Patent Publication Number: US-7221047-B2

Title: Pressure-contact type semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a division of application Ser. No. 10/765,158 filed Jan. 28, 2004 now U.S. Pat. No. 6,995,464, the entire contents of which are incorporated herein by reference. The present document also claims priority from Japanese application 2003-033325, filed Feb. 12, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a pressure-contact type semiconductor device such as a Gate Commutated Turn-off (referred to as “GCT” hereinafter) thyristor, which is applied to, for example, a BTB, a SVG and the like for electric power applications, an inverter for driving an iron manufacturing roller and the like for industrial applications, and other high-voltage, large-capacity switches and the like. 
   2. Description of the Related Art 
   A conventional GCT thyristor will be described below with reference to  FIG. 6 .  FIG. 6  is a cross-sectional view showing a main part of a GCT thyristor with a typical outer peripheral gate structure. Referring to the figure, an aluminum gate electrode  1   a  and an aluminum cathode electrode  1   b  are formed on an upper surface of a semiconductor substrate  1 , and an aluminum anode electrode  1   c  is formed on a back surface thereof. Reference numeral  1   d  denotes an insulation film made of polyimide or the like, and reference numeral  1   e  denotes an insulating rubber member formed on the outermost periphery of the semiconductor substrate  1 . 
   On the cathode electrode  1   b  on the top surface of the semiconductor substrate  1 , a cathode distortion buffer plate  2  is provided, and an external cathode electrode (see  FIGS. 1A and 1B ) is formed on the outer top portion of the cathode distortion buffer plate  2 . On the anode electrode  1   c , an anode distortion buffer plate  4  is provided, and outside of it, an external anode electrode is formed under the anode distortion buffer plate  4 . A ring gate portion  6  contacts the gate electrode  1   a , and the contacting top end portion is a plane of about 0.5 mm width. An external gate terminal (see  FIGS. 1A and 1B ) electrically connects to the ring gate portion  6 . The ring gate portion  6  is pressed to the gate electrode  1   a  by an annular elastic body having such a shape as a coned disc spring via an annular insulator, together with the external gate terminal. An insulator  10  insulates the ring gate portion  6  from the cathode distortion buffer plate  2  and the external cathode electrode. The GCT thyristor formed in this way has a sealed structure, the inside of which is substituted with an inert gas. 
   Next, an operation of the conventional GCT thyristor will be described. When the GCT thyristor is turned on, an electric current is flown from the external gate terminal to the external cathode electrode. A rising inclination of the gate current at this time is generally 1000 A/μs or more so as to speed up the turn-on expanding speed. When turned off, the current is flown from the external cathode electrode to the external gate terminal. At this time, it is required to supply a current with several thousands A/μs inclination for commutating the current equivalent to the main current to the gate. In order to supply such a large amount of current in an instant, a contact resistance of a current conducting path from the external gate terminal to the external cathode electrode is required to be reduced as much as possible. 
   Further, the GCT thyristor is generally used by applying a reverse bias voltage between the gate and the cathode. A gap between the cathode distortion buffer plate  2  and the gate electrode  1   a  formed on the surface of the semiconductor substrate has only several tens μm width. In order to prevent discharge in this gap, the insulation film  1   d  such as polyimide is formed on the innermost periphery and the surface of the gate electrode  1   a  to thereby coat the surface up to the position right below the insulator  10  (see, for example, Japanese Patent Unexamined Laid-open Publication No. 8-330572 (FIG. 1)). 
   However, in the conventional GCT thyristor as shown in  FIG. 6 , the ring gate portion  6  must contact the gate electrode  1   a  formed on the surface of the semiconductor substrate. Meanwhile, a portion of the gate electrode  1   a  positioned right below the outer periphery of the cathode distortion buffer plate  2 , which is adjacent to the ring gate portion  6 , must be coated with the polyimide insulation film  1   d  in order to prevent discharge. 
