A MOS TRANSISTOR

Abstract: A MOS transistor having a high resistance to a surge is shown. The device comprises a low-resistivity layer (32), a high-resistivity layer (33) of same conductivity type as the low-resistivity layer formed on the low-resistivity layer, and a region (35) of a conductivity type opposite that of the layers formed in the upper side of the high-resistivity layer. The value of the impurity concentration and thickness of the part (33a) the high-resistivity layer existing between the region and the low-resistivity layer is chosen so that reach-through breakdown may occur.

Description: TITLE OF THE INVENTION A MOS Transistor BACKGROUND OF THE INVENTION The present invention relates to a MOS transistor having improved resistance to a surge. In recent years, MOS power transistors have appeared. As a result, a MOS transistor 1 has been put into use as a switching element for a power load 2 as shown in Fig. 1. For a vehicle, for example, it has been proposed to apply a MOS transistor to switching of various power loads mounted on vehicles. Fig. 2 shows an example of structure of a conventional MOS transistor. The MOS transistor illustrated in Fig. 2 is a so-called lateral-type n-channel MOS transistor. The MOS transistor has on one main surface side of a p-type semiconductor substrate 4, an n+-type source diffused region 5, an n+-type drain diffused region 6, source and drain electrode 7, 8 made of aluminium respectively, and a gate electrode 9 placed between the source electrode 7 and the drain electrode 8. Under the above described source electrode 7, a p+-type substrate contact region 10 is formed. The source electrode 7 becomes a common electrode which also functions as a substrate electrode. For the MOS power transistor as described above, various contrivances for raising the resistance to the high voltage have been made under the necessity of switching of rather high voltage and high current. In the MOS transistor illustrated in Fig. 2, it is contrived to raise the resistance to the high voltage by forming a guard ring 12 and an n -type drift region 11 which extends from the side of the drain diffusion region 6 toward the gate. If the above described power load 2 is an inductive load having a large inductance such as a motor or a solenoid, however, since the resistance to the high voltage of a conventional MOS transistor is not large enough to withstand a surge such as generated when the load current is interrupted, breakdown might occur resulting in degradation in the characteristics of the device or damage thereto. This will now be described more concretely referring to Fig. 2. Assuming now that a high voltage surge is applied between the drain and the source, a depletion layer 13 is generated along the junction between the drain diffusion region 6 and the substrate 4. At this time, probability of occurrence of punch-through is lowered owing to the above described drift region 11 or guard ring 12. Due to a high voltage surge, breakdown might occur on a part having a comparatively large curvature (hereafter referred to as an edge portion) located on the periphery of the bottom of the drain diffusion region 6, especially on an edge portion 14 located near the gate G. This is because the electric field applied to the depletion layer converges at a portion having a small radius of curvature. Since the breakdown current BI flows through a narrow region in the vicinity of the edge portion 14, the heat convergence might cause damage to the device. Especially the gate G is often subjected to thermal damage because the breakdown current BI flows near the surface of the substrate 4. A similar problem is found in a vertical-type MOS transistor as well. Fig. 3 is a sectional view of an example of a vertical-type MOS transistor. This device basically includes a semiconductor substrate 19 composed of a lower n -type low-resistivity region 17 having a flat upper surface whereto a drain electrode 16 is joined and an upper n-type high-resistivity region 18, a plurality of p-type well regions 20 which have conductivity types opposite that of said substrate 19 and are formed at predetermined interval within said n-type region 18 located on the upper surface side of the substrate 19, an n+-type source region 21 formed within said p-type well region 20, and a gate electrode 23 formed on surfaces of the substrate 19 and the well regions 20 through a gate oxide film 22 which is an insulative film while extending over both the n+-type source region 21 and said n-type region 18 of the substrate 19 which substantially functions as a drain region. The upper surface of the gate electrode 23 except for the leading-out port is coated with a PSG (Phospho-Silicate glass) film 25. And a source electrode 24 is joined to the source region 21 as well as a p -type contact region 26 which is formed within a p-type well region 20. When a voltage is applied between the drain and the source in a reverse direction of the p-n junction, a depletion layer is produced along the junction face between the drain region 18 and the well region 20. In Fig. 2, the region of the depletion layer 27 is represented by dotted lines 27a. This depletion layer 27 is widened as the voltage between the drain and the source is increased. It should be noted for the conventional MOS transistor illustrated in Fig. 3 that avalanche breakdown occurs on the junction between the n-type drain region 18 and the well region 20 before the depletion layer 27 widening into the n-type drain region 18 reaches the n+-type drain region 17 and the current under the avalanche breakdown state is apt to converge to a limited part. When the avalanche breakdown is caused by a surge voltage, the breakdown current flowing between the drain and source converges to a limited part within the device even if the current is rather low. As a result, heat generation caused by current flow converges upon a part, therefor the device