Patent Publication Number: US-9853168-B2

Title: Diode

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 13/959,904 filed Aug. 6, 2013, which is based on and claims the benefit of priority from earlier Japanese Patent Applications No. 2012-174282 filed Aug. 6, 2012 and No. 2013-160707 filed Aug. 1, 2013, the descriptions of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to a diode having a plurality of PN junctions. 
     Related Art 
     JP-B-6-80313 discloses a technique for providing a constant voltage element between a secondary coil and a spark plug of an ignition device. According to this technique, when a discharge voltage of the spark plug reaches a breakdown voltage of the constant voltage element, part of a secondary current flows from the constant voltage element to the ground. Hence, the discharge voltage can be prevented from becoming higher than the breakdown voltage, thereby extending the lifetime of the spark plug. 
     One example of this kind of constant voltage element is an avalanche diode. However, since the breakdown voltage of the avalanche diode has extremely high temperature dependence, the discharge voltage of the ignition device varies depending on the temperature. 
     To solve this problem, the inventor studied the use of a diode element  32   x,  which is included in a semiconductor device of U.S. Pat. No. 5,365,099 (see  FIG. 13 ), as a constant voltage element of a ignition device. This diode  32   x  has a plurality of N-type regions  32   a,    32   c  and a plurality of P-type regions  32   b,    32   d.  The P-type regions and the N-type regions are alternately arranged in series on an oxide film  41  provided on a surface of a silicon substrate  40  to form PN junctions. Hence, the diode element  32   x  serves as an equivalent circuit in which a plurality of Zener diodes Da, Db, Dc are connected in series in a state where the adjacent diodes are opposite in direction to each other (see  FIG. 5 ). 
     There is an inverse relationship between a temperature characteristic of voltage (forward voltage VF) obtained when a voltage is applied in the forward direction of the Zener diodes Da, Db, Dc (see  FIG. 6A ) and a temperature characteristic of Zener voltage (backward withstand voltage VR) obtained when a voltage is applied in the backward direction of the Zener diodes Da, Db, Dc (see  FIG. 6B ). That is, as the temperature becomes higher, the forward voltage becomes lower and the backward withstand voltage becomes higher. Hence, a pair of Zener diodes Da, Db, which are connected so as to be opposite in direction to each other and opposed to each other, mutually cancels increase and decrease of the voltage value caused due to the temperature characteristic. Hence, the temperature characteristic of the Zener voltage of the diode element  32   x  can be moderate. 
     In the case of the ignition device, an extremely high Zener voltage is required for the constant voltage element. However, the diode element  32   x  installed in the semiconductor element of U.S. Pat. No. 5,365,099 is not assumed to use such a high Zener voltage. Hence, the present inventor studied increasing the number of PN junctions of the diode element  32   x  disclosed in U.S. Pat. No. 5,365,099 to make the temperature characteristic moderate while having a high Zener voltage. 
     Meanwhile, in the semiconductor element of U.S. Pat. No. 5,365,099, the diode element  32   x  is integrated together with a power MOSFET or an IGBT element illustrated by reference numeral  42  in  FIG. 13 . Hence, the semiconductor element has a configuration in which the diode element  32   x  is mounted on a silicon substrate  40  forming various semiconductor elements. 
     The diode element  32   x  is formed on a silicon dioxide film  41  and is insulated from the silicon substrate  40 . Hence, as studied above, if increasing the number of the PN junctions to increase the Zener voltage of the diode element  32   x,  the silicon dioxide film  41 , which is formed by thermal oxidation of silicon and has a thickness of about 1 μm, is limited to increasing withstand voltage (800 V at the most). Hence, it has found that the structure of the diode element  32   x  of U.S. Pat. No. 5,365,099 cannot sufficiently increase the Zener voltage. 
     In addition, if increasing the thickness of the silicon dioxide film  41  to increase the withstand voltage of the silicon dioxide film  41 , large stress is generated at an interface of the silicon dioxide film  41  due to thermal expansion difference because thermal expansion coefficients are largely different between the silicon substrate  40  and the silicon dioxide film  41 . In addition, it is basically difficult to form an oxide film having a thickness significantly exceeding 1 μm by thermal oxidation. 
     Note that the above problems similarly occur not only in a constant voltage element provided at the secondary side of the ignition device but also in diodes, for which high withstand voltage is required, such as one provided at the primary side of the ignition device and one provided in a device other than the ignition device. 
     SUMMARY 
     An embodiment provides a diode which has lower temperature dependence of Zener voltage and can make the Zener voltage sufficiently higher. 
