Patent Publication Number: US-6664607-B2

Title: Diode having breakdown voltage adjustable to arbitrary value without increase of parasitic capacitance and process for fabrication thereof

Description:
This application is a divisional of patent application Ser. No. 09/875,517, filed on Jun. 6, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a semiconductor device and, more particularly, to a diode incorporated in a semiconductor device and a process for fabricating a protective diode. 
     DESCRIPTION OF THE RELATED ARTS 
     Diodes have found a wide variety of applications in electric circuits. The diode permits electric current to flow therethrough only unidirectionally. When being focused on the diode characteristics that it “flows forward electric current”, the diode is used as an essential part of a rectifier. On the other hand, when being focused on the diode characteristics that it “blocks reverse electric current”, the diode serves as a protective element against abnormal voltage applied to a circuit component. 
     The later application is hereinbelow described. A semiconductor device has plural conductive terminals between an internal integrated circuit and an external electric circuit, and electric signals are transferred between the external circuit and the internal integrated circuit through the conductive terminals. Thus, the conductive terminals are indispensable components of a semiconductor device. However, the conductive terminals are exposed to the environment, and extremely high serge voltage is liable to be applied to the conductive terminals. If the extremely high serge voltage is directly applied to circuit components of the internal integrated circuit, the circuit components are seriously damaged, and the semiconductor device is to be replaced with a new one. 
     In order to protect the circuit components of the internal integrated circuit device against the extremely high serge voltage, protective diodes are formed in the semiconductor device, and are connected to conductive paths between the conductive terminals and the circuit components of the internal integrated circuit. 
     FIG. 1 shows a typical example of the protective diode. The prior art protective diode  1  is usually connected to a signal path between a conductive terminal and an input transistor of an integrated circuit. The protective diode  1  includes a heavily doped n-type well  2 , a p-type impurity region  3  and a p-type guard ring  4 . The heavily doped n-type well  2  is formed in an n-type semiconductor substrate, and the p-type impurity region  3  and the heavily doped n-type well  2  forms a p-n junction serving as a Zener diode. The p-type impurity region  3  is encircled by the p-type guard ring  4 , which is deeper than the p-type impurity region  3 . Thus, the p-type impurity region  3  and the p-type guard ring  4  form in combination a graft base structure. 
     The p-type guard ring expands the p-n junction, and the wide p-n junction allows the prior art protective diode  1  to flow a large amount of electric current due to the extremely high surge voltage or electrostatic discharge into the n-type semiconductor substrate. In other words, the wide p-n junction is desirable from the viewpoint of the anti-electrostatic discharge characteristics of the protective diode. However, the wide p-n junction has a large amount of parasitic capacitance. While the integrated circuit is operating, an input signal is supplied from the associated conductive terminal to the input transistor. A large amount of parasitic capacitance is causative of deformation of the waveform of the input signal, and the wide p-n junction is not desirable from the viewpoint of signal propagation characteristics. Thus, there is a trade-off between the anti-electrostatic discharge characteristics and the signal propagation characteristics. 
     The p-n junction between the p-type impurity region  3  and the heavily doped n-type well  2  has a strong influence on both of the parasitic capacitance and the breakdown voltage, and the designer suffers from the parasitic capacitance strongly tied up to the breakdown voltage in the prior art diode. In detail, the prior art diode is expected to be broken down at a certain voltage before the input transistor is damaged. However, the prior art protective diode is to be maintained in off-state while the electric signal is propagated between the conductive terminal and the input transistor. The breakdown phenomenon takes place at a critically strong electric field created across the depletion layer developed from the p-n junction between the p-type impurity region  3  and the heavily doped n-type well  2 . The heavier the dopant concentration, thinner the depletion layer. The electric field easily exceeds the critical value in a thin depletion layer. For this reason, the breakdown voltage is mainly dependent on the impurity concentration of the heavily doped n-type well  2 . The parasitic capacitance is dependent on the thickness of the depletion layer as well as the area of the p-n junction. The thin depletion layer undesirably increases the parasitic capacitance. When a designer gives a high breakdown voltage to the prior art diode  1 , the parasitic capacitance is unavoidably increased, and the electric signal tends to be deformed due to the large parasitic capacitance. Thus, the breakdown voltage is strongly tied up to the parasitic capacitance, and the designer feels it difficult to design an optimum diode. The obstacle for the designer is the parasitic capacitor strongly tied up to the breakdown voltage. 
