Abstract:
An electrostatic discharge (ESD) protection device and a layout thereof are provided. A bias conducting wire is mainly used to couple each base of a plurality of parasitic transistors inside ESD elements together, in order to simultaneously trigger all the parasitic transistors to bypass the ESD current, avoid the elements of a core circuit being damaged, and solve the non-uniform problem of bypassing the ESD current when ESD occurs. Furthermore, in the ESD protection layout, it only needs to add another doped region on a substrate neighboring to, but not contacting, doped regions of the ESD protection elements and use contacts to connect the added doped region, so as to couple each base of the parasitic transistors together without requiring for additional layout area.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ESD protection device and a layout thereof. More particularly, the present invention relates to an ESD protection device with equal-substrate-potential technology and a layout thereof. 
   2. Description of Related Art 
   Electronic products are often impacted by ESD in practical use. Generally speaking, an ESD voltage is much higher than a common supply voltage, and discharge models can be classified into human-body model (HBM), machine model (MM), and charge-device model (CDM) based on different voltage levels generated by ESD. When ESD occurs, the ESD current is likely to burn the elements, such that some ESD protection measures must be taken in the circuit to effectively isolate the ESD current, so as to prevent the elements from being damaged. 
   Commonly, a design of ESD protection device is disposed between a core circuit and a pad to protect internal circuits. There are several tests for ESD protection devices, which can be classified into PD, PS, ND, and NS modes. The PD/ND mode inputs a positive pulse/negative pulse via the pad to bypass the ESD current to the conducting wire of a system voltage VDD. The PS/NS mode inputs a positive pulse/negative pulse via the pad to bypass the ESD current to the conducting wire of a ground voltage VSS. 
     FIG. 1  is a block view of an ESD protection circuit. Referring to  FIG. 1 , the PD mode inputs a positive pulse  105  via a pad  101  and uses an ESD protection device  102  to bypass an ESD current to the system voltage trace VDD, so as to protect a core circuit  104 . The NS mode inputs a negative pulse  106  via the pad  101  and uses an ESD protection device  103  to bypass an ESD current to the ground voltage trace VSS, so as to protect the core circuit  104 . The operations of the PS, ND modes can be deduced in the same way. Further, electrostatic charges may be accumulated during the operation of the core circuit  104 , so the electrostatic charges generated by the core circuit  104  can also be bypassed and discharged by the ESD protection devices  102 ,  103 . 
   A conventional ESD protection circuit is usually implemented by a gate-grounded n-channel metal-oxide-semiconductor (GGNMOS) transistor.  FIG. 2  shows an ESD protection device implemented by a GGNMOS transistor. Referring to  FIG. 2 , when a core circuit  204  operates normally, as the gate of an NMOS transistor MN 1  is grounded, the NMOS transistor MN 1  is turned off and will not be conducted. When ESD occurs, a high voltage  205  enters via a pad  201 . When the high voltage  205  exceeds a drain/substrate breakdown voltage of the NMOS transistor, the drain/substrate of the NMOS transistor may be broken down and generate a bulk current which triggers parasitic transistors inside the NMOS transistor to bypass the ESD current. 
   As the ESD protection circuit withstands the high voltage ESD, a channel width of several hundreds of microns is required in the layout. Thus, a layout of multi-finger type is used to reduce the occupied silicon area. However, the above layout manner may result in a different base resistance of a lateral parasitic bipolar junction transistor (BJT) inside each finger of the NMOS transistor, i.e., the parasitic transistor closer to a central circuit has a higher base resistance. When a snapback breakdown of an NMOS transistor occurs, the ESD current may be concentrated and conducted to a ground terminal via the lateral parasitic BJT of the broken-down NMOS transistor. As the NMOS transistor that has been broken down lowers the potential of the conducting wire coupled thereto, the ESD pulse will not trigger other NMOS transistors, thus causing a non-uniform problem of bypassing the ESD current and weakening the ESD protection ability. In order to solve the above problems, the base resistances of the parasitic transistors must be substantially the same. 
     FIG. 3A  is a top view of an ESD protection circuit layout according to U.S. Pat. No. 5,811,856.  FIG. 3B  is a sectional view of the ESD protection circuit layout according to the U.S. Pat. No. 5,811,856. Referring to  FIGS. 3A and 3B , the ESD protection circuit can be regarded as the ESD protection device  103  in  FIG. 1 . The guard-ring formed by a P+ doped region  301  is used to avoid ESD current drain. A gate  302  and N+ doped regions  303 ,  304  form a GGNMOS transistor, and the N+ doped regions  303 ,  304  and a substrate  308  form a parasitic transistor  309 . N+ doped regions  307 ,  311 , and the substrate  308  form a parasitic transistor  312 . Moreover, the N+ doped regions  305 ,  307  and the substrate  308  form a parasitic transistor  310 . 
   A method of solving the non-uniform problem of bypassing the ESD current involves embedding a grounded P+ diffusion region  306  into the source  304  of a neighboring NMOS transistor, and making the base resistances of the parasitic transistors  309 ,  310 ,  312  being substantially the same, so as to simultaneously trigger the parasitic transistors to bypass the ESD current. However, the layout of embedding the P+ diffusion region  306  not only increases the layout area, but also results in an over low substrate resistance of the NMOS transistor in a deep-submicron complementary metal-oxide-semiconductor (CMOS) transistor process, thus making it difficult to trigger the internal parasitic transistors and bypass the ESD current in time to protect the core circuit. 