     As an aspect of the embodiment, a diode is provided which includes at least one diode element which has a plurality of N-type regions and a plurality of P-type regions, the N-type regions and the P-type regions being alternately arranged in series to form PN junctions; and an insulated substrate which has electric insulation. The N-type regions and the P-type regions are formed on the insulated substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a diagram showing an ignition system to which a diode according to a first embodiment is applied; 
         FIG. 2  is a perspective view showing a single diode; 
         FIG. 3  is a plan view showing the single diode of  FIG. 2 ; 
         FIG. 4  is a sectional view showing the single diode of  FIG. 3 ; 
         FIG. 5  is an equivalent circuit schematic of the diode according to the first embodiment; 
         FIGS. 6A, 6B and 6C  are diagrams showing temperature characteristics of the diode according to the first embodiment; 
         FIG. 7  is a diagram showing a diode according to a second embodiment; 
         FIG. 8  is a diagram showing a diode according to a third embodiment; 
         FIGS. 9A and 9B  are a diagram showing a diode according to a fourth embodiment; 
         FIGS. 10A and 10B  are diagrams showing a diode according to a fifth embodiment; 
         FIG. 11  is a diagram showing a diode according to a seventh embodiment; 
         FIG. 12  is a diagram showing a diode according to an eighth embodiment; and 
         FIG. 13  is a sectional view showing a conventional diode element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, hereinafter are described embodiments of a diode according to the present invention. In the embodiments, the components which are identical with or similar to each other are given the same reference numerals for the sake of omitting unnecessary explanation. 
     First Embodiment 
     As shown in  FIG. 1 , a diode D 1  according to the present embodiment is applied to an ignition system installed in an internal-combustion engine. This ignition system includes an ignition coil  10 , an igniter  13 , and a spark plug  20 . The ignition coil  10  has a primary coil  11  and a secondary coil  12 . The igniter  13  controls on-off of electric conduction to the primary coil  11 . The spark plug  20  is connected to the secondary coil  12 . In  FIG. 1 , the igniter  13  is incorporated into the ignition coil  10 . 
     The igniter  13  has a switching element such as an IGBT (insulated gate bipolar transistor), and controls on-off switching of electric conduction to the primary coil  11  by turning on and off the switching element on the basis of an ignition command signal received from the outside of the ignition coil  10 . Specifically, the igniter  13  first turns on the switching element to start electric conduction to the primary coil  11 . Thereafter, the igniter  13  turns off the switching element to interrupt the electric conduction to the primary coil  11 . Due to the interruption, the voltage of the secondary coil  12  (secondary voltage) is increased, which causes discharge in the spark plug  20 . 
     Meanwhile, as combustion is provided in an internal-combustion engine in a higher compression and leaner burning state, a required value of discharge voltage (secondary voltage) for the spark plug  20  becomes higher. During a discharge period, the discharge voltage becomes a peak value (peak voltage) immediately after the discharge starts. However, as the required value becomes higher, the peak voltage also becomes higher. In this case, due to the application of high peak voltage to the spark plug  20 , a concern rises that, for example, the spark plug  20  is damaged. 
     To overcome this concern, in the present embodiment, a diode D 1  is connected to the secondary side of the ignition coil  10 . Specifically, the diode D 1  is connected to the spark plug  20  in parallel. One end of the diode D 1  is connected to the secondary coil  12 , and the other end of the diode D 1  is grounded. The diode D 1  is configured so as to be an equivalent circuit in which a plurality of Zener diodes Da, Db, Dc (see  FIG. 5 ) are connected in series in a state where the adjacent diodes are opposite in direction to each other. 
     Hence, when the discharge voltage exceeds the Zener voltage of the diode D 1 , secondary current passing through the secondary coil  12  flows to the ground through the diode D 1 . Thereby, a voltage equal to or more than the Zener voltage is prevented from being applied to the spark plug  20 . That is, the peak value of the discharge voltage is limited so as not to exceed the Zener voltage. In short, the diode D 1  is a constant voltage diode which functions so as not to apply the voltage exceeding the Zener voltage to the spark plug  20 . 
     As shown in  FIG. 2 , the diode D 1  is configured by molding a semiconductor chip  30  and lead terminals  30   b,    30   c  connected to the semiconductor chip  30  with mold resin  30   a.  As shown in  FIGS. 3 and 4 , the semiconductor chip  30  includes an insulated substrate  31 , a diode element  32 , an insulating coating layer  33 , and electrodes  34 . The diode element  32  is formed on the insulated substrate  31 . The insulating coating layer  33  coats the diode element  32  from the opposite side of the insulated substrate  31 . The electrodes  34  are connected to the diode element  32 . 
