JP2000077682A - Schottky diode - Google Patents

Abstract

(57) [Summary] In a Schottky diode, device destruction due to overvoltage is prevented. A pn junction is formed in a Schottky barrier formation region, and a breakdown voltage is controlled by punch-through of the pn junction. [Effect] High breakdown voltage with excellent resistance to reverse overvoltage,
A large current Schottky diode is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a structure of a Schottky diode.

[0002]

2. Description of the Related Art As the operating frequency of power conversion devices such as inverters becomes higher, the speed of semiconductor switching elements and the speed of free-wheel diodes and free-wheel diodes connected in parallel to switching elements are strongly required. Since these diodes are required to have a function of rectifying a large current at a high voltage with a low loss, a pn junction diode is generally widely used. However, the pn junction diode has a property that a large reverse current flows during a turn-off transition due to minority carriers accumulated inside the semiconductor when a current flows, so that not only an excessive loss is generated when the switching element is turned on, but also an excessive noise is generated. It is a source and is a major factor that hinders high frequency operation of the converter. Although various pn junction diodes with improved recovery characteristics have been developed, this type of diode with minority carrier injection has an inherent limitation in reducing the reverse current during recovery.

There is a Schottky diode as a rectifier diode meeting such a demand. In the Schottky diode, the carrier that carries the current inside the semiconductor is only the majority carrier, and the minority carrier is not injected or accumulated even when the current is applied, so that the reverse current at the time of turn-off can be extremely reduced.

[0004] However, there are some problems when the conventional Schottky diode is applied to a high-voltage, large-current power converter. One is that the leakage current when a reverse voltage is applied is large. In particular, when a high voltage near the withstand voltage is applied at a high temperature, the leakage current increases, so that the loss generated in the reverse voltage blocking state increases. This has the disadvantage that it occurs locally in the diode element and the element is easily destroyed by partial thermal runaway. Another significant problem is that the resistance to reverse overvoltage is very weak. When a reverse voltage near the withstand voltage of the element is applied, the leakage current flowing over the Schottky barrier existing at the interface between the Schottky metal and the semiconductor increases sharply, so that local breakdown easily occurs. In the case of a Schottky diode, the reverse current at the time of recovery is only a very small displacement current generated due to the expansion of the depletion layer spreading in the semiconductor due to the reverse voltage applied between the barrier and the semiconductor. However, this minute change in the reverse current causes a large voltage fluctuation at the time of recovery, and a short-time noise-like overvoltage is applied to the element itself, resulting in instantaneous destruction of the element.

As a prior art for improving the surge withstand capability of such a Schottky diode when a reverse voltage is applied, there is an example of an integrated circuit clamp diode disclosed in Japanese Patent Application Laid-Open No. Sho 59-217354. FIG. 2 is a sectional view showing a schematic configuration of a clamp diode in the semiconductor integrated circuit device. In FIG. 2, reference numeral 10 denotes an input-side aluminum electrode, 20 denotes a GND-side aluminum electrode, 30 denotes a Schottky barrier diode used as an input clamp diode, 40 denotes an n-type epitaxial layer, 50 denotes an n ⁺ -type buried layer, and 60 denotes aluminum. is the contact electrode 10 and the ohmic and n ⁺ -type layer provided in order to lower the resistance between the aluminum electrode 10 and the n ⁺ -type buried layer 50, the p-type semiconductor substrate 70, 80 an oxide film of the element isolation, Reference numeral 90 denotes an oxide film for electrode separation, which is located in the n-type epitaxial
And ap ⁺ -type layer 120 in contact with the n ⁺ -type layer 60 and controlling the n ⁺ -type impurity concentration to reduce the pn junction diode 130 having a breakdown voltage lower than the breakdown voltage of the Schottky diode 30 to these n ⁺ -type layers. 60 and p ⁺ type layer 120
Between the input-side aluminum electrode 10 and the GND-side aluminum electrode 20.
This is obtained by connecting an n-junction diode 130 in parallel.
By controlling the impurity concentration of the p ⁺ -type layer, the breakdown voltage of the pn junction diode 130 can be set to about 8 to 20 V, while the breakdown voltage of the Schottky diode is about 25 to 30 V, and the input side aluminum When a positive surge voltage is applied to the electrode 10, most of the surge current is pn
It is described that the current flows through the junction diode 130 and does not flow through the Schottky diode 30. According to this prior art, a Schottky diode as a clamp diode and a pn junction diode having a lower breakdown voltage than this Schottky diode are connected in parallel in the same active region, so that the conventional performance is maintained as it is. It is described that there is a feature that the surge withstand voltage can be remarkably improved.

