SEMICONDUCTOR DEVICE INCLUDING AN ESD PROTECTION ELEMENT

Abstract

A semiconductor device includes an off transistor using an NMOS as an ESD protection element that has N-type drain region (102), and P-type drain region (103) in a drain active region (105) . The P-type drain region (103) has a potential same as a potential of a P well or a P-type semiconductor substrate (106), and the ESD protection element has a withstand voltage that is a junction withstand voltage of a PN junction in the drain active region (105) .

Claims

1. A semiconductor device including an ESD protection element, the semiconductor device comprising an element in an internal circuit region and having an operating voltage, the ESD protection element comprising an N-type MOS transistor provided on one of a P well and a P-type semiconductor substrate, the N-type MOS transistor including a gate electrode connected to one of the P well and the P-type semiconductor substrate such that the gate electrode has one of a well potential that is a potential of the P well and a ground potential that is a potential of the P-type semiconductor substrate, the N-type MOS transistor having a drain active region in which an N-type high-concentration drain region and a P-type drain region are formed so as to be adjacent to each other, to form a PN junction, the P-type drain region having a potential that comprises one of the potential of the P well and the potential of the P-type semiconductor substrate, the ESD protection element having a withstand voltage that comprises a junction withstand voltage of the PN junction in the drain active region.

2. A semiconductor device including an ESD protection element according to claim 1, wherein the junction withstand voltage of the PN junction is equal to or higher than the operating voltage and lower than withstand voltages of all elements in the internal circuit region.

3. A semiconductor device including an ESD protection element according to claim 1, further comprising a P-type MOS transistor, and wherein the P-type drain region has an impurity concentration that is equal to a concentration of a drain extension region of the P-type MOS transistor.

4. A semiconductor device including an ESD protection element according to claim 1, further comprising a P-type MOS transistor, and wherein the P-type drain region has an impurity concentration that is equal to a concentration of a high-concentration region of a drain region of the P-type MOS transistor.

5. A semiconductor device including an ESD protection element according to claim 1, wherein the P-type drain region is provided so as to be adjacent to an end portion of the drain active region in a W direction.

6. A semiconductor device including an ESD protection element according to claim 1, wherein the P-type drain region is provided in a region away from an end portion of the drain active region in a W direction.

7. A semiconductor device including an ESD protection element according to claim 1, wherein the P-type drain region is provided so as to be adjacent to an end portion of the drain active region in a W direction, and another P-type drain region is also provided in a region away from the end portion in the W direction.

8. A semiconductor device including an ESD protection element according to claim 1, wherein the P-type drain region is provided so as to be adjacent to each of two end portions of the drain active region in a W direction, along a rim of the gate electrode.

9. A semiconductor device including an ESD protection element according to claim 1, wherein the N-type high-concentration drain region is entirely surrounded by the P-type drain region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a schematic diagram for illustrating an ESD protection element according to a first embodiment of the present invention.

[0024] FIG. 2 is a schematic diagram for illustrating an ESD protection element according to a second embodiment of the present invention.

[0025] FIG. 3 is a schematic diagram for illustrating an ESD protection element according to a third embodiment of the present invention.

[0026] FIG. 4 is a schematic diagram for illustrating an ESD protection element according to a fourth embodiment of the present invention.

[0027] FIG. 5 is a schematic diagram for illustrating an ESD protection element according to a fifth embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0028] Now, embodiments of the present invention are described with reference to the drawings.

First Embodiment

[0029] FIG. 1 is a diagram for illustrating an ESD protection element according to a first embodiment of the present invention.

[0030] The ESD protection element using an NMOS is illustrated. The ESD protection element has an N-type high-concentration source region 101, an N-type high-concentration drain region 102 connected to an external output terminal, a gate electrode 104, and P-type drain regions 103 adjacent to the respective sides of the N-type high-concentration drain region 102. The N-type high-concentration source region 101 and the N-type high-concentration drain region 102 generally have an impurity concentration of about 10.sup.22 cm.sup.−3. Further, the above-mentioned NMOS is formed in a P well having an impurity concentration of from about 10.sup.15 cm.sup.−3 to about 10.sup.17 cm.sup.−3, or in a P-type semiconductor substrate 106, and has a gate insulating film having a thickness of from about 5 nm to about 30 nm. Here, the gate electrode 104 has the same potential as the P well or the P-type semiconductor substrate 106. Thus, the ESD protection element is a so-called off transistor. Further, the P-type drain regions 103 are also in contact with the P well or the P-type semiconductor substrate 106, and have the same potential as the P well or the P-type semiconductor substrate 106.

[0031] Here, first, how the ESD protection element operates when an ESD surge of positive polarity enters a semiconductor device from the outside thereof is described.

[0032] When the positive surge enters the semiconductor device from an external connection terminal, the potential of the N-type high-concentration drain region 102 of the ESD protection element increases. Meanwhile, since the P-type drain regions 103 are connected to the P well or the P-type semiconductor substrate 106 and have the same potential as the P well or the P-type semiconductor substrate 106, avalanche breakdown due to a PN junction thus occurs in the drain region to generate electron and hole pairs. The holes generated here flow to the source side at low potential through the P well or the P-type semiconductor substrate 106. At that time, voltage rise occurs due to the current that accordingly flows and the resistance of the P well or the P-type semiconductor substrate 106. When the voltage rise exceeds the potential of the N-type high-concentration source region 101 by a certain amount, electrons are injected from the N-type high-concentration source region 101 to the P well or the P-type semiconductor substrate 106 in the forward direction of the diode, and the electrons reach the N-type high-concentration drain region 102. In this way, current flows between the drain and the source of the NMOS. This operation is so-called parasitic bipolar operation. The semiconductor device can be protected by the ESD protection element through the parasitic bipolar operation in which an ESD surge is caused to flow from the drain region connected to the external connection terminal to a terminal on the source side so that the ESD surge does not reach the internal circuit region.

