Abstract:
In order to provide a reliable surge protective component with a straightforward manufacturing process, first and second buried layers are diffused over the entire inside surfaces of a semiconductor substrate, and first and second base layers are then diffused over the entire inside surfaces of the first and second buried layers. First and second emitter layers are then partially diffused at the inside of the first and second base layers. The peripheries of the first and second emitter layers are then surrounded by first and second moats, the bottoms of which reach the first and second buried layers. A PN junction formed between the first and second base layers and first and second buried layers is then simply a planar junction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the technical field of semiconductor devices, and in particular to the structure of surge protective component. 
     2. Description of Related Art 
     In the related art, two terminal surge protective components are widely used as semiconductor devices for protecting electronic circuits from surge voltages occurring due to lightening strikes, etc. 
     Numeral  101  in FIG. 8 is an example of a semiconductor device of the related art. Here, P-type base layers  113   a  and  113   b  are provided at a portion in the vicinity of both the inner side surface and the reverse side surface of an N-type substrate  109  so that a PN junction is formed between the substrate  109  and the base layers  113   a  and  113   b.    
     The base layers  113   a  and  113   b  are positioned substantially in the vicinity of the center of the semiconductor device  101 , and are patterned so as to be rectangular in shape, with rounded corners. N-type emitters  122   a  and  122   b  are arranged at a part in the vicinity of the surface of the inside of the base layers  113   a  and  113   b  so that a PN junction is formed between each of the emitter layers  122   a  and  122   b  and the base layers  113   a  and  113   b.    
     Ohmic layers  121   a  and  121   b  with a high P-type surface concentration are provided at a portion where the emitter layers  122   a  and  122   b  provided in the vicinity of the surface on the inside of the base layers  113   a  and  113   b  are not provided. 
     Metal films  127   a  and  127   b  are formed at the surfaces of the emitter layers  122   a  and  122   b  and the ohmic layers  121   a  and  121   b  on the surface side and rear surface side of the substrate  109 .The metal film  127   a  at the surface side is electrically connected to the surface side emitter layer  122   a  and the ohmic layer  121   a , but is not connected to the substrate  109 . The metal film  127   b  of the rear surface side is electrically connected to the surface side emitter layer  122   b  and the ohmic layer  121   b , but is not connected to the substrate  109 . 
     The semiconductor device  101  of this structure has a four-layer PNPN structure when viewed from the surface side or from the rear side. This means that the PN junction between the base layers  113   a  and  113   b  and the substrate  109  is reverse biased for whichever of the metal films  127   a  and  127   b  a voltage is applied to, wherein a current flows as a result of this PN junction experiencing an avalanche breakdown. 
     When a current flows, the PNPN structure latches up, and a voltage state lower than a voltage during avalanche breakdown is maintained between the metal films  127   a  and  127   b . Therefore, when the semiconductor device  101  is connected in parallel with an electronic circuit, when a surge voltage is applied to the electronic circuit, the semiconductor device  101  conducts in such a manner that the surge voltage is not applied to the electronic circuit. 
     There are, however, drawbacks with this semiconductor device  101  in that the PN junction formed between the base layers  113   a  and  113   b  and the substrate  109  are easily destroyed and reliability therefore becomes poor. 
     In order to improve the reliability of the semiconductor device  101 , there has been proposed a structure where a high-concentration N-type layer is provided within the substrate  110  so that a PN junction is formed between the N-type layer and base layers  113   a  and  113   b . However, this requires a complex process because the N-type layer is buried more deeply than the base layers  113   a  and  113   b.    
     As the present invention sets out to resolve the problems encountered in the related art, it is the object of the present invention to provide a surge protection semiconductor device that has a straightforward manufacturing process, a simple structure, and is highly reliable. 
     SUMMARY OF THE INVENTION 
     In order to resolve the aforementioned problems, a semiconductor device of the present invention having, when one of either an N-type or P-type is defined as a first conductivity type, and the other is defined as a second conductivity type, a semiconductor substrate of the first conductivity type, comprises first and second buried layers (in this invention, a buried layer may include a layer being partially exposed on the surface of a semiconductor substrate  9  as shown in FIG. 6) provided within the semiconductor substrate, being of the first conductivity type, and being of a higher concentration than the semiconductor substrate, first and second emitter layers of the first conductivity type, first and second base layers of the second conductivity type, and a substrate layer constituted by the semiconductor substrate. The substrate layer is sandwiched between the first and second buried layers. At least a part of the first and second base layers are positioned on one side surface and the other side surface of the semiconductor substrate so as to form PN junctions with the first and second buried layers. At least a part of the first and second emitter layers are located in a vicinity of a surface inside of the first and second base layers so as to form PN junctions with the first and second base layers. 