is damaged. This phenomenon will now described in detail. When the depletion layer 27 is not largely spread out as represented by the dotted lines 27a in Fig. 3, valleys V corresponding to two adjacent well regions 20 are generated in the depletion layer 27 existing in the n-type drain region 18. As indicated by an arrow E, the electric field applied to the depletion layer 27 is apt to converge upon a part located between the valleys V of the depletion layer 27 and a corner of the well region 20. If the electric field applied to the depletion layer 27 is thus concentrated on a limited part, the current flow under the avalanche state is also concentrated on that part. And the heat generated by the concentrated current damages the device. SUMMARY OF THE INVENTION An object of the present invention is to provide a verticaltype MOS transistor in which the breakdown current caused by a surge or the like does not converge to a limited part within the device but flows through a rather broad part uniformly, whereby the thermal breakdown of the device is not readily incurred, and surge withstanding ability of the device is increased. Briefly described, these and other objects, features and advantages are accomplished by provision of a MOS transistor formed on a semiconductor substrate of a first conductivity type composed of a laminated low-resistivity layer and high-resistivity layer, wherein the thickness value of a part of said high-resistivity layer existing between said low-resistivity layer and a region of a second conductivity formed within said high-resistivity layer is chosen so that reach-through breakdown may occur between said low-resistivity layer and said second conductivity region. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram of a switching circuit using a MOS transistor. Fig. 2 is a sectional view of a structure of a lateral-type MOS transistor according to the prior art. Fig. 3 is a sectional view of a structure of a vertical-type MOS transistor according to the prior art. Fig. 4 is a sectional view of an embodiment of a lateral-type MOS transistor according to the present invention. Fig. 5 is a sectional view of an embodiment of a vertical-type MOS transistor according to the present invention. DETAILED DESCRIPTION OF THE INVENTION An embodiment of a MOS transistor according to the present invention will now be described in detail by referring to Fig. 4. In the embodiment of the MOS transistor illustrated in Fig. 4, onto a p+-type low-resistivity layer 32 electrically connected to a substrate electrode 30, a p-type high-resistivity layer 33 which is lower than the p+-type layer 32 in impurity concentration is formed by means of epitaxial growth to provide a laminated substrate 31. On On the above described p-type layer 33, an n -type source diffused region 34 and an n+-type drain diffused region 35 are formed by means of diffusion. A source electrode 36 and a drain electrode 37 are provided by patterning of aluminium respectively on the source diffused region 34 and the drain diffused region 35. Between the source electrode 36 and the drain electrode 37, a gate electrode 39 is formed so as to be insulated from the p-type layer 33 and the source electrode 36 by an oxide film 38 and phosphorus glass 42. The thickness d and resistivity which depends upon the impurity concertration of a thin portion of the p-type region 33a existing between the drain diffusion region 35 and the p+ -type layer 32 are determined so as to bring about reach-through breakdown. An n -type drift region 40 in Fig. 4 is provided for the purpose of raising the punch-through voltage like the drift region 11 illustrated in Fig. 2. In general, the source electrode 36 is electrically connected to the substrate electrode 30. The source and substrate electrodes are used as a common electrode. The aforementioned reach-through breakdown will now be described. When a voltage VR is applied to a p-n junction of a semiconductor in the reverse direction, it is known that the width W of the spread of the depletion layer from the junction face may be expressed as EMI5.1 where g is the relative dielectric constant of the semiconductor, gO is the dielectric constant of vacuum, q is the electric charge of an electron, and Ni is the impurity concentration of the semiconductor. Assuming that the p+-type layer 32 is absent in the MOS transistor illustrated in Fig. 4 and the breakdown voltage at an edge portion 35a having impurity concentration N A on the periphery of the bottom of the drain diffusion region 35 is BV, it is apparent from the expression (1) that the spread width WB of the depletion layer 41 may be approximately expressed as EMI6.1 When the thickness d of the above described narrow portion 33a is chosen to be less than the spread width WB, i.e., d < WB, and a high voltage is applied to a drain D illustrated in Fig. 4, the depletion layer 41 does not spread out beyond the thickness d of the thin portion 33a of the p-type region 33. As the applied voltage is further raised, the depletion layer 41 is spread out along the upper surface of the p+-type region layer 32. And when the applied voltage has reached certain voltage which is lower than the above described breakdown voltage BV, breakdown or reach-through breakdown occurs on a bottom surface 35b of the above described drain diffused region 35. The breakdown current BI caused by the reach-through breakdown flows through a wide area within the bottom surface 35b of the drain diffused region 35 and through the p+-type layer 32. Accordingly, heat concentration is not incurred, preventing damage to the device due to breakdown. As evident from the expression (2), the spread width of the depletion layer in the above described MOS transistor depends upon the impurity concentration N A of the thin portion region 33a, i.e., the p-type layer 33. Accordingly, it is possible to construct so that the depletion layer in the p-type layer 33 