Load Switch Including Back-to-Back Connected Transistors

Abstract

An apparatus includes a first drain/source region and a second drain/source region over a substrate, and a first gate adjacent to the first drain/source region and a second gate adjacent to the second drain/source region, wherein the first gate and the second gate are separated from each other, wherein the first drain/source region, the second drain/source region, the first gate and the second gate form two back-to-back connected transistors.

Claims

1. An apparatus comprising: a first drain/source region and a second drain/source region over a substrate; and a first gate adjacent to the first drain/source region and a second gate adjacent to the second drain/source region, wherein the first gate and the second gate are separated from each other, and wherein the first drain/source region, the second drain/source region, the first gate and the second gate form two back-to-back connected transistors.

2. The apparatus of claim 1, wherein: the first drain/source region is a first source of the two back-to-back connected transistors; and the second drain/source region is a second source of the two back-to-back connected transistors.

3. The apparatus of claim 1, further comprising: an epitaxial layer having a first conductivity type over the substrate having the first conductivity type; a drift layer having a second conductivity type over the epitaxial layer; a first body region having the first conductivity type formed in the drift layer, wherein the first drain/source region is formed in the first body region and has the second conductivity type; a second body region having the first conductivity type formed in the drift layer, wherein the second drain/source region is formed in the second body region and has the second conductivity type; a high voltage oxide region over the drift layer; a first gate dielectric layer formed between the high voltage oxide region and the first drain/source region; and a second gate dielectric layer formed between the high voltage oxide region and the second drain/source region.

4. The apparatus of claim 3, wherein: the first conductivity type is P-type; and the second conductivity type is N-type.

5. The apparatus of claim 3, further comprising: a first body contact region formed in the first body region; and a second body contact region formed in the second body region, wherein: the first body contact region is coupled to the first drain/source region through a first source contact; and the second body contact region is coupled to the second drain/source region through a second source contact.

6. The apparatus of claim 3, wherein: the first drain/source region and the second drain/source region are arranged in a symmetrical manner with respect to a center line crossing the high voltage oxide region; and the first gate and the second gate are arranged in a symmetrical manner with respect to the center line crossing the high voltage oxide region.

7. The apparatus of claim 1, further comprising: an epitaxial layer having a first conductivity type over the substrate having the first conductivity type; a drift layer having a second conductivity type over the epitaxial layer; a first body region having the first conductivity type formed in the drift layer, wherein the first drain/source region is formed in the first body region and has the second conductivity type; a second body region having the first conductivity type formed in the drift layer, wherein the second drain/source region is formed in the second body region and has the second conductivity type; a first gate dielectric layer formed under the first gate; and a second gate dielectric layer formed under the second gate.

8. The apparatus of claim 1, further comprising: an epitaxial layer having a first conductivity type over the substrate having the first conductivity type; a well region having the first conductivity type over the epitaxial layer, wherein the first drain/source region and the second drain/source region are formed in the well region and have a second conductivity type; a first gate dielectric layer formed under the first gate; and a second gate dielectric layer formed under the second gate.

9. A method comprising: growing an epitaxial layer with a first conductivity type on a substrate with the first conductivity type; forming a drift layer having a second conductivity type over the epitaxial layer; forming a first body region and a second body region with the first conductivity type in the drift layer; implanting ions with the second conductivity type to form a first source region in the first body region and a second source region in the second body region; and forming a first gate adjacent to the first source region and a second gate adjacent to the second source region, wherein the first source region and the second source region are on opposing sides of the first gate and the second gate.

10. The method of claim 9, further comprising: forming a high voltage oxide region over the drift layer; forming the first gate along an edge of the first source region, wherein the first gate covers a first sidewall and a first edge portion of the high voltage oxide region; and forming the second gate along an edge of the second source region, wherein the second gate covers a second sidewall and a second edge portion of the high voltage oxide region.

11. The method of claim 10, wherein: the first source region, the first gate, the second source region and the second gate form shared-drain transistors; and an active region of the shared-drain transistors is orthogonal to the high voltage oxide region.

12. The method of claim 9, further comprising: forming a gate dielectric layer from an edge of the first source region to an edge of the second source region; forming the first gate covering a first edge portion of the gate dielectric layer; and forming the second gate covering a second edge portion of the gate dielectric layer.

13. The method of claim 9, further comprising: forming a shallow trench isolation (STI) region in the drift layer; forming the first gate extending from an edge of the first source region and covering a first edge region of the STI region; and forming the second gate extending from an edge of the second source region and covering a second edge region of the STI region.

