Semiconductor device equipped with a high-voltage misfet

Abstract: To realise a semiconductor device equipped with a high-voltage MISFET capable of forming a push-pull circuit on one chip by optimising a junction-separation structure. In the n-channel MOSFET (1), when a potential is applied to the gate electrode (15), to the source electrode (9), and across the drain electrode (5) and the semiconductor substrate (1a) to expand the depletion layer from the junction face of the semiconductor substrate (1a) and the well (3), the leading edge of this depletion layer does not reach the low-concentration drain diffusion region (4). When a potential is applied to the drain electrode (5), to the semiconductor substrate (1a), and across the source electrode (9) and the gate electrode (15) to expand the depletion layer from the junction face of the low-concentration drain diffusion region (4) and the well (3), and from the junction face of semiconductor substrate (1a) and the well (3), these depletion layers are connected with each other.

Description: The present invention relates to a semiconductor device equipped with a high-voltage MISFET, and more particularly to its voltage withstand structure. In NEC Research & Dev. No. 94 of July 1989, pages 29 to 35, there is described a series of high-voltage drive ICs wherein a 'self isolation' feature is used. This necessitates the formation of a deep and heavily doped N-well to support a high-voltage PMOS device, and uses a shallower lightly doped N-well to support low-voltage PMOS or CMOS devices. The formation of the deep and heavily doped N-well is time-consuming and leads to high production cost. 'Physics and Technology of Semiconductor Devices' by A.S. Grove (1967 John Wiley) pages 161 to 163 considers the electrostatic potential at a biased p-n junction, and predicts the width of depletion layers as a function of reverse bias voltage. 'Physics of Semiconductor Devices' by S.M. Sze (1969 John Wiley) considers the prediction of punch-through voltage as a function of doping concentration and layer width in a p-n-p-n device. 'MOS Field-Effect Transistors and Integrated Circuits' by P. Richman (1973 John Wiley) the drain voltage required to produce punch-through in an n-channel MOSFET is considered, punch-through occurring when drain and well depletion layers touch and current flows as a result. EP-A-0 319 047 discloses an integrated circuit structure utilising self-isolation techniques, and incorporating a high-voltage PMOS device. EP-A-0 387 999 discloses a process for forming transistors involving a high voltage LDD PMOS transistor with a gate layer on the surface of a gate oxide film in the region of a low concentration drain. Semiconductor devices equipped with a high-voltage MISFET for use in the area of power electronics require a voltage withstand performance of about 100V or higher, and various voltage withstand structures have been discussed. Among these MISFETs with a voltage withstand structure, a representative one is shown in Figure 7. In this Figure, the numeral 41 represents a p-channel type MOSFET, the surface of an n - -type semiconductor substrate 42 with a p - -type low concentration drain diffusion region 43a, a drain region 43 equipped with a p + -type high concentration drain diffusion region 43b as an ohmic contact, a p + -type source diffusion region 44, and a gate layer 46 formed on a gate oxide film 45. A field oxide film 47 on the surface of the low concentration drain diffusion region 43a has a thickness greater than the gate oxide film 45, and the boundary of the field oxide film 47 with the gate oxide film 45 forms a step. The gate layer 46 extends to directly above the low concentration drain diffusion region 43a beyond this step. Therefore, this extended part functions as a field plate, thereby raising the voltage between the source and the drain. At the same time, the source diffusion region 44 has an n + -type contact region 48 formed on its side, a source electrode 49 being electroconductively connected also to the n - -type semiconductor substrate 42. These arrangements prevent the occurrence of a latch-up phenomenon. In addition, a drain electrode 50 is electroconductively connected to the high concentration drain diffusion region 43b, and a gate electrode 51 is electroconductively connected to the gate layer 46. If the p-channel MOSFET 41 which has this structure has the conduction type of each part reversed, an n-channel MOSFET 61, as shown in Figure 8 will result. In this Figure, the structure of the n-channel MOSFET 61 is the same as that for the p-channel MOSFET 41, whereas each corresponding part is given the same numeral, and its explanation is omitted. As described above, the MOSFETs shown in Figures 7 and 8 are so structured so that they can resist the withstand voltage between the source and drain and prevent a latch-up phenomenon. Therefore, a