     Although another prior art diode is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 8-153887, the prior art diode is a photo diode. The technical target of the prior art photo diode is to improve the signal-to-noise ration as well as widen the dynamic range, and a heavily doped p-type impurity region is formed in the heavily doped n-type well by using an ion-implantation or a thermal diffusion technique. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide a diode, which is adjustable to an appropriate breakdown voltage without taking parasitic capacitance into account. 
     It is also an important object of the present invention to provide a process for fabricating the protective diode. 
     To accomplish the object, the present invention proposes to form plural junctions one of which has an influence on the breakdown voltage of a diode and the other of which dominates the parasitic capacitor. 
     In accordance with one aspect of the present invention, there is provided a diode comprising a first semiconductor layer having a first conductivity type, and a second semiconductor layer having a second conductivity type opposite to the first conductivity type and including plural regions different in dopant concentration so as to form plural junctions different in electric characterstics from one another. 
     In accordance with another aspect of the present invention, there is provided a process for fabricating a diode, comprising the steps of a) preparing a heavily doped semiconductor layer having one conductivity type, b) growing a lightly doped semiconductor layer of the aforesaid one conductivity type on the heavily doped semiconductor layer, c) forming a guard ring having the other conductivity type opposite to the aforesaid one conductivity type in the lightly doped semiconductor layer so as to form a first junction together with the lightly doped semiconductor layer, d) forming an impurity region of the aforesaid one conductivity type in a surface portion of the lightly doped semiconductor layer inside of the guard ring, and e) forming another impurity region of the other conductivity type in a surface portion of the impurity region and a surface portion of the guard ring so as to form a second junction different in electric characteristics from the first junction together with the impurity region. 
     In accordance with another aspect of the present invention, there is provided a process for fabricating a diode, comprising the steps of a) preparing a heavily doped semiconductor layer having one conductivity type, b) growing a lightly doped semiconductor layer of the aforesaid one conductivity type on the heavily doped semiconductor layer, c) forming an impurity region of the aforesaid one conductivity type in the lightly doped semiconductor layer and d) forming another impurity region having the other conductivity type opposite to the aforesaid one conductivity type in a surface portion of the impurity region and a surface region of the lightly doped semiconductor layer so as to form a first junction together with the lightly doped semiconductor layer and a second junction different in electric characteristics from the first junction together with the impurity region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the protective diode and the process will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross sectional view showing the structure of the prior art protective diode; 
     FIG. 2 is a cross sectional view showing the structure of a protective diode according to the present invention; 
     FIGS. 3A to  3 J are cross sectional views showing a process for fabricating the protective diode according to the present invention; 
     FIG. 4 is a cross sectional view showing the structure of another diode according to the present invention; 
     FIGS. 5A to  5 I are cross sectional views showing a process for fabricating the diode; and 
     FIG. 6 is a cross sectional view showing the structure of yet another diode according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 2 of the drawings, a protective diode  10  is integrated on a heavily doped n-type semiconductor substrate  11  together with an input field effect transistor T 1  forming a part of an internal electric circuit. A pin P 1  is connected to the gate electrode of the input field effect transistor T 1 , and the protective diode  10  is connected to the signal path between the pin P 1  and the gate electrode of the input field effect transistor T 1 . The heavily doped n-type semiconductor substrate  11  is doped at 1×10 18  cm −3  or greater than it, and has resistivity equal to or less than 20/1000 ohm-cm. 