     The first and second base layers are respectively provided between the first and second emitter layers and the first and second buried layers, and the first and second buried layers are located between the first and second base layers and the substrate layer. 
     With this semiconductor device of the present invention, the first and second metal films are formed on both sides of the semiconductor substrate, the first emitter layer and the first base layer are electrically short-circuited by the first metal film; and the second emitter layer and the second base layer are electrically short-circuited by the second metal film. 
     Further, with the semiconductor device of this invention, ring-shaped first and second moats with bottom surfaces reaching the buried layers are formed on both sides of the semiconductor substrate and the first and second emitter layers are located on inside of the first and second moats. 
     The insides of the first and second moats of this semiconductor device of the present invention are filled with oxide. 
     Moreover, at least a part of the first and second base layers are positioned at a region on outside of the first and second moats of surfaces of the semiconductor substrate. 
     Still further, at least a part of the first and second buried layers are positioned at a region on the outside of the first and second moats of the surfaces of the semiconductor substrate of the semiconductor device of the present invention. 
     A semiconductor device manufacturing method of the present invention comprises the steps of, when one of an N-type and P-type is defined as a first conductivity type and the other is defined as a second conductivity type: 
     forming a first buried layer of the first conductivity type in the vicinity of the surface at the inside of one side of the semiconductor substrate of the first conductivity type and forming a second buried layer of the first conductivity type in a vicinity of the surface of the other side, in such a manner that the first and second buried layers sandwich a substrate layer composed of a remaining portion of the semiconductor substrate, forming first and second base layers of the second conductivity type in the vicinity of surfaces at insides of the first and second buried layers in such a manner that bottom surfaces are positioned in the first and second buried layers; and forming first and second emitter layers of the first conductivity type in the vicinity of surfaces at insides of the first and second base layers so that the bottom surfaces thereof are positioned in the first and second base layers. 
     In the semiconductor device manufacturing method of the present invention, the first and second buried layers are formed by introducing an impurity of the first conductivity type into the semiconductor substrate with the surfaces of both sides of the semiconductor substrate completely exposed and diffusing impurity of the first conductivity type. 
     Further, in the semiconductor device manufacturing method of the present invention, the first and second base layers are formed by introducing an impurity of the second conductivity type into the first and second buried layers with the surfaces of the first and second buried layers completely exposed and diffusing impurity of the second conductivity type. 
     Moreover, the semiconductor device manufacturing method of the present invention further comprises a step of, with ring-shaped moats having bottom surfaces reaching positions deeper than the bottom surfaces of the base layers, forming moats including the first and second emitter layers inside of the ring-shaped moats on both sides of the semiconductor substrate. 
     Further, the semiconductor device manufacturing method of the present invention may comprise a further step of forming first and second metal films short-circuiting the first and second emitter layers positioned inside of the ring-shaped moats and the first and second base layers after filling the insides of the first and second moats with oxide and forming first and second passivation films. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) to FIG.  1 ( k ) illustrate steps for manufacturing a semiconductor device of a first example of the present invention. 
     FIG. 2 is a view (first of two) illustrating a structure for emitter layers of this semiconductor device. 
     FIG. 3 is a view (second of two) illustrating a structure for emitter layers of this semiconductor device. 
     FIG.  4 ( a ) and FIG.  4 ( b ) are views illustrating the planar shape of this semiconductor device. 
     FIG.  5 ( a ) to FIG.  5 ( d ) illustrate steps for manufacturing a semiconductor device of a second example of the present invention. 
     FIG. 6 is a view illustrating a structure for emitter layers of this semiconductor device. 
     FIG.  7 ( a ) and FIG.  7 ( b ) are views illustrating the planar shape of this semiconductor device. 
