Patent Publication Number: US-8125307-B2

Title: Aggregate substrate, production method of aggregate substrate, and varistor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an aggregate substrate, a production method of an aggregate substrate, and a varistor. 
     2. Related Background Art 
     There is a known varistor having a varistor part of a nearly rectangular parallelepiped shape to exhibit nonlinear voltage-current characteristics, a pair of internal electrodes located in this varistor part and opposed to each other with a portion of the varistor part in between, and a pair of terminal electrodes formed on an exterior surface of the varistor part and connected to the respective corresponding internal electrodes (e.g., cf. Japanese Patent Application Laid-open No. 2002-246207). 
     SUMMARY OF THE INVENTION 
     Incidentally, the varistor is connected in parallel to an electronic device such as a semiconductor light emitting device or FET (Field Effect Transistor) to protect the electronic device from an ESD (Electrostatic Discharge) surge. Some of such electronic devices generate heat during operation. When the electronic device becomes hot, the properties of the device itself become deteriorated to affect the operation thereof. For this reason, it is necessary to efficiently dissipate the heat generated. 
     Then the inventors considered that the heat could be dissipated from the varistor in such a manner that a heat dissipation part with a heat dissipation function was provided in contact with the varistor part and that the heat transferred to the varistor was dissipated form the heat dissipation part. However, this method has the following problem. 
     A conventional varistor production process involves making an aggregate substrate including a plurality of varistor parts. The aggregate substrate is obtained by laminating green sheets to become the varistor parts, electrode patterns to become the internal electrodes, etc. to form a multilayer green body, and firing this multilayer green body. 
     For producing the varistors with the heat dissipation part, the aggregate substrate is made by laminating green sheets to become the varistor parts, electrode patterns to become the internal electrodes, green sheets to become the heat dissipation part, etc. to form a multilayer green body, and firing it. When this multilayer green body is fired, there is difference between contraction caused by firing of the varistor parts and contraction caused by sintering of the heat dissipation part, which can cause warpage of the aggregate substrate. 
     An object of the present invention is therefore to provide a varistor capable of efficiently dissipating heat, and an aggregate substrate for production of this varistor Another object of the present invention is to provide a production method of an aggregate substrate capable of suppressing occurrence of warpage. 
     An aggregate substrate according to the present invention is an aggregate substrate comprising: a first varistor part comprising a first varistor element layer to exhibit nonlinear voltage-current characteristics, and a plurality of first internal electrodes juxtaposed in an extending direction of the first varistor element layer in the first varistor element layer, the first varistor part having a first principal face and a second principal face facing each other; a second varistor part comprising a second varistor element layer to exhibit nonlinear voltage-current characteristics, and a plurality of second internal electrodes juxtaposed in an extending direction of the second varistor element layer in the second varistor element layer, the second varistor part having a third principal face and a fourth principal face facing each other; and a heat dissipation layer having a fifth principal face and a sixth principal face facing each other, wherein the fifth principal face of the heat dissipation layer is in contact with the second principal face of the first varistor part and wherein the sixth principal face of the heat dissipation layer is in contact with the fourth principal face of the second varistor part. 
     In the aggregate substrate according to the present invention, the heat dissipation layer is sandwiched between the first varistor part and the second varistor part while being in contact with them. For this reason, warpage of the aggregate substrate is unlikely to occur. The use of the aggregate substrate according to the present invention facilitates production of varistors with high heat dissipation efficiency. 
     Preferably, the first varistor part further comprises a plurality of pairs of first surface electrodes formed on the first principal face, the second varistor part further comprises a plurality of pairs of second surface electrodes formed on the third principal face, each of the first surface electrodes in each pair is opposed at least in part to the corresponding first internal electrode, and each of the second surface electrodes in each pair is opposed at least in part to the corresponding second internal electrode. 
     More preferably, the aggregate substrate further comprises a plurality of first external electrodes each of which is electrically connected to one first surface electrode out of the first surface electrodes in each pair; and a plurality of second external electrodes each of which is electrically connected to the other first surface electrode out of the first surface electrodes in each pair. 
     Furthermore, preferably, the first varistor part further comprises a plurality of third internal electrodes, the second varistor part further comprises a plurality of fourth internal electrodes, each of the third internal electrodes is opposed to the corresponding first internal electrode in an opposing direction of the first principal face and the second principal face, and each of the fourth internal electrodes is opposed to the corresponding second internal electrode in the opposing direction of the first principal face and the second principal face. 
     More preferably, the aggregate substrate further comprises a plurality of first external electrodes electrically connected to the respective first internal electrodes, and a plurality of second external electrodes electrically connected to the respective second internal electrodes. 
     A production method of an aggregate substrate according to the present invention is a method comprising: a preparation step of preparing a first green sheet containing a varistor material, a second green sheet containing a varistor material and having a plurality of internal electrode patterns formed thereon, and a third green sheet containing a heat dissipation material; a laminating step of laminating the first to third green sheets prepared, to obtain a green laminated body having a first varistor green part, a second varistor green part, and a heat dissipation part; and a firing step of firing the green laminated body to obtain an aggregate substrate, wherein the laminating step comprises laying the third green sheet between a first portion made by at least laying the first green sheet on the second green sheet, and a second portion made by at least laying the first green sheet on the second green sheet, so as to be in contact with the first and second portions, thereby obtaining the green laminated body. 
     In the production method of the aggregate substrate according to the present invention, the third green sheet is sandwiched between the first and second portions, while being in contact with the first and second portions, in the green laminated body obtained. Therefore, it is feasible to suppress occurrence of warpage of the resultant aggregate substrate even if there is difference between contraction of the first and second green sheets and contraction of the third green sheet during firing the first to third green sheets. 
     Preferably, the preparation step comprises further preparing a fourth green sheet containing a varistor material and having a plurality of surface electrode patterns, and the laminating step comprises laying the fourth green sheet so that the plurality of surface electrode patterns are located on a surface of the green laminated body. 
     Preferably, the laminating step comprises laying at least two second green sheets so that the plurality of internal electrode patterns are opposed, in each of the first and second portions. 
     A varistor according to the present invention is a varistor comprising: a first varistor part having a first face and a second face facing each other; a second varistor part having a third face and a fourth face facing each other; a heat dissipation part located between the first and second varistor parts and being in contact with the second and fourth faces; and a pair of external electrodes arranged on the first varistor part, wherein the first varistor part comprises a first varistor element body to exhibit nonlinear voltage-current characteristics, a first internal electrode arranged in the first varistor element body, and a pair of first surface electrodes arranged on the first face and each opposed at least in part to the first internal electrode, wherein the second varistor part comprises a second varistor element body to exhibit nonlinear voltage-current characteristics, a second internal electrode arranged in the second varistor element body, and a pair of second surface electrodes arranged on the third face and each opposed at least in part to the second internal electrode, and wherein each external electrode is electrically connected to the corresponding first surface electrode. 
     Another varistor according to the present invention is a varistor comprising: a first varistor part having a first face and a second face facing each other; a second varistor part having a third face and a fourth face facing each other; a heat dissipation part located between the first and second varistor parts and being in contact with the second and fourth faces; and a pair of external electrodes arranged on the first varistor part, wherein the first varistor part comprises a first varistor element body to exhibit nonlinear voltage-current characteristics, and first and second internal electrodes arranged in the first varistor element body and opposed to each other in an opposing direction of the first and the second faces, wherein the second varistor part comprises a second varistor element body to exhibit nonlinear voltage-current characteristics, and third and fourth internal electrodes arranged in the second varistor element body and opposed to each other in an opposing direction of the third and the fourth faces, and wherein the pair of external electrodes are electrically connected to the first and the second internal electrodes, respectively. 
     Another aggregate substrate according to the present invention is an aggregate substrate comprising: a first varistor part comprising a first varistor element layer to exhibit nonlinear voltage-current characteristics, and a plurality of first internal electrodes juxtaposed in the first varistor element layer; a second varistor part comprising a second varistor element layer to exhibit nonlinear voltage-current characteristics, and a plurality of second internal electrodes juxtaposed in the second varistor layer; and a heat dissipation layer located between the first and second varistor parts and being in contact with the first and second varistor parts. 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a varistor according to the first embodiment. 
         FIG. 2  is a schematic sectional view of the varistor according to the first embodiment. 
         FIG. 3  is a partly enlarged view of the varistor shown in  FIG. 2 . 
         FIG. 4  is a flowchart showing production steps of the varistor according to the first embodiment. 
         FIG. 5  is a schematic plan view of a green laminated body according to the first embodiment. 
         FIG. 6  is schematic sectional views of the green laminated body and an aggregate substrate according to the first embodiment. 
         FIG. 7  is a drawing showing a procedure of forming insulator layers in the varistor according to the first embodiment. 
         FIG. 8  is a drawing showing a procedure of forming the insulator layers and external electrodes in the varistor according to the first embodiment. 
         FIG. 9  is a drawing showing a procedure of forming the external electrodes in the varistor according to the first embodiment. 