     The protective diode  10  comprises a lightly doped n-type semiconductor layer  12 , a heavily doped n-type impurity region  13 , a heavily doped p-type impurity region  14  and a p-type guard ring  15 . The lightly doped n-type semiconductor layer  12  is epitaxially grown on the heavily doped n-type semiconductor substrate  11 , and the heavily doped n-type impurity region  13  is formed in a surface portion of the lightly doped n-type semiconductor layer  12 . The heavily doped p-type impurity region  14  is partially overlapped with the heavily doped n-type impurity region  13 , and is shallower than the heavily doped n-type impurity region  13 . Thus, the heavily doped p-type impurity region  14  and the heavily doped n-type impurity region  13  form a major p-n junction a. The p-type guard ring  15  is formed around the heavily doped p-type impurity region  14  as well as around the heavily doped n-type impurity region  13 . The p-type guard ring  15  is deeper than the heavily doped n-type impurity region  13  so that the major p-n junction a is provided inside of the p-type guard ring  15 . The p-type guard ring  15  projects into the lightly doped n-type semiconductor layer  12 , and forms a p-n junction b at the boundary to the lightly doped n-type semiconductor layer  12 . The p-n junction b is much wider than the major p-n junction a. The lightly doped n-type semiconductor layer  12  may be smaller in dopant concentration than the heavily doped n-type well  2  by one order of magnitude. 
     The Zener breakdown voltage is dependent on the major p-n junction a. The other p-n junction b does not have any influence on the Zener breakdown voltage. On the other hand, the parasitic capacitance is dominated by the other p-n junction b. The influence of the major p-n junction a on the parasitic capacitance is limited as will be described herein later. The property of the p-n junction a is mainly dependent on the dopant concentration of the heavily doped n-type impurity region  13 , on the other hand, the property of the other p-n junction is dominated by the dopant concentration of the lightly doped n-type semiconductor layer  12  as well as the dopant concentration of the p-type guard ring  15 . Thus, the breakdown voltage is not strongly tied up to the parasitic capacitance. In this instance, even though the breakdown voltage is adjusted to the optimum value, the parasitic capacitance coupled to the protective diode  10  remains small. In fact, the parasitic capacitance coupled to the protective diode  10  is smaller than that of the prior art protective diode  1 , because the p-type guard ring  15  forms the p-n junction b together with the lightly doped n-type semiconductor layer  12 . 
     Subsequently, description is made on how the protective diode is designed. The diode  10  is expected to protect the input field effect transistor T 1  against extremely high serge voltage or electrostatic discharge. A certain voltage is applied to the diode  10  during the operation of the semiconductor device substantially at all times. For this reason, the protective diode  10  is designed to have a p-n junction directly affecting the breakdown voltage not on the surface but inside thereof. This means that the protective diode  10  has the graft base structure. 
     The present inventor investigated the ratio between the parasitic capacitance coupled to the main p-n junction a and the parasitic capacitance coupled to the other p-n junction b at the boundary of the guard ring. The present inventor analyzed a lot of diodes with the graft base structure, and concluded that most of the parasitic capacitance coupled to each diode was due to the p-n junction at the boundary of the guard ring. 
     As described hereinbefore, the parasitic capacitance is dominated by both of the area and the thickness of a depletion layer developed from a p-n junction. There is a limit to reduction of area. Therefore, the reduction of parasitic capacitance is to be achieved by increasing the thickness of depletion layer. This means that the dopant concentration is to be changed. However, the main p-n junction a is designed to exhibit the target breakdown voltage, and, accordingly, the dopant concentration can not be changed. For this reason, it is effective against the parasitic capacitance to design the p-n junction at the boundary of the guard ring in such a manner as to permit the depletion layer to be developed as wide as possible. 
     The guard ring  15  is expected to offer a wide current path to the surge current. In this circumstances, it is difficult to reduce the dopant concentration of the p-type guard ring  15 . The p-type guard ring  15  forms the p-n junction together with the n-type semiconductor layer. Any serious limit is not set on the n-type semiconductor layer. For this reason, the lightly doped n-type semiconductor layer  12  is employed in the protective diode  10  embodying the present invention. 
     The present inventor investigated the dopant concentration of the lightly doped n-type semiconductor layer  12  effective against the parasitic capacitance. The lightly doped n-type semiconductor layer  12  was analyzed by using the one-step junction approximation on the assumption of the thickness equal to or greater than 1 micron. The present inventor concluded that the parasitic capacitance was reduced on condition that the resistivity equal to or greater than 0.1 ohm-cm. For example, when the lightly doped n-type semiconductor layer  12  was designed to be 8 microns to 12 microns thick and 45 ohm-cm to 55 ohm-cm, the parasitic capacitance was reduced to a hundredth or less. 
