JPH10326900A - Diode for electric power - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a trade-off relation between the surge current dielectric strength of the diode for electric power and reverse recovery characteristic. SOLUTION: (1) A p<+> -source region 7 is formed at part of a p-anode layer 4 of a pin diode, an insulating film 3 such as an oxide film is formed at part of the surface of the p<+> -source region 7, and an anode electrode 1 is brought into contact with the surface of the p-anode layer 4 and the p<+> -source region 7 where the insulating film 3 is absent. (2) The insulating film 3 is formed at the part of the surface of the p-anode layer 4 of the pin diode, and the anode electrode 1 is brought into contact with the surface of the p-anode layer 4 where the insulating film 3 is absent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power rectifier.

[0002]

2. Description of the Related Art Power semiconductor devices are used in various applications such as inverters, and even now, the range of applications for applications with higher breakdown voltage and higher capacity is expanding. In recent years, switching elements, such as insulated gate bipolar transistors, which can operate at a high withstand voltage, a large capacity, and at a high frequency have been developed, and accordingly, high-speed recovery characteristics that enable a power diode to operate at a high frequency have been required. .

In such a background, an MPS (Merged P-IN Schottky) has been developed with a view to achieving a particularly fast reverse recovery characteristic.
Diode and SFD (Soft and Fast recovery Diode)
Various types of diodes having such a new structure have been reported, and improvements in the characteristics of power diodes have been promoted. One of the most widely used examples of power diodes is a pin diode. FIG. 4 shows a partial sectional view of the pin diode.

A p anode layer 4 is formed on one side of an n drift layer (also called i layer) 5 having a high specific resistance, and the anode electrode 1 is in contact with the surface thereof. An n ⁺ cathode layer 6 is formed on the other side of n drift layer 5, and its surface is in contact with cathode electrode 2. In order to improve the reverse recovery characteristic of the pin diode, a minority carrier lifetime control using electron beam irradiation or the like is currently performed.

[0005]

Conventionally, a freewheeling diode (FWD) has been used in an actual device such as an inverter.
Although an in-diode is used, when a load short circuit or the like occurs, a very large current, for example, 1000 A / cm ² or more may flow through the FWD. At this time, when the temperature of the diode rises and the intrinsic carrier density becomes higher than the injected carrier density, the diode exhibits a negative resistance, which causes destruction or deterioration [for example, Silver,
D. and Robertson, MJ, Solid State Electron. 16, p. 1
337 (1973)]. In such a case, the surge current withstand capability of the diode can be improved by increasing the impurity concentration of the p anode layer 4 and increasing the number of carriers injected from the p anode layer 4. However, if it does so, the normal reverse recovery characteristic will worsen. Thus, there is a trade-off between the surge current withstand capability and the reverse recovery characteristic. Therefore, only with the structure of the pin diode,
Both properties could not be improved simultaneously.

[0006] Application of diodes to applications requiring large capacity and high frequency operation, such as inverters for electric railways, requires not only high surge current resistance against a large current flow but also high-speed reverse recovery characteristics. Is done. In the future, the development of a new power diode that satisfies both at the same time will be more important. The inventors have previously disclosed in Japanese Patent Application No. 8-132466 that a thyristor is arranged in parallel with a pin diode, thereby achieving both high surge current resistance and good reverse recovery characteristics. However, according to the invention, although the surge current withstand capability was remarkably improved, the improvement of the reverse recovery characteristic was not satisfactory.

In view of the above problems, it is an object of the present invention to provide a power diode having both of the above two requirements, that is, both a large surge current resistance and a high-speed reverse recovery characteristic.