     FIG. 4  shows an ESD protection circuit disclosed in “Layout design on multi-finger MOSFET for on-chip ESD protection circuits in a 0.18-um Salicided CMOS process” (Proc. IEEE Int. Symp. Electronics, Circuits and Systems, 2001, pp. 361-364) published by Mr. M.-D. Ker, C.-H. Chuang, and W.-Y. Lo. Referring to  FIG. 4 , another method of solving the non-uniform problem of bypassing the ESD current involves coupling a sensing circuit to the gate of an MOS transistor. The sensing circuit is generally constituted by a resistor RP 1  (or RN 1 ) and a capacitor CP 1  (or CN 1 ). When the sensing circuit senses the occurrence of an ESD event, the sensing circuit provides a bias to the gates of MOS transistors MP 1 , MP 2  (or MN 1 , MN 2 ), so as to simultaneously turn on the transistors MP 1 , MP 2  (or MN 1 , MN 2 ) to bypass the ESD current. The PMOS transistors MP 1 , MP 2 , capacitor CP 1 , and resistor RP 1  can be regarded as internal elements of the ESD protection device  102  in  FIG. 1 . The NMOS transistors MN 1 , MN 2 , capacitor CN 1 , and resistor RN 1  can be regarded as internal elements of the ESD protection device  103  in  FIG. 1 . The resistors RN 1 , RP 1  and capacitors CN 1 , CP 1  can be adjusted to provide a bias to the gates of the NMOS transistors MN 1 , MN 2  and PMOS transistors MP 1 , MP 2  to reduce the trigger voltage of the NMOS transistors MN 1 , MN 2  and PMOS transistors MP 1 , MP 2 . Thus, when ESD occurs, a smaller trigger voltage can trigger the NMOS transistors MN 1 , MN 2  or PMOS transistors MP 1 , MP 2  in time to bypass the ESD current. However, the high bias applied on the gate of the NMOS transistor MN 1 /PMOS transistor MP 1  may generate a larger channel current, and a higher electric field may cause the breakdown of a thin gate-oxide layer, thus weakening the ESD protection ability. In addition, the impedance of the resistors RN 1 , RP 1  in a common sensing circuit is extremely high (approximately 100 kilo-ohm), which may also increase the layout area. 
     FIG. 5  shows an ESD protection circuit according to the U.S. Pat. No. 5,631,793. Referring to  FIG. 5 , the NMOS transistors MN 1 , resistor RN 1 , and capacitor CN 1  are internal elements of the ESD protection device  103  in  FIG. 1 . A method of solving the non-uniform problem of bypassing the ESD current involves electrically connecting a sensing circuit to the substrate of the GGNMOS transistors MN 1 , MN 2 . The sensing circuit is constituted by a resistor RN 1  and a capacitor CN 1 . The resistor RN 1  and the capacitor CN 1  can be adjusted to provide an appropriate voltage to the bodies of the parasitic transistors (i.e., the substrates of the GGNMOS transistors MN 1 , MN 2 ), so as to increase the base voltage of the parasitic transistors, i.e., reducing the trigger voltage of the GGNMOS transistors MN 1 , MN 2 , such that the internal parasitic transistors can be triggered simultaneously to solve the non-uniform problem of bypassing the ESD current. Therefore, it is not necessary to apply a bias to the gates of the NMOS transistors MN 1 , MN 2 , thus avoiding generating an extra channel current that weakens the ESD protection ability. However, the additional resistor RN 1  and capacitor CN 1  may also increase the layout area. 
     FIG. 6  shows an ESD protection circuit according to the U.S. Pat. No. 5,686,751. Referring to  FIG. 6 , a technology of solving the non-uniform problem of bypassing the ESD current involves triggering each finger of the NMOS transistor in a domino manner. In  FIG. 6 , Rd 1 -Rdi are respectively ballast resistors of the drains of NMOS transistors MN 1 -MNi, and Rs 1 -Rsi are respectively ballast resistors of the sources of the NMOS transistors MN 1 -MNi. The NMOS transistors MN 1 -MNi and resistors Rd 1 -Rdi, Rs 1 -Rsi are internal elements of the ESD protection device  103  in  FIG. 1 . When ESD occurs, as long as one of the NMOS transistors (for example, the NMOS transistor MN 1 ) is triggered, the ESD current provides a voltage to the gate of the NMOS transistor MN 2  via the ballast resistor Rs 1 . The triggered NMOS transistor MN 2  then allows the ESD current to pass through the ballast resistor Rs 2  to provide a voltage to the gate of the NMOS transistor MN 3 . The NMOS transistors MN 3 -MNi are triggered in the same way. However, though the non-uniform problem of bypassing the ESD current can be solved by the above conventional art, the complexity of the layout is increased. 
   SUMMARY OF THE INVENTION 
   An ESD protection device is provided by the present invention. Under a high voltage ESD, a plurality of ESD protection units can be triggered simultaneously to bypass the ESD current in time, so as to solve the non-uniform problem of bypassing the ESD current. In addition, when a core circuit under a small power supply operates together with an input/output interface (I/O interface) under a high voltage via an I/O pad, the ESD protection device can also work normally under a mixed-voltage operation. 
   An ESD protection device provided by the present invention can be applied to an output buffer with ESD protection ability which receives an output signal from the core circuit to control the ESD protection device to output an external signal, so as to enhance the output driving ability of the core circuit. 