     The Zener breakdown voltage was designed as follows. The Zener breakdown voltage was targeted of the order of 6 volts. Boron was to be diffused into an n-type region with the resistivity of the order of 10/1000 ohm-cm. The n-type region was coated with a boron source, and the boron was thermally diffused at 1100 degrees in centigrade for 60 minutes. N type dopant impurity has heavily doped, and the dopant concentration was targeted 1×10 19  cm −3  at 2.5 micron deep. In order to adjust the breakdown voltage at the target value, the impurity profile of born was to be flat until 3 microns deep. The flat impurity profile was achieved by prolonging the heat treatment for driving the boron thereinto. In this instance, the teat treatment was continued for 3.5 hours. 
     The present inventor investigated the diode characteristics of samples designed as described hereinbefore. The parasitic capacitance of 26 pF to 37 pF was coupled to the samples of the prior art diode. On the other hand, the parasitic capacitance coupled to the diode according to the present invention was only 5 pF. The diode withstood the electrostatic discharge at about 10 kilovolts, and the Zener breakdown voltage was 6.1 volts. 
     Subsequently, description is made on a process for fabricating the diode according to the present invention with reference to FIGS. 3A to  3 J. The Zener breakdown voltage is targeted at 6 volts. The process starts with preparation of the heavily doped n-type semiconductor substrate  11 . The heavily doped n-type semiconductor substrate  11  is formed of silicon. 
     Silicon is epitaxially grown to 800 nanometers thick on the heavily doped n-type semiconductor substrate  11 , and forms the lightly doped n-type semiconductor layer  12 . The lightly doped n-type semiconductor layer  12  is thermally oxidized at about 1000 degrees in centigrade for 180 minutes so as to grow a silicon oxide layer  16  to about 800 nanometers thick in a surface portion of the lightly doped n-type semiconductor layer  12  as shown in FIG.  3 A. 
     Photo-resist solution is spun onto the silicon oxide layer  16 , and is baked for forming a photo-resist layer. A pattern image for the p-type guard ring  15  is transferred from a photo mask (not shown), and forms a latent image in the photo-resist layer. The latent image is developed so as to form a photo-resist mask (not shown). Thus, the photo-resist mask is provided on the silicon oxide layer  16  by using photo-lithographic techniques. 
     Using the photo-resist mask, the silicon oxide layer  16  is partially etched away so as to expose an upper surface of the lightly doped n-type semiconductor layer  12  as shown in FIG.  3 B. The photo-resist mask is stripped off. The exposed surface is coated with a boron-containing diffusion source (not shown), and the boron is thermally diffused from the diffusion source into the lightly doped n-type semiconductor layer  12  at 1100 degrees in centigrade for 60 minutes. Thereafter, the boron is driven into the lightly doped n-type semiconductor layer  12  at about 1000 degrees in centigrade for 30 minutes. The p-type guard ring  15  is formed in the lightly doped n-type semiconductor layer  12 , and the exposed surface of the lightly doped n-type semiconductor layer  12  is thermally oxidized. As a result, the diffusion window is closed with the silicon oxide as shown in FIG.  3 C. 
     Subsequently, another photo-resist mask (not shown) is formed on the silicon oxide layer  16  by using the photo-lithographic techniques, and the silicon oxide layer  16  is partially etched away for forming a window for an ion-implantation. Phosphorous is ion implanted into the lightly doped n-type semiconductor layer  12  at dosage of 7.0×10 15  cm −2 . Ion-implanted phosphorous layer  17  is formed in the lightly doped n-type semiconductor layer  12  as shown in FIG.  3 E. After the ion-implantation, the phosphorous is driven into the lightly doped n-type semiconductor layer  12  at about 1100 degrees in centigrade for 180 minutes. The phosphorous forms the heavily doped n-type impurity region  13 , and the window for the ion-implantation is closed as shown in FIG.  3 F. 