semiconductor device independently constructed using these MOSFETs will exhibit a stable operation. However, because those MOSFETs form MOS sections directly on the semiconductor substrate, the semiconductor device has a problem in that it cannot meet the following requirement. That is, the circuitries have become more complex in recent years as the level of control has been elevated in the area of power electronics. As a result, a great burden has been imposed on system designs. Therefore, there have been attempts to reduce this burden by integrating MOSFETs with different conduction types and control circuits into one chip. However, as long as the MOS section with a conduction type corresponding to the substrate conduction type is formed, it is not possible to form the p-channel MOSFET 41 shown in Figure 7 and the n-channel MOSFET 61 shown in Figure 8 on one substrate, so that the above requirement cannot be met, and a smart device that can construct a push-pull output and a bridge output in one chip cannot be realised. Accordingly, one concept that eliminates the above problem calls for the adoption of a CMOS structure as in a low voltage MOSFET. However, in the high voltage MOSFET, it is necessary to have a structure that can handle a withstand voltage at a level not influenced by the operating conditions in the semiconductor region formed on one substrate, as well as a structure that can withstand voltages in the lateral direction. Therefore, an insulation separating structure using embedded layers utilising an epitaxial film-forming process has been considered although such a structure is not practical because of its excessive manufacturing costs. In view of the above problems, the present invention is intended to provide a semiconductor device equipped with both a high-voltage n-channel MISFET and high-voltage p-channel MISFET capable of creating a push-pull circuit in one chip by optimising the separation structure in addition to adopting a junction separation structure as in the low-voltage MOSFET. According to one aspect of the present invention there is provided a semiconductor device equipped with a high-voltage MISFET, comprising a semiconductor substrate of a first conduction type, having a well of a second conduction type formed in one of its surfaces and a MIS section of the first conduction type formed on the surface of the well, the MIS section including: a source region of the first conduction type; a source electrode electroconductively connected to the source region; a drain region of the first conduction type located opposite the source region, the drain region having a drain electrode connected thereto; the drain region comprising a low concentration drain region and a high concentration drain region, the drain electrode being connected to the high concentration drain region; a gate oxide film on the surface of the well and the drain region; a field oxide film adjacent the gate oxide film; and a gate layer on the surface of the gate oxide film and on the surface of the field oxide film in the region of the low concentration drain region; wherein a portion of the low concentration drain region adjacent the gate oxide film and the field oxide film is formed with an electric field strength relaxing region of the second conduction type. The MIS section may further include a contact region of the second conduction type formed on the surface of the well, the source electrode being electroconductively connected to the contact region. In an embodiment of the invention the semiconductor substrate has an impurity concentration of the first conduction type of about 3 x 10 14 /cm 3 or less, the well has an impurity dosage of the second conduction type from 1 x 10 13 to 3 x 10 13 /cm 2 , and the low concentration drain region has an impurity dosage of the first conduction type of 5 x 10 12 /cm 2 or less. In an embodiment of the invention the well has a diffusion depth in the range from 2 µm to 10 µm and the low concentration drain region has a diffusion depth in the range from 0.5 µm to 5 µm, the depth of the drain region being less than the depth of the well. The present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a cross-section of a semiconductor device equipped with a high-voltage MISFET for understanding the present invention; Figure 2 is a cross-section of a semiconductor device equipped with a high-voltage MISFET as in Figure 1, with a depletion layer expanded from the junction face of the well and the semiconductor substrate; Figure 3 is- a cross-section of a semiconductor device equipped with a high-voltage MISFET as in Figure 1, with a depletion layer expanded from the junction face of the well and the semiconductor substrate and from the junction face of the drain region and the well; Figure 4 is a graph showing the potential distribution directly below the drain region under the conditions