     FIG. 8 is a view illustrating a surge protective component of the related art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The semiconductor device of the present invention is a two terminal semiconductor device used for protecting electronic circuits from surge voltages. The semiconductor device includes, when one of either an N-type or P-type is defined as a first conductivity type and the other is defined as a second conductivity type, a semiconductor substrate of the first conductivity type, first and second buried layers provided within the semiconductor substrate, being of the first conductivity type, and being of a higher concentration than the semiconductor substrate, first and second emitter layers of the first conductivity type, first and second base layers of the second conductivity type, and a substrate layer constituted by the semiconductor substrate. The semiconductor layer is sandwiched between the first and second buried layers; and the first and second base layers are positioned on one side surface and the other side surface of the semiconductor substrate so as to form PN junctions with the first and second buried layers. The first and second emitter layers are located in the vicinity of the surface of the inside of the first and second base layers so as to form PN junctions with the first and second base layers; and the first and second base layers are respectively provided between the first and second emitter layers and the first and second buried layers, while the first and second buried layers are located between the first and second base layers and the substrate layer. 
     The first and second buried layers are formed by means of introducing impurities to within the semiconductor substrate with the surface side and rear surface side of the semiconductor being completely exposed. Processing to pattern the first and second buried layers is therefore unnecessary. 
     The first and second base layers are arranged so as to be positioned in the vicinity of the inside surfaces of the first and second buried layers so that the first and second base layers form PN junctions between the first and second buried layers and a PN junction is not formed between the first and second base layers and the substrate layer. 
     The first and second base layers are formed in such a manner that the entire surfaces of the first and second buried layers are exposed. When inner portions of first and second moats are then separated from the cross-section of the semiconductor device by the ring-shaped first and second moats with the bottom parts of the first and second base layers reaching the first and second buried layers, there are no spherical or cylindrical junction portions at the PN junction formed between the first and second base layers and the first and second buried layers; and this junction therefore consists only of a planar junction. 
     Further, when the first and second base layers are formed partially within the first and second buried layers, spherical junction portions and circular junction portions of a PN junction formed between the first and second base layers and the first and second buried layers are eliminated by the first and second moats so that only planar junction portions remain. 
     In either of these cases, the ends of a PN junction formed between the first and second base layers and the first and second buried layers are exposed within the first and second moats, but are not exposed at the surface of the semiconductor substrate. 
     The inside of the first and second moats is filled up with oxide etc. and baked so as to form first and second passivation films without causing the electrical characteristics of the PN junction to deteriorate. The oxide may include glass granules comprising lead oxide, aluminum oxide, silicon oxide etc. 
     A surge protective component of a preferred embodiment of a semiconductor device of the present invention and a method of manufacturing this surge protective component will now be described using the drawings. 
     FIG.  1 ( a ) to FIG.  1 ( k ) are views showing processes illustrating the method of manufacturing a semiconductor device of a first example of the present invention. Here, the first conductivity type is an N-type (including N − , N +  and N ++  types) and the second conductivity type is a P-type (including P − , P +  and P ++ -types). 
     Referring to FIG.  1 ( a ), first, a semiconductor substrate  9  consisting of high-resistivity N-type silicon single crystal is prepared. Here, a crystal with a resistivity of 20 Ωcm or more, and less than or equal to 60 Ωcm (less than or equal to 2×10 14 /cm 3  and greater than or equal to 7×10 13 /cm 3  in concentration conversions) was used as the semiconductor substrate  9 . 
     Next, phosphorous is implanted in with the entire surfaces of both sides of the semiconductor substrate  9  exposed; and as shown in FIG.  1 ( b ), high concentration N-type layers  11   a  and  11   b  are formed in the vicinity of the surfaces of both of the inner surfaces of the semiconductor substrate  9 . 
     Next, as shown in FIG.  1 ( c ), impurities contained in the high-concentration N-type layers  11   a  and  11   b  are diffused through heat processing so that first and second buried layers  12   a  and  12   b  of higher surface concentrations than the concentration of the semiconductor substrate  9  are formed. In this state, the surface of the first and second buried layers  12   a  and  12   b  coincide with the surface of the semiconductor substrate  9 ; and the depth of diffusion is substantially equal along the whole of the region for the first and second buried layers  12   a  and  12   b.    
     Numeral  10  in FIG.  1 ( c ) indicates a substrate layer  10  constituting a low N-type concentration portion, such portion being a portion of the semiconductor substrate  9 , except the first and second buried layers  12   a  and  12   b.    
     In this situation, the first and second buried layers  12   a  and  12   b  are formed over both surfaces of the semiconductor substrate  9  in their entireties; and therefore sandwich the substrate layer  10 . Numerals  19   a  and  19   b  show silicon oxide films formed on the surfaces of the buried layers  12   a  and  12   b  during a diffusion process. 
