80  and the ground T 81 . A signal S 80  is generated at the common node ND 80 . The inverter  81  is coupled to the common node ND 80  to receive the signal S 80 . The inverter  81  inverts the signal S 80  to generate a signal S 81  at the node ND 81 . The inverter  81  comprises a PMOS transistor P 83  and an NMOS transistor N 80 . The gate (control electrode terminal) of the PMOS transistor P 83  is coupled to the common node ND 80 , the source (electrode terminal) thereof is coupled to the power terminal T 80 , and the drain (electrode terminal) thereof is coupled to the node ND 81 . The bulk and the source of the PMOS transistor P 83  are coupled together. The gate of the NMOS transistor N 80  is coupled to the common node ND 80 , the drain thereof is coupled to the node ND 81 , and the source thereof is coupled to the ground terminal T 81 . The bulk and the source of the NMOS transistor N 80  are coupled together. The gate of the PMOS transistor P 80  is coupled to the node ND 80  to receive the signal S 80 , the source thereof is coupled to the power terminal T 80 , and the drain thereof is coupled to the ground terminal T 81 . The gate and the source of the PMOS transistor P 81  are coupled together at the node ND 80 , and the drain thereof is coupled to the bulk of the PMOS transistor P 80 . The bulk of the PMOS transistor P 81  is coupled to the power terminal T 80 . The gate of the PMOS transistor P 82  is coupled to the node ND 81  to receive the signal S 81 , the source thereof is coupled to the node ND 80  to receive the signal S 80 , and the drain thereof is coupled to the bulk of the PMOS transistor P 80 . The bulk of the PMOS transistor P 82  is coupled to the power terminal T 80 . 
         [0034]    When the core circuit  10  operates in the normal operation mode, an operation voltage VDD is applied to the pad PAD 10 , and the pad PAD 11  is coupled to the ground (such as 0V). At this time, the signal S 80  at the node ND 80  is at a high voltage level: that is, there is a high voltage at the node ND 80 , to turn off the PMOS transistors P 80  and P 81 . The inverter  81  inverts the signal S 80  with the high voltage level to generate the signal S 81  with a low voltage level. In detail, the high voltage at the node ND 80  turns off the PMOS transistor P 83  and turns on the NMOS transistor N 80 . Thus, the signal S 81  at the node ND 81  is at the low voltage level: that is, there is a low voltage at the node ND 81 , to turn on the PMOS transistor P 82 . Through the turned-on PMOS transistor P 82 , the bulk of the PMOS transistor P 80  is pulled to the high level of the node ND 80 . Accordingly, both the gate and the bulk of the PMOS transistor P 80  are at the same high voltage level. Thus, during the period when the core circuit  10  operates normally, the PMOS transistor P 80  can be in a stable turned-off state, so that there is no discharge path between the pads PAD 10  and PAD 11  in the ESD protection circuit  11 , and the operation of the core circuit  10  cannot be affected by any unexpected discharge path in the ESD protection circuit  11 . 
         [0035]    When the core circuit  10  does not operate in the normal operation mode, the operation voltage VDD is not applied to the pad PAD 10 . When an ESD event occurs at the pad PAD 10 , the voltage at the power terminal T 80  rises immediately. At this time, based on the element characteristics of the capacitor C 80 , the signal S 80  at the node ND 80  is at a low voltage level (that is, there is a low voltage at the node ND 80 ) to turn on the PMOS transistors P 80  and P 81 . The inverter  81  inverts the signal S 80  with the low voltage level to generate the signal S 81  with a high voltage level. In detail, the low voltage at the node ND 80  turns off the NMOS transistor N 80  and turns on the PMOS transistor P 83 . Thus, the signal S 81  at the node ND 8 I is at the high voltage level: that is, there is a high voltage at the node ND 81 , to turn off the PMOS transistor P 82 . Due to the turned-on PMOS transistor P 81 , there is a voltage difference between the gate and drain of the PMOS transistor P 81  (the voltage difference is V TH , V TH  is the threshold voltage of the PMOS transistor P 81 ). As described above, the gates of the PMOS transistors P 80  and P 81  are coupled together through the node ND 80 , and the drain of the PMOS transistor P 81  is coupled to the bulk of the PMOS transistor P 80 . In other words, the PMOS transistor P 81  is coupled between the gate and bulk of the PMOS transistor P 80 . Thus, there is a voltage difference between the gate and bulk of the PMOS transistor P 80 , so the gate-bulk voltage V GB  is not equal to zero, which ensures that the PMOS transistor P 80  is turned on. Due to the turned-on PMOS transistor P 80 , a discharge path is formed between the power terminal T 80  and the ground terminal T 81  (that is, between the pads PAD 10  and PAD 11 ). Accordingly, the electrostatic charges at the pad PAD 10  can be conducted to the pad PAD 11  through the discharge path, thereby protecting the elements in the core circuit  10  from being damaged by the electrostatic charges. 
         [0036]    In an embodiment, the speed of turning on the PMOS transistor P 80  can be increased by raising the gate-bulk voltage of the PMOS transistor P 80 . Thus, the ESD protection circuit  11  may further comprise at least one PMOS transistor which is coupled to the PMOS transistor P 81  in series. Referring to  FIG. 9 , the ESD protection circuit  11  further comprises a PMOS transistor P 90 . The gate and source of the PMOS transistor P 90  are coupled to the drain of the PMOS transistor P 81 , and the drain thereof is coupled to the bulk of the PMOS transistor P 80 . The bulk of the PMOS transistor P 90  is coupled to the power terminal T 80 . In the structure of  FIG. 9 , the source of the PMOS transistor P 81  is coupled to the bulk of the PMOS transistor P 80  through the PMOS transistor P 90 . In  FIGS. 8 and 9 , the elements with the same reference signs perform the same operation, thus, the related operations are omitted. In the embodiment, when the core circuit  10  does not operate in the normal operation mode and an ESD event occurs at the pad PAD 10 , both the PMOS transistors P 81  and P 90  are turned on. At this time, the gate-bulk voltage V GB  of the PMOS transistor P 80  in  FIG. 9  is two times the value of V TH , which is larger than the gate-bulk voltage V GB (=V TH ) of the PMOS transistor P 80  in  FIG. 8 . Compared with the embodiment of  FIG. 8 , the PMOS transistor P 80  in  FIG. 9  can be turned on more quickly when an ESD event occurs at the pad PAD 10 : that is, the PMOS transistor P 80  in  FIG. 9  can provide a discharge path in a short time. 
         [0037]    In the embodiment of  FIG. 9 , one PMOS transistor which is coupled to the PMOS transistor P 81  in series is given as an example for illustration. However, in other embodiments, the number of PMOS transistors coupled to the PMOS transistor P 81  in series can be determined according to the system requirements. The higher the number of PMOS transistors coupled to the PMOS transistor P 81  in series is, the more the gate-bulk voltage V GB  of the PMOS transistor P 80  is, so that the PMOS transistor P 80  can be turned on more quickly when an ESD event occurs at the pad PAD 10 . 
         [0038]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.