14. The method of claim 9, wherein: forming a local oxidation of silicon (LOCOS) structure having a lower portion in the drift layer and an upper portion over the drift layer; forming the first gate along an edge of the first source region, wherein the first gate covers a first sidewall and a first edge portion of the LOCOS structure; and forming the second gate along an edge of the second source region, wherein the second gate covers a second sidewall and a second edge portion of the LOCOS structure.

15. The method of claim 9, further comprising: forming a plurality of source regions; and forming a plurality of body contact regions, wherein the plurality of source regions and the plurality of body contact regions are arranged in an alternating manner.

16. The method of claim 15, further comprising: forming a plurality of contacts over the plurality of source regions and the plurality of body contact regions, wherein at least one contact couples one source region and one adjacent body contact region.

17. A load switch comprising: a first transistor; and a second transistor being back-to-back connected to the first transistor, wherein: a source of the first transistor and a source of the second transistor are formed over a substrate; a gate of the first transistor is adjacent to the source of the first transistor; and a gate of the second transistor is adjacent to the source of the second transistor, and wherein the gate of the first transistor and the gate of the second transistor are separated from each other.

18. The load switch of claim 17, further comprising: an epitaxial layer over the substrate; a drift layer over the epitaxial layer; a first body region and a second body region in the drift layer; and a high voltage oxide region over the drift layer, wherein: the gate of the first transistor covers a first sidewall and a first edge portion of the high voltage oxide region; and the gate of the second transistor covers a second sidewall and a second edge portion of the high voltage oxide region.

19. The load switch of claim 17, further comprising: an epitaxial layer over the substrate; a drift layer over the epitaxial layer; a first body region and a second body region in the drift layer; and an STI region in the drift layer, wherein: the gate of the first transistor extends from an edge of the source of the first transistor and covers a first edge region of the STI region; and the gate of the second transistor extends from an edge of the source of the second transistor and covers a second edge region of the STI region.

20. The load switch of claim 17, further comprising: an epitaxial layer over the substrate; a drift layer over the epitaxial layer; a first body region and a second body region in the drift layer; and an LOCOS structure having a lower portion in the drift layer and an upper portion over the drift layer, wherein: the gate of the first transistor along an edge of the source of the first transistor, wherein the gate of the first transistor covers a first sidewall and a first edge portion of the LOCOS structure; and the gate of the second transistor along an edge of the source of the second transistor, wherein the gate of the second transistor covers a second sidewall and a second edge portion of the LOCOS structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0013] FIG. 1 illustrates a schematic diagram of shared-drain transistors in accordance with various embodiments of the present disclosure;

[0014] FIG. 2 illustrates a simplified cross-sectional view of a load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure;

[0015] FIG. 3 illustrates a simplified top view of the load switch shown in FIG. 2 in accordance with various embodiments of the present disclosure;

[0016] FIG. 4 illustrates a simplified cross-sectional view of another load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure;

[0017] FIG. 5 illustrates a simplified cross-sectional view of another load switch formed by two transistors in accordance with various embodiments of the present disclosure;

[0018] FIG. 6 illustrates a simplified cross-sectional view of another load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure;

[0019] FIG. 7 illustrates a simplified cross-sectional view of another load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure; and

[0020] FIG. 8 illustrates a flow chart of a method for forming the shared-drain transistors shown in FIG. 1 in accordance with various embodiments of the present disclosure.

[0021] Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0022] The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosure, and do not limit the scope of the disclosure.

[0023] The present disclosure will be described with respect to embodiments in a specific context, a load switch including a pair of back-to-back connected lateral double-diffused metal oxide semiconductor (LDMOS) devices. The embodiments of the disclosure may also be applied, however, to a variety of metal oxide semiconductor field effect transistors (MOSFETs).

[0024] FIG. 1 illustrates a schematic diagram of shared-drain transistors in accordance with various embodiments of the present disclosure. A load switch may be implemented as two back-to-back connected LDMOS devices. These two LDMOS devices share a drain. Throughout the description, the load switch may be alternatively referred to as shared-drain transistors.

