Electrostatic discharge protection device

[0002] 1. Field of Invention

[0003] The present invention relates to an electrostatic discharge protection (ESD) device. More particularly, the present invention relates to an electrostatic discharge protection device fabricated through a complementary metal-oxide-semiconductor (CMOS) process that can be uniformly triggered during an electrostatic discharge.

[0004] 2. Description of Related Art

[0005] Electrostatic discharge is a phenomenon triggered by movement of static charges along a non-conducting surface. In general, an electrostatic discharge may produce serious damages to the semiconductors and other circuit components inside an integrated circuit. People walking on a carpet may generate a few hundred to a few thousand volts of static electricity even if the surrounding atmosphere has a high relative humidity. If the surrounding relative humidity is low, a static electricity up to ten thousand or more volts may be produced. Integrated circuit packaging machines or integrated circuit testing equipment may also generate static electricity from several hundred to several thousand volts too. When a charged body (such as the human body, machine or equipment) touches a silicon chip, static charges may discharge through the chip. The power produced by such transient electrostatic discharge may permanently damage the circuits inside the chip or render a loss in function of some of the devices.

[0006] To prevent damage to integrated circuits due to electrostatic discharge, various types of electrostatic discharge protection devices are introduced.

[0007] FIG. 1 is a top view showing a conventional electrostatic discharge protection device. FIG. 2 is a cross-sectional view along line I-I′ of FIG. 1. As shown in FIGS. 1 and 2, the conventional electrostatic discharge (ESD) protection device has a multi-finger structure. The ESD protection device is formed on a P-type substrate 10. The device comprises a metal-oxide-semiconductor (MOS) transistor region 100, a guard ring 30 and a shallow trench isolation (STI) region 32.

[0008] The MOS transistor region 100 accommodates a plurality of N-type metal-oxide-semiconductor (NMOS) transistors that includes a plurality of gate structures 12, 14, 16, 18 as well as a plurality of source terminals 20, 24, 28 and a plurality of drain terminals 22, 26 in the substrate 10 on each side of the gate structures 12, 14, 16 and 18. The source terminal 20 is the emitter of a parasitic bipolar junction transistor (parasitic BJT) 110 and the drain terminal 22 is the collector of the parasitic BJT 100. The substrate 10 between the source terminal 20 and the drain terminal 22 is the base of the parasitic BJT 110. The source terminal 24 is the emitter of a parasitic bipolar junction transistor 112 and the drain terminal 24 is the collector of the parasitic BJT 112. The substrate 10 between the source terminal 24 and the drain terminal 22 is the base of the parasitic BJT 112. The source terminal 24 is the emitter of a parasitic bipolar junction transistor 114 and the drain terminal 26 is the collector of the parasitic BJT 114. The substrate 10 between the source terminal 24 and the drain terminal 26 is the base of the parasitic BJT 114. Similarly, the source terminal 28 is the emitter of a parasitic bipolar junction transistor 116 and the drain terminal 26 is the collector of the parasitic BJT 116. The substrate 10 between the source terminal 28 and the drain terminal 26 is the base of the parasitic BJT 116.

[0009] The guard ring 30 is a circumscribing P+-doped region around the substrate 10. The guard ring 30 is connected to a ground (not shown). The guard ring 30 is isolated from the MOS transistor region 100 through shallow trench isolation (STI) regions 32. The source terminals 20, 24, 28 are also connected to the ground. The drain terminals 22 and 26 are connected to an input terminal (not shown).

[0010] However, for this type of ESD protection device, the device may be triggered non-uniformly during an electrostatic discharge. In other words, the parasitic BJTs 112, 114 near the central portion of the device may be turned on before the other parasitic BJTs 110 and 116. This occurs because the equivalent resistance of the base terminal for each of the parasitic transistors 110, 112, 114 and 116 may be different. That means, the parasitic BJTs 112 and 114 near the central portion of the device have a higher equivalent base resistance and hence open more readily. The two parasitic BJTs 110 and 116 on each side of the device have to open later due to a lower equivalent base resistance. In general, a BJT having a larger opening area has a higher electrostatic discharge capacity. Since the region having the maximum opening lies in the central portion of a conventional ESD protection device, such uneven opening across the device often leads to a drop in ESD capacity. ESD protection capacity is particularly compromised if the device occupies a large surface area.