shown in Figure 3; Figure 5 is a cross-section of a semiconductor device equipped with a high-voltage MISFET as in Figure 1, disposed with an electric field intensity relaxation layer; Figure 6 is a conceptual drawing of a common CMOS semiconductor device; Figure 7 is a cross-section of a semiconductor device formed with only a high-voltage p-channel MOSFET; and Figure 8 is a cross-section of a semiconductor device formed with only a high-voltage n-channel MOSFET. The effects of the present invention are explained using, as a representative example, a case in which said MIS section and a MIS section with a different conduction type, constructed on the surface of the same substrate, constitute a CMOS structure, with reference to Figure 6, a conceptional drawing of the construction. In Figure 6, the surface of the n-type semiconductor substrate 31 is formed with a p-well 32 and a p-channel MISFET 33. The surface of the p-well 32 is formed with an n-channel MISFET 34. The drain region 34a of the n-channel MISFET 34 and the drain region 33a of the p-channel MISFET 33 are electroconductively connected, the source region 34b of the n-channel MISFET 34 is grounded, and the source region 33b of the p-channel MISFET 33 is applied with a positive potential V DD . In such a semiconductor device, according to the present invention, when a potential, the potential V DD for example, is applied between the semiconductor substrate (31) and the gate layer 34c on the n-channel MISFET 34 (MIS section), the source region 34b (a source region of the first conduction type) and the drain region 34a (a drain region of the first conduction type) to expand the first depletion layer from the joint face 35 of the semiconductor substrate 31 with a p-well (a well), the leading edge of the first depletion layer is positioned between the joint face 35 and the drain region 34a, whereas the depletion layer does not reach the drain region 34a, and the drain region 34a, the p-well 32 and the semiconductor substrate 31 have n-p-n structure transistor bases that remain grounded. Therefore, the voltage withstand performance is so high that no breakdown can occur and the longitudinal voltage withstand can be assured even if the potential V DD , for example, remains applied to the drain region 34a and across the source region 34b and the semiconductor substrate 31, while the n-channel MISFET 34 is on and the p-channel MISFET 33 is off. Furthermore, when the potential V DD , which is positive with respect to the source region 34b and the gate layer 34c, is applied to the drain region 34a and the semiconductor substrate 31 to expand the second depletion layer from the joint face 35 of the drain region 34a with the p-well 32, and expand the third depletion layer from the joint face of the semiconductor substrate 31 with the well 32, if these depletion layers are arranged so that they are connected with each other, the p-well 32 directly below the drain region 34a, that is the p-well 32 between the drain region 34a and the semiconductor substrate 31, will be completely depleted even if the n-channel MISFET 34 has been turned off and the p-channel MISFET 33 has been turned on. Therefore, the p-well 32 in this region has no zero volt potential region, while only the perimeter of the source region 34b has a zero volt potential. Therefore, the electric field intensity is kept sufficiently low, so that no breakdowns can occur in the source region 34b or between the drain region 34a and the semiconductor substrate 31, and the voltage withstand performance can be assured. Thus, the voltage withstand performance is assured at the semiconductor substrate and the n-channel MISFET 34, and the p-channel MISFET 33 or a control can be formed in the other regions of-the semiconductor substrate. The semiconductor device equipped with high-voltage MOSFET is explained using the Figures. Figure 1 illustrates a cross section of the n-channel MOSFET. In this Figure, the numeral 1 represents the n-channel MOSFET, with an n - -type semiconductor substrate la surface formed with field oxide films 2 and p - -wells 3 extending to the other regions. The semiconductor substrate used has an impurity concentration of not more than 3 x 10 14 /cm 3 , and the p - -well 3 has a dosage that is set to 1 x 10 13 to 3 x 10 13 /cm 2 , and its diffusion depth is set within a range between 2 µm and 10 µm. The surface of this p - -well 3 has a drain region 7 disposed with a low concentration drain diffusion region 4 of the n-type at a relatively low concentration with a dosage of not more than 5 x 10 12 /cm 2 , for example, and a high concentration drain diffusion region 6 of n + -type at a relatively high concentration as an ohmic contact, as well as the n + -type source region 8 opposite the low concentration drain diffusion region 4 via the surface of the p - -well 3. The diffusion depth of the low