[0025] As shown in FIG. 1, the shared-drain transistors 100 comprise two back-to-back connected N-type transistors. A first transistor comprises a first drain D1, a first gate G1 and a first source S1. A second transistor comprises a second drain D2, a second gate G2 and a second source S2. The drains of these two N-type transistors are directly connected to each other as shown in FIG. 1. The back-to-back connected N-type transistors shown in FIG. 1 can be used as a load switch. The load switch is able to achieve bidirectional current blocking. As such, the load switch is also known as an isolation switch.

[0026] In operation, the source of the second transistor may be connected to a high voltage potential such as a bias voltage. The source of the first transistor may be connected to a low voltage potential. In this configuration, the second switch can be turned on when the second gate is driven by a voltage higher than the high voltage potential. A charge pump may be needed to provide a gate drive voltage higher than the high voltage potential.

[0027] In some embodiments, each transistor shown in FIG. 1 may be implemented as a lateral double-diffused MOS (LDMOS). The shared-drain transistors 100 may only have four terminals, namely a first gate terminal, a first source terminal, a second gate terminal and a second source terminal. As shown in FIG. 1, the drains of the two back-to-back connected N-type transistors are connected to each other. In other words, the drain is shared by the first transistor and the second transistor. The shared drain is floating as shown in FIG. 1. As such, the shared-drain transistors 100 do not comprise a drain terminal. The layout of the shared-drain transistors 100 may be improved through removing unnecessary drain regions such as drain diffusion regions, drain contacts, drain metal regions and the like. By removing unnecessary drain regions, the two transistors shown in FIG. 1 can be brought close to each other as possible while ensuring that the two gates (G1 and G2) are separated by the minimum spacing specified by the design rule. The detailed layout of the shared-drain transistors 100 will be described below with respect to FIGS. 2-3.

[0028] FIG. 2 illustrates a simplified cross-sectional view of a load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure. The shared-drain LDMOS transistors 200 include a substrate 102, a first layer 104, a drift layer 106 formed over the first layer 104, a first body region 112 and a second body region 122 formed in the drift layer 106. The shared-drain LDMOS transistors 200 further comprise a first drain/source region 114 formed in the first body region 112, a second drain/source region 124 formed in the second body region 122, a first gate dielectric layer 133, a high voltage oxide region 132, a second gate dielectric layer 135, a first source contact 116, a first gate 134, a second gate 136 and a second source contact 126.

[0029] In some embodiments, the substrate 102, the first layer 104, the first body region 112 and the second body region 122 have a first conductivity type. The drift layer 106, the first drain/source region 114 and the second drain/source region 124 have a second conductivity type. In some embodiments, the first conductivity type is P-type, and the second conductivity type is N-type. The shared-drain LDMOS transistors 200 are formed by two n-type transistors. Alternatively, the first conductivity type is N-type, and the second conductivity type is P-type. The shared-drain LDMOS transistors 200 are formed by two p-type transistors.

[0030] The substrate 102 may be formed of suitable semiconductor materials such as silicon, silicon germanium, silicon carbide and the like. Depending on different applications and design needs, the substrate 102 may be N-type or P-type. In some embodiments, the substrate 102 is a P-type substrate. Appropriate P-type dopants such as boron and the like are doped into the substrate 102. Alternatively, the substrate 102 is an N-type substrate. Appropriate N-type dopants such as phosphorous and the like are doped into the substrate 102.

[0031] The first layer 104 may be implemented as a P-type epitaxial layer. Throughout the description, the first layer 104 may be alternatively referred to as the P-type epitaxial layer 104. The P-type epitaxial layer 104 is grown from the substrate 102. The epitaxial growth of the P-type epitaxial layer 104 may be implemented by using any suitable semiconductor fabrication processes such as chemical vapor deposition (CVD) and the like. In some embodiments, the P-type epitaxial layer 104 is of a doping density in a range from about 10.sup.14/cm.sup.3 to about 10.sup.16/cm.sup.3.

[0032] The drift layer 106 is an N-type layer formed over the first layer 104. In some embodiments, the drift layer 106 may be doped with an N-type dopant such as phosphorous to a doping density of about 10.sup.15/cm.sup.3 to about 10.sup.17/cm.sup.3. It should be noted that other N-type dopants such as arsenic, antimony, or the like, could alternatively be used. It should further be noted that the drift layer 106 may be alternatively referred to as an extended drift region.