   An ESD protection layout provided by the present invention is an implementation of the above ESD protection device. In a limited layout area, a doped region is disposed in the substrate, and a bias conducting wire is used to electrically connect the doped region, thus the base coupling manner of the parasitic transistors in the ESD protection device is completed. Under a high voltage ESD, the parasitic transistors can be triggered simultaneously to bypass the ESD current in time, so as to avoid the non-uniform problem of bypassing the ESD current. 
   In order to solve the above problem, an ESD protection device comprising a plurality of ESD protection units and a bias conducting wire is provided. A plurality of ESD protection units is used to transmit an electrostatic current between a first conductive path and a second conductive path, wherein each ESD protection unit comprises a parasitic transistor and a parasitic resistor. A collector and an emitter of each parasitic transistor are respectively coupled to the first conductive path and the second conductive path, and each parasitic resistor is coupled between a base of the corresponding parasitic transistor and the second conductive path. The bias conducting wire is coupled to each base of the above parasitic transistors. 
   An ESD protection device comprising a plurality of output driving units and a bias conducting wire is further provided. The plurality of output driving units is used to generate an external output signal according to a core output signal and output the external output signal to a first conductive path, wherein each output driving unit comprises a parasitic transistor and a parasitic resistor. A collector and an emitter of each parasitic transistor are respectively coupled to the first conductive path and a second conductive path, and each parasitic resistor is coupled between the base of the corresponding parasitic transistor and the second conductive path. The bias conducting wire is coupled to each base of the above parasitic transistors. 
   An ESD protection device comprising a plurality of transistors, a plurality of resistors, and a bias conducting wire is still provided. A collector and an emitter of each transistor are respectively coupled to a first conductive path and a second conductive path, for transmitting an electrostatic current between the first conductive path and the second conductive path. The plurality of resistors is respectively coupled between the base of the corresponding transistor and the second conductive path. The bias conducting wire is coupled to each base of the above transistors. 
   An ESD protection layout comprising a substrate, a first doped region, a first conductive path, a second conductive path, a plurality of ESD protection units, a plurality of second doped regions, and a bias conducting wire is further provided. The substrate has a parasitic resistor. The first doped region is disposed on the substrate, and serves as an electrode of the substrate. The first conductive path is disposed above the substrate. The second conductive path is disposed above the substrate. Each of the above ESD protection units is disposed on the substrate without contacting the first doped region, for transmitting an electrostatic current between the first conductive path and the second conductive path, wherein each ESD protection unit has a parasitic transistor structure. The plurality of second doped regions is disposed on the substrate between the ESD protection units, wherein each second doped region does not contact any of the ESD protection units. The bias conducting wire is disposed above the substrate, and is electrically connected to each of the above second doped regions. 
   An ESD protection layout comprising a substrate, a first doped region, a first conductive path, a second conductive path, a plurality of ESD protection units, a plurality of third doped regions, and a bias conducting wire is also provided. The first doped region is disposed on the substrate, and serves as an electrode of the substrate. The first and second conductive paths are respectively disposed above the substrate. The plurality of ESD protection units is disposed on the substrate without contacting the first doped region, for transmitting an electrostatic current between the first conductive path and the second conductive path, wherein each ESD protection unit comprises a first MOS transistor and a second MOS transistor connected in series between the first conductive path and the second conductive path. The plurality of third doped regions is disposed in the substrate and between the first and second MOS transistors without contacting the two MOS transistors. The bias conducting wire is disposed above the substrate, and is electrically connected to each of the above third doped regions. 
   The present invention couples the bases of the parasitic transistors inside the ESD protection units together, for simultaneously triggering the ESD protection units to bypass the ESD current when a high voltage ESD passes through the ESD protection device. Moreover, when the devices operating under different voltages works together, the ESD protection device can work normally under the mixed-voltage operation. Further, the ESD protection device is coupled to a preceding driving device to discharge the charges generated by the preceding driving device. 
   In order to make the features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a block diagram of an ESD protection circuit. 
       FIG. 2  shows an ESD protection device implemented by a GGNMOS transistor. 
       FIG. 3A  is a top view of a conventional ESD protection circuit layout. 
       FIG. 3B  is a sectional view of a conventional ESD protection circuit layout. 
       FIG. 4  is a conventional ESD protection circuit. 
       FIG. 5  is a conventional ESD protection circuit. 
       FIG. 6  is a conventional ESD protection circuit. 
       FIG. 7  is an ESD protection device according to a preferred embodiment of the present invention. 
       FIG. 8  is an ESD protection device according to a preferred embodiment of the present invention. 
       FIG. 9  is an ESD protection device according to a preferred embodiment of the present invention. 
       FIG. 10  is an ESD protection device according to a preferred embodiment of the present invention. 
       FIG. 11  is an ESD protection device according to a preferred embodiment of the present invention. 
       FIG. 12  is an ESD protection device according to a preferred embodiment of the present invention. 
       FIG. 13A  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 13B  is a sectional view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 14  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 15  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 16A  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 16B  is a sectional view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 17  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 18  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
       FIG. 19  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. 
   

   DESCRIPTION OF EMBODIMENTS 
     FIG. 7  is an ESD protection device according to a preferred embodiment of the present invention. Referring to  FIG. 7 , an ESD protection device  703  is coupled between a pad  701  and a second conductive path (for example, a ground voltage trace)  702 . The pad  701  is coupled to a core circuit  706  via a first conductive path, and the pad  701  can be an input pad or an output pad. The ESD protection device  703  mainly includes ESD protection units  707 - 710  and a bias conducting wire  705 . This embodiment adopts, for example, a multi-finger type layout manner to implement the ESD protection device  703 , so as to reduce the occupied silicon area. Herein, only four ESD protection units  707 - 710  are taken as an example for illustration, and those of ordinary skill in the art can determine the number of the ESD protection unit as required. 