[0011] To switch on all the parasitic BJTs within an ESD protection device concurrently, a number of improvements to the structure of the ESD protection device have been introduced. FIG. 3 is a top view of a conventional electrostatic discharge protection device having an improved structure for countering non-uniform switching of parasitic devices. FIG. 4 is a cross-sectional view along line II-II′ of FIG. 3.

[0012] As shown in FIGS. 3 and 4, the device includes a metal-oxide-semiconductor (MOS) transistor region 100, a guard ring 30, a shallow trench isolation (STI) region 32 and a longitudinal P+-doped region 30aover a P-type substrate 10. The gate terminals 12, 14, 16, 18, the source terminals 20, 24, 28 and the drain terminals 22, 26 have been described with reference to FIGS. 1 and 2 and hence their description is omitted.

[0013] In this device, aside from having the guard ring 30 circumscribing the substrate 10, a longitudinal P+-doped region 30ais inserted into the mid-section of the source terminal 24. Hence, the source terminal 24 is divided into two source terminals, 24aand 24b. In addition, the STI region 32 is positioned at each end of the substrate 10 isolating the guard ring 30 from the MOS transistor region 100.

[0014] The design of this type of ESD protection device relies on connecting all the source terminals 20, 24a, 24band 26 of the MOS transistors within the device with a ground (the guard ring 30 and the longitudinal P+-doped region 30a). Thus, all the parasitic BJTs 110, 112, 114, 116 within the ESD protection device are triggered concurrently to produce an optimal discharging effect. Nevertheless, in deep sub-micron fabrication, design of the longitudinal P+-doped region 30alowers the base resistance of the parasitic BJTs 110, 112, 114, 116 so that the parasitic BJTs 110, 112, 114 and 116 are difficult to trigger hence reducing ESD capacity of the device.

[0015] In addition, the concentration of dopants in P-type and N-type wells will increase when integrated circuits are fabricated in the deep sub-micron regime. Since the equivalent base resistance of the parasitic BJTs within the ESD protection device is dependent on the resistance in the well region, triggering of the parasitic BJTs is increasingly difficult. Although the aforementioned method is capable of improving non-uniform triggering of parasitic BJTs, difficult triggering of the ESD protection device compromises its protective capacity.

[0016] Another method of improving non-uniform switching of a conventional ESD protection device is to set up special electronic circuits. Gate driven or substrate-triggered circuits are sometimes incorporated into the ESD protection device, but this type of design requires additional circuit layout leading to greater circuit complexity as well as an increase in production cost.

[0017] Accordingly, one object of the present invention is to provide an electrostatic discharge (ESD) protection device capable of switching internal parasitic bipolar transistors on concurrently during an electrostatic discharge.

[0018] A second object of this invention is to provide an electrostatic discharge protection device capable of sustaining an electrostatic discharge a little longer.

[0019] A third object of this invention is to provide an electrostatic discharge protection device that has an improved circuit protection capacity without any increase in circuit complexity and production cost.

[0020] To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an electrostatic discharge protection device. The device includes a metal-oxide-semiconductor (MOS) transistor region, a guard ring, an isolation region, a first longitudinal doped region and a second longitudinal doped region on a substrate. The guard ring circumscribes the substrate. The isolation region is inside the guard ring for isolating the devices within the MOS transistor region from the guard ring. The MOS transistor region further includes a first MOS transistor, a second MOS transistor, a third MOS transistor and a fourth MOS transistor. The first longitudinal doped region is between the drain terminal of the first MOS transistor and the drain terminal of the second MOS transistor. The first longitudinal doped region, the drain terminal of the first MOS transistor and the drain terminal of the second MOS transistor are isolated from each other by isolation regions. The second longitudinal doped region is between the drain terminal of the third MOS transistor and the drain terminal of the fourth MOS transistor. The second longitudinal doped region, the drain terminal of the third MOS transistor and the drain terminal of the fourth MOS transistor are isolated from each other by isolation regions. The first longitudinal doped region, the second longitudinal doped region and the guard ring are all connected to a ground. Furthermore, the source terminal and the gate terminal of each MOS transistor are connected to the ground while the drain terminal of each MOS transistor is connected to an input terminal.