   Comparing to the aluminum electrodes and the polyimide insulation film formed by a photoengraving technique, the other fabrication members have larger dimensional tolerances. Further, in order to ensure a clearance for positioning each member at the time of fabricating, the positions of the cathode distortion buffer plate  2  and the ring gate portion  6  with respect to the semiconductor substrate  1  are varied within the range of the integration tolerance which is the sum of the dimensional tolerances of the respective members. 
   Moreover, the ring gate portion  6  and the outer peripheral portion of the cathode distortion buffer plate  2  adjacent thereto must be fabricated in an extremely narrow region of the outer peripheral portion of the semiconductor substrate. Therefore, it was extremely difficult to satisfy the condition that “a portion of the gate electrode  1   a  positioned right below the outer periphery of the cathode distortion buffer plate  2  must be coated with the polyimide insulation film  1   d  to prevent discharge in the gap”, considering from aspects of the component processing accuracy and the positioning accuracy when fabricating. 
   As another measure for preventing the discharge in the conventional structure, there is a method to remove the gap itself by decreasing the diameter of the cathode distortion buffer plate  2  and retracting it up to the outer diameter equivalent to that of the cathode electrode  1   d  formed on the surface of the semiconductor substrate. In this case, however, there arises a problem that dynamic characteristics (for example, a serge resistance) of the GCT thyristor degrades. 
   SUMMARY OF THE INVENTION 
   The present invention is made to solve the aforementioned problems. An object of the present invention is to provide a pressure-contact type semiconductor device with a structure capable of easily preventing discharge with high accuracy in a gap between a cathode distortion buffer plate and a gate electrode without degrading dynamic characteristics. 
   In order to achieve the aforementioned object, according to the present invention, a pressure-contact type semiconductor device having an outer peripheral gate structure includes an external cathode electrode, an external anode electrode and an external gate terminal for flowing an electric current at a time of turn on/off operation, and the semiconductor device includes a semiconductor substrate having an outer peripheral step portion thinned in thickness thereof. The semiconductor substrate has a gate electrode and a cathode electrode formed on a top surface side of the substrate, and an anode electrode formed on a back surface of the substrate. The semiconductor device further includes a pressure-contact supporting block which is provided in an inner periphery of the external gate terminal and located on the gate electrode. 
   In this construction, the gate electrode is formed on the top surface of the outer peripheral step portion of the semiconductor substrate so as to face the pressure-contact supporting block, and a convex contacting portion is formed at a predetermined position of a top surface of the gate electrode so as to contact the pressure-contact supporting block, and an insulation film is formed on a surface area of the gate electrode, ranging from an inner periphery of the gate electrode to a position adjacent to the convex contacting portion. 
   With this configuration, a portion where a ring gate contacts the aluminum gate electrode can be positioned more accurately than that of the conventional structure shown in  FIG. 6 , so that the film formation area of the polyimide insulation film can be extended. Accordingly, the structure for preventing discharge in the gap between the cathode distortion buffer plate and the gate electrode formed on the semiconductor substrate can be achieved more accurately than the conventional device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will be readily understood from the following detailed description taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings, in which: 
       FIG. 1A  is a schematic diagram showing a GCT thyristor according to a first embodiment of the present invention; 
       FIG. 1B  is an enlarged cross-sectional view showing a main part of  FIG. 1A ; 
       FIG. 2  is an enlarged cross-sectional view showing a main part of a GCT thyristor according to a second embodiment of the present invention; 
       FIG. 3  is an enlarged cross-sectional view showing a main part of a GCT thyristor according to a third embodiment of the present invention; 
       FIG. 4A  is an enlarged cross-sectional view showing a main part of a GCT thyristor according to a fourth embodiment of the present invention; 
       FIG. 4B  is an enlarged cross-sectional view showing the main part of its modification; 
       FIG. 5A  is a schematic diagram showing a GCT thyristor according to a fifth embodiment of the present invention; 
       FIG. 5B  is an enlarged cross-sectional view showing a main part of  FIG. 5A ; and 
       FIG. 6  is an enlarged cross-sectional view showing a main part of a conventional GCT thyristor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the descriptions, the embodiments of the present invention will be given of GCT thyristors shown in the figures. However, the present invention is not limited thereto, and is also applicable to other pressure-contact type semiconductor devices. Note that the same reference numerals are used for the common elements in each figure, and the repetition thereof will be omitted for brevity. 