     The diode element  32  has a plurality of N-type regions  32   a,    32   c  and a plurality of P-type regions  32   b,    32   d.  The N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  are alternately arranged in series to form PN junctions. The length in the electric conduction direction (horizontal direction in  FIG. 4 ) of a set of the adjacent N-type region  32   a  and the P-type region  32   b  is preferably 1.5 to 5 μm. 
     In examples shown in  FIGS. 3 and 4 , the areas are arranged in a straight line. Hence, the mold resin  30   a  has a circular cylindrical shape extending in the longitudinal direction of the diode element  32  (horizontal direction in  FIGS. 2 to 4 ). The electrodes  34  are connected to the ends of the diode element  32 . Although the diode element  32  is sealed by being interposed between the insulated substrate  31  and the insulating coating layer  33 , a pair of electrodes  34  is exposed from the insulating coating layer  33 . Although not shown in  FIGS. 3 and 4 , the electrodes  34  are connected to the lead terminals  30   b,    30   c  shown in  FIG. 2 . 
     The insulated substrate  31  has a sufficient thickness and stiffness compared with the diode element  32 . Concrete examples of materials of the insulated substrate  31  include insulators such as quartz glass, sapphire, and ceramics. Specifically, quartz glass, sapphire, or alumina ceramics may be formed into a wafer so as to be suitably used for the insulated substrate  31 . 
     In the example shown in  FIG. 4 , the thickness of the diode element  32  is 1 μm, and the length in the electric conduction direction of the diode element  32  is 20 mm (3.5 μm×5700 sets). Polysilicon is used for the material of the diode element  32 . 
     As described above, in the diode element  32 , a plurality of Zener diodes Da, Db, Dc (see  FIG. 5 ) are connected in series in a state where the adjacent diodes are opposite in direction to each other. Each of the Zener diodes Da, Db, Dc is configured of a set of the N-type regions  32   a  and the P-type region  32   b.  Hereinafter, technical significance of this configuration will be described. 
     There is an inverse relationship between a temperature characteristic of voltage (forward voltage) obtained when a voltage is applied in the forward direction of the Zener diodes Da, Db, Dc (see  FIG. 6A ) and a temperature characteristic of Zener voltage (backward withstand voltage) obtained when a voltage is applied in the backward direction of the Zener diodes Da, Db, Dc (see  FIG. 6B ). That is, as the temperature becomes higher, the forward voltage becomes lower and the backward withstand voltage becomes higher. Hence, a pair of Zener diodes Da, Db, which are connected so as to be opposite in direction to each other and opposed to each other, mutually cancels increase and decrease of the voltage value caused due to the temperature characteristic. Hence, the temperature characteristic of the Zener voltage of the diode element  32   x  (diode D 1 ) can be moderate. 
       FIG. 6C  shows a result of a test of a relationship between peak values of discharge voltage of the spark plug  20  and environmental temperatures. According to this result, since the temperature characteristic of the diode D 1  is made moderate, discharge voltage (peak voltage) of the spark plug  20  can be stabilized without depending on the temperature. In addition, since a large number of sets of N-type and P-type regions (5700 sets in the example shown in  FIG. 4 ) are connected in series, Zener voltage of the diode D 1  is increased to a desired value (e.g. 36 kV). 
     In the example shown in  FIG. 4 , the thickness of the insulated substrate  31  is 0.5 mm. That is, the insulated substrate  31  is not a film, but has stiffness with which the diode element  32  can be supported so as not to be deformed. In addition, in the example shown in  FIG. 4 , the material and thickness of the insulated substrate  31  are set so that the insulated substrate  31  has dielectric voltage of 200 kV. That is, the material and thickness of the insulated substrate  31  are set so that the insulated substrate  31  has sufficient withstand voltage compared with the Zener voltage of the diode D 1  (e.g. 36 kV). 
     Next, one embodiment of a procedure (manufacturing method) of manufacturing the diode D 1  having the above structure will be described. 
     First, polysilicon is deposited on a quartz wafer, which serves as a base material of the insulated substrate  31 , by CVD (chemical vapor deposition) method. Next, B (boron) is ion-implanted in the whole surface of the polysilicon and thermal annealing is performed. Hence, P-type polysilicon is formed on the quartz wafer. Next, diode regions are etched. P (phosphorus) is ion-implanted by using a resist as a mask. In this state, thermal annealing is performed to form the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  in desired positions on the insulated substrate  31  so as to be alternately arranged. That is, the diode element  32  is formed on the insulated substrate  31 . 