reaches the p+-type layer 32 before a breakdown occurs by changing the impurity concentration NA instead of the thickness d of the p-type region 33a. An embodiment of a lateral-type MOS transistor according to the present invention has heretofore been described. However, the same principle can be applied to a vertical-type MOS transistor. Fig. 5 shows an embodiment of a vertical-type MOS transistor according to the present invention The basis structure of this MOS transistor is the same as that of the MOS transistor according to the prior art illustrated in Fig. 3. An identical component is denoted by the same numeral or symbol as that used in Fig. 3. This device basically includes a semiconductor substrate 19 composed of a lower n -type low-resistivity region 17 having a flat upper surface whereto a drain electrode 16 is joined and an upper n-type high-resistivity region 18, a plurality of p-type well regions 20 which have conductivity types opposite that of said substrate 19 and are formed at predetermined interval within said n-type region 18 located on the upper surface side of the substrate 19, an n+-type source region 21 formed within said p-type well region 20, and a gate electrode 23 formed on surfaces of the substrate 19 and the well regions 20 through a gate oxide film 22 which is an insulative film while extending over both the n+-type source region 21 and said n-type region 18 of the substrate 19 which substantially functions as a drain region. The upper surface of the gate electrode 23 except for the leading-out port is coated with a PSG (Phospho-Silicate glass) film 25. And a source electrode 24 is joined to the source region 21 as well as a p+-type contact region 26 which is formed within a p-type well region 20. A feature of the MOS transistor described in Fig. 5 different from conventional one described in Fig. 3 is that the impurity concentration ND and thickness d defining the resistivity of the n-type drain region 18 put between the p-type well region 20 and the n+ -type drain region 17 are chosen to satisfy the relation EMI7.1 so that reach-through breakdown may occur between the p-type well region 20 and the n -type drain region 17. The expression (3) and the foregoing description relating to the expression (1) and (2) reveal that avalanche breakdown of the p-n junction never occur in the MOS transistor according to the present invention when the boundary surface of a depletion layer 27 extending from the junction between the n-type drain region 18 and the well region 20 exists in the n-type drain region 18 having a thickness of d. That is to say, without causing the avalance breakdown, the depletion layer 27 extending from the above described p-n junction face passes through the n-type drain region 18 and reaches the n+-type drain region 17 (reach-through phenomenon}. Even if the voltage applied between the drain and source is further increased after the depletion layer 27 has reached the n+-type drain region 17, the depletion layer 27 hardly advances beyond the boundary face between the n-type drain region 18 and the n+-type drain region 17 into the region 17. If the voltage between the drain and source is further increased, breakdown occurs between the n+-type drain region 17 and the well-region 20. This is the so-called reach-through breakdown. It should be noted that when the depletion layer 27 has reached the n+-type drain region 17, the boundary surface of the depletion layer 27 forms a plane which nearly coincides with the boundary face between the n-type region 18 and the n+-type 17 and a valley V of the depletion layer 27 illustrated in Fig. 3 of the prior art is not produced. Therefore, the electric field applied to the depletin layer 27 is uniformly distributed over nearly entire surface of the bottom of the well region 20 as represented by arrows C. Thus, the electric field does not converge to a limited part unlike the prior art, Under such a state, breakdown occurs. Accordingly, the breakdown current flowing from the n+-type drain region 17 toward the bottom of the well region 20 is also distributed over a wide area nearly uniformly. Thus, the concentration of breakdown current upon the corner of the well region found in the prior art is prevented. Even if a comparatively large breakdown current flows, therefore, heat geneation is not limited to a part. As a result, thermal damage to the device is prevented. An example will now be described by using practical values. For a MOS transistor of the prior art having the surface concentration of the p-type well region 20 of 1x1018/cm3, its diffusion depth of 5,us, the space between two adjacent well regions 20 of 10 > w m, the resistivity value of the highresistivity drain region 18 of 2.5 n cm, and sufficiently large thickness d of the n-type drain region 18, it has been experimentally observed that the voltage BV at which avalanche breakdown is incurred between the n-type drain region 18 and the well region 20 is 160 volts and then the spread width WB of the depletion layer extending from the p-n junction face toward the drain region 18 is approximately 10)cm. For a MOS transistor according to the present invention having the same factors as the above described MOS transistor of the prior art excepting the thickness d of the n-type drain region 18 chosen to be 5 vym which is approximately the half of the the value of WB,the depletion layer 27 reaches the n -type drain region 17 when the voltage between the drain and the source is raised to approximately 40 volts, further extension being stopped. If the voltage is further increased, all of the valleys V of the depletion layer 27 as seen in Fig. 3 of the prior art reach the n -type drain region 17 and the depletion layer exhibits a plane at the boundary between the n+ -type drain region 17 and the n-type drain region 18. Raising the voltage to approximately 80 volts causes reach-through breakdown between the N+-type drain region 17 