   Each of the ESD protection units  707 - 710  in this embodiment has an NMOS transistor (i.e., M 1 -M 4  in  FIG. 7 ). As the NMOS transistors M 1 -M 4  are disposed in the substrate, each of the ESD protection units  707 - 710  has a parasitic transistor A 1 -A 4  and a parasitic resistor (substrate resistor) Ra 1 -Ra 4 . Resistors Rm 1 -Rm 4  are respectively coupled between the gates of the NMOS transistors M 1 -M 4  and the voltage trace  702 . Those of ordinary skill in the art can omit the resistors Rm 1 -Rm 4  as required, i.e., directly coupling the gates of the NMOS transistors M 1 -M 4  to the voltage trace  702 . In other embodiment, the gates of the NMOS transistors M 1 -M 4  is floating. 
   When ESD occurs, a high voltage  704  enters via the pad  701 . If the high voltage  704  exceeds the breakdown voltage between the drain and body of any (for example, the transistor M 2 ) of the NMOS transistors M 1 -M 4 , the interface between the drain and body of the NMOS transistor M 2  may be broken down to generate a bulk current. When the bulk current passes through a parasitic resistor Ra 2 , a bias voltage is generated. As the bias conducting wire  705  is used to connect the bases of the parasitic transistors A 1 -A 4 , the bias voltage not only triggers the parasitic transistor A 2 , but also simultaneously triggers other parasitic transistors A 1 , A 3 , A 4 . At this time, the parasitic transistors A 1 -A 4  bypass the ESD current through the first conductive path to the second conductive path (herein, a ground voltage trace)  702 , so as to prevent the ESD damaging the elements of the core circuit  706 , thus solving the non-uniform problem of bypassing the ESD current. 
   According to another embodiment of the present invention, the second conductive path  702  is a system voltage trace. If the second conductive path  702  is a system voltage trace, PMOS transistors can be used to substitute the NMOS transistors M 1 -M 4  in the ESD protection device  703 , in  FIG. 7 . 
     FIG. 8  shows an ESD protection device according to a preferred embodiment of the present invention. Referring to  FIG. 8 , the ESD protection device  803  can be used as a buffer. The ESD protection device  803  is coupled between a third conductive path (for example, a system voltage trace  801 ) and a second conductive path (for example, a ground voltage trace  802 ). The ESD protection device  803  mainly includes output driving units (or ESD protection units)  807 - 810 , a bias conducting wire  812 , and a bias conducting wire  813 . This embodiment uses, for example, a multi-finger type layout manner to implement the ESD protection device  803 . Herein, only four ESD protection units  807 - 810  are taken as an example for illustration, and those of ordinary skill in the art can determine the number of the ESD protection unit as required. 
   Each of the output driving units  807 - 810  in this embodiment has an NMOS transistor N 1 -N 4  and a PMOS transistor P 1 -P 4 . The transistors N 1 -N 4  and P 1 -P 4  are connected in series between the second conductive path (for example, the ground voltage trace  802 ) and the third conductive path (for example, the system voltage trace  801 ) (as shown in  FIG. 8 ). As the NMOS transistors N 1 -N 4  are disposed on the substrate, each of the output driving units  807 - 810  has a parasitic transistor C 1 -C 4  and a parasitic resistor Rc 1 -Rc 4 . As the PMOS transistors P 1 -P 4  are disposed on the substrate, each of the output driving units  807 - 810  also has a parasitic transistor B 1 -B 4  and a parasitic resistor Rb 1 -Rb 4 . 
   In this embodiment, the ESD protection device  803  serves as an output buffer of a core circuit  805 . Each of the output driving units  807 - 810  generates an external output signal according to a core output signal output by the core circuit  805  and outputs the external output signal to a pad  804  via a first conductive path  811 . As the bias conducting wire  813  couples the bases of the parasitic transistors C 1 -C 4  together, and the bias conducting wire  812  couples the bases of the parasitic transistors B 1 -B 4  together, when ESD occurs, if any of the output driving units  807 - 810  is broken down due to the ESD, the bias voltage generated by the ESD current passing through a parasitic resistor turns on the parasitic transistors B 1 -B 4  and the parasitic transistors C 1 -C 4  via the bias conducting wires  812 ,  813 . 
   For example, when the interface between the drain and body of the transistor N 2  (or P 2 ) is broken down due to the occurrence of ESD, the electrostatic current may pass through the parasitic resistor Rc 2  (or Rb 2 ) to generate a bias voltage. As the bias conducting wire  813  (or  812 ) is used to connect the bases of the parasitic transistors C 1 -C 4  (or B 1 -B 4 ), the bias voltage simultaneously triggers other parasitic transistors C 1 , C 3 , C 4  (or B 1 , B 3 , B 4 ). Therefore, when ESD occurs, all the output driving units  807 - 810  are triggered. The ESD current is bypassed to the third conductive path (for example, the system voltage trace  801 ) and/or the second conductive path (for example, the ground voltage trace  802 ) via each of the output driving units  807 - 810 , so as to prevent the electrostatic current damaging the elements inside the core circuit  805 , thus solving the non-uniform problem of bypassing the ESD current. 