     Next, the polysilicon is subjected to thermal oxidation, whereby silicon dioxide (SiO2) is formed on a surface of the diode element  32 . PSG (phosphorus silicate glass) is deposited on the silicon dioxide by CVD. Next, contact holes are formed in the PSG. The contact holes are formed at positions which are ends of the diode element  32  and where the electrodes  34  are formed. Next, Au/Ni/Ti is subjected to sputtering on the PSG. Next, the sputtered metal layer is subject to patterning by photolithography, whereby the electrodes  34  are formed on the ends of the diode element  32 . 
     Next, polyimide is coated on the diode element  32  and the electrodes  34  on the insulated substrate  31  to form an insulating layer  33 , in which openings are formed at positions corresponding to the electrodes  34 , by photolithography. Thereby, a Zener diode wafer is completed. Then, the completed Zener diode wafer is subject to dicing so that each semiconductor chip  30  is cut away. Next, lead terminals  30   b,    30   c  are connected to the electrodes  34  of the semiconductor chip  30  by soldering. Next, the whole of the semiconductor chip  30  and the lead terminals  30   b,    30   c  is coated with polyimide. Next, the whole of the coated semiconductor chip  30  is subjected to transfer molding. Thereby, the diode D 1  shown in  FIG. 2  is completed which has a shape in which the lead terminals  30   b,    30   c  extend from the mold resin  30   a  enclosing the semiconductor chip  30 . 
     As described above, according to the present embodiment, the PN junctions are alternately connected in series to form the diode element  32 . Hence, a pair of diodes Da, Db, which are connected so as to be opposite in direction to each other and opposed to each other, mutually cancels increase and decrease of the voltage value caused due to the temperature characteristic. Thereby, the temperature characteristic of the diode D 1  can apparently be moderate, and the temperature dependence of Zener voltage can be lower. Hence, discharge voltage (peak voltage) of the spark plug  20  can be stabilized without depending on the temperature. 
     In addition, according to the present embodiment, since N-type and P-type regions are formed on the insulated substrate  31 , withstand voltage of the insulated substrate  31  can be sufficiently and easily higher. Hence, while preventing the N-type regions  32   a ,  32   c  and the P-type regions  32   b,    32   d  from being short-circuited via the insulated substrate  31 , Zener voltage of the diode D 1  can be sufficiently higher by increasing the number of the PN junctions. 
     Furthermore, according to the present embodiment, the following advantages (a) to (c) can be obtained. 
     (a) In the present embodiment, the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  are formed in the polysilicon. Accordingly, the formation, in which the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  are alternately formed in series on the insulated substrate  31 , can be easily realized by using CVD. In addition, when forming N-type regions and P-type regions in single-crystal silicon, the time for heat treatment is required for diffusing impurities into the bottom of the single-crystal silicon. However, according to the present embodiment in which N-type regions and P-type regions are formed in polysilicon, the time for heat treatment can be shortened. This is because polysilicon has a diffusion coefficient of impurities higher than that of single-crystal silicon due to grain boundaries. 
     (b) Unlike the present embodiment, when the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  are arranged so as to be meandering as illustrated in  FIG. 8 , the clearance produced by the meander (see CL in  FIG. 8 ) is required to be sufficiently large to prevent dielectric breakdown from being caused by the meander. To solve this problem, in the present embodiment, the N-type and P-type regions are formed so as to be arranged in a straight line on the insulated substrate  31 . Hence, the concern about a short circuit due to the clearance CL can be resolved. 
     (c) In the present embodiment, the diode D 1  is applied as a constant voltage diode which is connected to the secondary side of the ignition coil  10  and functions so as to prevent the voltage exceeding the Zener voltage from being applied to the spark plug  20 . As described above, since an extremely high voltage (several tens of kV) is applied to the diode connected to the secondary side, high withstand voltage is required for the diode. On this point, the above-described advantage of the present embodiment is preferably exerted that the Zener voltage can be sufficiently made higher. 
     Second Embodiment 
     In the first embodiment, one semiconductor chip  30  (i.e. one diode element  32 ) is molded to form the diode D 1 . However, in the present embodiment shown in  FIG. 7 , a plurality of semiconductor chip  30  are integrally molded to form a diode D 2 . 