     Next, the silicon oxide films  19   a  and  19   b  are removed and boron is implated into the vicinity of the surfaces of the inner parts of the first and second buried layers  12   a  and  12   b  on the front and rear surfaces with the surfaces of the first and second buried layers  12   a  and  12   b  in an exposed state, as shown in FIG.  1 ( d ). High-concentration P-type layers  14   a  and  14   b  are formed over the entire surfaces of the first and second buried layers  12   a  and  12   b . In this state, the first and second buried layers  12   a  and  12   b  are buried in lower layers of the high-concentration P-type layers  14   a  and  14   b.    
     Next, when boron constituting the high-concentration P-type layers  14   a  and  14   b  is diffused by subjection to heat processing over a prescribed period of time, P-type first and second base layers  13   a  and  13   b  are formed in the vicinity of the inner part of both surfaces of the semiconductor substrate  9 , as shown in FIG.  1 ( f ). 
     As a result of this diffusion processing and diffusion processing described later, the positions of the bottom parts of the first and second base layers  13   a  and  13   b  exceed the bottom parts of the first and second buried layers  12   a  and  12   b  without being deep; and the first and second base layers  13   a  and  13   b  connect with the upper parts of the first and second buried layers  12   a  and  12   b  at this bottom part. The first and second base layers  13   a  and  13   b  therefore do not connect with the substrate layer  10 . 
     In this state, at the inner part of the semiconductor substrate  9 , the first and second buried layers  12   a  and  12   b  are positioned on either side of the substrate layer  10 ; and the first and second base layers  13   a  and  13   b  are positioned at the side surfaces of the first and second buried layers  12   a  and  12   b . PN junctions are formed between the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b.    
     The surface of the first and second base layers  13   a  and  13   b  and the surface of the semiconductor substrate  9  coincide; and the first and second buried layers  12   a  and  12   b  are not exposed at the surface of the semiconductor substrate  9  because the first and second buried layers  12   a  and  12   b  are sandwiched by the first and second base layers  13   a  and  13   b  and the substrate layer  10 . 
     Numerals  19   a  and  19   b  in FIG.  1 ( e ) show silicon oxide films formed on the surfaces of the base layers  13   a  and  13   b  during the diffusion process, and silicon oxide films  19   a  and  19   b  are positioned on both sides of the semiconductor substrate  9 . 
     An opening of a large surface area is made at prescribed positions on the silicon oxide films  19   a  and  19   b  on the surfaces on both sides of the semiconductor substrate  9  using photographic processes and etching processes. After exposing the surfaces of the base layers  13   a  and  13   b , boron is implanted into this exposed portion and a high-concentration P-type layer is formed. The numerals  15   a  and  15   b  in FIG.  1 ( f ) indicate openings of a large surface area formed at the silicon oxide films  19   a  and  19   b . Numerals  16   a  and  16   b  indicate high P-type concentration layers formed at the bottom surfaces of the openings  15   a  and  15   b.    
     Next, mesh-like openings are made in the planar pattern at positions where the high P-type concentration layers  16   a  and  16   b  of the silicon oxide films  19   a  and  19   b  on both sides are not formed. The surfaces of the base layers  13   a  and  13   b  are then exposed, phosphorous is introduced, and a high N-type concentration layer is formed. Numerals  17   a  and  17   b  in FIG.  1 ( g ) indicate mesh-like openings formed respectively at the silicon oxide films  19   a  and  19   b  on both sides and numerals  18   a  and  18   b  indicate high-concentration N-type layers formed at the bottom surfaces of the openings  17   a  and  17   b . The planar shape of the high N-type concentration layers  18   a  and  18   b  is mesh-like. 
     In this state, high-concentration P-type layers  16   a  and  16   b  and high-concentration N-type layers  18   a  and  18   b  are both formed in the vicinity of the inner surfaces of the base layers  13   a  and  13   b.    
     Next, the high-concentration P-type layers  16   a  and  16   b  and the high-concentration N-type layers  18   a  and  18   b  are diffused by heating. FIG.  1 (h) shows the situation after diffusion where the silicon oxide films formed during diffusion processing are removed. Here, first and second ohmic layers  21   a  and  21   b  are formed by diffusion of the high P-type concentration layers  16   a  and  16   b ; and first and second emitter layers  22   a  and  22   b  are formed by diffusion of the high-concentration N-type layers  18   a  and  18   b.    