[0033] The first body region 112 and the second body region 122 are P-type body regions. The P-type body regions may be formed by implanting P-type doping materials such as boron and the like. Alternatively, the P-type body regions can be formed by a diffusion process. In some embodiments, a P-type material such as boron may be implanted to a doping density of about 10.sup.16/cm.sup.3 to about 10.sup.18/cm.sup.3. The first body region 112 may be alternatively referred to as a first channel region. The second body region 122 may be alternatively referred to as a second channel region.

[0034] The first drain/source region 114 is a first N+ region formed in the first body region 112. The first drain/source region 114 may be alternatively referred to as the first N+ region 114. In accordance with an embodiment, the first N+ region 114 functions as a first source region of the shared-drain LDMOS transistors 200. The first source region may be formed by implanting N-type dopants such as phosphorous and arsenic at a concentration of between about 10.sup.19/cm.sup.3 and about 10.sup.20/cm.sup.3. Furthermore, a source contact 116 is formed over the first N+ region 114.

[0035] It should be noted that a P+ region (not shown but illustrated in FIG. 3) is formed adjacent to the first N+ region 114 in the first body region 112. The P+ region (e.g., P+ region 115 shown in FIG. 3) may be formed by implanting a P-type dopant such as boron at a concentration of between about 10.sup.19/cm.sup.3 and about 10.sup.20/cm.sup.3. The P+ region may contact the P-type body. In order to eliminate the body effect, the P+ region may be connected to the first source region (the first N+ region 114) directly through the first source contact 116.

[0036] The second drain/source region 124 is a second N+ region 124 formed in the second body region 122. The second drain/source region 124 may be alternatively referred to as the second N+ region 124. In accordance with an embodiment, the second N+ region 124 functions as a second source region of the shared-drain LDMOS transistors 200. The second source region may be formed by implanting N-type dopants such as phosphorous and arsenic at a concentration of between about 10.sup.19/cm.sup.3 and about 10.sup.20/cm.sup.3. Furthermore, a second source contact 126 is formed over the second N+ region 124.

[0037] It should be noted that a P+ region (not shown but illustrated in FIG. 3) is formed adjacent to the second N+ region 124 in the second body region 122. The P+ region (e.g., P+ region 125 shown in FIG. 3) may be formed by implanting a P-type dopant such as boron at a concentration of between about 10.sup.19/cm.sup.3 and about 10.sup.20/cm.sup.3. The P+ region may contact the p-type body. In order to eliminate the body effect, the P+ region may be connected to the second source region (the second N+ region 124) directly through the second source contact 126.

[0038] The first gate dielectric layer 133, the high voltage oxide region 132 and the second gate dielectric layer 135 are formed over the drift layer 106. As shown in FIG. 2, the first gate dielectric layer 133 is partially on top of the first body region 112, and partially on top of the drift layer 106. Likewise, the second gate dielectric layer 135 is partially on top of the second body region 122, and partially on top of the drift layer 106. The high voltage oxide region 132 is formed between the first gate dielectric layer 133 and the second gate dielectric layer 135. As shown in FIG. 2, the high voltage oxide region 132 is of a trapezoidal shape. It is within the scope and spirit of the present disclosure for the high voltage oxide region to comprise other shapes, such as, but not limited to square, rectangle and the like.

[0039] As shown in FIG. 2, the thickness of the high voltage oxide region 132 is much greater than the thickness of the first gate dielectric layer 133 and the second gate dielectric layer 135. In some embodiments, the first gate dielectric layer 133 and the second gate dielectric layer 135 are of a thickness of between about 100 Angstroms and about 200 Angstroms. The thickness of the high voltage oxide region 132 is about 5000 Angstroms. In some embodiments, the first gate dielectric layer 133, the high voltage oxide region 132 and the second gate dielectric layer 135 may be formed of suitable oxide materials such as silicon oxide, silicon oxynitride, hafnium oxide, zirconium oxide or the like.

[0040] The first gate 134 is formed on the first gate dielectric layer 133 and the high voltage oxide region 132. The second gate 136 is formed on the second gate dielectric layer 135 and the high voltage oxide region 132. The first gate 134 and the second gate 136 may be formed of polysilicon, polysilicon germanium, nickel silicide or other metal, metal alloy materials.

[0041] As shown in FIG. 2, the first gate 134 extends up onto the high voltage oxide region 132. The first gate 134 covers a first sidewall and a first edge portion of the high voltage oxide region 132. The combination of the first gate 134 and the high voltage oxide region 132 functions as a first field plate. This first field plate helps to maintain the breakdown voltage of the first transistor of the shared-drain LDMOS transistors 200. Likewise, the second gate 136 extends up onto the high voltage oxide region 132. The second gate 136 covers a second sidewall and a second edge portion of the high voltage oxide region 132. The combination of the second gate 136 and the high voltage oxide region 132 functions as a second field plate. This second field plate helps to maintain the breakdown voltage of the second transistor of the shared-drain LDMOS transistors 200.