This example teaches a technique for improving the surge withstand voltage when a reverse voltage is applied by arranging pn junction diodes having a breakdown voltage lower than that of a Schottky diode in parallel. However, the means for forming a pn junction diode having a low breakdown voltage can be applied only to an integrated circuit having a relatively low voltage, and cannot be applied to an element having a high breakdown voltage and a large current. This will be described below. Since the above-described example is an integrated circuit, the constituent elements are lateral, and the pn junction diode 130 having a low breakdown voltage is p ^{+ with a} relatively high impurity concentration.
A low-voltage p ⁺ n ⁺ junction having a breakdown voltage of about 8 to 20 V is formed in the mold layer 120 and the n ⁺ -type layer 60 having the same high impurity concentration by impurity diffusion from the semiconductor surface of the epitaxial layer 40. On the other hand, withstand voltage of several hundred to several thousand V,
In the case of a power element having a current capacity of several A to several hundred A, a so-called vertical element in which a current flows in the vertical direction of the semiconductor substrate, and the area of the semiconductor element is as large as about 1 cm ² . Due to this difference in basic structure, the above-described conventional technology cannot be applied as it is. That is, in the case of a power element, a p ⁺ or n ⁺ type high concentration layer is generally formed from the upper and lower surfaces of a semiconductor substrate having a relatively low impurity concentration and a high resistance to form a functional region. Therefore, p ⁺ and n
⁺ It is almost impossible to form a low breakdown voltage portion where the high concentration layer is in direct contact.

Further, in the case of a power element, in order to prevent the breakdown voltage from being lowered due to the electric field concentration at the peripheral edge of the Schottky barrier, a measure must be taken to make the breakdown voltage in the peripheral part higher than that in the barrier part. In order to reduce the increase in the leakage current of the Schottky barrier portion at a high temperature, a means for reducing the electric field intensity applied to the barrier portion is required. A region having a breakdown voltage lower than that of the Schottky barrier portion must be formed in a manner consistent with the formation of these means, but this cannot be realized by the conventional technology.

[0008]

As described above, a power Schottky diode with a high withstand voltage and a large current has a small size in addition to the conventional problem of the Schottky diode of reducing the leakage current at a high temperature and a high voltage. There is a need for a technical means for solving a new problem that it is necessary to prevent element destruction due to an overvoltage generated in the element itself due to a steep change in the recovery current. The present invention provides a new technical means made in consideration of these problems.

[0009]

In order to solve the above-mentioned problems, according to the present invention, a portion having a higher breakdown voltage than a Schottky barrier portion is provided in parallel with a Schottky barrier region, and a portion having a lower breakdown voltage is distributed. In addition, the breakdown voltage of the low breakdown voltage portion is controlled by punch-through of the pn junction.

By the above means, a portion where a locally high electric field is applied to the Schottky barrier is eliminated, and a surge reverse current due to an overvoltage is caused to flow in a portion having a lower breakdown voltage than the Schottky barrier, thereby making the reverse. Since the application of a high reverse voltage to the Schottky barrier region where the leakage current increases rapidly can be prevented, the resistance of the Schottky diode to overvoltage can be improved.