[0021] To provide a better electrostatic discharge protection capacity, this invention further includes a well. The well is formed within the source terminal of the first/fourth MOS transistor on each side of the substrate and adjacent isolation region and the substrate underneath the adjacent isolation region. Similarly, the well may also be formed in the drain terminal of the first MOS transistor and the substrate underneath the adjacent isolation region, in the drain terminal of the second MOS transistor and the substrate underneath the adjacent isolation region, in the drain terminal of the third MOS transistor and the substrate underneath the adjacent isolation region and in the drain terminal of the fourth MOS transistor and the substrate underneath the adjacent isolation region. The well may also be formed in all of the aforementioned areas.

[0022] This invention also provides a second electrostatic discharge (ESD) protection device. The ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region, a guard ring, an isolation region and a circular well over a substrate. The MOS transistor region accommodates a plurality of MOS transistors. The guard ring circumscribes the substrate. The isolation region is in the interior of the guard ring for isolating the MOS transistor region from the guard ring. The circular well is formed in the substrate underneath the isolation region and a portion of adjacent MOS transistors following the contour of the isolation region. The guard ring and the gate terminal of each MOS transistor are connected to a ground while the drain terminal of each MOS transistor is connected to an input terminal.

[0023] The ESD protection device according to this invention has an earth-connected longitudinal doped region within the drain terminal of the MOS transistor. Hence, the parasitic bipolar transistor of each MOS transistor within the device has equal switching power.

[0024] The ESD protection device of this invention also relies on proper positioning of the well to increase the base resistance of the parasitic bipolar transistor of each MOS transistor, thereby increasing switching capacity of the device.

[0025] Since the ESD protection device has an earth-connected longitudinal doped region inside the drain terminal of the MOS transistor and deep well in the appropriate region, the parasitic bipolar transistor of each MOS transistor has an identical and improved switching capability.

[0026] 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.

[0027] 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. In the drawings,

[0028] FIG. 1 is a top view showing a conventional electrostatic discharge protection device;

[0029] FIG. 2 is a cross-sectional view along line I-I′ of FIG. 1;

[0030] FIG. 3 is a top view of a conventional electrostatic discharge protection device having an improved structure for countering non-uniform switching of parasitic devices;

[0031] FIG. 4 is a cross-sectional view along line II-II′ of FIG. 3;

[0032] FIG. 5 is a top view showing an electrostatic discharge protection device fabricated according to a first embodiment of this invention;

[0033] FIG. 6 is a top view showing an alternative electrostatic discharge protection device fabricated according the first embodiment of this invention;

[0034] FIG. 7 is a cross-sectional view along line III-III′ of FIGS. 5 and 6;

[0035] FIG. 8 a top view showing an electrostatic discharge protection device fabricated according to a second embodiment of this invention;

[0036] FIG. 9 is a cross-sectional view along line IV-IV′ of FIG. 8;

[0037] FIG. 10 is a top view showing an electrostatic discharge protection device fabricated according to a third embodiment of this invention;

[0038] FIG. 11 is a top view showing an alternative electrostatic discharge protection device fabricated according the third embodiment of this invention;

[0039] FIG. 12 is a cross-sectional view along line V-V′ of FIGS. 10 and 11;

[0040] FIG. 13 a top view showing an electrostatic discharge protection device fabricated according to a fourth embodiment of this invention;

[0041] FIG. 14 is a cross-sectional view along line VI-VI′ of FIG. 13;

[0042] FIG. 15 is a top view showing an electrostatic discharge protection device fabricated according to a fifth embodiment of this invention;

[0043] FIG. 16 is a top view showing an alternative electrostatic discharge protection device fabricated according the fifth embodiment of this invention; and

[0044] FIG. 17 is a cross-sectional view along line VI-VI′ of FIGS. 15 and 16.