[0042] The first gate 134 and the second gate 136 may be formed by depositing a polysilicon layer with a thickness of about 4000 Angstroms over the gate dielectric layers and the high voltage oxide region, depositing a photoresist layer over the polysilicon layer, developing the photoresist layer to define the first gate 134 and the second gate 136, etching the polysilicon layer to form gates 134 and 136.

[0043] As shown in FIG. 2, the first drain/source region 114 and the second drain/source region 124 are arranged in a symmetrical manner with respect to a center line 101 crossing the high voltage oxide region 132. The first gate 134 and the second gate 136 are arranged in a symmetrical manner with respect to the center line 101 crossing the high voltage oxide region 132.

[0044] In the conventional shared-drain LDMOS transistors, two LDMOS transistors are placed in a symmetrical manner with respect to the shared drain. Each LDMOS transistor has its own high voltage oxide region. The two high voltage oxide regions are separated by the drain contact. In accordance with the design rule of the 24 V LDMOS devices, the dimension of a single LDMOS transistor is about 2.16 um. The dimension of the two LDMOS transistors is about 4.32 um. It should be noted that the dimensions used in the previous example are selected purely for demonstration purposes and are not intended to limit the various embodiments of the present invention to any particular size dimensions. A skilled person in the art will appreciate that depending on different fabrication processes, there can be many variations of the dimension of the LDMOS.

[0045] As shown in FIG. 2, a single high voltage oxide region 132 is employed to replace the two high voltage oxide regions in the conventional shared-drain LDMOS transistors. Furthermore, the drain region and the drain contact have been removed. As a result of merging two high voltage oxide regions into one high voltage oxide region and removing the drain region and the drain contact, the two LDMOS transistors are brought close to each other.

[0046] As shown in FIG. 2, the dimension of the shared-drain LDMOS transistors 200 is denoted as D. In particular, D is the distance between the first source contact 116 and the second source contact 126. In accordance with the design rule of the 24 V LDMOS devices, D is about 3.15 um. The dimension of the shared-drain LDMOS transistors 200 is much smaller than the dimension (4.32 um) of the conventional shared-drain LDMOS transistors.

[0047] For the shared-drain LDMOS transistors formed by 24 V LDMOS devices, the area of the shared-drain LDMOS transistors shown in FIG. 2 is about 1.46 times the area of a single transistor. Rsp is equal to an area of a semiconductor device times the on resistance of the semiconductor device. The effective Rsp is obtained through a normalization process. Based on the normalization process, the effective Rsp of a single 24 V LDMOS device is equal to 1. According to the same normalization process, the effective Rsp of the shared-drain LDMOS transistors shown in FIG. 2 is equal to 2.92. In the conventional device, the effective Rsp is equal to 4. As such, the shared-drain LDMOS transistors shown in FIG. 2 represent a 27% Rsp improvement relative to the conventional device.

[0048] In operation, when a first gate voltage and a second gate voltage are applied to the first gate 134 and the second gate 136, respectively, and the gate voltages are greater than the threshold of the corresponding transistors. A first inversion layer is formed in the first body region 112. The first inversion layer couples the first N+ region 114 to the drift layer 106. A second inversion layer is formed in the second body region 122. The second inversion layer couples the second N+ region 124 to the drift layer 106. As a result of having the first inversion layer and the second inversion layer, a conductive channel is established between the first source region and the second source region. A current flows between the first source region and the second source region of the shared-drain LDMOS transistors. On the other hand, when the gate voltages are less than the threshold of the transistors, the shared-drain LDMOS transistors are turned off accordingly.

[0049] FIG. 3 illustrates a simplified top view of the load switch shown in FIG. 2 in accordance with various embodiments of the present disclosure. Two U-shaped N+ regions are placed adjacent to each other. On the left side, a terminal portion of the first U-shaped N+ region and a terminal portion of the second U-shaped N+ region form the first source region 114 shown in FIG. 2. The first source region 114 is formed within the first body region 112. Likewise, on the right side, a terminal portion of the first U-shaped N+ region and a terminal portion of the second U-shaped N+ region form the second source region 124 shown in FIG. 2. The second source region 124 is formed within the second body region 122.