   First Embodiment 
   Hereinafter, a first embodiment of the present invention will be described with reference to  FIGS. 1A and 1B .  FIG. 1A  is a schematic diagram showing a GCT thyristor having an outer peripheral gate structure according to the first embodiment of the present invention, and  FIG. 1B  is an enlarged cross-sectional view showing a main part indicated as “A part” of the embodiment shown in  FIG. 1A . In  FIGS. 1A and 1B , reference numeral  1  denotes a semiconductor substrate, and a portion in the vicinity of the outer periphery thereof is formed as a thin step portion  1 ′ where the thickness of the semiconductor substrate is thinned by a predetermined value. On the surface of the step portion  1 ′, a first gate electrode  1   a  made of aluminum is formed, and a second gate electrode  1   a ′ is convexly formed at a predetermined position on the surface of the first gate electrode  1   a . Reference numeral  1   b  denotes a cathode electrode made of aluminum,  1   c  denotes an anode electrode made of aluminum,  1   d  denotes an insulation film made of polyimide or the like for preventing discharge in a gap, and  1   e  denotes an insulating rubber member holding an outermost peripheral end of the semiconductor substrate  1 . 
   On the top surface of the step portion in the vicinity of the outer periphery of the semiconductor substrate, the polyimide insulation film  1   d  is formed so as to overlay an internal top surface portion of the first gate electrode  1   a  facing a cathode distortion buffer plate  2 . The top surface level of the polyimide insulation film  1   d  is made coincident with the height level of the top surface of the semiconductor substrate  1 . 
   It should be noted that, in this embodiment, the film thicknesses of the first gate electrode, the cathode electrode, the anode electrode and the polyimide insulation film are several tens μm order. However, the present invention is not limited to those film thickness. 
   The cathode distortion buffer plate  2  is disposed on the surface of the cathode electrode  1   b  formed on the semiconductor substrate  1 , and an external cathode electrode  3  is formed outside the cathode distortion buffer plate  2 . An anode distortion buffer plate  4  is disposed on the surface of the anode electrode  1   c , and an external anode electrode  5  is formed outside the anode distortion buffer plate  4  below the semiconductor substrate  1  in the figure. A ring gate portion  6  which serves as a pressure-contact supporting block is disposed on the second gate electrode  1   a ′ so as to directly contact the top surface of the second gate electrode  1   a ′. An external gate terminal  7  is not fixed to, but is electrically connected to the ring gate portion  6 . Reference numeral  8  denotes an annular elastic body having such a shape as a coned disc spring which presses the ring gate portion  6  together with the external gate terminal  7  to the second gate electrode  1   a ′ via an annular insulator  9 . In this way, the convex second gate electrode  1   a ′ has a function of positioning a portion of the ring gate portion  6  to contact and abut the aluminum gate electrode. 
   An insulator body  10  is provided between the cathode distortion buffer plate  2  and the ring gate portion  6  so as to insulate the ring gate portion  6  from the cathode distortion buffer plate  2  and the external cathode electrode  3 . The structure also includes a first flange  11  affixed to the outer peripheral end of the external cathode electrode  3 , a second flange  12  affixed to the outer peripheral end of the external anode electrode  5 , and an insulation cylinder  13  made of ceramic or the like which is held between the first and the second flanges. The GCT thyristor configured in this manner has a sealed structure, and the inside thereof is filled with an inert gas. 