     FIG. 9  shows an ESD protection device according to a preferred embodiment of the present invention. The ESD protection device  903  is coupled between a third conductive path (for example, a system voltage trace  901 ) and a second conductive path (for example, a ground voltage trace  902 ). The ESD protection device  903  mainly includes output driving units  907 ,  908 , ESD protection units  909 ,  910 , and bias conducting wires  912 ,  913 . This embodiment uses, for example, a multi-finger type layout manner to implement the ESD protection device  903 . Herein, only two output driving units  907 ,  908  and two ESD protection units  909 ,  910  are taken as an example for illustration, and those of ordinary skill in the art can determine the number of the output driving unit and ESD protection unit as required. 
   Each of the output driving units  907 ,  908  in this embodiment has a PMOS transistor P 5 , P 6  and an NMOS transistor N 5 , N 6 . Each of the ESD protection units  909 ,  910  has an NMOS transistor N 7 , N 8  and a PMOS transistor P 7 , P 8 . The transistors N 5 -N 8  and P 5 -P 8  are connected in series between the second conductive path and the third conductive path (as shown in  FIG. 9 ). Each of the transistors N 5 -N 8  has a parasitic transistor C 5 -C 8  and a parasitic resistor Rc 5 -Rc 8 . Each of the transistors P 7 , P 8  has a parasitic transistor B 5 -B 8  and a parasitic resistor Rb 5 -Rb 8 . The bias conducting wire  913  couples the bases of the parasitic transistors C 5 -C 8  together, and the bias conducting wire  912  couples the bases of the parasitic transistors B 5 -B 8  together. 
   In this embodiment, the ESD protection device  903  serves as an output buffer of a core circuit  905 . Each of the output driving units  907 ,  908  generates an external output signal according to a core output signal output by the core circuit  905  and outputs the external output signal to a pad  904  via a first conductive path  911 . Referring to  FIG. 9 , as the bias conducting wire  912  couples the bases of the parasitic transistors B 5 -B 8  together, and the bias conducting wire  913  couples the bases of the parasitic transistors C 5 -C 8  together. When ESD occurs, if any of the output driving units  907 ,  908  or ESD protection units  909 ,  910  is broken down due to the ESD, the bias voltage generated by the ESD current passing through a parasitic resistor turns on other parasitic transistors via the bias conducting wires  912 ,  913 . 
   For example, when the interface between the drain and body of the transistor N 7  (or P 7 ) is broken down due to the occurrence of ESD, the electrostatic current may pass through the parasitic resistor Rc 7  (or Rb 7 ) to generate a bias voltage. As the bias conducting wire  913  (or  912 ) is used to connect the bases of the parasitic transistors C 5 -C 8  (or B 5 -B 8 ), the bias voltage simultaneously triggers other parasitic transistors C 5 , C 6 , C 8  (or B 5 , B 6 , B 8 ). Therefore, when ESD occurs, all the output driving units  907 ,  908  and the ESD protection units  909 ,  910  are triggered. The ESD current is bypassed to the third conductive path (for example, the system voltage trace  901 ) and/or the second conductive path (for example, the ground voltage trace  902 ) via the output driving units  907 ,  908  and the ESD protection units  909 ,  910 , so as to prevent the electrostatic current damaging the elements inside the core circuit  905 , thus solving the non-uniform problem of bypassing the ESD current. 
   Moreover, along with the progress of semiconductor transistor process, the supply voltage required by a core circuit becomes smaller, so as to reduce the power consumption and heat dissipation. However, the core circuit operating under a low voltage is still likely to work together with other I/O interfaces operating under a high supply voltage. In such a mixed-voltage operation, the ESD protection device must maintain the ESD protection ability still remains when the core circuit works under a high voltage, so as to improve the voltage tolerance of the ESD protection device. 
     FIG. 10  shows an ESD protection device according to a preferred embodiment of the present invention. Referring to  FIG. 10 , a pad  1001  is coupled to a core circuit  1006  via a first conductive path. The ESD protection device  1003  is coupled between the first conductive path and a second conductive path (for example, a ground voltage trace  1002 ). The pad  1001  can be an input pad or an output pad. The ESD protection device  1003  mainly includes ESD protection units  1007 - 1010  and a bias conducting wire  1005 . This embodiment uses, for example, a multi-finger type layout manner to implement each of the ESD protection units, so as to reduce the occupied silicon area. Herein, only four ESD protection units  1007 - 1010  are taken as an example for illustration, and those of ordinary skill in the art can determine the number of the ESD protection unit as required. 
   Each of the ESD protection units  1007 - 1010  in this embodiment has a first MOS transistor (for example, an NMOS transistor Q 1 -Q 4 ) and a second MOS transistor (for example, an NMOS transistor D 1 -D 4 ). The first and second MOS transistors are connected in series between the first conductive path and the second conductive path (for example, the ground voltage trace  1002 ) (as shown in  FIG. 10 ). The gates of the transistors Q 1 -Q 4  are coupled to the second conductive path. The gates of the transistors D 1 -D 4  are coupled to a third conductive path (for example, a system voltage trace VDD). As the NMOS transistors Q 1 -Q 4 , D 1 -D 4  are disposed on the substrate, each of the ESD protection units  1007 - 1010  has a parasitic transistor E 1 -E 4  and a parasitic resistor Re 1 -Re 4 . Resistors Rq 1 -Rq 4  are respectively coupled between the gates of the NMOS transistors Q 1 -Q 4  and the ground voltage trace  1002 , and those of ordinary skill in the art can omit the resistors Rq 1 -Rq 4  as required, i.e., directly coupling the gates of the NMOS transistors Q 1 -Q 4  to the ground voltage trace  1002 . In other embodiment, the gates of the NMOS transistors Q 1 -Q 4  is floating. 