concentration diffusion region 4 is set within a range between about 0.5 µm and about 5 µm. The source electrode 9 electroconductively connected to the source diffusion region 8 is also electroconductively connected to the p - -well 3 via the p + -type contact region 10 formed on the surface of the p - -well 3, which has a structure that prevents the occurrence of the latch-up phenomenon. The surface of the p - -well 3 between the source diffusion region 8 and the low concentration drain diffusion region 4 forms the n-channel formed region 11, which has a surface with the gate layer 13 disposed on the opposite side via the thin gate oxide film 12. The gate oxide film 12 is formed next to the field oxide film 14 that is thicker than the gate oxide film 12. The gate layer 13 extends over the surface of this field oxide film 14 to directly above the low concentration drain diffusion region 4 and functions as a field plate raising the withstand voltage for the source diffusion region 8 and the drain region 7. This gate layer 13 is electroconductively connected with the gate electrode 15. The numeral 16 shows an inter-layer insulation film. The semiconductor substrate la of this configuration has its other regions formed with a low voltage CMOS section (not shown) which constitutes a control circuit, as well as a p-channel MOSFET with the same construction as the p-channel MOSFET 41 shown in Figure 7, constituting the n-channel MOSFET 1 and the CMOS construction (hereinafter referred to as the p-channel MOSFET 41). The withstand voltage in the longitudinal direction for the n-channel MOSFET 1 is assured to be high under the following conditions. First, as shown in Figure 2, when the gate electrode 15, the source electrode 9, and the drain electrode 5 in the n-channel MOSFET 1 are all grounded, and a potential is applied to the semiconductor substrate la, the positive potential V DD corresponding to the power source voltage, for example, to expand the first depletion layer 18 (shown by the diagonal hatching in Figure 2) from the p-n junction face 17 of the semiconductor substrate la with the p - -well 3, the leading edge 18a of the first depletion layer 18 is positioned between the p-n junction face 17 and the low concentration drain diffusion region 4, and the depletion layer 18 does not reach the p-n junction face 19 of the low concentration drain diffusion region 4 with the p - -well 3. The spacing between the junction face 17 and the junction face 19 is so arranged that the leading edge 18a of the first depletion layer lies between the junction faces 17 and 19, and preferably lies in the range from 0.5 µ m to 9.5 µ m. Therefore, even under these conditions, the breakdown voltage at the low concentration drain diffusion region 4 and between the source diffusion region 8 and the semiconductor substrate la, that is, the withstand voltage in the longitudinal direction of the n-channel MOSFET 1, can be maintained at a high level because the low concentration drain diffusion region 4, the p - -well 3, and the semiconductor substrate la have their n-p-n transistor bases grounded. Therefore, even when the n-channel MOSFET 1 and the p-channel MOSFET 41 of Figure 7 are formed on the same semiconductor substrate la to form a CMOS construction, no breakdown voltage is generated among them, and the withstand voltage at the semiconductor substrate is maintained at a high level when the n-channel MOSFET 1 is turned on and the p-channel MOSFET 41 is turned off even if the potential V DD , for example, is applied directly to the semiconductor substrate la against the low concentration drain diffusion region 4 and the source diffusion region 8. Furthermore, as shown in Figure 3, when the source electrode 9 and the gate electrode 15 are grounded, and the drain electrode 5 and the semiconductor substrate la have a positive potential V DD (corresponding to the drive voltage) applied to them for example, to expand the second depletion layer from the p-n junction face 35 of the low concentration diffusion region 4 with the p - -well 3, and to expand the third depletion layer from the p-n junction face 17 of the semiconductor substrate la with the p - -well 3, the second depletion layer and the third depletion layer are connected with each other, and set in a way that they form an integrated depletion layer 20, with the depletion layer 20 spreading thereon except on the source diffusion region 8 and the perimeter 20a of the contact region 10. Therefore, the potential distribution directly below the drain region 7 under these conditions takes the form as shown in Figure 4. In Figure 4, the axis of the abscissa represents the distance from the surface (indicated by the conduction type of each region), while the axis of the ordinate shows the potential. The curve A indicates the potential distribution of the n-channel MOSFET 1 with this embodiment, and