         FIG. 10  is a drawing showing a procedure of forming the external electrodes in the varistor according to the first embodiment. 
         FIG. 11  is a schematic sectional view of an aggregate substrate with external electrodes according to the first embodiment. 
         FIG. 12  is a schematic sectional view of a varistor according to the second embodiment. 
         FIG. 13  is schematic sectional views of a green laminated body and an aggregate substrate according to the second embodiment. 
         FIG. 14  is a schematic sectional view of an aggregate substrate with external electrodes according to the second embodiment. 
         FIG. 15  is a schematic sectional view of a varistor according to the third embodiment. 
         FIG. 16  is schematic sectional views of a green laminated body and an aggregate substrate according to the third embodiment. 
         FIG. 17  is a schematic sectional view of a varistor according to the fourth embodiment. 
         FIG. 18  is schematic sectional views of a green laminated body and an aggregate substrate according to the fourth embodiment. 
         FIG. 19  is a schematic sectional view of a varistor according to the fifth embodiment. 
         FIG. 20  is schematic sectional views of a green laminated body and an aggregate substrate according to the fifth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings the same elements will be denoted by the same reference symbols, without redundant description. 
     First Embodiment 
       FIG. 1  is a schematic perspective view of the varistor according to the first embodiment.  FIG. 2  is a schematic sectional view of the varistor according to the first embodiment. As shown in  FIGS. 1 and 2 , the varistor V 1  of the first embodiment has an element body  3  of a nearly rectangular parallelepiped shape, insulating layers  4 ,  5  formed on the top and bottom surfaces of the element body  3 , and a pair of external electrodes  6 ,  7 . The element body  3  has a heat dissipation part  8  of a nearly rectangular parallelepiped shape, and first and second varistor parts  10 ,  20  laid on the top and bottom surfaces of the heat dissipation part  8 . The vertical direction of the element body  3  is defined as a Z-direction in an XYZ orthogonal coordinate system. 
     The first varistor part  10  includes a varistor element body  11 , an internal electrode  12 , and a pair of surface electrodes  13 ,  14 . The varistor element body  11  is of a nearly rectangular parallelepiped shape and has faces  11   a  and  11   b  facing each other in the Z-direction. The varistor element body  11  is a laminated body formed by laminating a plurality of varistor layers in the Z-direction. Each varistor layer exhibits the nonlinear voltage-current characteristics and contains ZnO as a main component and Pr or Bi as an accessory component. The accessory component is present in the form of simple metal or oxide in the varistor layers. The varistor layers are integrally formed in practical varistor V 1  so that no border can be visually recognized between the varistor layers. 
     The internal electrode  12  is a layer of a nearly rectangular shape and is arranged in an approximately central region in the varistor element body  11  so that its principal faces are parallel to the first face  11   a . The pair of surface electrodes  13 ,  14  are layers of a nearly rectangular shape and are arranged in juxtaposition in the X-direction on the face  11   a  of the varistor element body  11 . The pair of surface electrodes  13 ,  14  are arranged apart from each other and electrically isolated from each other. A portion on the surface electrode  14  side in the surface electrode  13  and a portion on the surface electrode  13  side in the surface electrode  14  are opposed to the internal electrode  12  in the Z-direction. 
     The second varistor part  20  includes a varistor element body  21 , an internal electrode  22 , and a pair of surface electrodes  23 ,  24 . The varistor element body  21  is of a nearly rectangular parallelepiped shape and has faces  21   a  and  21   b  facing each other in the Z-direction. 
     The varistor element body  21  is a laminated body formed by laminating a plurality of varistor layers in the Z-direction as the varistor element body  11  is. The internal electrode  22  is a layer of a nearly rectangular shape and is arranged in an approximately central region in the varistor element body  21  so that its principal faces are parallel to the first face  21   a . The pair of surface electrodes  23 ,  24  are layers of a nearly rectangular shape and are arranged in juxtaposition in the X-direction on the face  21   a  of the varistor element body  21 . A portion on the surface electrode  24  side in the surface electrode  23  and a portion on the surface electrode  23  side in the surface electrode  24  are opposed to the internal electrode  22  in the Z-direction. 
     The heat dissipation part  8  is of a nearly rectangular parallelepiped shape and has faces  8   a  and  8   b  facing each other in the Z-direction. The heat dissipation part  8  has a pair of side faces  8   c ,  8   d  facing each other in the X-direction and a pair of side fades  8   e ,  8   f  facing each other in the Y-direction. The face  8   a  of the heat dissipation part  8  is in contact with the face  11   b  in the first varistor part  10 . The face  8   b  of the heat dissipation part  8  is in contact with the face  21   b  in the second varistor part  20 . 
     The heat dissipation part  8  is made of a composite material of metal and metal oxide. Examples of the metal applicable herein include Ag, Ag—Pd, Pd, and so on and the metal is preferably Ag in terms of thermal conductivity. Examples of the metal oxide applicable herein include Al 2 O 3 , ZnO, SiO 2 , and ZrO 2 . The heat dissipation part  8  may be made of particles obtained by coating particles of metal oxide with metal. For example, it is possible to use particles obtained by plating particles of Al 2 O 3  with Ag by electroless deposition. 
     Since the heat dissipation part  8  contains Ag which is metal, heat dissipation paths are established between the face  8   a  in contact with the first varistor part  10  and the side faces  8   c - 8   f . Therefore, heat in the first varistor part  10  is efficiently dissipated from the side faces  8   c - 8   f  of the heat dissipation part  8 . The first varistor part  10  and the second varistor part  20  are arranged in symmetry with respect to the heat dissipation part  8 . 
     The insulating layer  4  is arranged so as to cover the face  11   a  of the varistor element body  11  and the pair of surface electrodes  13 ,  14  in the element body  3 . The insulating layer  5  is arranged so as to cover the face  21   a  of the varistor element body  21  and the pair of surface electrodes  23 ,  24  in the element body  3 . The insulating layers  4 ,  5  are made of polyimide. The insulating layer  4  is provided with apertures  4   a ,  4   b  which are formed at positions corresponding to the pair of surface electrodes  13 ,  14 , respectively. This makes the surfaces of the pair of surface electrodes  13 ,  14  exposed in part from the insulating layer  4 . 
     The pair of external electrodes  6 ,  7  are arranged in juxtaposition and apart from each other in the X-direction on the insulating layer  4 . The external electrode  6  covers the aperture  4   a  of the insulating layer  4  and extends into the aperture  4   a  to come into physical contact with the surface electrode  13  so as to be electrically connected thereto. The external electrode  7  covers the aperture  4   b  of the insulating layer  4  and extends into the aperture  4   b  to come into physical contact with the surface electrode  14  so as to be electrically connected thereto. Each of the external electrodes  6 ,  7 , as shown in  FIG. 3 , is composed of four layers of Cr layer  6   a ,  7   a , Cu layer  6   b ,  7   b , Ni layer  6   c ,  7   c , and Au layer  6   d ,  7   d . This pair of external electrodes  6 ,  7  function as connecting terminals to an electronic device (e.g., a semiconductor light emitting device or the like). 
     Next, a production process of the above-described varistor V 1  will be described. The production process of the varistor V 1  involves first producing an aggregate substrate. A production method of this aggregate substrate, as shown in  FIG. 4 , includes a preparation step S 1  of varistor green sheets, a preparation step S 2  of internal electrode pattern sheets, a preparation step S 3  of surface electrode pattern sheets, a preparation step S 4  of heat dissipation green sheets, a laminating step S 5 , and a firing step S 6 . Each of these steps will be described below. 
     The preparation step S 1  of varistor green sheets is to prepare a predetermined number of varistor green sheets to become varistor layers. First, a varistor material of powder is prepared by mixing ZnO as a main component of the varistor element bodies  11 ,  21 , and metals or oxides of Pr, Co, Cr, Ca, Si, Bi, etc. as accessory components, at a predetermined ratio. Thereafter, an organic binder, an organic solvent, an organic plasticizer, etc. are added into this varistor material to obtain a slurry. This slurry is applied onto film and thereafter dried to obtain varistor green sheets. 
     The preparation step S 2  of internal electrode pattern sheets is to form a plurality of internal electrode patterns on two varistor green sheets. An internal electrode pattern formed on one varistor green sheet out of the two becomes the internal electrode  12  and an internal electrode pattern formed on the other varistor green sheet becomes the internal electrode  22 . The internal electrode patterns are formed by printing an electroconductive paste obtained by mixing an organic binder and an organic solvent in a metal powder consisting primarily of Ag particles, onto the varistor green sheets and drying it. 
     The preparation step S 3  of surface electrode pattern sheets is to form plural pairs of surface electrode patterns on two varistor green sheets. Each of the plural pairs of surface electrode patterns formed on one varistor green sheet becomes the surface electrodes  13 ,  14  and each of the plural pairs of surface electrode patterns formed on the other varistor green sheet becomes the surface electrodes  23 ,  24 . The surface electrode patterns can be formed with the same electroconductive paste and in the same manner as the internal electrode patterns. 