[0033] In order to protect an IC by the above-mentioned operating principle, the impurity concentration of the P-type drain regions 103 is required to be determined such that a junction withstand voltage between the P-type drain regions 103 and the N-type high-concentration drain region 102 takes a desired withstand voltage that is equal to or higher than the operating voltage of the IC and lower than the withstand voltage of an internal element.

[0034] For example, an internal element which forms an IC, whose operating voltage is 8 V or lower, has, in case of an NMOS, a withstand voltage that is determined, in many cases, by the junction withstand voltage between an N-type high-concentration region, and a P-type field region formed through ion implantation to the lower part of a field insulating film for element isolation. On the other hand, in case of a PMOS, an internal element has a withstand voltage that is determined, in many cases, by the junction withstand voltage similar to the above or a junction withstand voltage between a P-type high-concentration region and an N well.

[0035] Here, a drain of a PMOS of an IC whose operating voltage is 8 V or lower has, in many cases, an LDD structure or a drain extension structure including a drain electric field reducing layer. The impurity concentration of a P-type region that is formed in this drain electric field reducing layer through ion implantation is from about 10.sup.17 cm.sup.−3 to about 10.sup.20 cm.sup.−3 in general.

[0036] When the P-type drain region 103 is formed by ion implantation for the drain electric field reducing layer of the PMOS, the withstand voltage of the ESD protection element according to the present invention can be set to a value that is equal to or higher than the operating voltage and lower than the withstand voltage of the internal element without the addition of a mask or a step.

[0037] In the case of an IC whose operating voltage is 5 V or lower, the P-type drain region 103 may be subjected to ion implantation for P-type high-concentration source and drain regions used in the PMOS. Also in this case, desired characteristics can be obtained without the addition of a mask or a step.

[0038] In the first embodiment of the present invention, the P-type drain regions 103 are formed in end portions (W ends) in a W direction, which is the channel width of a drain active region 105, so as to be adjacent to a channel region located under the gate electrode 104, and the N-type high-concentration drain region 102 is formed between the two P-type drain regions 103. Each of the two P-type drain regions 103 extends, along the rim of the drain active region 105 that is parallel to a direction connecting the source region and the drain region, from one rim of the drain active region 105 immediately below the gate electrode 104 to another rim of the drain active region 105 that is opposite to the gate electrode 104. In this case, the P-type drain region 103 is required to have a sufficient P-type region even when an ion implantation mask for the N-type high-concentration drain region 102 is not correctly aligned, or thermal diffusion occurs in the N-type high-concentration drain region 102.

[0039] Meanwhile, when the P-type drain region 103 is too large, the balance between the P-type drain region 103 and the N-type high-concentration source region 101 may be lost, or the effective width of the N-type high-concentration drain region 102 may greatly reduce to lower ESD tolerance. Thus the P-type drain region 103 has a width of from 1 μm to 3 μm.

[0040] The P-type regions 103 are formed in both of the W ends in the above description, but the P-type drain region 103 may be formed only in one of the W ends.

Second Embodiment

[0041] FIG. 2 is a diagram for illustrating an ESD protection element according to a second embodiment of the present invention. The P-type region 103 may be formed in the drain active region 105. In the case of an ESD protection element having a small unit of W, when the two P-type drain regions 103 are provided, the effective N-type high-concentration drain region 102 reduces to lower ESD tolerance. To cope with this, as illustrated in FIG. 2, the P-type drain region 103 is provided between the N-type high-concentration drain regions 102 formed in both of the end portions (both of W ends) in the channel width direction.

Third Embodiment

[0042] FIG. 3 is a diagram for illustrating an ESD protection element according to a third embodiment of the present invention. The P-type regions 103 may be provided in the middle of the drain active region 105 in addition to both of the W ends. In this case, the two N-type high-concentration drain regions 102 are respectively formed on the inner sides of the two P-type drain regions 103 formed in both of the W ends, and the P-type drain region 103 is provided between the two N-type high-concentration drain regions 102. In the case of an ESD protection element having a large channel width W, when only the two P-type drain regions 103 are provided in the W ends, parasitic bipolar operation which starts from the W ends rarely occurs, with the result that desired characteristics may not be obtained. Thus, the arrangement of FIG. 3 may also be employed.

Fourth Embodiment

[0043] FIG. 4 is a diagram for illustrating an ESD protection element according to a fourth embodiment of the present invention. The P-type regions 103 are formed in an L direction of the drain active region 105. That is, the drain active region 105 is divided into three, and in order from the gate electrode 104, the P-type drain region 103, the N-type high-concentration drain region 102, and the P-type drain region 103 are formed parallel to the channel width direction. With this arrangement, the drain active region 105 is increased in size. However, the PN junction in the drain region is in the same direction as the N-type high-concentration source region 101, and the junction surface is large. As a result, stable parasitic bipolar operation can be expected.

FIFTH EMBODIMENT

[0044] FIG. 5 is a diagram for illustrating an ESD protection element according to a fifth embodiment of the present invention. In the drain active region 105, the P-type drain region 103 surrounds the N-type high-concentration drain region 102. With this arrangement, the junction surface of the PN junction in the drain region can be larger than that of the fourth embodiment, and hence parasitic bipolar operation more stable than that of the fourth embodiment can be expected.