and the well region 20. At this time, the breakdown current flows through the nearly entire region of the bottom of the well region 20. Accordingly, current distribution is significantly improved as compared with the prior art, sufficient surge durability being attained. In the above example, only the thickness d of the n-type drain region 18 between the n+-type drain region 17 and the well region 20 is reduced without changing the impurity concentration of the n-type drain region 18 to satisfy the expression (3). When the resistance to the voltage is not desired to be lowered, it is possible to satisfy the expression (3) by reducing the impurity concentration ND in order to increase the resistivity without changing the thickness d. As a result, reach-through breakdown is readily incurred. Both the prior art and embodiments of the present invention have heretofore been described by referring to n-channel MOS transistors. However, it is a matter of course that the present invention may be similarly applied to a p-channel, MOS transistor as well. In addition, the present invention is not restricted to a MOS transistor having the illustrated structure, but may be applied to MOS transistors having other structures such as a VMOS transistor. As heretofore been described in detail, even if a high voltage due to a surge or the like is applied between the drain and source of a MOS transistor according to the present invention and thereby breakdown is incurred, the breakdown current does not converge to a limited part within the device but flows through a wide area nearly uniformly. Therefore, the device is not readily subjected to thermal damage, sufficient surge durability being attained. Thus, the present invention provides a device suitable for switching an inductive load. In addition, the necessity of providing an external zener diode for protecting the device is eliminated.

Claims: What is claimed is; 1. A MOS transistor comprising; a low-resistivity layer of a first conductivity type provided with a substrate electrode on a lower surface thereof, a high-resistivity layer of said first conductivity type formed on said low-resistivity layer, a source region of a second conductivity type opposite said first conductivity type formed in said high-resistivity layer at upper side thereof, a drain region of said second conductivity type formed in said high-resistivity layer apart from said source region at upper side thereof, said source and drain regions provided with electrodes respectively on upper surface thereof, a gate electrode formed on a surface of said highresistivity layer between said source and drain regions through an insulative film extending over both said source and drain regions, the value of the impurity concentration and the thickness of a part of said high-resistivity layer existing between the bottom surface of said drain region and the upper surface of said low-resistivity layer is chosen so that reach- through breakdown may occur. 2. A MOS transistor claimed in claim 1, wherein, said impurity concentration and thickness of part of said high-resistivity layer existing between the bottom surface of said drain region and the upper surface of said low-resistivity layer satisfy the condition, EMI11.1 wherein ND is said impurity concentration, d is said thickness, 6 is relative dielectric constant of said high- resistivity layer, 1, is the dielectric constant of vacuum, BV is the breakdown voltage of said high-resistivity layer, and q is the electric charge of an electron. 3. A MOS transistor claimed in claim 2, wherein, said source electrode is electrically coanceted to said substrate electrode. 4. A MOS transistor claimed in claim 2, wherein, said high-resistivity layer is formed by means of epitaxial growth on said low-restivity layer, and said source and drain regions are formed by means of diffusion on said high-resistivity layer. 5. A MOS transistor claimed in claim 2, wherein, said drain region is provided with a drift region on said second conductivity type having impurity concentration lower than that of said drain region, and said drift region extends from the side of said drain region toward the gate region. 6. A MOS transistor comprising; a low-resistivity layer of a first conductivity type provided with a drain electrode on a lower surface thereof, a high-resistivity layer of said first conductivity type formed on an upper surface of said low-resistivity layer, a well region of second conductivity type opposite said first conductivity type formed in said high-resistivity layer at an upper side thereof, a source region of said first conductivity type formed in said well region at an upper side thereof and provided with a source electrode, a gate electrode formed on the surface of said well region through an insulative layer, the value of the impurity concentration and thickness of a part of said high-resistivity layer exiting between the bottom surface of said well region and the upper surface of said low-resistivity layer is chosen so that reach- through breakdown may occur. 7. A MOS transistor claimed in claim 6, wherein, said impurity concentration and thickness of part of said high-resistivity layer existing between the bottom surface of said well region and the upper surface of said low-resistivity layer satisfy the condition EMI13.1 wherein ND is said impurity concentration, d is said thickness is is relative dielectric constant of said high- resistivity layer, gO is the dielectric constant of vacuum, BV is the breakdown voltage of said high-resistivity layer, and q is the electric charge of an electron. 8. A MOS transistor claimed in claim 7 wherein; said source electrode comes in contact with a contact region of said second conductivity type which is formed in said well region.

Inventors: MIHARA, TERUYOSHI C/O TATENOSHATAKU 13-305
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1983
Main classification: H01L29
IPC codes: H01L29