   In the structure of the first embodiment, on the surface of the first gate electrode  1   a , the second gate electrode  1   a ′ is convexly formed by applying a photoengraving technique, so that the approximate center of the bottom surface of the ring gate electrode  6  accurately contacts the top surface of the convex second gate electrode  1   a ′. That is, the first gate electrode  1   a  is formed on the top surface of the outer peripheral step portion  1 ′ of the semiconductor substrate so as to face the ring gate portion  6  provided as a pressure-contact supporting block. At the predetermined position of the top surface of the first gate electrode  1   a , the second gate electrode  1   a ′ is formed as a convex contacting portion to thereby contact the predetermined position of the ring gate portion  6  provided as a pressure-contact supporting block. An inner side portion and a top surface portion of the first gate electrode, ranging from the inner periphery to the position adjacent to the second gate electrode  1   a ′ of a convex contacting portion, is coated with the polyimide insulation film  1   d.    
   With this configuration, the present embodiment can achieve more accurate positioning of the portion at which the ring gate portion  6  contacts the aluminum gate electrode, compared to that of the conventional configuration shown in  FIG. 6 . According to the effect in which the positioning can be performed more accurately than the conventional configuration, the film formation area of the polyimide insulation film  1   d  formed on the surface of the first gate electrode  1   a  can be extended outward up to the position adjacent to the second gate electrode  1   a ′, from the position right below the insulator body  10 . 
   Accordingly, it becomes possible to satisfy the condition that “the gate electrode  1   a  positioned right below the outer periphery of the cathode distortion buffer plate  2  must be coated with the polyimide  1   d ” in order to prevent discharge in the gap between the cathode distortion buffer plate  2  and the gate electrode  1   a  formed on the surface of the semiconductor substrate. Further, it becomes possible to achieve the positioning of the cathode distortion buffer plate  2  and the ring gate portion  6  with respect to the semiconductor substrate  1  more accurately than the conventional structure. 
   Preferably, in the case where the second gate electrode  1   a ′ is formed by evaporating a metal such as nickel (Ni) which is little oxidative, it is possible to suppress oxidation during the manufacturing process, to thereby reduce the contact resistance against the ring gate portion  6 . Moreover, if the second gate electrode  1   a ′ is made of a metallic material such as gold (Au) which is less oxidative than nickel (Ni), oxidation during the manufacturing process can be further suppressed. Since gold (Au) and aluminum (Al) are not easily adhered to each other, the second gate electrode  1   a ′ may be formed on the first gate electrode  1   a , to have a multilayer structure with nickel (Ni) and gold (Au) laminated in turn in this order. With this structure, oxidation of the second gate electrode  1   a ′ during the manufacturing process can be further suppressed and adherence performance of the second gate electrode  1   a ′ to the first gate electrode  1   a  is improved, so that the contact resistance between the second gate electrode  1   a ′ and the ring gate portion  6  may be further reduced. 
   Second Embodiment 
   Hereinafter, a second embodiment of the present invention will be described with reference to  FIG. 2 .  FIG. 2  is an enlarged cross-sectional view showing a main part indicated as “A part” of the GCT thyristor having an outer peripheral gate structure according to the second embodiment of the present invention. In the second embodiment as shown in  FIG. 2 , the second gate electrode  1   a ′ is disposed on the surface of the thin step portion  1 ′ of the semiconductor substrate  1 , and the first gate electrode  1   a  is formed on the surface of the semiconductor substrate  1  so as to overlay the second gate electrode  1   a ′. In this configuration, a part of the first gate electrode  1   a  overlying the second gate electrode  1   a ′ is convexly formed. 
   More specifically, the second gate electrode  1   a ′ is formed on the outer peripheral step portion  1 ′ of the semiconductor substrate  1  so as to face the pressure-contact supporting block  6 , and the first gate electrode  1   a  is formed to entirely overlay the second gate electrode  1   a ′ The first gate electrode  1   a  has a convex portion  1   g  integrally protruded therefrom so that the top surface of the convex portion  1   g  abuts the bottom surface of the pressure-contact supporting block  6 . 
   With this configuration of the present embodiment, it becomes possible to more accurately position the contacting portion of the ring gate portion  6  to abut the aluminum gate electrode, compared to that of the conventional configuration shown in  FIG. 6 . Accordingly, the film formation area of the polyimide insulation film  1   d  formed on the surface of the first gate electrode  1   a  can be extended outward up to the position adjacent to the convex portion of the first gate electrode  1   a.    