     Yet another photo-resist mask (not shown) is formed on the silicon oxide layer  16  by using the photo-lithographic techniques, and the silicon oxide layer  16  is partially etched away for forming a window for a thermal diffusion as shown in FIG.  3 G. The heavily doped n-type impurity region  13  and a part of the p-type guard ring  15  are exposed to the window. The photo-resist mask is stripped off. The exposed surface is coated with a boron-containing diffusion source (not shown), and the boron s thermally diffused at about 1100 degrees in centigrade for 60 minutes. The boron is further driven at about 1000 degrees in centigrade for 30 minutes. The boron forms the heavily doped p-type impurity region  14 , and the window is closed, again, as shown in FIG.  3 H. 
     Still another photo-resist mask (not shown) is formed on the silicon oxide layer  16  by using the photo-lithographic techniques. Using the photo-resist mask, the silicon oxide layer  16  is partially etched away for forming a contact hole. The photo-resist mask is stripped off. The heavily doped p-type impurity region  14  and a part of he p-type guard ring  15  are exposed to the contact hole as shown in FIG.  3 I. 
     Aluminum is evaporated onto the resultant structure, and forms an aluminum layer. A photo-resist mask is formed on the aluminum layer by using the photo-lithographic techniques, and the aluminum layer is patterned into an electrode  18 . Finally, another electrode  19  of gold or silver is formed on the reverse surface of the heavily doped n-type semiconductor layer  11  as shown in FIG.  3 J. Thus, the protective diode  10  shown in FIG. 2 is fabricated on the heavily doped n-type semiconductor substrate  11 . 
     As will be understood from the foregoing description, the protective diode according to the present invention has the major p-n junction a and the other p-n junction b. The Zener breakdown voltage is dominated by the major p-n junction, and most of the parasitic capacitance is coupled to the other p-n junction b. The two p-n junctions a and b are independently designed so that the protective diode  10  is adjusted to the target breakdown voltage without serious large amount of parasitic capacitance. 
     Second Embodiment 
     Turning to FIG. 4 of the drawings, another diode  20  embodying the present invention is fabricated on a heavily doped n-type semiconductor substrate  11 . The diode  20  comprises a lightly doped n-type semiconductor layer  12 , a heavily doped n-type impurity region  13  and a heavily doped p-type impurity region  14 . The lightly doped n-type semiconductor layer  12  is epitaxially grown on the heavily doped n-type semiconductor substrate  11 , and has the dopant concentration of the order of 1×10 14  cm −3 . The heavily doped n-type impurity region  13  is so deep that the heavily doped n-type impurity region  13  projects into the heavily doped n-type semiconductor substrate  11 . This feature is desirable, because the resistance against electric current flowing between the heavily doped n-type impurity region  13  and the heavily doped n-type semiconductor substrate  11  is surely reduced. The heavily doped n-type impurity region  13  is formed by using an ion-implantation, and has the dopant concentration of the order of 1×10 19  cm −3 . The heavily doped p-type impurity region  14  is shallower than the heavily doped n-type impurity region  13 , and is wider than the heavily doped n-type impurity region  13 . For this reason, the heavily doped p-type impurity region  14  and the heavily doped n-type impurity region  13  form a major p-n junction a, and the heavily doped p-type impurity region  14  and the lightly doped n-type semiconductor layer  12  form another p-n junction c. Thus, the diode  20  is similar to the protective diode  10  except the p-type guard ring  15 . 
     The Zener breakdown voltage is dominated by the major p-n junction a, and the p-n junction a is designed to have the Zener breakdown voltage at 6 volts. The other p-n junction c achieves a high withstanding voltage against the electrostatic discharge. The withstanding voltage is to be higher than the Zener breakdown voltage, i.e., higher than 6 volts. The withstanding voltage may be of the order of 10 volts. Thus, the combination of the lightly doped n-type semiconductor layer  12  and the heavily doped p-type impurity region  14  serves as the combination of the heavily doped n-type semiconductor layer  12  and the p-type guard ring  15 . In case where the lightly doped n-type semiconductor layer  12  has the resistivity of 50 ohm-cm, the withstanding voltage is equal to or greater than 300 volts. 