the curve B shows the potential distribution in the comparative example, where the second depletion layer expanded from the p-n junction face 19 of the low concentration drain diffusion region 4 with the p - -well 3 does not connect with the second depletion layer expanded from the p-n junction face 17 of the semiconductor substrate la with the p - -well 3. In other words, the comparative example refers to a case there the p - -region (p - -well 3) remains directly below the low concentration drain diffusion region 4. In the Figure, the high concentration drain diffusion region 6 and the semiconductor substrate la have the potential V DS corresponding to the source to drain voltage (V DD in this embodiment). Because in the comparative example a region that has not been depleted exists in the p - -well 3 directly below the drain region 7 as shown by the curve B in the Figure, the potential in that region decreases to zero volts, raising in turn the potential intensity (corresponding to the gradient of the curve). As opposed to this embodiment, because the p - -well 3 is depleted completely as shown by the curve A, the potential there does not decrease to zero volts, thereby making the potential intensity low. For this reason, because the potential intensity is low in this embodiment, the breakdown voltage at the source region 8 and across the drain region 7 against the semiconductor substrate la is high, thereby assuring the withstand voltage of this semiconductor device 1. Therefore, in the case where the n-channel MOSFET 1 and the p-channel MOSFET 41 are formed on the same semiconductor substrate la resulting in a CMOS construction, even if the voltage V DD , for example, is applied to the source diffusion region 8 and the drain region 7 against the semiconductor substrate la, when the n-channel MOSFET 1 is off and the p-channel MOSFET 41 is on, the depletion layer 20 expands to maintain a high breakdown voltage and the withstand voltage at the semiconductor device 1 high. Next, the method used to manufacture this semiconductor device is explained hereunder, applying the assumption that the p-channel MOSFET 41 is formed simultaneously on the surface of the semiconductor substrate la, in addition to the n-channel MOSFET 1. First, the semiconductor substrate la with an n - -type impurity at a concentration of 3 x 10 13 to 3 x 10 14 /cm 3 and a thickness of about 500 µ m is subjected to steam oxidation in an atmosphere at a temperature of about 1100°C for about four hours. This forms a thermal oxide film of about 1 µm on the surface of the semiconductor substrate la. Then, this is etched by photolithography and buffered hydrofluoric acid is used to open windows on the predetermined regions. The thermal oxide film that remains on the surface as a result of this process is the field oxide film 2. (The first process step). Next, an accelerated voltage injects ions from above the resist mask formed by photolithography under approximately 100 keV, and injects boron ions at a dosage of 1 x 10 13 to 3 x 10 13 /cm 2 into the semiconductor substrate la. After this injection the resist mask is removed, and the semiconductor substrate la is heat-treated in an atmosphere at a temperature between 1100 and 1150°C for two to ten hours to diffuse the ions and form a low-concentration drain diffusion layer 43a on the p-channel MOSFET 41 and the p - -well 3 on the n-channel MOSFET 1. The depth of the diffusion is set within a range of 2 to 10 µm. (The second process step). Then, the semiconductor substrate la is subjected to steam oxidation in an atmosphere at a temperature at about 1100°C for about 2.5 hours to form a thermal oxide film measuring about 0.8 µm, followed by etching by photolithography and with buffered hydrofluoric acid to form a resist mask which has windows opened around the area selected for the formation of the low-concentration drain diffusion region 4. Under these conditions, an accelerated voltage injects ions from above the resist mask under approximately 100 keV, and injects phosphorus ions at a dosage of 1 x 10 12 to 5 x 10 12 /cm 2 into the semiconductor substrate. After this injection, the resist mask is removed, and the semiconductor substrate la is heat-treated in an atmosphere at a temperature of 1100°C for two to ten hours to form a low-concentration drain diffusion layer 4 on the n-channel MOSFET 1. The depth of the diffusion is set to within a range of 0.5 to 5.0 µm. (The third process step). Next, the semiconductor substrate la is subjected to dry oxidation in an atmosphere at a temperature of about 1100°C for about one hour to form a thermal oxide film measuring about 0.5 µm, then, this is etched by photolithography and using buffered hydrofluoric acid to form field oxide films 14, 47. (The fourth process step). Next, the semiconductor substrate la is subjected to dry oxidation in an atmosphere at a temperature