     The preparation step S 4  of heat dissipation green sheets is to prepare a predetermined number of heat dissipation green sheets to constitute the heat dissipation part  8 . First, a heat dissipation material (e.g., Ag powder) is mixed in the aforementioned varistor material and an organic binder, an organic solvent, an organic plasticizer, etc. are added therein to obtain a slurry. This slurry is applied onto film and then dried to obtain heat dissipation green sheets. The above preparation steps result in preparing the predetermined numbers of varistor green sheets, internal electrode pattern sheets, surface electrode pattern sheets, and heat dissipation green sheets. 
     The subsequent laminating step S 5  is to laminate the varistor green sheets, internal electrode pattern sheets, surface electrode pattern sheets, and heat dissipation green sheets to form a green laminated body. Specifically, the green laminated body shown in  FIGS. 5 and 6  ( a ) is made by laminating the varistor green sheets with neither of the internal electrode patterns and the surface electrode patterns, the varistor green sheets with the internal electrode patterns thereon, the varistor green sheets with the surface electrode patterns thereon, and the heat dissipation green sheets in a predetermined order, pressing them, and cutting the laminate in the lamination direction (Z-direction). 
       FIG. 5  is a schematic plan view of the green laminated body and  FIG. 6(   a ) a schematic sectional view of the green laminated body. The green laminated body  300  contains a plurality of green element assemblies  30  to become element assemblies  3  after fired.  FIGS. 5 and 6  show the green laminated body  300  containing thirty green element assemblies arranged in a matrix of five columns in the X-direction and six rows in the Y-direction, for convenience’ sake of illustration, but a practical green laminated body  300  contains a larger number of green element assemblies  30 . 
     The green laminated body  300  has a heat dissipation green part  308  to become the heat dissipation part  8 , a first varistor green part  310  to become the first varistor part  10 , and a second varistor green part  320  to become the second varistor part  20 . 
     The first varistor green part  310  is formed by laminating a varistor green sheet with a plurality of internal electrode patterns  312 , a varistor green sheet with plural pairs of surface electrode patterns  313 ,  314 , and varistor green sheets without any electrode pattern in a predetermined order in the Z-direction. This leads the first varistor green part  310  to have a varistor green layer  311 , a plurality of internal electrode patterns  312 , and plural pairs of surface electrode patterns  313 ,  314 . 
     The varistor green layer  311  is composed of a lamination of varistor green sheets and has a principal face  311   a  and a principal face  311   b  facing each other in the Z-direction. The plurality of internal electrode patterns  312  are arranged in the varistor green layer  311  and are juxtaposed in extending directions of the varistor green sheets (the X-direction and Y-direction). 
     The varistor green sheet constituting the principal face  311   a  of the varistor green layer  311  is the one with plural pairs of surface electrode patterns  313 ,  314  thereon. This allows the plural pairs of surface electrode patterns  313 ,  314  to be arranged on the principal face  311   a  of the varistor green layer  311 . These plural pairs of surface electrode patterns  313 ,  314  are arranged so that a pair of surface electrode patterns  313 ,  314  are opposed each to one internal electrode pattern  312 . These surface electrode patterns  313 ,  314  are located on a surface of the green laminated body  300 . 
     The second varistor green part  320  is formed by laminating a varistor green sheet with a plurality of internal electrode patterns  312  thereon, a varistor green sheet with plural pairs of surface electrode patterns  313 ,  314  thereon, and varistor green sheets without any electrode pattern in a predetermined order in the Z-direction. This leads the second varistor green part  320  to have a varistor green layer  321 , a plurality of internal electrode patterns  312 , and plural pairs of surface electrode patterns  313 ,  314 . These surface electrode patterns  313 ,  314  are also located on a surface of the green laminated body  300 . 
     The varistor green layer  321  is composed of a lamination of varistor green sheets and has a principal face  321   a  and a principal face  321   b  facing each other in the Z-direction. The plurality of internal electrode patterns  312  are arranged in the varistor green layer  321  and juxtaposed in the extending directions of the varistor green sheets (the X-direction and Y-direction). 
     The varistor green sheet constituting the principal face  321   a  of the varistor green layer  321  is the one with plural pairs of surface electrode patterns  313 ,  314  thereon. This allows the plural pairs of surface electrode patterns  313 ,  314  to be arranged on the principal face  321   a  of the varistor green layer  321 . These pairs of surface electrode patterns  313 ,  314  are arranged so that a pair of surface electrode patterns  313 ,  314  are opposed each to one internal electrode pattern  312 . 
     The heat dissipation green part  308  is formed by laminating the heat dissipation green sheets in the Z-direction, and has a principal face  308   a  and a principal face  308   b  facing each other in the Z-direction. The principal face  308   a  of the heat dissipation green part  308  is in contact with the principal face  311   b  of the first varistor green part  310 . Furthermore, the principal face  308   b  of the heat dissipation green part  308  is in contact with the principal face  321   b  of the second varistor green part  320 . The first varistor green part  310  and the second varistor green part  320  are arranged in symmetry with respect to the heat dissipation green part  308 . 
     The next firing step S 6  is to perform a debindering process of the resultant green laminated body  300 . The green laminated body  300  is heated, for example, at the temperature of 180° C.-400° C. and for about 0.5 hour to 24 hours, so as to be debindered. After completion of the debindering process of the green laminated body  300 , it is fired at the temperature of not less than 800° C. in an O 2  atmosphere to form an aggregate substrate  31  shown in  FIG. 6(   b ). 
     The aggregate substrate  31  has a heat dissipation layer  9  made by firing of the heat dissipation green part  308 , a first varistor part  19  made by firing of the first varistor green part  310 , and a second varistor part  29  made by firing of the second varistor green part  320 . 
     The first varistor part  19  includes a varistor element layer  18  made by firing of the varistor green layer  311 , a plurality of internal electrodes  12  made by firing of the plurality of internal electrode patterns  312 , and plural pairs of surface electrodes  13 ,  14  made by firing of the plural pairs of surface electrode patterns  313 ,  314 . The varistor element layer  18  has a principal face  18   a  made by firing of the varistor green layer  311 , and a principal face  18   b  made by firing of the varistor green layer  311 . 
     The second varistor part  29  includes a varistor element layer  28  made by firing of the varistor green layer  321 , a plurality of internal electrodes  22  made by firing of the plurality of internal electrode patterns  312 , and surface electrodes  23 ,  24  made by firing of the surface electrode patterns  313 ,  314 . The varistor element layer  28  has a principal face  28   a  made by firing of the varistor green layer  321 , and a principal face  28   b  made by firing of the varistor green layer  321 . 
     The heat dissipation layer  9  has a principal face  9   a  made by firing of the heat dissipation green part  308 , and a principal face  9   b  made by firing of the heat dissipation green part  308 . The heat dissipation green sheets and the varistor green sheets contain the common component ZnO. Since the debindering and firing are carried out in the state in which the principal face  308   a  of the heat dissipation green part  308  is in contact with the principal face  311   b  of the first varistor green part  310 , the heat dissipation layer  9  and the first varistor part  19  are more firmly joined together. Similarly, since the debindering and firing are carried out in the state in which the principal face  308   b  of the heat dissipation green part  308  is in contact with the principal face  321   b  of the second varistor green part  320 , the heat dissipation layer  9  and the second varistor part  29  are more firmly joined together. The first varistor part  19  and the second varistor part  29  are arranged in symmetry with respect to the heat dissipation layer  9 . 
     There is difference between contraction caused by firing of the heat dissipation green part  308  and contraction caused by firing of the first and second varistor green parts  310 ,  320 . However, since the heat dissipation green part  308  is sandwiched between the first varistor green part  310  and the second varistor green part  320  with the first varistor green part  310  being in contact with the principal face  308   a  of the heat dissipation green part  308  and with the second varistor green part  320  being in contact with the principal face  308   b  of the heat dissipation green part  308 , the aggregate substrate  31  of planar shape can be formed while preventing occurrence of warpage during the firing. 
     After the aggregate substrate  31  is formed through the above steps, an insulating layer forming step S 7  and an external electrode forming step S 8  are carried out to produce an aggregate substrate with external electrodes. The insulating layer forming step S 7  and the external electrode forming step S 8  will be described with reference to  FIGS. 7 to 10 .  FIGS. 7 to 10  show only a part corresponding to one element body  3  in the aggregate substrate  31 , for convenience sake of illustration, but it should be noted that the whole aggregate substrate  31  is subjected to the same processing in fact. 
     First, the insulating layer forming step S 7  includes forming an insulating layer on each of the principal face  18   a  of the first varistor part  19  and the principal face  28   a  of the second varistor part  29  shown in  FIG. 7(   a ). As shown in  FIG. 7(   b ), a raw solution of photosensitive polyimide is applied onto the principal face  18   a  of the first varistor part  19  and onto the principal face  28   a  of the second varistor part  29  by spin coating, and then precured and dried to form precured polyimide layers  41 ,  42 . 