[0008]

In order to solve the above problems, the present invention provides a first conductive type cathode layer, a first conductive type drift layer having a lower impurity concentration than the first conductive type cathode layer, and a second conductive type anode layer. In any of the above, three layers of semiconductors are stacked in this order, and in a power diode having an anode electrode in contact with the surface of the anode layer of the second conductivity type and a cathode electrode in contact with the back surface of the cathode layer of the first conductivity type, An insulating film is formed on a part of the surface of the anode layer, and the anode electrode is in contact with the surface of the second conductive type anode layer where the insulating film is not formed. In the following description, the first conductivity type is assumed to be n-type and the second conductivity type is assumed to be p-type, but this can be reversed.

In such a power diode, when a forward bias is applied to the anode electrode, holes are injected from the second conductivity type anode layer into the first conductivity type drift layer, while the first conductivity type cathode layer is Then, electrons are injected into the second conductivity type anode layer through the first conductivity type drift layer. By increasing the forward bias and increasing the current density,
The carrier density of the first conductivity type drift layer is increased by the conductivity modulation. In the second conductive type anode layer below the insulating film on the surface, the electron concentration increases as compared with the portion without the insulating film. This is considered for the following reason.
In the portion where the insulating film is formed, an effect is produced such that minority carriers in the semiconductor (electrons in the second conductivity type anode layer) are blocked by the insulating film. Therefore, the higher the injection state, the higher the electron concentration in the second conductivity type anode layer near the insulating film. Further, the hole concentration in the first conductivity type drift layer also increases so as to satisfy the neutral condition. Therefore, when an insulating film is formed on a part of the surface of the second conductivity type anode layer, the forward voltage increases at a higher forward current as compared with a case where the anode electrode is in contact without a normal insulating film. Become smaller.

Preferably, a plurality of striped or dot-shaped insulating films are evenly arranged. By doing so, many points of high injection state occur, and the current is evenly distributed. In addition, a second conductive type source region having a higher concentration than the second conductive type anode layer is formed in a part of the second conductive type anode layer, and an insulating film is formed on a part of the surface of the second conductive type source region. The anode electrode may be in contact with the surface of the formed second conductive type source region and the second conductive type anode layer where the insulating film is not formed.

In this case, since the second conductive type anode layer has a low concentration, it has a normal rated current density (up to 100 A).
/ Cm ² ), the injection of holes is suppressed and the normal pi
The reverse recovery characteristic is improved as compared with the n diode. On the other hand, in the surge current region, as described above, a large number of holes are injected into the first conductivity type drift layer from the second conductivity type source region having a higher concentration than the second conductivity type anode layer in the portion where the insulating film is formed. Conversely, many electrons are injected from the first conductivity type drift layer into the second conductivity type anode layer, and the carrier concentration increases.

In particular, the diffusion depth of the anode layer of the second conductivity type is larger than that of the source region of the second conductivity type. By doing so, the pn junction between the second-conductivity-type anode layer and the first-conductivity-type drift layer has no bent portion, and thus is suitable for a high breakdown voltage element. Further, the diffusion depth of the second conductivity type anode layer may be smaller than that of the second conductivity type source region.

In this case, the total amount of impurities in the anode layer of the second conductivity type is small, carriers injected into the drift layer of the first conductivity type are reduced, and the reverse recovery characteristic is improved. It is assumed that a plurality of source layers and insulating films are evenly arranged. Further, it is assumed that these are in the form of dots and stripes. By doing so, the current distribution is uniform, there is no current concentration, and the characteristics are stable.

It is assumed that the insulating film is an oxide film. An oxide film can be easily obtained in an element forming process.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A power diode according to an embodiment of the present invention will be described below with reference to the drawings. In the following description, layers, regions, and the like bearing n and p mean layers, regions, and the like, each having electrons and holes as majority carriers. In addition, the first conductivity type is all n-type and the second conductivity type is p-type, but this can be reversed.

FIG. 1 is a partial cross-sectional view of a unit portion near the center of a chip of a power diode according to a first embodiment of the present invention. The main part of the power diode is formed by repeating unit parts as shown in many figures, and a guard ring structure is formed in a peripheral part of the chip as a withstand voltage part, which is related to the essence of the present invention. It is omitted because it is not a part.