   Thus, by forming a convex portion integrally protruded from the first gate electrode, the same effect as the first embodiment can be achieved in preventing discharge in the gap between the cathode distortion buffer plate  2  and the gate electrode  1   a  formed on the surface of the semiconductor substrate. Moreover, in the first embodiment, the second gate electrode  1   a ′ is exposed to the surface of the gate electrode as a fine protrusion. In the second embodiment, in contrast, the second gate electrode  1   a ′ is covered and protected by the first gate electrode  1   a . Therefore, this configuration can reduce damages caused by the mechanical stress at the time of pressure contact by the ring gate portion  6 , which improves the reliability of the GCT thyristor. 
   Third Embodiment 
   Hereinafter, a third embodiment of the present invention will be described with reference to  FIG. 3 . FIG.  3  is an enlarged cross-sectional view showing a main part indicated as “A part” of the GCT thyristor having an outer peripheral gate structure according to the third embodiment of the present invention. In the third embodiment as shown in  FIG. 3 , a portion of the surface of the semiconductor substrate  1  is chemically etched so as to form a protrusion  1   f , and subsequently the first gate electrode  1   a  is formed so as to entirely overlay the protrusion  1   f . The other parts of the structure are the same as those of the second embodiment. In this fabrication, the protrusion  1   f  is formed by performing the etching at least two times. 
   In the first etching process, in order to align the height of the protrusion  1   f  to be a prescribed height, an area of the semiconductor substrate  1  for forming the first gate electrode  1   a  thereon is etched from the surface. Next, a protection film (not shown) for maintaining a non-etched portion is formed on a position of the surface of the etched substrate which is to be left as the top surface of the protrusion  1   f . In the condition having the protection film formed, the second etching is performed so that the non-etched portion remains as the protrusion  1   f  formed on the corresponding portion of the surface of the step portion  1 ′ which is formed by etching the outer peripheral portion of the semiconductor substrate  1 . Subsequently, the first gate electrode  1   a  is formed on surface of the step portion  1 ′ of the semiconductor substrate  1  so as to overlay the protrusion  1   f.    
   More specifically, the first gate electrode  1   a  contains the protrusion  1   f  which is integrally formed with the semiconductor substrate and is protruded on the outer peripheral step portion  1 ′ of the semiconductor substrate  1 . In this configuration, a convex portion  1   g  is integrally formed with the first gate electrode  1   a  to overlay the protrusion  1   f , so that the top surface of the convex portion  1   g  abuts the bottom of the ring gate portion  6 . 
   With this configuration, it becomes possible to more accurately position the contacting portion of the ring gate portion  6  to contact the aluminum gate electrode, compared to that of the conventional configuration shown in  FIG. 6 . Accordingly, the film formation area of the polyimide insulation film  1   d  formed on the surface of the first gate electrode  1   a  can be extended outward up to the position adjacent to the convex portion  1   g  of the first gate electrode  1   a.    
   Thus, by forming a portion of the semiconductor substrate  1  to be a protrusion ( 1   f ) and a portion of the first gate electrode  1   a  overlaying the protrusion  1   f  to be a convex portion ( 1   g ), the same effect as that of the first and the second embodiments can be achieved in preventing discharge in the gap between the cathode distortion buffer plate  2  and the first gate electrode  1   a . Moreover, in the first and the second embodiments, the second gate electrode  1   a ′ is formed as a convex portion through vapor-depositing a metal such as aluminum (Al). However, if adherence of the electrodes are not well performed, there is a possibility that the second gate electrode is peeled off from the semiconductor substrate. 
   In the third embodiment, the protrusion  1   f  is integrally formed with the semiconductor substrate  1  by partially etching the substrate as described above, instead of using the second gate electrode  1   a ′. Accordingly, the third embodiment can surely avoid the problem of peel-off of the gate electrode, to thereby further improve the reliability of the GCT thyristor than that of the second embodiment. 