[0045] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

[0046] FIG. 5 is a top view showing an electrostatic discharge protection device fabricated according to a first embodiment of this invention. FIG. 6 is a top view showing an alternative electrostatic discharge protection device fabricated according the first embodiment of this invention. FIG. 7 is a cross-sectional view along line III-III′ of FIG. 5.

[0047] As shown in FIGS. 5 and 7, the ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region 100, a P+-doped guard ring 30, a shallow trench isolation (STI) region 32 and a pair of longitudinal P+-doped regions 34 and 36 formed on a P-type substrate 10. The MOS transistor region 100 accommodates a plurality of NMOS structures having gate terminals 12, 14, 16, 18, source terminals 20, 24, 28 and drain terminals 22, 26. Since the positioning of these gate, source and drain terminals is similar to the ones in FIGS. 1 and 2, detailed description of them is omitted. The P+-doped guard ring 30 circumscribes the substrate 10. The STI region 32 is formed between the P+-doped guard ring 30 and the MOS transistor region 100 as well as within the MOS transistor region 100.

[0048] A longitudinal P+-doped region 34 is inserted in the source terminal 22 between the gate terminal 12 and the gate terminal 14. The longitudinal P+-doped region 34 is isolated from the source terminal 22 by a circumscribing shallow trench isolation region 32a. Similarly, another longitudinal P+-doped region 36 is inserted in the source terminal 26 between the gate terminal 16 and the gate terminal 18. The longitudinal P+-doped region 36 is isolated from the source terminal 26 by a circumscribing shallow trench isolation region 32b.

[0049] In an alternative ESD protection device design as shown in FIGS. 6 and 7, the ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region 100, a P+-doped guard ring 30, a shallow trench isolation (STI) region 32 and a pair of longitudinal P+-doped regions 34 and 36 formed on a P-type substrate 10. The MOS transistor region 100 accommodates a plurality of NMOS structures having gate terminals 12, 14, 16, 18, source terminals 20, 24, 28 and drain terminals 22, 26. Since the positioning of these gate, source and drain terminals is similar to the ones in FIGS. 1 and 2, detailed description of them is omitted. The P+-doped guard ring 30 circumscribes the substrate 10. The STI region 32 is formed between the P+-doped guard ring 30 and the MOS transistor region 100 as well as within the MOS transistor region 100.

[0050] The source region 22 between the gate terminal 12 and the gate terminal 14 is divided into a source region 22aand another source region 22bby the STI region 32. The longitudinal P+-doped region 34 is formed between the source region 22aand the source region 22b. The longitudinal P+-doped region 34 is isolated from the source regions 22aand 22bby the STI region 32. Similarly, the source region 26 between the gate terminal 16 and the gate terminal 18 is divided into a source region. 26aand another source region 26bby the STI region 32. The longitudinal P+-doped region 36 is formed between the source region 26aand the source region 26b. The longitudinal P+-doped region 36 is isolated from the source regions 26aand 26bby the STI region 32.

[0051] The P+-doped guard ring 30, the longitudinal P+-doped regions 34, 36 and the source terminals 20, 24, 28 are all connected to a ground terminal (not shown). The drain terminals 22, 26 are connected to an input terminal (not shown). If the gate terminals 12, 14, 16 and 18 are not designed to connect with other circuits, the gate terminals 12, 14, 16, 18 are also connected to the ground terminal.

[0052] The ESD protection device in the first embodiment of this invention has a rather symmetrical design. Consequently, the parasitic bipolar transistors 110, 112, 114 and 116 each have identical base resistance values. Hence, the parasitic bipolar transistors 110, 112, 114, 116 within the MOS transistor region switch on concurrently and uniformly.