of about 1000°C for about one hour to form the gate oxide films 12, 45 with a thickness of about 200 Å. (The fifth process step). Next, polycrystalline silicon is deposited on the semiconductor substrate via the CVD process, then injected with ions under approximately 50 keV, and phosphorus ions are injected at a dosage of about 5 x 10 15 /cm 2 . After this injection, the substrate is activated by being heat-treated in an atmosphere at a temperature of about 1000°C for about 30 minutes. Thereafter, the gate layers 13, 46 are formed through photolithography and dry etching, with poly-crystalline silicon left on only the predetermined areas. (The sixth process step). Subsequently, after the resist mask is formed, windows are opened and an accelerated voltage selectively injects boron ions at a dosage of about 5 x 10 15 /cm 2 under approximately 50 keV, and the resist mask is removed. (The seventh process step). Furthermore, after the resist mask is formed, windows are opened and an accelerated voltage selectively injects arsenic ions at a dosage of about 5 x 10 15 /cm 2 , and the resist mask is removed. Thereafter, the substrate is activated by being heat-treated in an atmosphere at a temperature of about 1000°C for about 30 minutes. (The eighth process step). As a result, the contact region 10, the source diffusion region 8, and the high-concentration drain diffusion region 6 on the n-channel MOSFET 1 side, and the contact region 48, the source diffusion region 44, and the high-concentration drain diffusion region 43b on the p-channel MOSFET 41 side are all formed sequentially. Next, a BPSG (boron phosphorous glass) film is deposited, and is then heat-treated in an atmosphere at a temperature of about 1000°C for about 30 minutes. Subsequently, windows are opened in predetermined regions to leave the inter-layer insulation film 16, through photolithography and dry etching. (The ninth process step). Thereafter, an aluminium-silicon film of about 1 µ m is deposited, and the electrodes 9, 15, 5, 49, 51, 50 and the wiring layer are left in predetermined regions, through photolithography and dry etching. (The tenth process step). The above-mentioned process steps simultaneously form the n-channel MOSFET 1, and the p-channel MOSFET 41 which constitute this MOSFET 1 and CMOS on the surface of the semiconductor substrate la. Since expensive processes such as the epitaxial film formation process are not applied, this manufacturing method can be practically applied while costs are kept low. The n-channel MOSFET 21 shown in Figure 5 according to the invention may be formed during fourth and fifth process steps, by forming a resist mask which leaves windows opened in predetermined regions on the surface of the low-concentration drain diffusion region 14 on the n-channel MOSFET 1 using photolithography and etching, removing the resist mask after an accelerated voltage has injected boron ions at a dosage of 1 x 10 13 /cm 2 from above this resist mask at about 30 keV, and activating the semiconductor substrate la by heat-treating it in an atmosphere at a temperature at about 1000°C for about 30 minutes. Since this n-channel MOSFET 21 has the same configuration as the n-channel MOSFET 1 shown in Figure 1, the corresponding parts are shown with the same numerals, and their explanation is omitted, except that the surface of the low concentration drain diffusion region 4 corresponding to the stepping 22 between the thin gate oxide film 12 and the field oxide film 14 thicker than the gate oxide film has a p-type electric field relaxation layer 23. Therefore, a high electric field intensity is generated in the low-concentration drain diffusion region 4 by the gate layer 13 at the stepping 23 between the gate oxide film 12 and the field oxide film 14, that is, the edge of a region functioning substantially as a gate layer. This results in the maintenance of a high withstand voltage across the source diffusion region 8 and the drain region 7. As described, because the optimised junction and separation structure in this embodiment realises the maintenance of a high withstand voltage at the perimeter of the MOS section even with a well construction, high voltage MOSFETs with different conduction types can be formed on one substrate. Therefore, a smart device will result, in which one chip will be capable of both the push-pull output and the bridge output. The MOSFET with a CMOS construction shown in this embodiment is an example of adopting the high voltage MOSFET construction in this embodiment, with no limitations in its circuitry. As described above, the device of the present invention is constructed so that, even when a potential is applied between the semiconductor substrate and the gate layer, the source region of the first conduction type and the drain region of the first conduction type in the MIS section of the first conduction type formed