Furthermore, a Schottky diode with a high breakdown voltage can be manufactured in which the breakdown voltage of the low breakdown voltage portion can be accurately controlled in a high voltage region of several hundred volts or more by the punch-through phenomenon of the pn junction or the avalanche phenomenon due to the curvature of the junction. .

Further, by disposing a large number of the low breakdown voltage portions in parallel with the Schottky barrier, the loss absorption region due to the surge overcurrent can be made substantially uniform inside the semiconductor having a relatively large area. Surge absorption capacity can be improved.

If the Schottky diode manufactured by the above method and having a high resistance to overvoltage at the time of recovery is used as a freewheeling diode or a freewheel diode of a converter, a reverse voltage having a function of absorbing circuit loss at the time of switching is provided. By the action as the clamp diode, a high-frequency converter having a simplified circuit configuration can be realized.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to specific embodiments.

FIG. 1 shows a first embodiment of the present invention, and is a cross-sectional view of a high breakdown voltage Schottky diode having improved overvoltage resistance. A parallel-plate-shaped semiconductor substrate 1 having upper and lower main surfaces is composed of an n ⁺ -type layer 2 having a high impurity concentration and a low resistance.
When, n ⁺ -type layer of high resistivity lower impurity concentration than 2 n ^- -type layer 3 consists, n ⁺ -type layer 2 cathode electrode 6 ohmic contact of low resistance on one major surface is exposed, A Schottky metal 5 serving as an anode electrode is provided on the other main surface where n ⁻ type layer 3 is exposed, and a Schottky barrier 5 is provided at a portion where n ⁻ type layer 3 and Schottky metal 5 are in contact.
1 is formed. The portion where the Schottky metal 5 terminates, that is, the end of the semiconductor substrate, is n ⁻ from the other main surface.
A relatively high-concentration p ⁺ -type layer 4 is provided in the mold layer 3, and the surface thereof is in ohmic contact with the Schottky electrode 5 at low resistance. Then, in a portion where the inner major functional area than the p ⁺ -type layer 4 is formed, n from the other main surface ^- at a relatively high concentration type layer 3, and p ⁺
There are provided a plurality of p ⁺ -type layers 7 that extend deeper than the mold layer 4, and each has a pn junction 7 with the n ⁻ -type layer 3.
1 is formed, and the other main surface is in ohmic contact with the Schottky metal 5 with low resistance.

The operation of each part in this embodiment will be described below. The rectifying action of the current as a diode operates in the portion of the Schottky barrier 51 formed between the n ⁻ -type layer 3 and the Schottky metal 5. That is, when a voltage in a direction in which the Schottky metal 5 becomes a positive potential with respect to the cathode electrode 6 is applied, the Schottky barrier 51 has a potential of approximately 0.1 to 0.1.
Electrons flow from the Schottky metal 5 to the n ⁻ -type layer 3 beyond the relatively low barrier of 0.5 V, and further flow toward the cathode electrode 6 to conduct. When a voltage in the opposite direction is applied, the flow of electrons is stopped by the Schottky barrier 51, and the flow of current is blocked. At this time, since the voltage is held by the depletion layer extending from the Schottky barrier 51 into the n ⁻ -type layer 3, the n ⁻ -type layer 3 is a relatively high resistance and thick semiconductor layer in a high breakdown voltage element. The p ⁺ -type layer 4 provided at the terminal end of the Schottky metal 5 prevents a breakdown voltage from being reduced due to a locally concentrated electric field applied to the Schottky barrier 51 in a state where a reverse voltage is applied, and has a high reverse voltage of the p ⁺ n junction. Utilizes blocking characteristics. In this example, a so-called guard ring structure which is usually used is shown, but other structures such as a field limiting ring (FL)
R), field plate (FP), or junction termination extension (JTE)
Etc. can also be applied.