     Next, as shown in  FIG. 7(   c ), a negative mask  43  of glass is placed on the polyimide layer  41 , in order to form apertures in the polyimide layer  41  formed on the principal face  18   a , and exposure is performed. Subsequently, as shown in  FIG. 8(   a ), the entire aggregate substrate  31  is immersed in a Na-base aqueous solution  44  to effect development, thereby forming apertures  41   a ,  41   b . The surface electrodes  13 ,  14  are exposed in part through the apertures  41   a ,  41   b . The apertures  41   a ,  41   b  correspond to the apertures  4   a ,  4   b  of the varistor V 1 . 
     Thereafter, the substrate is washed with pure water and then the polyimide layers  41 ,  42  are subjected to main curing/drying, thereby forming insulating layers  45 ,  46 , as shown in  FIG. 8(   b ). The above process forms the insulating layers  45 ,  46  to become the insulating layers  4 ,  5 . 
     The external electrode forming step S 8  is to form plural pairs of external electrodes  6 ,  7 . First, as shown in  FIG. 8(   b ), a Cr layer  47 , which covers the insulating layer  45 , and the exposed portions of the surface electrodes  13 ,  14  exposed from the apertures  45   a ,  45   b  of the insulating layer  45 , is formed by sputtering. Subsequently, a Cu layer  48  is formed on the Cr layer  47  by sputtering. Then, as shown in  FIG. 8(   c ), dry film  49  is pasted onto the Cu layer  48 . 
     As shown in  FIG. 9(   a ), a mask  50  corresponding to the shape of the external electrodes  6 ,  7  is placed on the dry film  49  and exposure is performed. Subsequently, as shown in  FIG. 9(   b ), the aggregate substrate  31  is immersed in a developer solution  51  to effect development, whereby the dry film  49  is shaped corresponding to the shape of the external electrodes  6 ,  7 . After the development, as shown in  FIG. 9(   c ), the aggregate substrate  31  is immersed in an etching solution  59  to etch the Cu layer  48  to form Cu layers  6   b ,  7   b , followed by washing with pure water. 
     Subsequently, as shown in  FIG. 10(   a ), the aggregate substrate  31  is immersed in a remover solution  53  to remove the dry film  49 . Then, as shown in  FIG. 10(   b ), the aggregate substrate  31  is immersed in an etching solution  54  to etch the Cr layer  47 , thereby forming Cr layers  6   a ,  7   a . Thereafter, the aggregate substrate  31  is washed with pure water and then dried. 
     Thereafter, the surfaces of the Cu layers  6   b ,  7   b  are plated with Ni to form Ni layers  6   c ,  7   c , and then the aggregate substrate is immersed in a plating solution  55  to effect flash plating, thereby forming Au layers  6   d ,  7   d . This step results in forming the external electrodes  6 ,  7  composed of the Cr layer  6   a ,  7   a , Cu layer  6   b ,  7   b , Ni layer  6   c ,  7   c , and Au layer  6   d ,  7   d.    
     The aggregate substrate  32  with external electrodes shown in  FIG. 11  is obtained through the above steps. The aggregate substrate  32  with external electrodes has the aggregate substrate  32 , the insulating layers  45 ,  46 , and plural pairs of external electrodes  6 ,  7 . The insulating layers  45 ,  46  correspond to the insulating layers  4 ,  5 , respectively. The aggregate substrate  32  with external electrodes is then cut to obtain a plurality of varistors V 1  (cutting step S 9 ). 
     In the varistors V 1  formed as described above, the heat dissipation part  8  contains ZnO being the main component of the varistor element bodies  11 ,  21 . During the firing, Ag in the heat dissipation part  8  diffuses into grain boundaries of ZnO in the varistor element bodies  11 ,  21  near the interface between the face  11   b  and the face  8   a  and near the interface between the face  21   b  and the face  8   b . This leads the first varistor part  10  and the heat dissipation part  8  to be firmly joined together and the second varistor part  20  and the heat dissipation part  8  to be firmly joined together. 
     In the varistors V 1 , therefore, there is little cracking between the first varistor part  10  and the heat dissipation part  8  and between the second varistor part  20  and the heat dissipation part  8  during the firing (or during the debindering), which ensures sufficient joint strength between the first varistor part  10  and the heat dissipation part  8  and sufficient joint strength between the second varistor part  20  and the heat dissipation part  8 . Therefore, heat transferred from an electronic device through the external electrodes  6 ,  7  to the first varistor part  10  is efficiently dissipated through conduction paths formed from the face  8   a  to the side faces  8   c - 8   f  in the heat dissipation part  8  by Ag particles and coating portions of Al 2 O 3 . 
     In the production process of the varistors V 1 , the first and second varistor parts  10 ,  20  and the heat dissipation part  8  are simultaneously fired. This realizes simplification of the production process and achieves improvement in production efficiency of the varistors V 1  and reduction of cost thereof. 
     There is the difference due to the difference of composition between the contraction caused by firing of the heat dissipation green part  308  (heat dissipation part  8 ) and the contraction caused by the firing of the first and second varistor green parts  310 ,  320  (first varistor part  10  and second varistor part  20 ). However, since the heat dissipation green part  308  is sandwiched between the first varistor green part  310  and the second varistor green part  320  with the first varistor green part  310  being in contact with the principal face  308   a  of the heat dissipation green part  308  and with the second varistor green part  320  being in contact with the principal face  308   b  of the heat dissipation green part  308 , the aggregate substrate  31  of planar shape can be formed while suppressing occurrence of warpage during the firing. Since the individual varistors V 1  are obtained by forming the external electrodes  6 ,  7  on the planar aggregate substrate  31  and cutting it, the plurality of varistors V 1  with good heat dissipation efficiency can be readily produced. 
     Second Embodiment 
     The varistor according to the second embodiment of the present invention will be described.  FIG. 12  is a schematic sectional view showing the varistor according to the second embodiment of the present invention. The varistor V 2  shown in  FIG. 12  has no surface electrode and is different in a configuration of internal electrodes from the varistor V 1  of the first embodiment. The varistor V 2  has an element body  3 A instead of the element body  3  and this element body  3 A has first and second varistor parts  60 ,  70  instead of the first and second varistor parts  10 ,  20 . 
     The first varistor part  60  includes a varistor element body  61  of a nearly rectangular parallelepiped shape, a pair of internal electrodes  62 ,  63  facing each other in the varistor element body  61 , and penetrating conductors  64 ,  65 . The varistor element body  61  has a face  61   a  and a face  61   b  facing each other in the Z-direction. An insulating layer  4  is arranged on the face  61   a  and the face  61   b  is in contact with the face  8   a  of the heat dissipation part  8 . The internal electrodes  62 ,  63  are opposed in part to each other in the Z-direction as shifted relative to each other in the X-direction. 
     The penetrating conductor  64  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  62  and the other end of which is exposed from the face  61   a . The other end of the penetrating conductor  64  is located in the aperture  4   a  of the insulating layer  4  and is physically and electrically connected to the external electrode  6 . The penetrating conductor  65  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  63  and the other end of which is exposed from the face  61   a . The other end of the penetrating conductor  65  is located in the aperture  4   b  of the insulating layer  4  and is physically and electrically connected to the external electrode  7 . Namely, the internal electrode  62  is electrically connected through the penetrating conductor  64  to the external electrode  6  and the internal electrode  63  is electrically connected through the penetrating conductor  65  to the external electrode  7 . 
     The second varistor part  70  includes a varistor element body  71  of a nearly rectangular parallelepiped shape, a pair of internal electrodes  72 ,  73  facing each other in the varistor element body  71 , and penetrating conductors  74 ,  75 . The varistor element body  71  has a face  71   a  and a face  71   b  facing each other in the Z-direction. An insulating layer  5  is arranged on the face  71   a  and the face  71   b  is in contact with the face  8   b  of the heat dissipation part  8 . The internal electrodes  72 ,  73  are opposed in part to each other in the Z-direction as shifted relative to each other in the X-direction. 
     The penetrating conductor  74  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  72  and the other end of which is exposed from the face  71   a . The other end of the penetrating conductor  74  is covered by the insulating layer  5 . The penetrating conductor  75  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  73  and the other end of which is exposed from the face  71   a . The other end of the penetrating conductor  75  is covered by the insulating layer  5 . The first varistor part  60  and the second varistor part  70  are arranged in symmetry with respect to the heat dissipation part  8 . 
     A production method of this varistor V 2  will be described. The varistor V 2  is produced by a production method similar to that of the varistor V 1  in the first embodiment, but, because of the difference in the configuration of the internal electrodes  62 ,  63 ,  72 ,  73  in the first and second varistor parts  60 ,  70 , the process is partly different in the green laminated body formed in the laminating step S 5  and in the configuration of the aggregate substrate formed in the firing step S 6 . The difference will be explained with reference to  FIGS. 13 and 14 . 