on the surface of the well of a second conduction type to expand a first depletion layer from a junction face of the semiconductor substrate with the well, the leading edge of the depletion layer is positioned between the junction face and the drain region. In other words, the leading edge does not reach the junction face of the drain region with the well. Therefore, the invention has the following effects: (1) Because the breakdown voltage at the semiconductor substrate and between the source region and the drain region is kept high, the withstand voltage is assured even if a MIS section of the first conduction type or a control part is formed on one substrate, regardless of their operating conditions. Therefore, these high voltage MISFETs can realise a semiconductor device capable of both a push-pull output and bridge output from one chip. (2) When a potential is applied to a drain region and a semiconductor substrate against a source region and a gate layer so that the second depletion layer from the junction face of the drain region and the well are connected together with the third depletion layer from the junction face of the semiconductor substrate and the well, the well directly below the drain region is depleted, which means that no region with high electric field intensity is generated. Therefore, the withstand voltage between the source region and the drain region is kept high, thereby enhancing the reliability of the above-mentioned semiconductor device. (3) When the drain region is equipped with a low-concentration drain region of the first conduction type located opposite the source region, and a high-concentration drain region of the first conduction type, and the gate layer is extended to the surface of a field oxide film thicker than the gate oxide film, and the source electrode electroconductively connected to the source region is also electroconductively connected to the high concentration contact region of the second conduction type formed on the surface of the well, and when the electric field intensity relaxation layer of the second conduction type is disposed on the surface of the low concentration drain region corresponding to the boundary between the gate oxide film and the field oxide film, the reliability of the semiconductor device can be improved in terms of its construction.

Claims: A semiconductor device equipped with a high-voltage MISFET, comprising a semiconductor substrate (la) of a first conduction type, having a well (3) of a second conduction type formed in one of its surfaces and a MIS section of the first conduction type formed on the surface of the well, the MIS section including: a source region (8) of the first conduction type; a source electrode (9) electroconductively connected to the source region (8); a drain region (4, 6) of the first conduction type located opposite the source region (8), the drain region having a drain electrode (5) connected thereto; the drain region comprising a low concentration drain region (4) and a high concentration drain region (6), the drain electrode (5) being connected to the high concentration drain region (6); a gate oxide film (12) on the surface of the well (3) and the drain region (4, 6); a field oxide film (14) adjacent the gate oxide film (12); and a gate layer (13) on the surface of the gate oxide film and on the surface of the field oxide film (14) in the region of the low concentration drain region (4); characterised in that a portion of the low concentration drain region (4) adjacent the gate oxide film (12) and the field oxide film (14) is formed with an electric field strength relaxing region (23) of the second conduction type. A semiconductor device as claimed in claim 1, characterised in that the MIS section further includes a contact region (10) of the second conduction type formed on the surface of the well (3), the source electrode (9) being electroconductively connected to the contact region (10). A semiconductor device as claimed in claim 1 or 2, characterised in that the semiconductor substrate (la) has an impurity concentration of the first conduction type of about 3 x 10 14 /cm 3 or less, the well (3) has an impurity dosage of the second conduction type from 1 x 10 13 to 3 x 10 13 /cm 2 , and the low concentration drain region (4) has an impurity dosage of the first conduction type of 5 x 10 12 /cm 2 or less. A semiconductor device as claimed in any preceding claim, characterised in that the well (3) has a diffusion depth in the range from 2 µm to 10 µm and in that the low concentration drain region (4) has a diffusion depth in the range from 0.5 µm to 5 µm, the depth of the drain region (4) being less than the depth of the well.

Inventors: FUJIHIRA TATSUHIKO, FUJIHIRA, TATSUHIKO
Applicants: FUJI ELECTRIC CO LTD, FUJI ELECTRIC CO. LTD.
Application year: 1992
Main classification: H01L27
IPC codes: H01L21, H01L29, H01L27