The plurality of deep p ⁺ -type layers 7 are the main feature of this embodiment. The deepest position of the p ⁺ n junction 71 and the length W _D1 between the n ⁺ type layer 2 and the p ⁺ type layer 4 and n
The length W _D2 with the ⁺ type layer 2 and the Schottky barriers 51 and n
⁺ The length of the mold layer 2 and the length W _D3 are made longer. That is, W _D1 <W
_D2 < _WD3 . By doing so, the breakdown voltage of the p ⁺ -type layer 7 becomes the lowest when a reverse voltage is applied.
For example, n ⁻ type layer 3 having a resistivity of 40 Ωcm, W _D2 = 20 μm
If m, W _D1 = 15 μm, the breakdown voltage of the p ⁺ -type layer 4 is about 500 V, and the breakdown voltage of the p ⁺ -type layer 7 is about 400 V
Becomes Therefore, when the applied reverse voltage is increased, p
Breakdown occurs in the ⁺ -type layer 7, and a higher voltage is not applied to the element. Since all of the surge reverse current flows in this portion, the conduction of the Schottky barrier portion that is weak to the reverse current can be prevented beforehand. As a result, if the time during which the overvoltage is applied is short, thermal breakdown does not occur, and the diode has a high resistance to the overvoltage.

FIG. 3 is a sectional view of a Schottky diode according to a second embodiment of the present invention. The same reference numerals as those in the first embodiment shown in FIG. 1 indicate the same components, structures, conduction types, and functions as those of the first embodiment shown in FIG. In FIG. 3, a plurality of p ⁺ -type layers 7 having a relatively high concentration and deeper than the p ⁺ -type layer 4 are provided in the n ⁻ -type layer 3 from the other main surface in a portion serving as a main functional region. This is the same as in the first embodiment. In this embodiment, a new point is that a plurality of p ⁺ -type layers 11 having a relatively high concentration are provided in the n ⁻ -type layer 3. The p ⁺ -type layer 11 is provided between the p ⁺ -type layer 4 and the p ⁺ -type layer 7 and between the p ⁺ -type layers 7. The impurity concentration and depth of the p ⁺ -type layer 11 are the same as those of the p ⁺ -type layer 4 and can be formed simultaneously. The length _WD1 between the deepest position of the plurality of deep p ⁺ -type layers 7 and the n ⁺ -type layer 2 is defined as p ^{+ serving} as a guard ring.
The length W _D2 between the mold layer 4 and the n ⁺ -type layer 2 and the newly provided p
The length between the ⁺ type layer 11 and the n ⁺ type layer 2 is longer than the length W _D3 . That is, the relationship of W _D1 > W _D2 ≒ W _D3 is established.

In this example, the main operation of the Schottky diode and the operation of each part are the same as described above. When a reverse voltage is applied to the cathode electrode 6 so that the cathode electrode 6 has a positive potential with respect to the Schottky metal, the depletion layer is formed from the respective junctions of the p ⁺ -type layer 4, the p ⁺ -type layer 7, and the p ⁺ -type layer 11 by n. ^The depletion layers spread in the ⁻ type layer 3, but if the distance between the respective p ⁺ type layers is set to be narrow, the depletion layers spreading there will be connected to each other, resulting in a so-called pinch-off state. As a result, the electric field applied to the Schottky barrier 51 is reduced, and the leakage current when a reverse voltage is applied can be significantly reduced. Further, when the reverse voltage is applied, the breakdown voltage of the p ⁺ -type layer 7 becomes the lowest, and the resistance to the overvoltage is improved as in the first embodiment. Therefore, in the present embodiment, a Schottky diode is obtained in which the reverse voltage characteristics and the overvoltage resistance are simultaneously improved.

FIG. 4 is a sectional view of a Schottky diode according to a third embodiment of the present invention. In the figure, the same reference numerals as those in the first embodiment shown in FIG. 1 denote constituent parts having the same structure, conduction type and function. In Figure 4, the portion to be the inner major functional region than the p ⁺ -type 4, n from the other main surface ^- -type layer 3 p ⁺
A plurality of p ⁻ -type layers 9 having a lower concentration than the mold layer 4 are provided. The main operation of the Schottky diode and the operation of each part are the same as described above. In this embodiment, a plurality of p ⁻ -type layers 9 having a relatively low concentration is a novel point.