A high resistivity n drift layer (also called i layer)
5 is formed on one side of p anode layer 4 by diffusion.
An insulating film is provided on a part of the surface on the side of the p anode layer 4.
3 is formed, and the insulating film 3 of the p anode layer 4 is formed.
The anode electrode 1 is provided in contact with the surface
You. On the other side of the n drift layer 5, n⁺Cathode layer 6
Are formed by diffusion, and n⁺N-doped cathode layer 6
The cathode electrode 2 is provided on the surface opposite to the lift layer 5.
Have been killed. Unlike the conventional pin diode of FIG.
The p anode layer 4 is located on the surface of the p anode layer 4.
High concentration of p ⁺A source region 7 is formed,
p⁺The point that the insulating film 3 is formed on the surface of the source region 7
It is. p⁺Source region 7 and insulating film 3 are shown in plan view.
Has a stripe shape. In the figure, the anode electrode 1
Is extended on the insulating film 3, but not necessarily
You don't have to extend it.

The power rectifying element of the present invention is a conventional pin rectifier.
The p anode layer 4, n
After forming a three-layer structure including the drift layer 5 and the n ⁺ cathode layer 6 and selectively forming the p ⁺ source region 7 on the surface layer of the p anode layer 4, a thermal oxide film obtained by a normal process; the insulating film 3 such as a CVD oxide film partially leaving by photolithography, the upper and the n ⁺ cathode layer 6
A metal film is sputtered or vapor-deposited on the back surface of the substrate to produce an anode electrode 1 and a cathode electrode 2. The lifetime control is performed by electron beam irradiation. In some cases, the insulating film 3 may be formed before the formation of the three-layer structure.

In this example, for example, the p-anode layer 4 is formed.
The amount of boron ion implantation for p-anode layer
4 is higher than that of the conventional pin diode.
Reduce by about one digit. Where p⁺About source area 7
Is about the same as the surface impurity concentration of a conventional pin diode.
Or several times larger. Impurity concentration of each layer
Degree or surface impurity concentration, thickness or junction depth
For example, the values are as follows. n⁺Cathode layer: 1 × 10
²⁰/ Cm^Three , 80 μm, n drift layer: 3 × 10 ¹³/ C
m^Three , 320 μm, p anode layer: 1 × 10^Fifteen/ Cm
^Three , 6 μm, p⁺Source area 7: 7 × 10¹⁶/ Cm^Three,
3.4 μm, width, 11.4 μm, oxide film width: 6 μm, film
Thickness: 0.5 μm, width of region where oxide film is not formed:
9 μm.

FIG. 5 is a forward characteristic diagram of the power diode (A) according to the first embodiment of the present invention. As a comparative example,
The configuration of the semiconductor substrate is the same as that of the first embodiment, and the forward characteristics of the comparative diode (B) without the insulating film 3 on the surface and the conventional pin diode (C) are also described. The surface impurity concentration of the p anode layer 4 of the pin diode (C) is 3
× 10 ¹⁶ / cm ³ , and the junction depth was 3.4 μm. Lifetime control was performed by electron beam irradiation. The breakdown voltage of the power diode (A), the comparison diode (B), and the pin diode (C) exceeds 3000 V. When the forward voltage is compared at 100 A / cm ² which is usually used as a rated current, the forward voltage of the pin diode (C) is 2.9 V
However, the power diode (A) and the comparison diode (B) are not much different from 3.1 to 3.2 V.

On the other hand, in FIG. 5, when the forward voltage is compared in the surge current region of 1000 A / cm ² or more, the power diode is compared with 9.6 V of the comparison diode (B) and 9.1 V of the pin diode (C). In (A), 8.9
V and the on-state voltage have decreased. Therefore, heat generation during surge current application is reduced, and an increase in intrinsic carrier concentration is suppressed, so that surge current withstand capability is improved.