     The p-n junction c wider in area than the major p-n junction a so as to be conducive to reduction of the parasitic capacitance coupled to the diode  20 . The lightly doped n-type semiconductor layer  12  reduces the parasitic capacitance coupled to the p-n junction equal to or less than 1 pF. The parasitic capacitance coupled to the prior art diode is of the order of 30 pF. Thus, the parasitic capacitance is drastically reduced. 
     The dopant concentration of the lightly doped n-type semiconductor layer  12  is determined regardless of the Zener breakdown voltage, and the parasitic capacitance is not strongly tied up. Thus, a designer can design the breakdown voltage without increase of the parasitic capacitance. 
     FIGS. 5A to  5 I shows a process for fabricating the diode  20 . The process starts with preparation of the heavily doped n-type semiconductor substrate  11  of silicon. Silicon is epitaxially grown on the heavily doped n-type semiconductor substrate  11 , and forms the lightly doped n-type semiconductor layer  12 . A surface portion of the lightly doped n-type semiconductor layer  12  is thermally oxidized at 1000 degrees in centigrade for 180 minutes. Then, the lightly doped n-type semiconductor layer  12  is covered with a silicon oxide layer  16  of the order of 800 nanometers thick as shown in FIG.  5 A. 
     Subsequently, a photo-resist mask (not shown) is prepared on the silicon oxide layer  16  by using the photo-lithographic techniques, and the silicon oxide layer  16  is partially etched away for forming a window for an ion-implantation as shown in FIG.  5 B. 
     Phosphorous is ion implanted into the lightly doped n-type semiconductor layer  12  through the window at dosage of 7.0×10 15  cm −2 . Thus, the phosphorous  17  is introduced into the lightly doped n-type semiconductor layer  12  as shown in FIG.  5 C. 
     The phosphorous  17  is driven into the lightly doped n-type semiconductor layer  12  through a heat treatment at about 1100 degrees in centigrade for 180 minutes. When the heat treatment is finished, the heavily doped n-type impurity region  13  is formed in the lightly doped n-type semiconductor layer  12 , and the window is closed with the silicon oxide grown in the heat treatment as shown in FIG.  5 D. 
     Another photo-resist mask (not shown) is patterned on the silicon oxide layer  16 , and the silicon oxide layer  16  is partially etched away for forming another window as shown in FIG.  5 E. 
     A boron-containing diffusion source (not shown) is spread over the exposed area, and the boron is thermally diffused at about 1100 degrees in centigrade for 60 minutes as shown in FIG.  5 F. The boron-containing diffusion source is removed, and the boron is driven into the lightly doped n-type semiconductor layer  12  at about 1000 degrees in centigrade for 30 minutes. While the boron is being driven, the phosphorous is also diffused. The phosphorus reaches the heavily doped n-type semiconductor substrate  11 , and the heavily doped n-type impurity region  13  is continued to the heavily doped n-type semiconductor substrate  11  as shown in FIG.  5 G. The window is also closed during the heat treatment. 
     Yet another photo-resist mask (not shown) is patterned on the silicon oxide layer  16  by using the photo-lithographic techniques. The silicon oxide layer  16  is partially etched away so that the heavily doped p-type impurity region  14  is exposed to a window formed in the silicon oxide layer  16  a shown in FIG.  5 H. 
     Subsequently, aluminum is evaporated, and forms an aluminum layer. Still another photo-resist mask (not shown) is patterned on the aluminum layer by using the photo-lithographic techniques. Using the photo-resist mask, the aluminum layer is partially etched away so as to form an electrode  18  on the silicon oxide layer  16 . 
     Finally, gold or silver is evaporated on the reverse surface of the heavily doped n-type semiconductor substrate  11 , and forms another electrode  19  as shown in FIG.  5 I. 
     The heat treatment is repeated in the process, and the n-type dopant impurity is thermally diffused in the heat treatments. Accordingly, the lightly doped n-type semiconductor layer  12  is reduced in thickness. The minimum thickness of the lightly doped n-type semiconductor layer is the sum of the depth of the heavily doped p-type impurity region  14 , the width of the depletion layer developed from the p-n junction c on the normal operating conditions and the decrement of thickness due to the thermal diffusion of n-type dopant impurity from the heavily doped n-type semiconductor substrate  11 . In this instance, the lightly doped n-type semiconductor layer  12  of the order of 11 microns thick. 