[0050] A first P+ region 115 is between two N+ regions of the first source region 114. The first P+ region 115 may contact the first p-type body region shown in FIG. 2. A plurality of first source contacts 116 are formed over the first P+ region 115 and the adjacent N+ regions. At least one source contact 116 couples the first P+ region 115 to the adjacent N+ regions.

[0051] It should be noted that while on the left side, there is one P+ region 115, the semiconductor device may comprise a plurality of P+ regions 115. More particularly, the N+ regions and the plurality of P+ regions may be formed in an alternating manner.

[0052] A second P+ region 125 is between two N+ regions of the second source region 124. The second P+ region 125 may contact the second p-type body region shown in FIG. 2. A plurality of second source contacts 126 are formed over the second P+ region 125 and the adjacent N+ regions. At least one source contact 126 couples the second P+ region 125 to the adjacent N+ regions.

[0053] It should be noted that while on the right side, there is one P+ region 125, the semiconductor device may comprise a plurality of P+ regions 125. More particularly, the N+ regions and the plurality of P+ regions may be formed in an alternating manner.

[0054] The high voltage oxide region 132 is placed between the first body region 112 and the second body region 122. The active region 120 of the shared-drain LDMOS transistors is orthogonal to the high voltage oxide region 132 as shown in FIG. 3. The first gate 134 is placed adjacent to the first source region 114. A plurality of first gate contacts 137 is formed over the first gate 134. The second gate 136 is placed adjacent to the second source region 124. A plurality of second gate contacts 139 is formed over the second gate 136. As shown in FIG. 3, the first gate 134 and the second gate 136 are separated from each other.

[0055] FIG. 4 illustrates a simplified cross-sectional view of another load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure. The shared-drain LDMOS transistors 400 shown in FIG. 4 are similar to the shared-drain LDMOS transistors 200 shown in FIG. 2 except that the shared-drain LDMOS transistors 400 do not include a high voltage oxide region. As shown in FIG. 4, the first gate 134 and the second gate 136 are formed over the first gate dielectric layer 133 and the second gate dielectric layer 135 respectively. The first gate dielectric layer 133 and the second gate dielectric layer 135 are formed of a same material. The first gate dielectric layer 133 and the second gate dielectric layer 135 are collectively referred to as a gate dielectric layer. As shown in FIG. 4, the first gate 134 covers a first edge portion of the gate dielectric layer. The second gate 136 covers a second edge portion of the gate dielectric layer. The first gate 134 and the second gate 136 are separated from each other. The shared-drain LDMOS transistors 400 shown in FIG. 4 may be used in medium voltage applications such as 12 V applications.

[0056] FIG. 5 illustrates a simplified cross-sectional view of another load switch formed by two transistors in accordance with various embodiments of the present disclosure. The load switch 500 shown in FIG. 5 is formed by the same principle as that shown in FIG. 2 except that the load switch shown in FIG. 5 is used in low voltage applications such as 5 V applications.

[0057] In FIG. 5, an epitaxial layer 104 is formed over a substrate 102. A well 106 is formed over the epitaxial layer 104. A first drain/source region 114 and a second drain/source region 124 are formed in the well 106. In some embodiments, the substrate 102, the epitaxial layer 104, the well 106 have a first conductivity type. The first drain/source region 114 and the second drain/source region 124 have a second conductivity type. In some embodiments, the first conductivity type is P-type. The second conductivity type is N-type.

[0058] It should be noted that when the first drain/source region 114 and the second drain/source region 124 are implemented as source regions, the load switch is formed by two shared-drain transistors. On the other hand, when the first drain/source region 114 and the second drain/source region 124 are implemented as drain regions, the load switch is formed by two shared-source transistors.

[0059] FIG. 6 illustrates a simplified cross-sectional view of another load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure. The shared-drain LDMOS transistors 600 shown in FIG. 6 are similar to the shared-drain LDMOS transistors 200 shown in FIG. 2 except that the high voltage oxide region is replaced by a shallow trench isolation (STI) region. The LDMOS device with the STI region for improving the breakdown voltage is well known in the art, hence is not discussed again herein. As shown in FIG. 6, the STI region is of an inverted trapezoidal shape. The first gate 134 extends from an edge of the first drain/source region 114 and covers a first edge region of the STI region. The second gate 136 extends from an edge of the second drain/source region 124 and covers a second edge region of the STI region. The shared-drain LDMOS transistors 600 shown in FIG. 6 may be used in medium voltage applications such as 24 V applications.