       FIG. 13(   a ) is a schematic sectional view of the green laminated body. The green laminated body  300 A of the second embodiment includes a plurality of green element assemblies  30 A. This green laminated body  300 A includes a heat dissipation green part  308  to become the heat dissipation part  8 , a first varistor green part  360  to become the first varistor part  60 , and a second varistor green part  370  to become the second varistor part  70 . 
     The first varistor green part  360  is formed by laminating a varistor green sheet with internal electrode patterns  362  thereon, a varistor green sheet with internal electrode patterns  363  thereon, and varistor green sheets without any electrode pattern in a predetermined order in the Z-direction. 
     In the varistor green sheets, through holes are preliminarily formed at positions corresponding to the penetrating conductors and these through holes are filled with a conductor paste. Penetrating conductor patterns  364 ,  365  are formed by laminating the varistor green sheets with the conductor paste in the through holes, as well as the internal electrode patterns  362 ,  363  thereon. 
     This leads the first varistor green part  360  to have a varistor green layer  361 , a plurality of internal electrode patterns  362 , a plurality of internal electrode patterns  363 , a plurality of penetrating conductor patterns  364 , and a plurality of penetrating conductor patterns  365 . 
     The varistor green layer  361  is composed of a lamination of varistor green sheets and has a principal face  361   a  and a principal face  361   b  facing each other in the Z-direction. The plurality of internal electrode patterns  362  are arranged in the varistor green layer  361  and juxtaposed in the extending directions of the varistor green sheets (the X-direction and Y-direction). The plurality of internal electrode patterns  363  are arranged as opposed in the Z-direction to the respective internal electrode patterns  362 . 
     The plurality of penetrating conductor patterns  364  extend in the Z-direction, one end of each of which is in physical contact with the corresponding one of the plurality of internal electrode patterns  362  and the other end of each of which is exposed from the principal face  361   a . The plurality of penetrating conductor patterns  365  extend in the Z-direction, one end of each of which is in physical contact with the corresponding one of the plurality of internal electrode patterns  363  and the other end of each of which is exposed from the principal face  361   a.    
     The second varistor green part  370  has a varistor green layer  371 , a plurality of internal electrode patterns  372 , a plurality of internal electrode patterns  373 , a plurality of penetrating conductor patterns  374 , and a plurality of penetrating conductor patterns  375 . The varistor green layer  371  has a principal face  371   a  and a principal face  371   b  facing each other in the Z-direction. The plurality of internal electrode patterns  372  are arranged in the varistor green layer  371  and juxtaposed in the extending directions of the varistor green sheets (the X-direction and Y-direction). The plurality of internal electrode patterns  373  are arranged as opposed in the Z-direction to the respective internal electrode patterns  372 . 
     The plurality of penetrating conductor patterns  374  extend in the Z-direction, one end of each of which is in physical contact with the corresponding one of the plurality of internal electrode patterns  372  and the other end of each of which is exposed from the principal face  371   a . The plurality of penetrating conductor patterns  375  extend in the Z-direction, one end of each of which is in physical contact with the corresponding one of the plurality of internal electrode patterns  373  and the other end of each of which is exposed from the principal face  371   a.    
     The principal face  308   a  of the heat dissipation green part  308  is in contact with the principal face  361   b  of the first varistor green part  360 . The principal face  308   b  of the heat dissipation green part  308  is in contact with the principal face  371   b  of the second varistor green part  370 . The first varistor green part  360  and the second varistor green part  370  are arranged in symmetry with respect to the heat dissipation green part  308 . 
     An aggregate substrate  31 A according to the second embodiment will be explained below with reference to  FIG. 13(   b ). The aggregate substrate  31 A includes a plurality of element assemblies  3 A. This aggregate substrate  31 A has a heat dissipation layer  9  made by firing of the heat dissipation green part  308 , a first varistor part  69  made by firing of the first varistor green part  360 , and a second varistor part  79  made by firing of the second varistor green part  370 . 
     The first varistor part  69  includes a varistor element layer  68  made by firing of the varistor green layer  361 , a plurality of internal electrodes  62  made by firing of the plurality of internal electrode patterns  362 , a plurality of internal electrodes  63  made by firing of the plurality of internal electrode patterns  363 , a plurality of penetrating conductors  64  made by firing of the plurality of penetrating conductor patterns  364 , and a plurality of penetrating conductors  65  made by firing of the plurality of penetrating conductor patterns  365 . The varistor element layer  68  has a principal face  68   a  made by firing of the varistor green layer  361 , and a face  68   b  made by firing of the varistor green layer  361 . 
     The second varistor part  79  includes a varistor element layer  78  made by firing of the varistor green layer  371 , a plurality of internal electrodes  72  made by firing of the plurality of internal electrode patterns  372 , a plurality of internal electrodes  73  made by firing of the plurality of internal electrode patterns  373 , a plurality of penetrating conductors  74  made by firing of the plurality of penetrating conductor patterns  374 , and a plurality of penetrating conductors  75  made by firing of the plurality of penetrating conductor patterns  375 . The varistor element layer  78  has a principal face  78   a  made by firing of the varistor green layer  371 , and a principal face  78   b  made by firing of the varistor green layer  371 . 
     An aggregate substrate  32 A with external electrodes shown in  FIG. 14  is obtained by forming insulating layers  45 ,  46  on the aggregate substrate  31 A and forming plural pairs of external electrodes  6 ,  7 . Each of the plural pairs of external electrodes  6 ,  7  are physically and electrically connected to the corresponding penetrating conductors  64 ,  65 , respectively. A plurality of varistors V 2  are obtained by cutting the aggregate substrate  32 A with external electrodes. 
     In the varistors V 2 , the varistor element bodies  61 ,  71  contain ZnO as a main component and the heat dissipation part  8  is made of a composite material of metal Ag and metal oxide including ZnO as the main component of the varistor element bodies  61 ,  71 . Therefore, as in the first embodiment, sufficient joint strength is ensured between the first varistor part  60  and the heat dissipation part  8  and heat transferred from an electronic device through the external electrodes  6 ,  7  to the varistor part  60  is efficiently dissipated through conduction paths formed from the face  8   a  to the side faces  8   c - 8   f  in the heat dissipation part  8 . Sufficient joint strength is also ensured between the second varistor part  70  and the heat dissipation part  8 . 
     There is difference between contraction caused by firing of the heat dissipation green part  308  (heat dissipation part  8 ) and contraction caused by firing of the first and second varistor green parts  360 ,  370  (first and second varistor parts  60 ,  70 ). However, since the heat dissipation green part  308  is sandwiched between the first varistor green part  360  and the second varistor green part  370  with the first varistor green part  360  being in contact with the principal face  308   a  of the heat dissipation green part  308  and with the second varistor green part  370  being in contact with the principal face  308   b  of the heat dissipation green part  308 , the aggregate substrate  31 A of planar shape can be formed while suppressing occurrence of warpage during the firing. Since the individual varistors V 2  are obtained by forming the external electrodes  6 ,  7  on the planar aggregate substrate  31 A and cutting it, the plurality of varistors V 2  with good heat dissipation efficiency can be readily produced. 
     Third Embodiment 
     The varistor according to the third embodiment of the present invention will be described below.  FIG. 15  is a schematic sectional view showing the varistor according to the third embodiment of the present invention. The varistor V 3  shown in  FIG. 15  has an element body  3 B, insulating layers  4 ,  5 , a 300 pair of external electrodes  6 ,  7 , and a pair of external electrodes  76 ,  77 . The element body  3 B has a first varistor part  60 , a second varistor part  70 , and a heat dissipation part  80 . 
     The first varistor part  60  includes penetrating conductors  85 ,  86 , in addition to the aforementioned internal electrodes  62 ,  63  and penetrating conductors  64 ,  65 . The penetrating conductor  85  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  62  and the other end of which is exposed from the face  61   b . The penetrating conductor  86  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  63  and the other end of which is exposed from the face  61   b.    
     The second varistor part  70  includes penetrating conductors  87 ,  88 , in addition to the aforementioned internal electrodes  72 ,  73  and penetrating conductors  74 ,  75 . The penetrating conductor  87  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  72  and the other end of which is exposed from the face  71   b . The penetrating conductor  88  extends in the Z-direction, one end of which is physically and electrically connected to the internal electrode  73  and the other end of which is exposed from the face  71   b.    
     Apertures  5   a ,  5   b  are formed at positions corresponding to the penetrating conductors  74 ,  75  in the insulating layer  5 . The external electrode  76  is formed so as to cover the aperture  5   a  and is physically and electrically connected to the penetrating conductor  74 . The external electrode  77  is formed so as to cover the aperture  5   b  and is physically and electrically connected to the penetrating conductor  75 . 
     The heat dissipation part  80  has a face  80   a  and a face  80   b  facing each other in the Z-direction. The heat dissipation part  80  is made of a material similar to that of the heat dissipation part  8 . The heat dissipation part  80  includes two penetrating conductors  81 ,  82  penetrating the face  80   a  and the face  80   b , and electrically insulating layers  83 ,  84  formed around the penetrating conductors  81 ,  82 , respectively. 