     As will be understood, the diode  20  has two p-n junctions a and c, and the electric characteristics of the two junctions a and c are independently designed. The Zener breakdown voltage is dominated by the major p-n junction a, and the other p-n junction c has the strong influence on the withstanding voltage against the electrostatic discharge and the amount of parasitic capacitance. Thus, the diode  20  has the Zener breakdown voltage adjusted to the target value without increase the parasitic capacitance. 
     Third Embodiment 
     FIG. 6 shows yet another diode  21  embodying the present invention. The diode  21  is similar to the diode  20  except that the heavily doped n-type impurity region  13  is spaced from the heavily doped n-type semiconductor layer  11 . For this reason, the layers and regions of the diode  21  are labeled with the same references designating corresponding layers and regions of the diode  20  without detailed description for the sake of simplicity. 
     The gap between the heavily doped n-type impurity region  13  and the heavily doped n-type semiconductor layer  11  is dependent on the resistivity of the lightly doped n-type semiconductor layer  12 . The gap is designed in such manner that the resistance after the breakdown is sharply increased. In other words, the design work for the part of the epitaxial layer between the heavily doped n-type impurity region  13  and the heavily doped n-type semiconductor substrate  11  aims at substantial reduction in resistance. 
     Even though the heavily doped n-type impurity region  13  is opposed to the heavily doped n-type semiconductor substrate  11  through the part of the lightly doped n-type semiconductor layer  12 , the diode  21  achieves the high withstanding voltage against the electrostatic discharge and a small amount of parasitic capacitance. The drive-in step for the heavily doped n-type impurity region  13  is shorter than that of the second embodiment, and a turn around time is shortened. 
     When the diode  21  is reversely biased with the surge voltage, the Zener breakdown takes place, and a large amount of electric current flows vertically. The part of the lightly doped n-type semiconductor layer  12  serves as a resistor connected in series, and the diode suffers from the large resistance. 
     The two p-n junctions a and c are formed in the diode  21 , and the lightly doped n-type semiconductor layer  12  eliminates the undesirable influences of the heavily doped n-type impurity region  13  from the diode  21 . As a result, the parasitic capacitance is reduced to a quarter with respect to the prior art diode without increase of the semiconductor substrate  11 . Moreover, the Zener breakdown voltage is adjustable without changing the dopant concentration of the semiconductor substrate  11 . 
     If the lightly doped n-type semiconductor layer  12  is thin, if the heavily doped n-type impurity region  13  is deep, the resistance between the electrodes  18  and  19  is reduced. 
     As will be appreciated from the foregoing description, the p-type impurity region  14  forms the p-n junctions a/b or a/c together with the n-type impurity regions different in dopant concentration from each other in the diode according to the present invention. The breakdown voltage is dependent on the electric characteristics of the major p-n junction a, and the electric characteristics of the major p-n junction a are controlled with the dopant concentration in the p-type/n-type impurity regions on both sides of the major p-n junction. The other p-n junction does not have any influence on the breakdown voltage. On the other hand, the parasitic capacitance coupled to the diode is dominated by the other p-n junction b or c. For this reason, the breakdown voltage is adjustable without consideration of the parasitic capacitance. 
     Although particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. 
     The diode according to the present invention is never limited to the protective usage. The advantage of the present invention is directed to the breakdown voltage not strongly tied up to the parasitic capacitance. For this reason, the diode according to the present invention is available for any kind of electric circuit where the designer thinks it important to adjust the breakdown voltage regardless of the parasitic capacitance. One of the applicable electric circuits is a clamp circuit. 
     If the n-type dopant impurity is ion implanted into the lightly doped n-type semiconductor layer at high acceleration energy, the projection range of the n-type dopant impurity is long. This means that the drive-in step is shortened. 
     A gas phase diffusion or an ion-implantation may be used in the process according to the present invention.