     The depth of diffusion of the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b  is shallower than that of the first and second base layers  13   a  and  13   b  so that the first and second ohmic layers  21   a  and  21   b  and the outer portion of the surface shape of the first and second emitter layers  22   a  and  22   b  are located further towards the inside than the outer portion of the surface shape of the first and second base layers  13   a  and  13   b.    
     The first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b  are formed on the insides of the first and second base layers  13   a  and  13   b , respectively. 
     A PN junction is therefore formed between the first and second emitter layers  22   a  and  22   b  and the first and second base layers  13   a  and  13   b  because these layers are of opposite conductivity types. 
     On the other hand, the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b  do not make contact with the first and second buried layers  12   a  and  12   b  or with the substrate layer  10 . 
     The pattern on one side of the semiconductor substrate  9  and on the other side is substantially the same in this state. This planar shape is shown in FIG.  4 ( a ). The first and second emitter layers  22   a  and  22   b  are formed in the shape of a mesh. This means that the surfaces of the first and second base layers  13   a  and  13   b  are exposed in dotted state within the region where the surfaces of the first and second emitter layers  22   a  and  22   b  are positioned. 
     FIG.  1 ( h ) is a cross-section of a portion that does not include the first and second base layers  13   a  and  13   b  in a dotted state, corresponding to a cross-section taken along line A—A of FIG.  4 ( a ). FIG. 2 is a cross-section of a portion that includes the surfaces of the first and second base layers  13   a  and  13   b  in a dotted state that corresponding to a cross-section taken along line B—B of FIG.  4 ( b ). 
     Next, as shown in the situation in FIG.  1 ( h ) and FIG. 2, resist films having ring-shaped openings are formed on the front and rear surfaces of the semiconductor substrate  9 , the semiconductor substrate  9 , with the bottom surfaces of the openings exposed, is etched, and moats are formed. 
     In FIG.  1 ( i ), after moats are formed by etching, the situation is such that the resist film is removed. Here, numeral  25   a  shows a first moat formed at one side surface of the semiconductor substrate  9  and numeral  25   b  shows a second moat formed in the surface on the other side. 
     The ring-shaped openings of the resist films are located along a boundary of the first and second emitter layers  22   a  and  22   b  and the first and second base layers  13   a  and  13   b , and the boundary of the first and second ohmic layers  21   a  and  21   b  and the first and second base layers  13   a  and  13   b.    
     In other words, the ring-shaped openings of the resist films are positioned above the edge portions of a region constituted by the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b . The first and second moats  25   a  and  25   b  are formed in the same ring shape as the pattern for the openings for the resist films and are positioned above the edge portions of regions constituted by the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b . That is, the first and second emitter layers  22   a  and  22   b  and the first and second ohmic layers  21   a  and  21   b  are positioned to the inside of the rings of the first and second moats  25   a  and  25   b.    
     The first and second moats  25   a  and  25   b  are deeper than the depth of the PN junction formed by the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b ; and therefore, the first and second moats  25   a  and  25   b  are only dug to depths that do not reach the substrate layer  10 . The bottom parts of the first and second moats  25   a  and  25   b  are positioned in the first and second buried layers  12   a  and  12   b . Therefore, an inner region and outer region of the first and second moats  25   a  and  25   b  of the first and second base layers  13   a  and  13   b  are electrically separated by the PN junction formed between the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b . 
     The PN junction formed between the first and second base layers  13   a  and  13   b  on the inside of the first and second moats  25   a  and  25   b  and the first and second buried layers  12   a  and  12   b  is therefore a planar junction. 
     Next, oxide is affixed to both sides of the semiconductor substrate  9  and an inside portion of a ring for the first and second moats  25   a  and  25   b  (and a scribe line portion for the semiconductor substrate  9  (not shown)) are exposed using photographic and etching processes after baking. 
     In this state, as shown in FIG.  1 ( j ), first and second passivation films  26   a  and  26   b  are formed by silicon oxide that fills up the inside of the first and second moats  25   a  and  25   b  and silicon oxide positioned at the surface of the region on the outside of the first and second moats  25   a  and  25   b.    
     Next, as shown in FIG.  1 ( k ), a first metal film  27   a  consisting of a metal such as nickel is formed at the surface of the first ohmic layer  21   a  and first emitter layer  22   a  on one side of the semiconductor substrate  9 . On the other side, a second metal film  27   b  of the same substance as the first metal film  27  is formed at the surface of the second ohmic layer  21   b  and the second emitter layer  22   b  so as to subsequently form the semiconductor device  1  of the present invention. 