     Specifically, a plurality of semiconductor chips  30  having the same structure as those shown in  FIGS. 3 and 4  are arranged side by side on a lead frame  30   d.  In the example shown in  FIG. 7 , the semiconductor chips  30  are arranged side by side so that the longitudinal directions thereof are the same. In addition, the electrodes  34  of the semiconductor chips  30  are connected by wires W so that the semiconductor chips  30  are electrically connected in series. The lead frame  30   d  is made of metal such as copper and has electrical conductivity. 
     The number of the PN junctions is increased to make the Zener voltage of the diode D 2  sufficiently high, as in the case of the first embodiment. However, if configuring the diode with one semiconductor chip  30  as shown in  FIGS. 3 and 4 , the entire length of the diode increases due to the increased number of the PN junctions, which can make it difficult to install the diode in a predetermined portion. 
     According to the present embodiment, in consideration of the above point, a plurality of semiconductor chips  30  are provided which have the diode elements  32  formed so that the P-type regions and the N-type regions are arranged in a predetermined direction (vertical direction in  FIG. 7 ) and in a line. The semiconductor chips  30  are electrically connected in series by wire bonding. In addition, the semiconductor chips  30  are arranged on the lead frame  30   d  so that the N-type and P-type regions  32   a,    32   c,    32   b ,  32   d  meander. 
     Hence, the external shape of the diode D 2  can be rectangular while preventing the diode D 2  from having a long bar shape extending in a predetermined direction. Thereby, the diode D 2  can be easily installed even in a predetermined portion where a bar-shaped one cannot be easily installed. 
     Third Embodiment 
     In the second embodiment, a plurality of semiconductor chips  30 , in which the N-type and P-type regions  32   a,    32   c,    32   b,    32   d  are arranged in the predetermined direction (vertical direction in  FIG. 7 ) and in a line, are arranged in the direction perpendicular to the predetermined direction (horizontal direction in  FIG. 7 ) and then are connected by wire bonding. Thereby, the N-type and P-type regions  32   a,    32   c,    32   b,    32   d  are arranged so as to be meandering. In contrast, in the present embodiment shown in  FIG. 8 , a plurality of semiconductor chips  30 , in which the N-type and P-type regions  32   a,    32   c ,  32   b,    32   d  form PN junctions in series so as to be in a line, are meandering and are arranged on the lead frame  30   d.  Note that, in the example shown in  FIG. 8 , a plurality of semiconductor chips  30  are arranged in parallel and are also electrically connected in parallel. 
     According to the present embodiment configured as described above, the external shape of the diode D 3  can be rectangular as in the case of the second embodiment. Hence, the diode D 3  can be easily installed even in a predetermined portion where a bar-shaped one cannot be easily installed. 
     Fourth Embodiment 
     In the second embodiment, the electrodes  34  of the semiconductor chips  30  are directly and electrically connected by wires W. However, in the present embodiment shown in  FIGS. 9A and 9B , the electrodes  34  of a plurality of semiconductor chips  30  are connected to the lead frame  30   d,  whereby the semiconductor chips  30  are connected in series via the lead frame  30   d.  In the example shown in  FIGS. 9A and 9B , a diode D 4  includes a first diode element  301  and a second diode element  302 . Note that a positive terminal electrically connected to one end of the first diode element  301  corresponds to the lead terminal  30   b,  and a negative terminal electrically connected to one end of the second diode element  302  corresponds to the lead terminal  30   c.  The other end of the first diode element  301  and the other end of the second diode element  302  are connected via the lead frame  30   d  and the wires W. 
     The number of the PN junctions included in the first diode element  301  and the number of the PN junctions included in the second diode element  302  are the same. This means that the first diode element  301  and the second diode element  302  are connected to the lead frame  30   d  at the intermediate position thereof. Hence, an intermediate value between a potential of the positive terminal  30   b  and a potential of the negative terminal  30   c  is a potential of the lead frame  30   d.    
     Although not shown in  FIG. 9B , the insulated substrate  31  is interposed between the diode element  32  and the lead frame  30   d.  Hence, as a potential difference between the lead frame  30   d  and the positive terminal  30   b  and a potential difference between the lead frame  30   d  and the negative terminal  30   c  become larger, withstand voltage required for the insulated substrate  31  becomes higher. 
     According to the present embodiment, in consideration of the above point, the first diode element  301  and the second diode element  302  are connected to the lead frame  30   d  at the intermediate position thereof. Hence, in both the first diode element  301  and the second diode element  302 , the potential difference can be restrained from being larger. Hence, withstand voltage required for the insulated substrate  31  can be lower. 