   Fourth Embodiment 
   Hereinafter, a fourth embodiment of the present invention will be described with reference to  FIGS. 4A and 4B .  FIG. 4A  is an enlarged cross-sectional view showing a main part (indicated as “A part”) of the GCT thyristor having an outer peripheral gate structure according to the fourth embodiment of the present invention, and  FIG. 4B  is a view showing a modification thereof. In the fourth embodiment as shown in  FIG. 4A , the semiconductor substrate  1  is chemically etched so as to form the protrusion  1   f  as same as the third embodiment, and the second gate electrode  1   a ′ is disposed on the surface of the protrusion  1   f , and the first gate electrode  1   a  is formed on the surface of the semiconductor substrate  1  so as to overlay the protrusion  1   f  and the second gate electrode  1   a′.    
   In this way, the second gate electrode  1   a ′ is formed after the protrusion  1   f  is formed, then the first gate electrode  1   a  is formed to protect them. In this configuration, the first gate electrode  1   a  has a partially convex portion  1   g  the top surface of which contacts the bottom surface of the ring gate portion  6 . That is, the first gate electrode contains the protrusion  1   f  which is integrally formed on the outer peripheral step portion  1 ′ of the semiconductor substrate, and the convex portion  1   g  of the first gate electrode  1   a  is formed to cover the protrusion  1   f.    
   With this configuration, it becomes possible to more accurately position the contacting portion of the ring gate portion  6  to contact the aluminum gate electrode, compared to the conventional configuration shown in  FIG. 6 . According to the improvement of the accuracy in positioning, the film formation area of the polyimide insulation film  1   d  formed on the surface of the first gate electrode  1   a  can be extended outward up to the position adjacent to the convex portion  1   g  of the first gate electrode  1   a.    
   With the configuration, since the second gate electrode  1   a ′ disposed on the surface of the protrusion  1   f  of the semiconductor substrate  1  is covered and protected by the first gate electrode  1   a , even when the convex portion  1   g  of the first gate electrode  1   a  contacting the bottom end of the ring gate portion  6  is suffered from damages such as breakage caused by friction with the ring gate portion  6 , the second gate electrode  1   a ′ functions as a protective member, to thereby further improve the reliability of the GCT thyristor than that of the third embodiment. 
   Further, it is preferable that the protrusion  1   f  of the semiconductor substrate  1  formed by etching have a taper shape, that is, trapezoid in cross-section. This is because the protrusion  1   f  is a fine protrusion and may be chipped or broken when any mechanical damage or the like is applied after the protrusion  1   f  is formed. By forming the protrusion  1   f  to be tapered having a trapezoid shape in cross-section, the bottom of the protrusion  1   f  can be made larger so that breakage can be effectively prevented. 
   It should be noted here that in the present embodiment, the formation order of the first and the second gate electrodes  1   a  and  1   a ′ may be changed. In the modification shown in  FIG. 4B , the protrusion  1   f  is first formed by etching the surface of the semiconductor substrate  1 . Then, the first gate electrode  1   a  is formed so as to overlay the protrusion  1   f  in the same manner as in the third embodiment. Subsequently, the second gate electrode  1   a ′ is formed on the top surface of the convex portion of the first gate electrode  1   a  above the forming position of the protrusion  1   f . Thus, it may be configured that the top surface of the second gate electrode  1   a ′ contacts the bottom end surface of the ring gate portion  6  in the same manner as in the first embodiment. That is, the first gate electrode  1   a  contains the protrusion  1   f  integral with the semiconductor substrate is formed on the outer peripheral step portion thereof, and the second gate electrode  1   a ′ is formed as the convex contacting portion ( 1   g ) contacting the bottom of the ring gate portion  6 . 
   With this configuration, the second gate electrode  1   a ′ formed as a fine convex portion may be subject to breakage and abrasion because of direct contact with the bottom surface of the ring gate portion  6 . However, even when the second gate electrode  1   a ′ is broken, the first gate electrode  1   a  functions as a protective member, so that the reliability of the GCT thyristor is further improved than that of the third embodiment. 