[0060] FIG. 7 illustrates a simplified cross-sectional view of another load switch formed by shared-drain LDMOS transistors in accordance with various embodiments of the present disclosure. The shared-drain LDMOS transistors 700 shown in FIG. 7 are similar to the shared-drain LDMOS transistors 200 shown in FIG. 2 except that the high voltage oxide region is replaced by a local oxidation of silicon (LOCOS) structure. The LOCOS structure has an upper portion over the drift layer 106, and a lower portion extending into the drift layer 106. As shown in FIG. 7, the first gate 134 is formed along an edge of the first drain/source region 114. The first gate 134 covers a first sidewall and a first edge portion of the LOCOS structure. The second gate 136 is formed along an edge of the second drain/source region 124. The second gate 136 covers a second sidewall and a second edge portion of the LOCOS structure.

[0061] The LDMOS device with the LOCOS structure for improving the breakdown voltage is well known in the art. As such, the detailed operating principle of this LDMOS device is not discussed again herein. The shared-drain LDMOS transistors 700 shown in FIG. 7 may be used in medium voltage applications such as 24 V applications.

[0062] FIG. 8 illustrates a flow chart of a method for forming the shared-drain transistors shown in FIG. 1 in accordance with various embodiments of the present disclosure. This flowchart shown in FIG. 8 is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps illustrated in FIG. 8 may be added, removed, replaced, rearranged and repeated.

[0063] Referring back to FIG. 1, the shared-drain transistors comprise two back-to-back connected N-type transistors. A first transistor comprises a first drain, a first gate and a first source. A second transistor comprises a second drain, a second gate and a second source. The drains of these two N-type transistors are directly connected to each other as shown in FIG. 1. The back-to-back connected N-type transistors shown in FIG. 1 can be used as a load switch. The load switch is able to achieve bidirectional current blocking. The shared-drain transistors may be fabricated through the following steps.

[0064] At step 802, an epitaxial layer with a first conductivity type is grown over a substrate with the first conductivity type. In some embodiments, the first conductivity type is p-type. In other words, a P-type epitaxial layer is grown on a P-type substrate.

[0065] At step 804, a drift layer having a second conductivity type is formed over the epitaxial layer. In some embodiments, the second conductivity type is N-type. In other words, an N-type drift layer is formed over the P-type epitaxial layer.

[0066] At step 806, a first body region and a second body region with the first conductivity type are formed in the drift layer.

[0067] At step 808, ions with the second conductivity type are implanted in the first body region to form a first source region, and in the second body region to form a second source region.

[0068] At step 810, a first gate is formed adjacent to the first source region. A second gate is formed adjacent to the second source region. The first source region and the second source region are on opposing sides of the first gate and the second gate.

[0069] Referring back to FIG. 2, the method further comprises forming a high voltage oxide region over the drift layer, forming the gate of the first transistor along an edge of the source of the first transistor, wherein the gate of the first transistor covers a first sidewall and a first edge portion of the high voltage oxide region, and forming the gate of the second transistor along an edge of the source of the second transistor, wherein the gate of the second transistor covers a second sidewall and a second edge portion of the high voltage oxide region.

[0070] Referring back to FIG. 4, the method further comprises forming a gate dielectric layer from an edge of the first source to an edge of the second source, forming the gate of the first transistor covering a first edge portion of the gate dielectric layer, and forming the gate of the second transistor covering a second edge portion of the gate dielectric layer.

[0071] Referring back to FIG. 6, the method further comprises forming a shallow trench isolation (STI) region in the drift layer, forming the gate of the first transistor extending from an edge of the first source region and covering a first edge region of the STI region, and forming the gate of the second transistor extending from an edge of the second source region and covering a second edge region of the STI region.

[0072] Referring back to FIG. 7, the method further comprises forming a local oxidation of silicon (LOCOS) structure having a lower portion in the drift layer and an upper portion over the drift layer, forming the gate of the first transistor along an edge of the source of the first transistor, wherein the gate of the first transistor covers a first sidewall and a first edge portion of the LOCOS structure, and forming the gate of the second transistor along an edge of the source of the second transistor, wherein the gate of the second transistor covers a second sidewall and a second edge portion of the LOCOS structure.

[0073] Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

[0074] Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.