     Fifth Embodiment 
     In the fourth embodiment, both the first and second diode elements  301 ,  302  are mounted on one surface (front surface) of the lead frame  30   d.  However, in the present embodiment shown in  FIGS. 10A and 10B , the first diode element  301  is mounted on a front surface of the lead frame  30   d,  and the second diode element  302  is mounted on a rear surface of the lead frame  30   d.  According to this configuration, a diode D 5  can be miniaturized in the width direction (horizontal direction in  FIGS. 9A, 9B, 10A and 10B ). 
     Sixth Embodiment 
     In the present embodiment, the diode D 1  shown in  FIG. 2  is installed in a coil mounted on the spark plug  20 , so-called a plug top coil (hereinafter, referred to as “PTC  10 S”) (see  FIG. 11 ). 
     The PTC  10 S is a structure in which the primary coil  11 , the secondary coil  12 , and the igniter  13  are integrally formed together with the diode D 1  by using resin. Note that the PTC  10 S includes a socket  10   a  to which the spark plug  20  is connected. In addition, the PTC  10 S includes a connector  10   b  having a power supply line connected to the primary coil  11  and a ignition command signal line connected to the igniter  13 . 
     The diode D 1  according to the present embodiment has a cylindrical shape. The cylindrical diode D 1  is arranged so as to be adjacent to the primary coil  11  and the secondary coil  12 . That is, the diode D 1  is included in the PTC  10 S so that center line of the coils  11  and  12  and a center line of the diode D 1  are in parallel. 
     Seventh Embodiment 
     In the present embodiment, the diodes D 2  to D 5  shown in  FIGS. 7 to 10  are included in the PTC  10 S (see  FIG. 12 ). 
     The diodes D 2  to D  5  according to the present embodiment have a cubic shape. The cubic diodes D 2  to D  5  are arranged so as to be adjacent to the primary coil  11  and the secondary coil  12 . That is, the diodes D 2  to D  5  are arranged at a portion opposite to the primary coil  11  with respect to the secondary coil  12 . 
     Eighth Embodiment 
     Extremely high discharge voltage of the spark plug  20  is not desirable for restraining electrode wear of the spark plug  20 . In contrast, extremely low discharge voltage of the spark plug  20  is not desirable for maintaining stable discharge in the spark plug  20 . In consideration of the above points, it is desirable that discharge voltage of the spark plug  20  is 32 to 38 kV (more preferably, 33 to 37 kV) in a range of operating temperature between −30° C. and 120° C. 
     Zener voltage Vz of the diodes D 1  to D 5  are equal to the discharge voltage of the spark plug  20 . Hence, it is desirable that Zener voltage Vz is 32 to 38 kV (more preferably, 33 to 37 kV) in a range of operating temperature between −30° C. and 120° C. 
     The Zener voltage Vz changes substantially linearly with respect to the temperature, and is expressed as the equation: Vz=V 0 ×(1+K×T). Vo is Vz (33.8 kV) at 0° C. K is a Zener voltage temperature coefficient. Based on this equation, if K is set to 700 ppm/° C. or less, discharge voltage can be fallen within the range of 32 to 38 kV described above. 
     In consideration of the above points, in the present embodiment, the diode is manufactured so that the Zener voltage temperature coefficient is 700 ppm/° C. or less. Specifically, by controlling carrier densities of P-type regions and N-type regions, the Zener voltage temperature coefficient is adjusted so as to be 700 ppm/° C. or less. Hence, electrode wear of the spark plug  20  can be restrained, and stable discharge can be maintained. 
     Ninth Embodiment 
     In the first embodiment, N-type regions and P-type regions are formed in polysilicon. In contrast, in the present embodiment, N-type regions and P-type regions are formed in single-crystal silicon. Note that the configuration of the present embodiment is the same as those of the above embodiments in that an equivalent circuit is formed in which a plurality of diodes are connected in series in a state where the adjacent diodes are opposite in direction to each other. 
     Hereinafter, a concrete example of a method of manufacturing a diode will be described. First, an SOS (Silicon On Sapphire) wafer is manufactured in which single-crystal silicon is subject to epitaxial growth on a sapphire wafer. Next, the single-crystal silicon layer of the SOS wafer is subject to processes such as photolithography, ion implantation, diffusion, and electrode formation, as in the case of the first embodiment. Thereby, a Zener diode wafer is completed. Furthermore, as in the case of the first embodiment, the Zener diode wafer is subject to dicing so that each semiconductor chip  30  is cut away. Next, soldering for the lead terminals  30   b,    30   c  and transfer molding and the like are performed, thereby completing the diode D 1 . 