   Fifth Embodiment 
   Hereinafter, a fifth embodiment of the present invention will be described with reference to  FIGS. 5A  and  5 B.  FIG. 5A  is a schematic diagram showing a GCT thyristor having a center gate structure according to the fifth embodiment of the present invention, and  FIG. 5B  is an enlarged cross-sectional view showing a main part indicated as “A part” in  FIG. 5A . 
   In contrast to the first to the fourth embodiments each having an outer gate structure in which the ring gate portion  6  is provided on the outer periphery of the semiconductor substrate  1 , in the fifth embodiment as shown in  FIGS. 5A and 5B , the ring gate portion  6  is provided at a center part of the semiconductor substrate  1  which is referred to as “a center gate structure” in the description. 
   More specifically, in the fifth embodiment, a concave step portion  1   h  is formed in the center part of the surface of the semiconductor substrate  1  by etching. Then, the first gate electrode  1   a  is disposed on the concave step portion  1   h , and the second gate electrode  1   a ′ is disposed at a center part of the top surface of the first gate electrode  1   a , so that the second gate electrode  1   a ′ which serves as the convex contacting portion contacts the center part of the bottom surface of the ring gate portion  6 . 
   That is, in the fifth embodiment, the ring gate portion  6  serving as the pressure contact supporting block  6  is provided on the convex contacting portion  1   a ′ disposed in the center part of the semiconductor substrate  1  and between a pair of the insulator bodies  10  in the inner periphery of the outer gate terminal  7 . The first gate electrode  1   a  is disposed in the concave step portion  1   h  formed at the center part of the semiconductor substrate so as to face the pressure contact supporting block  6 . At the center part of the top surface of the first gate electrode  1   a , the second gate electrode  1   a ′ is formed as the convex contacting portion  1   g  which contacts the bottom of the ring gate portion  6 . 
   It is noted here that, instead of forming the second gate electrode  1   a ′, the convex contacting portion  1   g  may be formed of a convex portion integral with the first gate electrode  1   a  as shown in  FIGS. 2 ,  3  and  4 A, so that the convex portion of the first gate electrode  1   a  contacts the pressure contact supporting block  6 . 
   Referring back to  FIG. 5B , the top surface of the first gate electrode except the area of forming the convex contacting portion  1   a ′ (or  1   g ) is coated with the polyimide insulation film  1   d.    
   In this example, the top surface of the first gate electrode other than the second gate electrode  1   a ′ is coated with the polyimide insulation film  1   d , and the film formation area of the polyimide insulation film  1   d  is expanded up to the position adjacent to the convex second gate electrode  1   a ′. In this way, it is also possible to achieve the same effects as those of the first to the fourth embodiments, even with the center gate structure in which the ring gate portion  6  is provided at the center part of the semiconductor substrate  1 . 
   For example, in the GCT thyristor according to the fifth embodiment of the present invention shown in  FIGS. 5A and 5B , the outer peripheral gate structure of the GCT thyristor of the first embodiment shown in  FIGS. 1A and 1B  is changed to the center gate structure. Similarly, the outer peripheral gate structures of the GCT thyristors according to the second to the fourth embodiments shown in  FIGS. 2 to 4A  and  4 B may be changed to the center gate structure. Further, it is noted that the outer peripheral gate structures of the GCT thyristors according to the first to the fourth embodiments or the center gate structure according to the fifth embodiment may be changed to a middle gate structure in which the ring gate portion  6  is provided at a middle part of the semiconductor substrate  1 . 
   As described above, according to the present invention, it becomes possible to position a contacting portion of the ring gate portion to contact the aluminum gate electrode more accurately than the conventional device. According to the improvement of the positioning accuracy, the film formation region of the polyimide insulation film can be expanded. Therefore, the present invention can satisfy, more easily and accurately than the conventional device, the condition that “the gate electrode  1   a  positioned right below the outer periphery of the cathode distortion buffer plate  2  must be coated with the polyimide insulation film  1   d  so as to prevent the discharge in the gap”, thereby improving the reliability of the GCT thyristor. 
   Although the present invention has been described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.