   Referring to  FIGS. 7 and 10 , the difference between  FIGS. 7 and 10  is that the NMOS transistors Q 1 -Q 4  are respectively connected in series with the NMOS transistors D 1 -D 4 , so as to improve the trigger voltage of each of the high ESD protection units  1007 - 1010 , thus making the ESD protection device  1003  have a high voltage tolerance. As the bias conducting wire  1005  couples the bases of the parasitic transistors E 1 -E 4  together, when ESD occurs, if any of the ESD protection units  1007 - 1010  is broken down due to the ESD, the bias voltage generated by the ESD current turns on the parasitic transistors E 1 -E 4  via the bias conducting wire  1005 . The gates of the NMOS transistors D 1 -D 4  are coupled to the system voltage VDD and conducted. Those of ordinary skill in the art should understand that the same purpose can be achieved by coupling the gates of the PMOS transistors to the ground voltage VSS, and this embodiment will not be limited herein. 
     FIG. 11  shows the equivalent circuit of aqual-substrate-potential stacked-NMOS used as an ESD protection device according to a preferred embodiment of the present invention. Referring to  FIG. 11 , the ESD protection device  1103  can be used as a self-protecting output buffer. The ESD protection device  1103  is coupled between a third conductive path (for example, a system voltage trace  1101 ) and a second conductive path (for example, a ground voltage trace  1102 ). The ESD protection device  1103  mainly includes a plurality of output driving units (only four output driving units  1107 - 1110  are illustrated in  FIG. 11 ), and bias conducting wires  1113 . 
   Each of the output driving units  1107 - 1110  in this embodiment has an NMOS transistor W 1 -W 4 , an NMOS transistor X 1 -X 4 , and a PMOS transistor Y 1 -Y 4 . Each of the output driving units  1107 - 1110  has a parasitic transistor F 1 -F 4 , and a parasitic resistor Rf 1 -Rf 4 . The bias conducting wire  1113  couples the bases of the parasitic transistors F 1 -F 4  together. 
   In this embodiment, the ESD protection device  1103  serves as an output buffer of a core circuit  1105 . Each of the output driving units  1107 - 1110  generates an external output signal according to a core output signal output by the core circuit  1105  and outputs the external output signal to a pad  1104  via a first conductive path  1111 . Referring to  FIGS. 8 and 11 , the circuit operation manner of this embodiment is similar to that of the embodiment in  FIG. 8 , and the details will not be described herein again. One of the difference between  FIG. 11  and  FIG. 8  involves that in  FIG. 11 , NMOS transistors X 1 -X 4  are respectively connected in series between the NMOS transistors W 1 -W 4  and the PMOS transistors Y 1 -Y 4 , so as to respectively raise the trigger voltage of each of the high ESD protection units  1107 - 1110 , thus making the ESD protection device  1103  have a high voltage tolerance. 
     FIG. 12  shows an ESD protection device according to a preferred embodiment of the present invention. The ESD protection device  1203  is coupled between a third conductive path (for example, a system voltage trace  1201 ) and a second conductive path (for example, a ground voltage trace  1202 ). The ESD protection device  1203  mainly includes output driving units  1207 ,  1208 , ESD protection units  1209 ,  1210 , and a bias conducting wire  1213 . Herein, only two output driving units  1207 ,  1208  and two ESD protection units  1209 ,  1210  are taken as an example for illustration, and those of ordinary skill in the art can determine the number of the output driving unit and ESD protection unit as required. 
   The output driving units  1207 ,  1208  respectively have parasitic transistors F 5 , F 6  and parasitic resistors Rf 5 , Rf 6 . The ESD protection units  1209 ,  1210  respectively have parasitic transistors F 7 , F 8  and parasitic resistors Rf 7 , Rf 8 . The bias conducting wire  1213  couples the bases of the parasitic transistors F 5 -F 8  together. 
   Referring to  FIGS. 9 and 12 , the circuit operation manner of this embodiment is similar to that of the embodiment in  FIG. 9 , and the details will not be described herein again. One of the difference between  FIG. 12  and  FIG. 9  is that, in  FIG. 12 , NMOS transistors X 5 -X 8  are respectively connected in series between the NMOS transistors W 5 -W 8  and the PMOS transistors Y 5 , so as to respectively raise the trigger voltage of each of the output driving units  1207 ,  1208 , and each of the ESD protection units  1209 ,  1210 , thus making the ESD protection device  1203  have a high voltage tolerance. It should be noted that though a possible configuration of the ESD protection device has been described in the above embodiment of the present invention, those of ordinary skill in the art should understand that the adopted ESD protection elements are different. For example, NMOS transistors are taken as an example of the ESD protection elements for illustration in the above embodiment, while PMOS transistors can also be used as ESD protection elements to substitute the NMOS transistors. Therefore, the application of the present invention is not limited to this possible configuration. In other words, any configuration that couples the bases of a portion of or all the parasitic transistors inside the ESD protection device together, and provides the parasitic transistors in the ESD protection device with an equal-substrate-potential, for simultaneously triggering the parasitic transistors to bypass the ESD current conforms to the spirit of the present invention. 