[0053] FIG. 8 a top view showing an electrostatic discharge protection device fabricated according to a second embodiment of this invention. FIG. 9 is a cross-sectional view along line IV-IV′ of FIG. 8.

[0054] As shown in FIGS. 8 and 9, the ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region 100, a P+-doped guard ring 30, a shallow trench isolation (STI) region 32 and an N-doped well region 40 formed on a P-type substrate 10. The MOS transistor region 100 accommodates a plurality of NMOS structures having gate terminals 12, 14, 16, 18, source terminals 20, 24, 28 and drain terminals 22, 26. Since the positioning of these gate, source and drain terminals is similar to the ones in FIGS. 1 and 2, detailed description of them is omitted. The P+-doped guard ring 30 circumscribes the substrate 10. The STI region 32 is formed between the P+-doped guard ring 30 and the MOS transistor region 100.

[0055] The ESD protection device according to the second embodiment includes a circumscribing N-doped well 40 just interior to the guard ring 30 following the contour of the STI region 32. The N-doped well region 40 is formed in the substrate 10 underneath the source terminals 20, 28 and adjacent STI region 32.

[0056] Similarly, the guard ring 30 and the source terminals 20, 24, 28 are all connected to a ground terminal (not shown) while the drain terminals 22, 26 are connected to an input terminal (not shown). If the gate terminals 12, 14, 16, 18 are not designed to connect with other circuits, the gate terminals 12, 14, 16, 18 are also connected to the ground terminal.

[0057] The ESD protection device according to the second embodiment of this invention relies on the N-doped well 40 formed in CMOS fabrication process to increase the base resistance value of the parasitic bipolar transistors 110 and 116 on each side of the device. Hence, the parasitic bipolar transistors 110 and 116 are more readily switched on. With the switching of the transistors 110 and 116 facilitated, each of the parasitic bipolar transistors 110, 112, 114, 116 can be turned on concurrently. In addition, the presence of the N-doped well 40 permits the channeling of current deeper into the substrate 10 and enhances the capacity for current discharge.

[0058] FIG. 10 is a top view showing an electrostatic discharge protection device fabricated according to a third embodiment of this invention. FIG. 11 is a top view showing an alternative electrostatic discharge protection device fabricated according the third embodiment of this invention. FIG. 12 is a cross-sectional view along line V-V′ of FIGS. 10 and 11.

[0059] As shown in FIGS. 10 and 12, the ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region 100, a P+-doped guard ring 30, a pair of longitudinal P+-doped regions 34, 36, a shallow trench isolation (STI) region 32 and a pair of N-doped wells 42, 44 formed on a P-type substrate 10. Since the positioning of the gate terminals 12, 14, 16, 18, source terminals 20, 24, 28 and drain terminals 22a, 22b, 26a, 26bwithin the MOS transistor region 100 as well as the P+-doped guard ring 30, the STI region 32 and the longitudinal P+-doped regions 34, 36 has already been explained with reference to FIGS. 6 and 7, detailed description of them is omitted.

[0060] In the third embodiment of this invention, longitudinal N-doped wells 42, 44 are formed in the substrate 10 on each side of the ESD protection device. The wells 42, 44 are positioned interior to the guard ring 30 adjacent to a strip of STI region 32. In other words, the longitudinal N-doped wells 42 and 44 are formed within the substrate 10 underneath the respective source terminals 20 and 28 and adjacent STI regions 32 (as shown in FIG. 12).

[0061] In the third embodiment, the guard ring 30 and the source terminals 20, 24, 28 are all connected to a ground terminal (not shown) while the drain terminals 22a, 22b, 26a, 26bare all connected to an input terminal (not shown). If the gate terminals 12, 14, 16, 18 are not designed to connect with other circuits, the gate terminals 12, 14, 16, 18 are also connected to the ground terminal.