     The penetrating conductor  81  extends in the Z-direction, one end of which is physically and electrically connected to the penetrating conductor  85  and the other end of which is physically and electrically connected to the penetrating conductor  87 . This causes the external electrode  6  and external electrode  76  to be electrically connected through the penetrating conductors  64 ,  85 ,  81 ,  87 ,  74 . The penetrating conductor  82  extends in the Z-direction, one end of which is physically and electrically connected to the penetrating conductor  86  and the other end of which is physically and electrically connected to the penetrating conductor  88 . This causes the external electrode  7  and external electrode  77  to be electrically connected through the penetrating conductors  65 ,  86 ,  82 ,  88 ,  75 . The first varistor part  60  and the second varistor part  70  are arranged in symmetry with respect to the heat dissipation part  8 . 
     The varistor V 3  operates as follows: when the external electrodes  6 ,  7  are connected to an electronic device, the second varistor part  70 , as well as the first varistor part  60 , is also connected in parallel to the electronic device and the second varistor part  70  also exercises the function to protect the electronic device from the ESD surge. In the varistor V 3 , the external electrodes  6 ,  7  may be used as connecting terminals to the electronic device, or the external electrodes  76 ,  77  may be used as connecting terminals to the electronic device. It is also possible to use the external electrodes  6 ,  7  as connecting terminals to an electronic device and the external electrodes  76 ,  77  as connecting terminals to a substrate. 
     A production method of this varistor V 3  will be explained. The varistor V 3  is produced by a production method similar to that of the varistor V 2  according to the second embodiment, but, because of the presence of the penetrating conductors  81 ,  82  and layers  83 ,  84  in the heat dissipation part  80 , the process is partly different in the green laminated body formed in the laminating step S 5  and the configuration of the aggregate substrate formed in the firing step S 6 . The difference will be explained below with reference to  FIG. 16 . 
       FIG. 16(   a ) is a schematic sectional view of the green laminated body. The green laminated body  300 B of the third embodiment includes a plurality of green element assemblies  30 B. The green laminated body  300 B includes a heat dissipation green part  380  to become the heat dissipation part  80 , a first varistor green part  360 , and a second varistor green part  370 . 
     The heat dissipation green part  380  is formed by laminating heat dissipation green sheets in the Z-direction. Through holes are preliminarily formed in the heat dissipation green sheets and the interior of the through holes is filled with an insulating material to form layers  383 ,  384 . Thereafter, through holes are formed in the central regions of the respective portions filled with the insulating material and a conductor paste is charged into the through holes. The heat dissipation green sheets are laminated to form a plurality of penetrating conductor patterns  381 ,  382  covered by the respective layers  383 ,  384 . 
     The heat dissipation green part  380  has a principal face  380   a  and a principal face  380   b  facing each other in the Z-direction. The principal face  380   a  of this heat dissipation green part  380  is in contact with the principal face  361   b  of the first varistor green part  360 . The penetrating conductor patterns  381 ,  382  in the heat dissipation green part  380  are physically connected to the penetrating conductor patterns  385 ,  386 , respectively, in the first varistor green part  360 . The principal face  380   b  of the heat dissipation green part  380  is in contact with the principal face  371   b  of the second varistor green part  370 . The penetrating conductor patterns  381 ,  382  in the heat dissipation green part  380  are physically connected to the penetrating conductor patterns  387 ,  388 , respectively, in the second varistor green part  370 . The first varistor green part  360  and the second varistor green part  370  are arranged in symmetry with respect to the heat dissipation green part  380 . 
     Subsequently, an aggregate substrate  31 B of the third embodiment will be explained with reference to  FIG. 16(   b ). The aggregate substrate  31 B includes a plurality of element assemblies  3 B. The aggregate substrate  31 B includes a heat dissipation layer  89  made by firing of the heat dissipation green part  380 , a first varistor part  69 , and a second varistor part  79 . The first varistor part  69  and the second varistor part  79  are arranged in symmetry with respect to the heat dissipation layer  89 . 
     An aggregate substrate with external electrodes is obtained by forming insulating layers  45 ,  46  on the aggregate substrate  31 B and forming plural pairs of external electrodes  6 ,  7  and plural pairs of external electrodes  76 ,  77 . A plurality of varistors V 3  are obtained by cutting the aggregate substrate with external electrodes thus obtained. 
     In the varistors V 3 , the varistor element bodies  61 ,  71  also contain ZnO as a main component and the heat dissipation part  8  is made of a composite material of metal Ag and metal oxide including ZnO as the main component of the varistor element bodies  61 ,  71 . Therefore, sufficient joint strength is ensured between the first varistor part  60  and the heat dissipation part  80  and heat transferred from an electronic device through the external electrodes  6 ,  7  to the varistor part  60  is efficiently dissipated through conduction paths formed from the face  80   a  to the exposed side faces in the heat dissipation part  80 . Sufficient joint strength is also ensured between the second varistor part  70  and the heat dissipation part  80  and heat transferred from an electronic device through the external electrodes  76 ,  77  to the varistor part  70  is efficiently dissipated through conduction paths formed from the face  80   b  to the exposed side faces in the heat dissipation part  80 . 
     There is difference between contraction caused by firing of the heat dissipation green part  380  (heat dissipation part  80 ) and contraction caused by firing of the first and second varistor green parts  360 ,  370  (first varistor part  60  and second varistor part  70 ). However, since the heat dissipation green part  380  is sandwiched between the first varistor green part  360  and the second varistor green part  370  with the first varistor green part  360  being in contact with the principal face  380   a  of the heat dissipation green part  380  and with the second varistor green part  370  being in contact with the principal face  380   b  of the heat dissipation green part  380 , the aggregate substrate  31 B of planar shape can be formed while suppressing occurrence of warpage during the firing. Since the individual varistors V 3  are obtained by forming the external electrodes  6 ,  7 ,  76 ,  77  on the planar aggregate substrate  31 B and cutting it, the plurality of varistors V 3  with good heat dissipation efficiency can be readily produced. 
     Fourth Embodiment 
     The varistor according to the fourth embodiment of the present invention will be described,  FIG. 17  is a schematic sectional view showing the varistor according to the fourth embodiment of the present invention. The varistor V 4  shown in  FIG. 17  is different in the configuration of internal electrodes in the first and second varistor parts from the varistor V 1 . The varistor V 4  has an element body  3 C instead of the element body  3  and the element body  3 C has a first varistor part  90 , a second varistor part  100 , and a heat dissipation part  8 . 
     The first varistor part  90  includes a varistor element body  91 , internal electrodes  92   a - 94   a ,  92   b - 94   b ,  95 - 97 , a pair of surface electrodes  98   a ,  98   b , and penetrating conductors  99   a ,  99   b . The varistor element body  91  has a face  91   a  and a face  91   b  facing each other in the Z-direction. 
     The internal electrodes  92   a - 94   a ,  92   b - 94   b ,  95 - 97  are arranged in the varistor element body  91 . The internal electrodes  92   a ,  92   b  are arranged in juxtaposition in the X-direction. The internal electrode  95  is arranged above the internal electrodes  92   a ,  92   b  so that the internal electrode  95  is opposed in the Z-direction through a varistor layer to center-side portions of the internal electrodes  92   a ,  92   b . Similarly, each pair of the internal electrodes  93   a ,  93   b  and the internal electrodes  94   a ,  94   b  are also arranged in juxtaposition in the X-direction, the internal electrodes  93   a ,  93   b  are arranged through a varistor layer above the internal electrode  95 , the internal electrode  96  is arranged through a varistor layer above them, the internal electrodes  94   a ,  94   b  are arranged through a varistor layer above it, and the internal electrode  97  is arranged above them. 
     The surface electrodes  98   a ,  98   b  are arranged on the face  91   a  of the varistor element body  91  and the center-side portions of the respective surface electrodes  98   a ,  98   b  are opposed to the internal electrode  97 . When viewed from the Z-direction, the internal electrodes  92   a - 94   a  and the surface electrode  98   a  overlap with each other, the internal electrodes  92   b - 94   b  and surface electrode  98   b  overlap with each other, and the internal electrodes  95 - 97  overlap with each other. 
     Each of the internal electrodes  92   a - 94   a  and the surface electrode  98   a  is physically and electrically connected to the penetrating conductor  99   a  extending in the Z-direction. Each of the internal electrodes  92   b - 94   b  and the surface electrode  98   b  is physically and electrically connected to the penetrating conductor  99   b  extending in the Z-direction. Since the surface electrodes  98   a ,  98   b  are electrically connected to the external electrodes  6 ,  7 , respectively, the internal electrodes  92   a - 94   a  and the internal electrodes  92   b - 94   b  are electrically connected to the external electrodes  6 ,  7 , respectively. 
     The second varistor part  100  includes a varistor element body  101 , internal electrodes  102   a - 104   a ,  102   b - 104   b ,  105 - 107 , a pair of surface electrodes  108   a ,  108   b , and penetrating conductors  109   a ,  109   b . The varistor element body  101  has a face  101   a  and a face  101   b  facing each other in the Z-direction. 