   Next, another embodiment is given below to enable those of ordinary skill in the art to implement the above embodiment.  FIG. 13A  is a top view of the ESD protection layout according to the embodiment in  FIG. 7 .  FIG. 13B  is a sectional view of the ESD protection layout according to the embodiment in  FIG. 7 . Referring to  FIGS. 13A and 13B , the ESD protection layout of this embodiment includes a P-type substrate  1303 , a first doped region  1304 , ESD protection units  1307 - 1310 , second doped regions  1320 - 1321 , a first conductive path  1301 , a second conductive path  1302 , and a bias conducting wire  1305 . Each of the ESD protection units  1307 - 1310  has an NMOS transistor and a parasitic transistor structure. The substrate  1303  has parasitic resistors inside. The first doped region  1304  is a P+ doped region, which is disposed on the substrate  1303  and coupled to a ground voltage, serving as an electrode of the P-type substrate  1303 . 
   The ESD protection unit  1307  has an NMOS transistor formed by N+ doped regions  1311 ,  1312  and a gate  1316 , and has a parasitic transistor formed by N+ doped regions  1311 ,  1312  and the P-type substrate  1303 . The ESD protection unit  1308  has an NMOS transistor formed by N+ doped regions  1312 ,  1313  and a gate  1317 , and has a parasitic transistor formed by N+ doped regions  1312 ,  1313  and the P-type substrate  1303 . The ESD protection unit  1309  has an NMOS transistor formed by N+ doped regions  1313 ,  1314  and a gate  1318 , and has a parasitic transistor formed by N+ doped regions  1313 ,  1314  and the P-type substrate  1303 . The ESD protection unit  1310  has an NMOS transistor formed by N+ doped regions  1314 ,  1315  and a gate  1319 , and has a parasitic transistor formed by N+ doped regions  1314 ,  1315  and the P-type substrate  1303 . 
   The ESD protection units  1307 - 1310  are used to transmit an ESD current between the first conductive path  1301  and the second conductive path  1302 . Therefore, the N+ doped regions  1312 ,  1314  (the drains of the NMOS transistors) are coupled to the first conductive path  1301 , wherein the first conductive path  1301  is electrically connected to a pad  1306  (also, an output pad or input pad herein). The N+ doped regions  1311 ,  1313 ,  1315  (the sources of the NMOS transistors) and the gates  1316 - 1319  are coupled to the second conductive path  1302  (also, a ground voltage trace herein). 
   This embodiment couples the bases of the internal parasitic transistors together via the bias conducting wire  1305 , so as to simultaneously trigger the parasitic transistors to bypass the ESD current. In order to electrically connect the bias conducting wire  1305  and the bases of the parasitic transistors, the second doped regions  1320 ,  1321  are respectively disposed in the N+ doped regions  1312 ,  1314 . The second doped regions  1320 ,  1321  are respectively isolated from the N+ doped regions  1312 ,  1314  by a field oxide layer (or other isolation techniques). The second doped regions  1320 ,  1321  are P+ doped regions, and the bias conducting wire  1305  is electrically connected to the second doped regions  1320 ,  1321 . 
   In another embodiment of the present invention, each of the ESD protection units  1307 - 1310  can be implemented by a PMOS transistor, such that the substrate  1303  is an N-type substrate (or an N-type well disposed in a P-type substrate), the first doped region is an N+ doped region and coupled to the system voltage, the second doped regions  1311 - 1312  are N+ doped regions, and the second conductive path  1302  is a system voltage trace. 
     FIG. 14  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. Referring to  FIGS. 14 and 13A , the difference between  FIGS. 14 and 13A  lies in that second doped regions  1322 - 1324  are respectively disposed in the N+ doped regions  1311 ,  1313 ,  1315 . The second doped regions  1322 - 1324  are respectively isolated from the N+ doped regions  1311 ,  1313 ,  1315  by a field oxide layer (or other isolation techniques). The second doped regions  1322 - 1324  are P+ doped regions, and the bias conducting wire  1305  is electrically connected to the second doped regions  1322 - 1324 , so as to couple the bases of the internal parasitic transistors together. 
     FIG. 15  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. Referring to  FIGS. 15 and 13A , in this embodiment, the second doped regions  1320 - 1324  are respectively disposed in the N+ doped regions  1311 - 1315 . The second doped regions  1320 - 1324  are respectively isolated from the N+ doped regions  1311 - 1315  by a field oxide layer (or other isolation techniques). The second doped regions  1320 - 1324  are P+ doped regions, and the bias conducting wire  1305  is electrically connected to the second doped regions  1320 - 1324 , so as to couple the bases of the internal parasitic transistors together. 
     FIG. 16A  is a top view of the ESD protection layout according to the embodiment in  FIG. 10 .  FIG. 16B  is a sectional view of the ESD protection layout according to the embodiment in  FIG. 10 . Referring to  FIGS. 16A and 16B  together, the ESD protection layout of this embodiment includes a P-type substrate  1603 , a first doped region  1604 , ESD protection units  1607 - 1610 , second doped regions  1630 - 1631 , N+ doped regions  1611 - 1619 , a first conductive path  1601 , a second conductive path  1602 , and a bias conducting wire  1605 . The P-type substrate  1603  has parasitic resistors inside. The ESD protection units  1607 - 1610  are implemented by NMOS transistors. The first doped region  1604  is a P+ doped region, which is disposed in the P-type substrate  1603  and coupled to a ground voltage trace, serving as an electrode of the substrate  1603 . 