[0062] The alternative ESD protection device design in FIGS. 11 and 12, aside from the aforementioned MOS transistor region 100, the P+-doped guard ring 30, the STI region 32, the longitudinal P+-doped regions 34, 36 and the longitudinal N-doped wells 42, 44, further includes a pair of U-shaped N-doped wells 46 between the drain terminals 22aand 22b. The N-doped wells 46 are formed symmetrically at each end of the longitudinal P+-doped region 34. Similarly, a pair of U-shaped N-doped wells 48 is also formed between the drain terminals 26aand 26b. The N-doped wells 48 are formed symmetrically at each end of the longitudinal P+-doped region 36.

[0063] The ESD protection device in the third embodiment is actually a design that combines the characteristics of the first and the second embodiment. In other words, the third embodiment includes both longitudinal P+-doped regions and N-doped wells. To minimize uneven conduction due to a non-symmetrical side MOS transistor structure, N-doped wells 42, 44, 46, 48 are formed inside the guard ring 30 as shown in FIGS. 10 and 11 having a cross-sectional structure as shown in FIG. 12 in a CMOS process. In this way, the base resistance of the parasitic bipolar transistors 110, 116 on each side of the device is connected in parallel with another resistor. In addition, the N-doped wells 46 between the drain terminals 22aand 22band at the ends of the longitudinal P+-doped region 34 as well as the N-doped wells 48 between the drain terminals 26aand 26band at the ends of the longitudinal P+-doped region 36 as shown in FIG. 11 increase the parasitic resistance values at both ends of the finger structure. Therefore, all the parasitic bipolar transistors are switched on concurrently and uniformly.

[0064] FIG. 13 a top view showing an electrostatic discharge protection device fabricated according to a fourth embodiment of this invention. FIG. 14 is a cross-sectional view along line VI-VI′ of FIG. 13.

[0065] As shown in FIGS. 13 and 14, the ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region 100, a P+-doped guard ring 30, a pair of longitudinal P+-doped regions 34, 36, a shallow trench isolation (STI) region 32 and a pair of N-doped well regions 50, 52 formed on a P-type substrate 10. Since the positioning of the gate terminals 12, 14, 16, 18, source terminals 20, 24, 28 and drain terminals 22a, 22b, 26a, 26bwithin the MOS transistor region 100 as well as the P+-doped guard ring 30, the STI region 32 and the longitudinal P+-doped regions 34, 36 has already been explained with reference to FIGS. 6 and 7, detailed description of them is omitted.

[0066] In the fourth embodiment, a ring of N-doped well 50 is formed in the substrate 10 underneath the drain terminals 22a, 22band adjacent to the STI region 32. The ring of N-doped well 50 circumscribes the longitudinal P+-doped region 34. Similarly, a ring of N-doped well 52 is formed in the substrate 10 underneath the drain terminals 26a, 26band adjacent to the STI region 32. The ring of N-doped well 52 circumscribes the longitudinal P+-doped region 36.

[0067] The guard ring 30 as well as the source terminals 20, 24, 28 are all connected to a ground terminal (not shown) while the drain terminals 22a, 22b, 26a, 26bare all connected to an input terminal (not shown). If the gate terminals 12, 14, 15, 18 are not designed to connect with other circuits, the gate terminals 12, 14, 16, 18 are also connected to the ground terminal.

[0068] The fourth embodiment is actually a variation through a combination of the first and the second embodiment. With a suitable layout as shown in FIG. 14, the parasitic bipolar transistors 110, 112, 114, 116 all have a greater base resistance value. Hence, this type of design is able to improve overall switching capacity of the device.

[0069] FIG. 15 is a top view showing an electrostatic discharge protection device fabricated according to a fifth embodiment of this invention. FIG. 16 is a top view showing an alternative electrostatic discharge protection device fabricated according the fifth embodiment of this invention. FIG. 17 is a cross-sectional view along line VI-VI′ of FIGS. 15 and 16.