     As described above, according to the present embodiment, since N-type and P-type regions are formed in single-crystal silicon, the high-performance diode D 1  having lower operation resistance can be manufactured. 
     Tenth Embodiment 
     In the ninth embodiment, single-crystal silicon is formed on a sapphire wafer. However, in the present embodiment, single-crystal silicon is formed in an SOI (Silicon On Insulator) layer of a laminated wafer including an insulating film. Next, the whole wafer is immersed in hydrofluoric acid aqueous solution. Then, only the SOI layer in which a diode is formed is removed (lift-off). Thereby, a thin-film diode wafer is formed. Next, the thin-film diode wafer is mounted on a quartz glass substrate, and is subject to heat treatment. Next, electrodes are formed on the substrate, thereby completing a diode element formed of single-crystal silicon formed on the quartz glass substrate. The diode element is an equivalent circuit in which a plurality of diodes are connected in series in a state where the adjacent diodes are opposite in direction to each other. Furthermore, as in the case of the first embodiment, the Zener diode wafer is subject to dicing so that each semiconductor chip  30  is cut away. Next, soldering for the lead terminals  30   b,    30   c  and transfer molding and the like are performed, thereby completing the diode D 1 . 
     As described above, according to the present embodiment, since N-type and P-type regions are formed in single-crystal silicon, the high-performance diode D 1  having lower forward voltage VF can be manufactured. 
     Eleventh Embodiment 
     In the above embodiments, polysilicon or single-crystal silicon is formed on the insulated substrate  31 , and the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  are formed in the silicon. In contrast, according to the present embodiment, polycrystalline silicon carbide or single-crystal silicon carbide is formed on the insulated substrate  31 , and the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  are formed in the silicon carbide. For example, in the diode D 1  shown in  FIGS. 2 to 4 , silicon used for forming the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d  is replaced with silicon carbide to manufacture the diode of the present embodiment. 
     When using polycrystalline silicon carbide, the polycrystalline silicon carbide is formed on the insulated substrate  31  by high temperature CVD. Hence, it is desirable to use sapphire having a high heat resistance property for the insulated substrate  31 . Then, impurity ions are implanted into the polycrystalline silicon carbide to form the N-type regions  32   a,    32   c  and the P-type regions  32   b,    32   d.  Concrete examples of the impurities include N (nitrogen) used when the N-type regions  32   a,    32   c  are formed, Al (aluminum) and B (boron) used when the P-type regions  32   b,    32   d  are formed. 
     The diffusion coefficient of impurities of silicon carbide is extremely lower than that of silicon. Hence, it is desirable that ion implantation (multistep ion implantation) is performed with multistep high acceleration voltage, and thereafter activation annealing is performed. Accordingly, ion-implanted impurities are electrically activated, thereby uniformizing the impurity concentration in the depth direction. 
     As described in the eighth embodiment, the Zener voltage temperature coefficient K is desirably 700 ppm/° C. or less. In a case of silicon carbide, if a unit diode including a set of an N-type region and a P-type region is manufactured so that Zener voltage thereof is between 20 V and 28V, the Zener voltage temperature coefficient K becomes substantially zero. Hence, if the Zener voltage of a unit diode is set to 24 V, a 36 kV Zener diode can be formed by connecting 1500 sets of unit diodes in series. In contrast, according to the first embodiment in which silicon is used, it is required to connect 5700 sets of unit diodes in series to form a 36 kV Zener diode. 
     If the length of a unit diode in the electric conduction direction (line and space of a PN junction) is too short, a punch through phenomenon is caused in which a depletion layer extending from the P-type region and a depletion layer extending from the N-type region are connected. Thereby, a desired Zener voltage cannot be obtained. The minimum length, in which the punch through phenomenon is not caused, obtained when using silicon carbide is smaller than that obtained when using silicon. For example, in a case of a unit diode manufactured by using polycrystalline silicon carbide, the minimum length, in which the punch through phenomenon is not caused, is 1.0 μm. In contrast, in a case of a unit diode manufactured by using polysilicon, the minimum length is 1.5 μm. 
     As described above, according to the present embodiment, the number of sets in which unit diodes are connected in series can be smaller. In addition, the whole length of a unit diode can be shortened. Hence, the whole length of the diode can be shortened, thereby miniaturizing the diode. 