As described in the previous section, in the power diode (A) in which the oxide film is formed, minority carriers (electrons in the p anode layer 4) of the semiconductor surface layer are insulated in the region where the insulating film 3 is formed. The effect of being blocked by the film occurs, and the higher the injection state, the higher the electron concentration in the p-anode layer 4 near the insulating film, and further, the higher the hole concentration in the n-drift layer 5 so as to satisfy the neutral condition. Is also thought to increase. Therefore, when the insulating film 3 is formed on a part of the surface of the p anode layer 4, the forward voltage increases as the forward current increases as compared with the comparative diode (B) and the pin diode (C) without the insulating film 3. The rate becomes smaller. Normally, there is no operation in the reverse recovery mode in the surge current region. Therefore, it is not necessary to consider the influence on the reverse recovery characteristic due to the promotion of hole injection in such a current region.

FIG. 6 is a characteristic diagram comparing the recovery characteristics of the power diode (A), the comparison diode (B), and the conventional pin diode (C) of the second embodiment. The horizontal axis represents the forward voltage at 100 A / cm ² , and the vertical axis represents the reverse recovery peak current. The condition of the reverse recovery operation is I _F = 100
_{^{A / cm 2, V R =}} 1300V, -dI F / dt = 25
0 A / (cm ² · μs). As shown in this figure, there is a trade-off relationship between the forward voltage and the reverse recovery peak current. Although the power diode (A) of this embodiment is slightly outside the comparison diode (B), it is a pin diode. It can be seen that trade-off characteristics superior to (C) are exhibited. That is, when compared with diodes having the same forward voltage, the power diode (A) of the present embodiment has a small reverse recovery peak current, and therefore has a high switching speed.
Switching loss is small.

As described in the previous section, the structure is such that the p ⁺ source region 7 having a higher concentration than the p anode layer 4 is formed and the anode electrode 1 is in contact with a part of the surface thereof. At the rated current density (〜100 A / cm ² ), current mainly flows from the p anode layer 4 in contact with the anode electrode 1. This is because, by lowering the surface concentration of the p anode layer 4, the injection of holes is suppressed lower than in the conventional pin diode, so that the reverse recovery characteristic is improved.

That is, the power diode of this embodiment has improved forward characteristics in the surge current region and improved reverse recovery characteristics in the rated current region as compared with the conventional pin diode, and has a higher speed and severe operation. To withstand. It can be said that the effect of the formation of the insulating film 3 on the characteristics is negligible. The shapes of the insulating film 3 and the p ⁺ source region 7 are as follows:
Stable characteristics can be obtained if the structure is dispersed without current concentration, such as stripes and dots. The thickness and width of the insulating film 3 and the surface concentration, width, and junction depth of the p anode layer 4 and the p ⁺ source region 7 can be controlled by existing process technology, and can be optimized according to the device structure and the like. In this example, the insulating film 3 is an oxide film which is most easily formed. However, an insulating material other than the oxide film, for example, a nitride film or polyimide may be used.

Embodiment 2 FIG. 2 is a partial sectional view of a power diode according to a second embodiment of the present invention. In this example,
The diffusion depth of the p anode layer 4 is different from that of the first embodiment in FIG.
The case where it is smaller than that of the p ⁺ source region 7 is shown.
Insulating film 3 is provided in contact with the surface of p ⁺ source region 7. By doing so, the p anode layer 4 to n
The effect of further suppressing the injection of holes into the drift layer 5 can be obtained, and the reverse recovery characteristics can be improved.

[Embodiment 3] FIG. 3 is a partial sectional view of a unit portion near the center of a chip of a power diode according to a third embodiment of the present invention. The difference from the first embodiment shown in FIG. 1 is that the p ⁺ source region 7 having a higher concentration than the p anode layer 4 is not formed. Insulating film 3 is provided in contact with the surface of p anode layer 4. However, in this example, the boron ion implantation amount for forming the p anode layer 4 is increased so that the surface concentration of the p anode layer 4 is about one order of magnitude higher than in the case of the first embodiment. Surface impurity concentration,
The diffusion depths are 3 × 10 ¹⁶ / cm ³ and 3.4 μm, respectively.