According to this structure, when a reverse voltage is applied to the cathode electrode 6 so that the cathode electrode 6 has a positive potential with respect to the Schottky metal, the depletion layer is formed on the n ⁻ -type layer 3 at the portion of the Schottky barrier 51. Only the p ^- type layer 9 extends to the p ⁻ -type layer 9 in the p ⁻ -type layer 9 together with the n ⁻ -type layer 3. If the impurity concentration of the p ⁻ -type layer 9 is set to be low, when the tip of the depletion layer spreading there reaches the other main surface in ohmic contact with the Schottky metal, a voltage breakdown occurs due to a so-called punch-through phenomenon. The impurity amount of the p ⁻ -type layer 9 is set such that the punch-through start voltage is lower than the breakdown voltage of the p ⁺ -type layer 4 of the Schottky barrier 51 and the guard ring. For example, the n ⁻ -type layer 3 has a resistivity of 40 Ωcm and a thickness of 2
If the depth of the p ⁻ -type layer 9 is about 3 μm and the average impurity concentration is 3.0 × 10 ¹⁵ cm ⁻³ , the breakdown voltage of the Schottky barrier is about 500 V and the p ⁻ -type layer 9 has Has a punch-through voltage of about 400V. Therefore, when the applied reverse voltage is increased, breakdown occurs in the p ⁻ -type layer 9 portion,
Higher voltages are not applied to the device. Since all of the surge reverse current flows in this portion, the conduction of the Schottky barrier portion that is weak to the reverse current can be prevented beforehand. As a result, if the time during which the overvoltage is applied is short, thermal breakdown does not occur, and the diode has a high resistance to the overvoltage.

FIG. 5 is a sectional view of a Schottky diode according to a fourth embodiment of the present invention. The same reference numerals as those in the third embodiment shown in FIG. 4 denote the same parts as those of the third embodiment shown in FIG. In FIG. 5, the primary n in the functional areas to become part of the other main surface ^- than the p ⁺ -type layer 4 into a mold layer 3 low concentration p
^{Although a} plurality of-type layers 9 are provided as in the third embodiment, a plurality of p ⁺ -type layers 11 having a relatively high concentration are further provided in the n ^- type layer 3. Is a new point. The p ⁺ type layer 11 is provided between the p ⁺ type layer 4 and the p ⁻ type layer 9 and between the p ⁻ type layers 9. The impurity concentration and depth of the p ⁺ -type layer 11 are the same as those of the second embodiment shown in FIG. The functions and effects of the p ⁺ -type layer 11 and the p ⁻ -type layer 9 are the same as those described in the second and third embodiments. In this embodiment, the reverse voltage characteristics and the overvoltage resistance can be simultaneously improved. .

FIG. 6 is a sectional view of a Schottky diode according to a fifth embodiment of the present invention. The same reference numerals as those in the fourth embodiment shown in FIG. 5 denote the same parts in the structure, conduction type, and action as those in the fourth embodiment. In FIG. 6, a relatively high-concentration p ⁺ -type layer 11 is formed in n ⁻ -type layer 3 from the other main surface in a portion serving as a main functional region.
Although but the provided plurality is the same as the fourth embodiment of the further low impurity concentration than the p ⁺ -type layer 11 between the p ⁺ -type layer 11 adjacent p ^- type layer 91 Is a novel point that a plurality of are provided.