     Other Embodiments 
     It will be appreciated that the present invention is not limited to the configurations described above, but any and all modifications, variations or equivalents, which may occur to those who are skilled in the art, should be considered to fall within the scope of the present invention. In addition, characteristic configurations of the embodiments may optionally be combined. 
     In the first embodiment, a quartz wafer is used as a base material of the insulated substrate  31 . However, a sapphire wafer may be used. In addition, although a sapphire wafer is used as a base material of the insulated substrate  31  in the ninth and tenth embodiments, a quartz wafer may be used. In addition, as the insulated substrate, an alumina wafer may be used which is formed of ceramic whose main component is Al2O3 (aluminum oxide) as in the case of sapphire. In this case, the diode can be manufactured with manufacturing cost lower than that of a diode manufactured by using single-crystal sapphire. 
     The diodes D 1  to D 5  are configured so as to be an equivalent circuit in which a plurality of unit diodes are connected in a state where the adjacent diodes are opposite in direction to each other. However, it is desirable that the diodes D 1  to D 5  are manufactured so that withstand voltage of the unit diodes is between 6 V to 7 V. Accordingly, the temperature characteristic of discharge voltage (Zener voltage) shown in  FIG. 6C  can be moderate. That is, the discharge voltage can be further stabilized without depending on the temperature. 
     It is desirable that the length in the electric conduction direction of the unit diode is between 1.5 μm and 5 μm. Hereinafter, technical significance of these values will be described. If Zener voltage of the unit diode is set to 6.3 V, temperature dependence of Zener voltage of the whole diode element can be eliminated. In addition, in a case where the Zener voltage is set to 6.3 V as described above, if the length in the longitudinal direction of the unit diode is set to less than 1.5 μm, a punch through phenomenon is caused in which a depletion layer extending from the P-type region and a depletion layer extending from the N-type region are connected. Thereby, desired Zener voltage cannot be obtained. Hence, as described above, the length in the longitudinal direction of the unit diode is preferably 1.5 μm or more. In addition, if the length of the unit diode is longer than 5 μm, not only the whole length of the diode becomes longer but also operation resistance thereof becomes larger. Hence, the length in the longitudinal direction of the unit diode is preferably 5 μm or less. 
     Although the diode is formed in single-crystal silicon in the ninth embodiment, the diode may be formed in polysilicon which is grown on sapphire by CVD. 
     According to the eleventh embodiment, silicon carbide is formed on the insulated substrate  31  by CVD, and impurities are added into the silicon carbide by ion implantation. In contrast, when depositing silicon carbide (e.g. polycrystalline silicon carbide) on the insulated substrate  31  by CVD, impurities may be added into a source gas, and carriers may be formed with the impurities while forming the silicon carbide. In this case, it is desirable that sapphire is used for the insulated substrate  31 , and polycrystalline silicon carbide is used for the silicon carbide. 
     In addition, when growing a silicon carbide film on the sapphire substrate, the growth temperature may be increased to form single-crystal silicon carbide by epitaxial growth. If the single-crystal silicon carbide is formed, operation resistance can be lower compared with when using the polycrystalline silicon carbide. 
     Hereinafter, aspects of the above-described embodiments will be summarized. 
     As an aspect of the embodiment, a diode is provided which includes at least one diode element which has a plurality of N-type regions and a plurality of P-type regions, the N-type regions and the P-type regions being alternately arranged in series to form PN junctions; and an insulated substrate which has electric insulation. The N-type regions and the P-type regions are formed on the insulated substrate. 
     According to the above diode, since the PN junctions are alternately connected in series to form the diode element, the diode element serves as an equivalent circuit in which a plurality of diodes Da, Db, Dc (see  FIG. 5 ) are connected in series in a state where the adjacent diodes are opposite in direction to each other. Hence, a pair of diodes Da, Db, which are connected so as to be opposite in direction to each other and opposed to each other, mutually cancels increase and decrease of the voltage value caused due to the temperature characteristic. Hence, the temperature dependence of the Zener voltage can be lower. 
     In contrast, for example, when N-type regions and P-type regions are formed on a semiconductor substrate via an insulating film (see U.S. Pat. No. 5,365,099), it is difficult to sufficiently increase the withstand voltage of the insulating film. To solve this problem, in the above embodiment, since the N-type regions and P-type regions are formed on the insulated substrate  31 , the above insulating film is not necessary. In addition, the withstand voltage of the insulated substrate can make higher easily. Hence, while ensuring the withstand voltage of the insulated substrate sufficiently, the number of the PN junctions can be increased, thereby making the Zener voltage of the diode sufficiently higher.