     The surface concentration of the first and second ohmic layers  21   a  and  21   b  is higher than the concentration of the surfaces of the first and second base layers  13   a  and  13   b . When the first and second metal films (described later) are formed at the surface of the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b , the first and second metal films are ohmically connected with the first and second ohmic layers  21   a  and  21   b , and the first and second metal films are therefore electrically connected with the first and second base layers  13   a  and  13   b  via the first and second ohmic layers  21   a  and  21   b.    
     The first and second metal films and the first and second emitter layers  22   a  and  22   b  are ohmically connected because the surface concentrations of the first and second emitter layers  22   a  and  22   b  are sufficiently high. 
     The bottom parts of the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b  do not make contact with the first and second buried layers  12   a  and  12   b.    
     A plan view of the front surface and rear surface in the state, where the first and second passivation films  26   a  and  26   b  and the first and second metal films  27   a  and  27   b  are peeled, (i.e., in the same state as shown in FIG.  1 ( i )) is shown in FIG.  4 ( b ). FIG. 3 is a cross-sectional view taken along line B—B in FIG.  4 ( b ). 
     With the semiconductor device  1  of the present invention, the concentration of the first and second buried layers  12   a  and  12   b  is higher than the concentration of the substrate layer  10 ; and these layers have a resistivity of 4 to 6 Ωcm. The withstand voltage of the PN junction formed between the first and second base layers  13   a  and  13   b ; and the first and second buried layers  12   a  and  12   b  is therefore low at approximately 200 to 500 Volts. 
     The depth of the first and second base layers  13   a  and  13   b  is approximately 20 to 30 μm and the depth of the first and second buried layers  12  is 30 to 40 μm. The thickness of the substrate layer  10  is approximately 200 to 300 μm. 
     At the semiconductor device  1 , the first and second base layers  13   a  and  13   b  and the substrate layer  10  do not make contact; and a PN junction is therefore not formed between them. 
     A PN junction having a large surface area and low withstand voltage is formed between the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b . Therefore, when a surge voltage is applied so that the semiconductor device  1  conducts, a current flows in the PN junction formed between the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b  in a uniform manner without this current partially converging; and resistance to destruction is therefore high. 
     Further, the PN junction, formed between the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b , is only a planar junction and no spherical junctions or cylindrical junctions exist. There are therefore no portions within the PN junction where the withstand voltage is low and current convergence therefore does not occur. 
     The first and second buried layers  12   a  and  12   b  are formed by diffusion from the surface of the semiconductor substrate  9  and are therefore of a high concentration. The avalanche breakdown voltage of the PN junction between the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b  can therefore be easily controlled. 
     Manufacture can be completed with few photographic processes because the first and second base layers  13   a  and  13   b  and the first and second buried layers  12   a  and  12   b  are not patterned. 
     The present invention does, however, also include semiconductor devices where the first and second base layers  13   a  and  13   b  are patterned. 
     The manufacturing processes for this semiconductor device are now described in a second embodiment of a semiconductor device of the present invention, with the diffusion layers and thin films being the same as those for the semiconductor device of the first example and with all items being given the same numerals. 
     First, FIG.  5 ( a ) is a view corresponding to FIG.  1 ( e ). Referring to FIG.  5 ( a ), points of difference with the above embodiment are that the P-type first and second base layers  33   a  and  33   b  are positioned at the centers of the first and second buried layers  12   a  and  12   b  and are partially arranged in the vicinity of the surface of the inside thereof. The surfaces of the first and second base layers  33   a  and  33   b  and the surfaces of the first and second buried layers  12   a  and  12   b  are also exposed at the surfaces of the semiconductor substrate  9 . 
     FIG.  5 ( a ) shows the situation when silicon oxide films  19   a  and  19   b  are formed at the surfaces of the first and second base layers  33   a  and  33   b  and the surfaces of the first and second buried layers  12   a  and  12   b.    
     The planar shapes of the first and second base layers  33   a  and  33   b  are in the shape of quadrilaterals with rounded corners, with first and second buried layers  12   a  and  12   b  remaining at portions at the periphery of the first and second base layers  33   a  and  33   b  at the surfaces of the semiconductor substrate  9 . 