In this case, the distance between the two p ⁺ -type layers 11 sandwiching the p ^-- type layer, that is, the width of the p ^-- type layer 91 is set wider than in the fourth embodiment. Rather, the distance between the two adjacent p ⁺ -type layers 11 sandwiching the p ^-- type layer 9 in the fourth embodiment is reduced so that the structure is in contact with the intervening p ^-- type layer 9. is there. p ⁺ -type layer 4, p ⁺ -type layer 11 and the p ^- impurity concentration of each portion of the mold layer 91, the depth each p ⁺ -type layer of the fourth embodiment 4, the p ⁺ -type layer 11 and the p
^{-Same as the} mold layer 9. The effect of reducing the leakage current due to the pinch-off action of the p ⁺ -type layer 11 is the same as described above.
The effect of the low breakdown voltage due to the punch-through of the p ⁻ -type layer 91 is the same as that of the fourth embodiment. In this embodiment,
Further, the leakage current at the time of application of the reverse voltage can be more reliably reduced. In the fourth embodiment, the depletion layers extending to the p ⁺ -type layer 11 and the p ⁻ -type layer 9 are connected to each other to form a so-called pinch-off state, and the leakage current is reduced by reducing the electric field applied to the Schottky barrier 51. Reduced.
However, since the depletion layer also extends into the p ⁻ -type layer 9, the width of the depletion layer extending between the adjacent p ⁺ -type layer 11 is reduced, and the expected pinch-off effect is likely to be impaired. In this embodiment, the p ⁺ -type layer 11 and the p ^-- type layer 91 are connected to each other by contact with each other, so that the pinch-off action is not impaired.

FIG. 7 shows an example in which an inverter for driving a motor is constituted by using a Schottky diode to which the present invention is applied. Six switching elements, S
W11, SW12, SW21, SW22, SW31, SW
32 and six diodes SD11, S to which the present invention is applied.
This is an example in which a three-phase induction motor is controlled by D12, SD21, SD22, SD31, and SD33. The applied diode acts as a rectifier diode, has the function of clamping overvoltage in the reverse direction, and has the function of absorbing the loss due to the LC of the circuit and preventing the occurrence of abnormal voltage. An inverter device that operates at high speed with low electromagnetic noise can be realized with a simple circuit configuration without providing a passive circuit for suppressing overvoltage in the element.
In the present embodiment, an example of application to an inverter device for driving a motor has been described. However, another conversion device used by connecting a diode in parallel to a switching element, for example, AC-DC
The present invention is also applicable to power converters such as converters, DC-DC converters, and choppers.

In the above embodiment, the conductivity type of the semiconductor substrate 1 is n
Although the case of the p-type has been described, the present invention is also applicable to the case of the p-type if all the described conduction types are the opposite conduction types.

[0027]

According to the present invention, a Schottky diode having a high withstand voltage and a large current having excellent resistance to overvoltage in the reverse direction can be obtained. Therefore, if the Schottky diode according to the present invention is applied to a power conversion circuit such as an inverter device, it is possible to realize a high-frequency low-loss converter with a simple circuit configuration in which generation of voltage noise is suppressed.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a first embodiment of a Schottky diode to which the present invention is applied.

FIG. 2 is a sectional view showing a conventional technique.

FIG. 3 is a sectional view showing a second embodiment of a Schottky diode to which the present invention is applied.

FIG. 4 is a sectional view showing a third embodiment of the Schottky diode to which the present invention is applied.

FIG. 5 is a sectional view showing a fourth embodiment of the Schottky diode to which the present invention is applied.

FIG. 6 is a sectional view showing a fifth embodiment of the Schottky diode to which the present invention is applied.

FIG. 7 is a circuit diagram of an example in which a Schottky diode to which the present invention is applied is used for an inverter for driving a motor.

[Explanation of symbols]

1 ... semiconductor substrate, 2 ... n ⁺ -type layer, 3 ... n ^- -type layer, 4 ... p ⁺ -type layer serving as a guard ring, the Schottky metal is 5 ... anode electrode, 6 ... cathode electrode, 7 ... relatively deep the high concentration p ⁺ -type layer, 9 and 91 ... p ^- -type layer, 11 ... p ⁺ -type layer,
51: Schottky barrier.