   The ESD protection unit  1607  has two serially connected NMOS transistors formed by the N+ doped regions  1611 - 1613  and gates  1620 - 1621 , and has a parasitic transistor formed by the N+ doped regions  1611 ,  1613  and the substrate  1603 . The ESD protection unit  1608  has two serially connected NMOS transistors formed by the N+ doped regions  1613 - 1615  and gates  1622 - 1623 , and has a parasitic transistor formed by the N+ doped regions  1613 ,  1615  and the substrate  1603 . The ESD protection unit  1609  has two serially connected NMOS transistors formed by the N+ doped regions  1615 - 1617  and gates  1624 - 1625 , and has a parasitic transistor formed by the N+ doped regions  1615 ,  1617  and the substrate  1603 . The ESD protection unit  1610  has two serially connected NMOS transistors formed by the N+ doped regions  1617 - 1619  and gates  1626 - 1627 , and has a parasitic transistor formed by the N+ doped regions  1617 ,  1619  and the substrate  1603 . 
   The ESD protection units  1607 - 1610  are used to transmit an ESD current between the first conductive path  1601  and the second conductive path  1602 . Therefore, the N+ doped regions  1613 ,  1617  are coupled to the first conductive path  1601 , wherein the first conductive path  1601  is electrically connected to a pad  1606  (an output pad or input pad herein). The N+ doped regions  1611 ,  1615 ,  1619  and the gates  1620 ,  1623 ,  1624 ,  1627  are coupled to the second conductive path  1602  (a ground voltage trace herein). In addition, the gates  1621 ,  1622 ,  1625 ,  1626  are coupled to the system voltage VDD. 
   This embodiment couples the bases of the internal parasitic transistors together via the bias conducting wire  1605 , so as to simultaneously trigger the parasitic transistors to bypass the ESD current. In order to electrically connect the bias conducting wire  1605  and the bases of the parasitic transistors, the second doped regions  1630 - 1631  are respectively disposed in the N+ doped regions  1613 ,  1617 . The second doped regions  1630 - 1631  are respectively isolated from the N+ doped regions  1613 ,  1617  by a field oxide layer (or other isolation techniques). The second doped regions  1630 - 1631  are P+ doped regions, and the bias conducting wire  1605  is electrically connected to the second doped regions  1630 - 1631 . 
   In another embodiment of the present invention, each of the ESD protection units  1607 - 1610  can be implemented by two serially connected PMOS transistors, such that the substrate  1603  is an N-type substrate (or an N-type well disposed in a P-type substrate), the first doped region is an N+ doped region and coupled to the system voltage, the second doped regions  1630 - 1631  are N+ doped regions, and the second conductive path  1602  is a system voltage trace. 
     FIG. 17  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. Referring to  FIGS. 17 and 16A , the difference between  FIGS. 17 and 16A  lies in that third doped regions  1632 - 1635  are respectively disposed in the N+ doped regions  1612 ,  1614 ,  1616 ,  1618 . The third doped regions  1632 - 1635  are respectively isolated from the N+ doped regions  1612 ,  1614 ,  1616 ,  1618  by a field oxide layer (or other isolation techniques). The third doped regions  1632 - 1635  are P+ doped regions, and the bias conducting wire  1605  is electrically connected to the third doped regions  1632 - 1635 , so as to couple the bases of the internal parasitic transistors together. 
     FIG. 18  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. Referring to  FIGS. 18 and 16A , in this embodiment, P+ doped regions  1636 - 1638  are respectively disposed in the N+ doped regions  1611 ,  1615 ,  1619 . The second doped regions  1636 - 1638  are respectively isolated from the N+ doped regions  1611 ,  1615 ,  1619  by a field oxide layer (or other isolation techniques). The bias conducting wire  1605  is electrically connected to the second doped regions  1636 - 1638 , so as to couple the bases of the internal parasitic transistors together. 
     FIG. 19  is a top view of an ESD protection layout according to a preferred embodiment of the present invention. Referring to  FIGS. 19 and 16A , in this embodiment, the second doped regions  1636 ,  1630 ,  1637 ,  1631 ,  1638  are respectively disposed in the N+ doped regions  1611 ,  1613 ,  1615 ,  1617 ,  1619 . The second doped regions  1636 ,  1630 ,  1637 ,  1631 ,  1638  are respectively isolated from the N+ doped regions  1611 ,  1613 ,  1615 ,  1617 ,  1619  by a field oxide layer (or other isolation techniques). The second doped regions  1630 - 1631 ,  1636 - 1638  are P+ doped regions, and the bias conducting wire  1605  is electrically connected to the second doped regions  1630 - 1631 ,  1636 - 1638 , so as to couple the bases of the internal parasitic transistors together. 
   In view of the above, the ESD protection device provided by the present invention couples the bases of the parasitic transistors inside the ESD protection elements together, for simultaneously triggering the parasitic transistors to bypass the ESD current when the ESD occurs, thus solving the non-uniform problem of bypassing the ESD current. Moreover, the ESD protection device can be used as an output buffer to enhance the output driving ability of the core circuit. As for the layout of the ESD protection device, another doped region is added onto the substrate neighboring to the doped regions of the ESD protection elements. However, the added doped region cannot contact the doped regions of the ESD protection element, but is electrically connected thereto, so as to make the bases of the parasitic transistors coupled together without using extra layout area. 
   Though the present invention has been disclosed above by the preferred embodiments, they are not intended to limit the present invention. Anybody skilled in the art can make some modifications and variations without departing from the spirit and scope of the present invention. Therefore, the protecting range of the present invention falls in the appended claims.