     P-type and N-type impurities are implanted into portions in the vicinity of the surface at the inside surfaces of the first and second base layers  33   a  and  33   b  are diffused. P + type first and second ohmic layers  21   a  and  21   b  and N + type first and second emitter layers  22   a  and  22   b  are then formed, as shown in FIG.  5 ( b ). 
     FIG.  7 ( a ) shows the condition of both sides of the semiconductor substrate  9  in this state. FIG.  5 ( b ) corresponds to a cross-sectional view taken along line C—C of FIG.  7 ( a ). 
     When first and second moats  25   a  and  25   b  are formed along exposed edge portions of the surface of the semiconductor substrate  9  of the first and second base layers  33   a  and  33   b  (i.e., along portions of the PN junction formed between the first and second base layers  33   a  and  33   b  and the first and second buried layers  12   a  and  12   b ), exposed at the surface of the semiconductor substrate  9 , as shown in FIG.  5 ( c ), the first and second moats  25   a  and  25   b  form ring-shapes. 
     Cylindrical and spherical junction portions of a PN junction formed between the first and second base layers  33   a  and  33   b  and the first and second buried layers  12   a  and  12   b  by the first and second moats  25   a  and  25   b  are removed. 
     The first and second base layers  33   a  and  33   b  are therefore not arranged at the region on the outside of the first and second moats  25   a  and  25   b  and portions at the surfaces side of the first and second buried layers  12   a  and  12   b  are exposed. 
     Next, as shown in FIG.  5 ( d ), oxide is baked so as to form first and second passivation films  26   a  and  26   b  at a surface portion of the first and second buried layers  12   a  and  12   b , respectively, positioned at regions on the inside of the first and second moats  25   a  and  25   b  and on the outside of the first and second moats  25   a  and  25   b.    
     Next, first and second metal films  27   a  and  27   b  are then formed on the inside of the first and second moats  25   a  and  25   b  with the surfaces of the first and second ohmic layers  21   a  and  21   b  and the first and second emitter layers  22   a  and  22   b  exposed in order to obtain a semiconductor device  2  of the present invention. 
     With this semiconductor device  2 , the depths of the first and second moats  25   a  and  25   b  are formed to be deeper than the depths of the first and second base layers  33   a  and  33   b  and spherical and cylindrical junction portions of the PN junction formed between the first and second base layers  33   a  and  33   b  and the first and second buried layers  12   a  and  12   b  are removed by the first and second moats  25   a  and  25   b.    
     The first metal film  27   a  is electrically connected to the first emitter layer  22   a  and the first ohmic layer  21   a  at a surface of one side of the semiconductor substrate  9 ; and the second metal film  27   b  is electrically connected to the second emitter layer  22   b  and the second ohmic layer  21   b  at a surface on the other side. 
     FIG.  7 ( b ) shows surface conditions for the front surface side and the rear surface side of the semiconductor  2  of the present invention with the first and second passivation films  26   a  and  26   b  and the first and second metal films  27   a  and  27   b  peeled away. 
     As with the semiconductor device  1  of the first embodiment, the planar shape of the first and second emitter layers  22   a  and  22   b  of this semiconductor device  2  is also mesh-shaped; and portions where the first and second base layers  33   a  and  33   b  are exposed at the surface of a region where the first and second emitter layers  22   a  and  22   b  are located are in dotted state. 
     FIG.  5 ( c ) is a cross-sectional view taken along a line C—C not passing portions where the first and second base layers  33   a  and  33   b  exist. FIG. 6 is a cross-sectional view taken along line D—D passing portions where the first and second base layers  33   a  and  33   b  exist. 
     With the semiconductor device  2  of the second embodiment, the first and second base layers  33   a  and  33   b  and the substrate layer  10  also do not make contact and a PN junction is therefore not formed between them. Further, the PN junction formed between the first and second base layers  33   a  and  33   b  and the buried layers  12   a  and  12   b  is a planar junction and no spherical junctions or cylindrical junctions exist. There are therefore no portions within the PN junction where the withstand voltage is low and current convergence does not occur. 
     In the above, a description is given where the first conductivity type is taken to be an N-type and the second conductivity type is taken to be a P-type, but conversely, a P-type may be taken to be the first conductivity type and an N-type may be taken to be a second conductivity type. In this case, the semiconductor substrate is P-type, the first and second buried layers are P + -type, the first and second base layers are N-type, the first and second emitter layers are P + -type, and the first and second ohmic layers are N + -type. 
     A component for protecting against highly destructive surges can be obtained with few photographic processes.