[0070] As shown in FIGS. 15 and 17, the ESD protection device includes a metal-oxide-semiconductor (MOS) transistor region 100, a P+-doped guard ring 30, a pair of longitudinal P+-doped regions 34, 36, a shallow trench isolation (STI) region 32, a pair of N-doped well regions 42, 44 and a pair of ring-like N-doped wells 50, 52 formed on a P-type substrate 10. Since the positioning of the gate terminals 12, 14, 16, 18, source terminals 20, 24, 28 and drain terminals 22a, 22b, 26a, 26bwithin the MOS transistor region 100 as well as the P+-doped guard ring 30, the STI region 32 and the longitudinal P+-doped regions 34, 36 has already been explained with reference to FIGS. 6 and 7, detailed description of them is omitted.

[0071] In the fifth embodiment, the longitudinal N-doped wells 42 and the longitudinal N-doped wells 44 are formed in the substrate 10 on each side of the ESD protection device. The N-doped wells 42 and 44 are positioned just inside the guard ring 30 following the contour of the STI region 32. As shown in FIG. 17, the longitudinal N-doped wells 42 and 44 are formed within the substrate 10 between the respective source terminals 20, 28 and the STI region 32. Furthermore, the ring-like N-doped well 50 is formed between the drain terminals 22aand 22benclosing the longitudinal P+-doped region 34 while the ring-like N-doped well 52 is formed between the drain terminals 26aand 26benclosing the longitudinal P+-doped region 36.

[0072] The guard ring 30 as well as the source terminals 20, 24, 28 are all connected to a ground terminal (not shown) while the drain terminals 22a, 22b, 26a, 26bare all connected to an input terminal (not shown). If the gate terminals 12, 14, 15, 18 are not designed to connect with other circuits, the gate terminals 12, 14, 16, 18 are also connected to the ground terminal.

[0073] The alternative ESD protection device design in FIGS. 16 and 17, aside from the aforementioned MOS transistor region 100, the P+-doped guard ring 30, the STI region 32, the longitudinal P+-doped regions 34, 36 and the longitudinal N-doped wells 42, 44, further includes a longitudinal N-doped well region 54 between the drain terminal 22aand the longitudinal P+-doped region 34. The longitudinal N-doped well region 54 is formed within the substrate 10 underneath the drain terminal 22aand adjacent STI region 32. Similarly, the ESD protection device further includes a longitudinal N-doped well region 56 between the drain terminal 22band the longitudinal P+-doped region 34. The longitudinal N-doped well region 56 is formed within the substrate 10 underneath the drain terminal 22band adjacent STI region 32. The ESD protection device also includes a longitudinal N-doped well region 58 between the drain terminal 26aand the longitudinal P+-doped region 36. The longitudinal N-doped well region 58 is formed within the substrate 10 underneath the drain terminal 26aand adjacent STI region 32. Similarly, the ESD protection device further includes a longitudinal N-doped well region 60 between the drain terminal 26band the longitudinal P+-doped region 36. The longitudinal N-doped well region 60 is formed within the substrate 10 underneath the drain terminal 26band adjacent STI region 32.

[0074] The ESD protection device in the fifth embodiment is actually a design that combines the characteristics of the third and the fourth embodiment. In other words, the fifth embodiment includes both N-doped wells on each side of the device and ring-like N-doped wells in the interior. To provide a better symmetry for the device, the N-doped wells are positioned at suitable positions just inside the guard ring as shown in FIGS. 15 and 16 with a cross-sectional structure as shown in FIG. 17. In this way, each of the parasitic bipolar transistors 110, 112, 114, 116 has a greater but identical base resistance value. Thus, all the parasitic bipolar transistors 110, 112, 114, 116 have identical but improved switching capacity.

[0075] In conclusion, major advantages of this invention include:

[0076] 1. By forming an earth-connected longitudinal doped region within the drain terminal of a MOS transistor, the parasitic bipolar transistor of each MOS transistor has equal switching capacity.

[0077] 2. By forming well regions in suitable locations, base resistance value of the parasitic bipolar transistor of each MOS transistor is increased. Hence, switching capacity of the electrostatic discharge protection device is enhanced.

[0078] 3. By forming an earth-connected longitudinal doped region within the drain terminal of a MOS transistor as well as forming well regions in suitable locations, the parasitic bipolar transistor of each MOS transistor has identical but improved switching capacity.

[0079] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.