     The internal electrodes  102   a - 104   a ,  102   b - 104   b ,  105 - 107  are arranged in the varistor element body  101 . The internal electrodes  102   a ,  102   b  are arranged in juxtaposition in the X-direction. The internal electrode  105  is arranged below the internal electrodes  92   a ,  92   b  so that the internal electrode  105  is opposed in the Z-direction through a varistor layer to center-side portions of the internal electrodes  102   a ,  102   b . Similarly, each pair of the internal electrodes  103   a ,  103   b  and the internal electrodes  104   a ,  104   b  are arranged in juxtaposition in the X-direction, the internal electrodes  103   a ,  103   b  are arranged through a varistor layer below the internal electrode  105 , the internal electrode  106  is arranged through a varistor layer below them, the internal electrodes  104   a ,  104   b  are arranged through a varistor layer below it, and the internal electrode  107  is arranged below them. 
     The surface electrodes  108   a ,  108   b  are arranged on the face  101   a  of the varistor element body  101  and the center-side portions of the respective surface electrodes  108   a ,  108   b  are opposed to the internal electrode  107 . When viewed from the Z-direction, the internal electrodes  102   a - 104   a  and the surface electrode  108   a  overlap with each other, the internal electrodes  102   b - 104   b  and the surface electrode  108   b  overlap with each other, and the internal electrodes  105 - 107  overlap with each other. 
     Each of the internal electrodes  102   a - 104   a  and the surface electrode  108   a  is physically and electrically connected to the penetrating conductor  109   a  extending in the Z-direction. Each of the internal electrodes  102   b - 104   b  and the surface electrode  108   b  is physically and electrically connected to the penetrating conductor  109   b  extending in the Z-direction. 
     The face  91   b  of the first varistor part  90  is in contact with the face  8   a  of the heat dissipation part  8  and the face  101   b  of the second varistor part  100  is in contact with the face  8   b  of the heat dissipation part  8 . The first varistor part  90  and the second varistor part  100  are arranged in symmetry with respect to the heat dissipation part  8 . 
     A production method of this varistor V 4  will be explained. The varistor V 4  is produced by a production method similar to that of the varistor V 1  according to the first embodiment, but, because of the difference in the configuration of the internal electrodes in the first and second varistor parts, the process is partly different in the green laminated body formed in the laminating step S 5  and the configuration of the aggregate substrate formed in the firing step S 6 . The difference will be explained below with reference to  FIG. 18 . 
       FIG. 18(   a ) is a schematic sectional view of the green laminated body. The green laminated body  300 C of the fourth embodiment includes a plurality of green element assemblies  30 C. This green laminated body  300 C includes a heat dissipation green part  308 , a first varistor green part  390 , and a second varistor green part  400 . 
     The first varistor green part  390  includes a varistor green layer  391 , a plurality of internal electrode patterns  392   a - 394   a ,  392   b - 394   b ,  395 - 397 , plural pairs of surface electrode patterns  398   a ,  398   b , and a plurality of penetrating conductor patterns  399   a ,  399   b . The plurality of internal electrode patterns  392   a - 394   a ,  392   b - 394   b ,  395 - 397  correspond to the internal electrodes  92   a - 94   a ,  92   b - 94   b ,  95 - 97 , respectively. The plural pairs of surface electrode patterns  398   a ,  398   b  correspond to the pair of surface electrodes  98   a ,  98   b , respectively. The plurality of penetrating conductor patterns  399   a ,  399   b  correspond to the penetrating conductors  99   a ,  99   b , respectively. 
     The first varistor green part  390  is formed by laminating the varistor green sheets with the aforementioned electrode patterns and others in a predetermined order. The varistor green layer  391  has a principal face  391   a  and a principal face  391   b  facing each other in the Z-direction. The principal face  391   b  is in contact with the principal face  308   a  of the heat dissipation green part  308 . 
     The second varistor green part  400  includes a varistor green layer  401 , a plurality of internal electrode patterns  402   a - 404   a ,  402   b - 404   b ,  405 - 407 , plural pairs of surface electrode patterns  408   a ,  408   b , and a plurality of penetrating conductor patterns  409   a ,  409   b . The plurality of internal electrode patterns  402   a - 404   a ,  402   b - 404   b ,  405 - 407  correspond to the internal electrodes  102   a - 104   a ,  102   b - 104   b ,  105 - 107 , respectively. The plural pairs of surface electrode patterns  408   a ,  408   b  correspond to the pair of surface electrodes  108   a ,  108   b , respectively. The plurality of penetrating conductor patterns  409   a ,  409   b  correspond to the penetrating conductors  109   a ,  109   b , respectively. 
     The second varistor green part  400  is formed by laminating the varistor green sheets with the aforementioned electrode patterns and others in a predetermined order. The varistor green layer  401  has a principal face  401   a  and a principal face  401   b  facing each other in the Z-direction. The principal face  401   b  is in contact with the principal face  308   a  of the heat dissipation green part  308 . The first varistor green part  390  and the second varistor green part  400  are arranged in symmetry with respect to the heat dissipation green part  308 . 
     Subsequently, an aggregate substrate  31 C of the fourth embodiment will be described with reference to  FIG. 18(   b ). The aggregate substrate  31 C includes a plurality of element assemblies  3 C. This aggregate substrate  31 C includes a heat dissipation layer  9 , a first varistor part  298  made by firing of the first varistor green part  390 , and a second varistor part  299  made by firing of the second varistor green part  400 . The first varistor green part  390  and the second varistor green part  400  are arranged in symmetry with respect to the heat dissipation layer  9 . 
     An aggregate substrate with external electrodes is obtained by forming insulating layers  45 ,  46  on the aggregate substrate  31 C and forming plural pairs of external electrodes  6 ,  7 . A plurality of varistors V 4  are obtained by cutting the aggregate substrate with external electrodes thus obtained. 
     In the varistors V 4 , the varistor element bodies  91 ,  101  also contain ZnO as a main component and the heat dissipation part  8  is made of a composite material of metal Ag and metal oxide including ZnO as the main component of the varistor element bodies  91 ,  101 . Therefore, as in the first embodiment, sufficient joint strength is ensured between the first varistor part  90  and the heat dissipation part  8  and heat transferred from an electronic device through the external electrodes  6 ,  7  to the first varistor part  90  is efficiently dissipated through conduction paths formed from the face  80   a  to the exposed side faces in the heat dissipation part  8 . Sufficient joint strength is also ensured between the second varistor part  100  and the heat dissipation part  8 . 
     There is difference between contraction caused by firing of the heat dissipation green part  308  (heat dissipation part  8 ) and contraction caused by firing of the first and second varistor green parts  390 ,  400  (first varistor part  90  and second varistor part  100 ). However, since the heat dissipation green part  308  is sandwiched between the first varistor green part  390  and the second varistor green part  400  with the first varistor green part  390  being in contact with the principal face  308   a  of the heat dissipation green part  308  and with the second varistor green part  400  being in contact with the principal face  308   b  of the heat dissipation green part  308 , the aggregate substrate  31 C of planar shape can be formed while suppressing occurrence of warpage during the firing. Since the individual varistors V 4  are obtained by forming the external electrodes  6 ,  7  on the planar aggregate substrate  31 C and cutting it, the plurality of varistors V 4  with good heat dissipation efficiency can be readily produced. 
     Fifth Embodiment 
     The varistor according to the fifth embodiment of the present invention will be explained.  FIG. 19  is a schematic sectional view showing the varistor according to the fifth embodiment of the present invention. The varistor V 5  shown in  FIG. 19  is different from the varistor V 2  of the second embodiment in that paired internal electrodes are formed in plural pairs (three pairs in the present embodiment). The varistor V 5  has an element body  3 D instead of the element body  3 , and the element body  3 D has first and second varistor parts  110 ,  120  instead of the first and second varistor parts  10 ,  20 . 
     The first varistor part  110  includes a varistor element body  111  of a nearly rectangular parallelepiped shape, three pairs of internal electrodes  112 ,  113  facing each other in the varistor element body  111 , and penetrating conductors  114 ,  115 . The varistor element body  111  has a face  111   a  and a face  111   b  facing each other in the Z-direction. The face  111   b  is in contact with the face  8   a  of the heat dissipation part  8 . The internal electrodes  112 ,  113  are opposed in part in the Z-direction to each other as shifted relative to each other in the X-direction. The internal electrodes  112  and the internal electrodes  113  are alternately laminated with a varistor layer in between. 
     The penetrating conductor  114  extends in the Z-direction and is physically and electrically connected to the three internal electrodes  112 , and the tip thereof is exposed from the face  111   a . The tip of the penetrating conductor  114  is located in the aperture  4   a  of the insulating layer  4  and physically and electrically connected to the external electrode  6 . The penetrating conductor  115  extends in the Z-direction and is physically and electrically connected to the three internal electrodes  113 , and the other end thereof is exposed from the face  111   a . The tip of the penetrating conductor  115  is located in the aperture  4   b  of the insulating layer  4  and physically and electrically connected to the external electrode  7 . Namely, the internal electrodes  112  are electrically connected to the external electrode  6  through the penetrating conductor  114  and the internal electrodes  113  are electrically connected to the external electrode  7  through the penetrating conductor  115 . 