Claims (10)

[Claims] A first conductive type semiconductor substrate having a pair of main surfaces; a Schottky metal formed on one main surface of the semiconductor substrate and forming a Schottky barrier between the semiconductor substrate and the semiconductor substrate; A first semiconductor layer of a second conductivity type that makes ohmic contact with the Schottky metal at an end portion of the Schottky metal and forms a pn junction with the semiconductor substrate;
A cathode electrode in ohmic low-resistance contact with the semiconductor substrate on the other main surface of the semiconductor substrate; and a pn between the Schottky metal on the one main surface of the semiconductor substrate and contact with the Schottky metal. A plurality of second semiconductor layers of a second conductivity type forming a junction, wherein a breakdown voltage of a pn junction of the second semiconductor layer is lower than other portions. 2. A semiconductor substrate of a first conductivity type having a pair of main surfaces, a Schottky metal formed on one main surface of the semiconductor substrate and forming a Schottky barrier between the semiconductor substrate and the semiconductor substrate, A first semiconductor layer of a second conductivity type that makes ohmic contact with the Schottky metal at an end portion of the Schottky metal and forms a pn junction with the semiconductor substrate;
On the other main surface of the semiconductor substrate, a cathode electrode in ohmic low resistance contact with the semiconductor substrate, and in contact with the Schottky metal on the one main surface of the semiconductor substrate, between the semiconductor substrate A plurality of second semiconductor layers of a second conductivity type forming a pn junction deeper than a pn junction formed by the first semiconductor layer. 3. The semiconductor device according to claim 2, further comprising: a plurality of second conductivity-type semiconductor elements, wherein said plurality of second conductivity type contacts said Schottky metal on said one main surface of said semiconductor base to form a pn junction with said semiconductor base. A Schottky diode having three semiconductor layers. 4. The semiconductor device according to claim 3, wherein the distance between the first, second and third semiconductor layers is set such that the depletion layers extending from the respective semiconductor layers are connected to each other when a reverse voltage is applied. Schottky diode. 5. A semiconductor substrate of a first conductivity type having a pair of main surfaces, a Schottky metal formed on one main surface of the semiconductor substrate and forming a Schottky barrier between the semiconductor substrate and the semiconductor substrate, A first semiconductor layer of a second conductivity type that makes ohmic contact with the Schottky metal at an end portion of the Schottky metal and forms a pn junction with the semiconductor substrate;
On the other main surface of the semiconductor substrate, a cathode electrode in ohmic low resistance contact with the semiconductor substrate, and in ohmic contact with the Schottky metal on the one main surface of the semiconductor substrate, between the cathode electrode and the semiconductor substrate. And a plurality of second semiconductor layers of a second conductivity type having a lower impurity concentration than the first semiconductor layer, forming a pn junction. 6. The punch-through voltage according to claim 5, wherein the impurity concentration of said second semiconductor layer is such that a depletion layer whose reverse withstand voltage value of the element extends into said second semiconductor layer reaches said one main surface. A Schottky diode characterized by being set to be limited by: 7. The semiconductor device according to claim 6, further comprising a second semiconductor layer which is in contact with said Schottky metal on said one main surface of said semiconductor substrate and forms a pn junction with said semiconductor substrate. A Schottky diode comprising a plurality of third semiconductor layers of a second conductivity type having an impurity concentration. 8. The semiconductor device according to claim 7, wherein an interval between the first, second and third semiconductor layers is set so that a depletion layer extending from each semiconductor layer is connected to each other when a reverse voltage is applied. Schottky diode. 9. The Schottky diode according to claim 7, wherein said second semiconductor layer and said third semiconductor layer have portions that are in contact with each other. 10. A power converter comprising a semiconductor switching element and a rectifier diode connected in parallel, wherein the rectifier diode is the Schottky diode according to any one of claims 1 to 9. converter.