Therefore, in the power diode of the third embodiment, the number of steps can be reduced by the amount that the p ⁺ source region 7 is not formed as compared with the first embodiment. In the power diode of the third embodiment, a lower forward voltage than that of the first embodiment was obtained in the surge current region.

[0029]

As described above, according to the present invention, an insulating film is provided on the surface of the second conductive type anode layer or a part of the surface of the second conductive type source region, and the anode electrode is provided on the part without the insulating film. The contact reduces the on-voltage in the surge current region, suppresses heat generation, and improves the surge current capability. In particular, if the impurity concentration of the anode layer of the second conductivity type is reduced, the reverse recovery characteristic at the rated current can be improved, and a large surge current withstand capability and high-speed reverse recovery characteristic, which were difficult with the conventional pin diode, can be achieved. A power diode is provided which is compatible with the above.

[Brief description of the drawings]

FIG. 1 is a partial sectional view of a power diode according to a first embodiment of the present invention.

FIG. 2 is a partial sectional view of a power diode according to a second embodiment of the present invention.

FIG. 3 is a partial sectional view of a power diode according to a third embodiment of the present invention.

FIG. 4 is a partial cross-sectional view of a conventional power pin diode.

FIG. 5 is a diagram showing forward static characteristics of a power diode according to an example of the present invention and a comparative example.

FIG. 6 is a trade-off characteristic diagram of the reverse recovery peak current and the on-state voltage of the power diode according to the embodiment of the present invention and the comparative example.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 anode electrode 2 cathode electrode 3 insulating film 4 p anode layer 5 n drift layer 6 n ⁺ cathode layer 7 p ⁺ source region

Claims (11)

[Claims] A first conductive type cathode layer, a first conductive type drift layer having a lower impurity concentration than the first conductive type cathode layer, and a second conductive type anode layer are each formed by stacking three semiconductor layers in this order. In a power diode having an anode electrode in contact with the surface of the two-conductivity type anode layer and a cathode electrode in contact with the back surface of the first conductivity type cathode layer, an insulating film is formed on a part of the surface of the second conductivity type anode layer Wherein the anode electrode is in contact with the surface of the second conductivity type anode layer on which the insulating film is not formed. 2. The power diode according to claim 1, wherein the plurality of insulating films are arranged substantially evenly. 3. The power diode according to claim 2, wherein the insulating film has a stripe shape. 4. The power diode according to claim 2, wherein the insulating film has a dot shape. 5. A first conductive type cathode layer, a first conductive type drift layer having a lower impurity concentration than the first conductive type cathode layer, and a second conductive type anode layer each having three semiconductor layers stacked in this order. In a power diode having an anode electrode in contact with the surface of the two-conductivity type anode layer and a cathode electrode in contact with the back surface of the first conductivity type cathode layer, a part of the second conductivity type anode layer is A second conductive type source region having a high concentration is also formed, an insulating film is formed on a part of the surface of the second conductive type source region, and the second conductive type source region and the second conductive type where the insulating film is not formed are formed. A power diode, wherein the anode electrode is in contact with the surface of a mold anode layer. 6. The power diode according to claim 5, wherein the diffusion depth of the anode layer of the second conductivity type is larger than that of the source region of the second conductivity type. 7. The power diode according to claim 5, wherein the diffusion depth of the anode layer of the second conductivity type is smaller than that of the source region of the second conductivity type. 8. The power diode according to claim 6, wherein the plurality of second conductivity type source regions and the insulating film are arranged evenly. 9. The power diode according to claim 8, wherein the source region of the second conductivity type and the insulating film have a stripe shape. 10. The power diode according to claim 8, wherein the source region of the second conductivity type and the insulating film are dot-shaped. 11. The power diode according to claim 1, wherein the insulating film is an oxide film.