     The second varistor part  120  includes a varistor element body  121  of a nearly rectangular parallelepiped shape, three pairs of internal electrodes  122 ,  123  facing each other in the varistor element body  121 , and penetrating conductors  124 ,  125 . The varistor element body  121  has a face  121   a  and a face  121   b  facing each other in the Z-direction. The insulating layer  5  is arranged on the face  121   a  and the face  121   b  is in contact with the face  8   b  of the heat dissipation part  8 . The internal electrodes  122 ,  123  are opposed in part in the Z-direction to each other as shifted relative to each other in the X-direction. The internal electrodes  122  and the internal electrodes  123  are alternately laminated with a varistor layer in between. 
     The penetrating conductor  124  extends in the Z-direction and is physically and electrically connected to the three internal electrodes  122 , and the tip thereof is exposed from the face  121   a  and covered by the insulating layer  5 . The penetrating conductor  125  extends in the Z-direction and is physically and electrically connected to the three internal electrodes  123 , and the tip thereof is exposed from the face  121   a  and covered by the insulating layer  5 . The first varistor part  110  and the second varistor part  120  are arranged in symmetry with respect to the heat dissipation part  8 . 
     A production method of the varistor V 5  will be explained. The varistor V 5  is produced by a production method similar to that of the varistor V 2  of the second embodiment, but, because of the difference in the configuration of the internal electrodes in the first and second varistor parts, the process is partly different in the green laminated body formed in the laminating step S 5  and the configuration of the aggregate substrate formed in the firing step S 6 . The difference will be explained below with reference to  FIG. 20 . 
       FIG. 20(   a ) is a schematic sectional view of the green laminated body. The green laminated body  300 D of the fifth embodiment includes a plurality of green element assemblies  30 D. This green laminated body  300 D includes a heat dissipation green part  308 , a first varistor green part  410 , and a second varistor green part  420 . 
     The first varistor green part  410  includes a varistor green layer  411 , a plurality of internal electrode patterns  412 ,  413 , and a plurality of penetrating conductor patterns  414 ,  415 . The plurality of internal electrode patterns  412 ,  413  correspond to the internal electrodes  112 ,  113 , respectively. The plurality of penetrating conductor patterns  414 ,  415  correspond to the penetrating conductors  114 ,  115 , respectively. 
     The first varistor green part  410  is formed by laminating the varistor green sheets with the aforementioned electrode patterns and others in a predetermined order. The varistor green layer  411  has a principal face  411   a  and a principal face  411   b  facing each other in the Z-direction. The principal face  411   b  is in contact with the principal face  308   a  of the heat dissipation green part  308 . 
     The second varistor green part  420  includes a varistor green layer  421 , a plurality of internal electrode patterns  422 ,  423 , and a plurality of penetrating conductor patterns  424 ,  425 . The plurality of internal electrode patterns  422 ,  423  correspond to the internal electrodes  122 ,  123 , respectively. The plurality of penetrating conductor patterns  424 ,  425  correspond to the penetrating conductors  124 ,  125 , respectively. 
     The second varistor green part  420  is formed by laminating the varistor green sheets with the electrode patterns and others in a predetermined order. The varistor green layer  421  has a principal face  421   a  and a principal face  421   b  facing each other in the Z-direction. The principal face  421   b  is in contact with the principal face  308   a  of the heat dissipation green part  308 . The first varistor green part  410  and the second varistor green part  420  are arranged in symmetry with respect to the heat dissipation green part  308 . 
     The aggregate substrate  31 D of the fifth embodiment will be described below with reference to  FIG. 20(   b ). The aggregate substrate  31 D includes a plurality of element assemblies  3 D. This aggregate substrate  31 D includes a heat dissipation layer  9 , a first varistor part  110  made by firing of the first varistor green part  410 , and a second varistor part  120  made by firing of the second varistor green part  420 . The first varistor part  110  and the second varistor green part  120  are arranged in symmetry with respect to the heat dissipation layer  9 . 
     An aggregate substrate with external electrodes is obtained by forming the insulating layers  45 ,  46  on the aggregate substrate  31 D and forming plural pairs of external electrodes  6 ,  7 . A plurality of varistors V 5  are obtained by cutting the aggregate substrate with external electrodes thus obtained. 
     In the varistors V 5 , the varistor element bodies  111 ,  121  also contain ZnO as a main component and the heat dissipation part  8  is made of a composite material of metal Ag and metal oxide including ZnO as the main component of the varistor element bodies  111 ,  121 . Therefore, as in the first embodiment, sufficient joint strength is ensured between the first varistor part  110  and the heat dissipation part  8  and heat transferred from an electronic device through the external electrodes  6 ,  7  to the first varistor part  110  is efficiently dissipated through conduction paths formed from the side face  8   a  to the exposed side faces in the heat dissipation part  8 . Sufficient joint strength is also ensured between the second varistor part  120  and the heat dissipation part  8 . 
     There is difference between contraction caused by firing of the heat dissipation green part  308  (heat dissipation part  8 ) and contraction caused by firing of the first and second varistor green parts  410 ,  420  (first and second varistor parts  110 ,  120 ). Since the heat dissipation green part  308  is sandwiched between the first varistor green part  410  and the second varistor green part  420  with the first varistor green part  410  being in contact with the principal face  308   a  of the heat dissipation green part  308  and with the second varistor green part  420  being in contact with the principal face  308   b  of the heat dissipation green part  308 , the aggregate substrate  31 D of planar shape can be formed while suppressing occurrence of warpage during the firing. Since the individual varistors V 5  are obtained by forming the external electrodes  6 ,  7  on the planar aggregate substrate  31 D and cutting it, the plurality of varistors V 5  with good heat dissipation efficiency can be readily produced. 
     The present invention is not limited to the above embodiments, but can be modified in many ways. 
     In the above first to fifth embodiments, the first varistor green part  310 ,  360 ,  390 ,  410  and the second varistor green part  320 ,  370 ,  400 ,  420  are arranged in symmetry with respect to the heat dissipation green part  308 ,  380  in the green laminated body  300 ,  300 A- 300 D, but the present invention is not limited to this configuration. The first varistor green part  310 ,  360 ,  390 ,  410  and the second varistor green part  320 ,  370 ,  400 ,  420  in the green laminated body  300 ,  300 A- 300 D may be shifted relative to each other in the X-direction, and the thicknesses of the respective constituent elements may be different between them. In connection with the aforementioned configuration, the first varistor part  19 ,  69 ,  298 ,  419  and the second varistor part  29 ,  79 ,  299 ,  429  are arranged in symmetry with respect to the heat dissipation layer  9 ,  89  in the aggregate substrate  31 ,  31 A- 31 D, but the present invention is not limited to this configuration. The first varistor part  19 ,  69 ,  298 ,  419  and the second varistor part  29 ,  79 ,  299 ,  429  in the aggregate substrate  31 ,  31 A- 31 D may be shifted relative to each other in the X-direction, and the thicknesses of the respective constituent elements may be different between them. Furthermore, the first varistor part  10 ,  60 ,  90 ,  110  and the second varistor part  20 ,  70 ,  100 ,  120  are arranged in symmetry with respect to the heat dissipation part  8 ,  80  in the varistor V 1 -V 5 , but the present invention is not limited to this configuration. The first varistor part  10 ,  60 ,  90 ,  110  and the second varistor part  20 ,  70 ,  100 ,  120  in the varistor V 1 -V 5  may be shifted relative to each other in the X-direction, and the thicknesses of the respective constituent elements may be different between them. 
     In the first and fourth embodiments the surface electrodes  13 ,  14 ,  23 ,  24 ,  98   a ,  98   b ,  108   a ,  108   b  are formed by firing the electroconductive paste in the firing step S 6 , but the present invention is not limited to this method. The surface electrodes  13 ,  14 ,  23 ,  24 ,  98   a ,  98   b ,  108   a ,  108   b  may be formed as follows: after the firing step S 6 , an electroconductive paste is applied on the resultant aggregate substrate and it is then sintered. 
     Each of the above embodiments exemplified ZnO as a semiconductor ceramic being the main component of the varistor element body  11 ,  21 ,  61 ,  71 ,  91 ,  101 ,  111 ,  121 , but it is also possible to use any one of semiconductor ceramics other than ZnO, e.g., SrTiO 3 , BaTiO 3 , SiC, and so on. 
     The devices to be connected to the varistor V 1 -V 5  can be nitride-base semiconductor LEDs except for the GaN type, e.g., InGaNAs-base semiconductor LEDs, or semiconductor LEDs and LDs except for the nitride type. Besides the LEDs, the varistor may be connected to a variety of electronic devices that generate heat during operation, e.g., field effect transistors (FETs), bipolar transistors, and so on. 
     From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.