JP3807023B2 - Power diode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a power rectifying element.
[0002]
[Prior art]
Power semiconductor elements are used in various applications including inverters, and even today, the application range is expanding to applications with higher breakdown voltage and larger capacity. In recent years, switching elements capable of operating at high voltage, large capacity, and high frequency, such as insulated gate bipolar transistors, have been developed, and power diodes are also required to have high-speed recovery characteristics capable of high-frequency operation. .
[0003]
Against this background, various diodes with new structures such as MPS (Merged PIN Schottky) diodes and SFD (Soft and Fast recovery Diodes) have been reported, particularly with the aim of achieving high-speed reverse recovery characteristics. It is being advanced.
One representative example of the most widely used power diode is a pin diode. FIG. 4 shows a partial cross-sectional view of the pin diode.
[0004]
A p anode layer 4 is formed on one side of an n drift layer (also referred to as 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 the n drift layer 5, and the cathode electrode 2 is in contact with the surface thereof. In order to improve the reverse recovery characteristic of this pin diode, the lifetime control of minority carriers using electron beam irradiation or the like is currently performed.
[0005]
[Problems to be solved by the invention]
Conventionally, a pin diode is used as a freewheel diode (FWD) in an actual device such as an inverter. However, when a load short-circuit occurs, a very large current of, for example, 1000 A / cm ² or more may flow through the FWD. . If the temperature of the diode rises and the intrinsic carrier density becomes higher than the injected carrier density at this time, the diode exhibits a negative resistance, resulting in breakdown and deterioration [for example, Silber, D. and Robertson, MJ, Solid State Electron. 16, p.1337 (1973)]. In such a case, the surge current resistance of the diode can be improved by increasing the impurity concentration of the p anode layer 4 and increasing the number of injected carriers from the p anode layer 4. However, in this case, the normal reverse recovery characteristic is deteriorated. Thus, there is a trade-off relationship between the surge current withstand capability and the reverse recovery characteristic. Therefore, the characteristics of both cannot be improved at the same time only by the structure of the pin diode.
[0006]
In the application of a diode to an application that requires a large capacity and operation at a high frequency, such as an electric railway inverter, not only a high surge current resistance against a large current flow but also a high-speed reverse recovery characteristic is required. In the future, the development of new power diodes that satisfy both of these requirements will increase in importance.
The inventors previously made it possible to achieve both high surge current resistance and good reverse recovery characteristics by arranging a thyristor in parallel with a pin diode in Japanese Patent Application No. 8-132466. However, in the invention, although the surge current withstand capability has been remarkably improved, it cannot be said that the reverse recovery characteristic has been improved.
[0007]
In view of the above problems, an object of the present invention is to provide a power diode having both of the above two requirements, that is, a large surge current withstand capability and a fast reverse recovery characteristic.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides a semiconductor layer including a first conductivity type cathode layer, a first conductivity type drift layer having a lower impurity concentration than the first conductivity type cathode layer, and a second conductivity type anode layer. In a power diode having an anode electrode in contact with the surface of the second 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 anode on the surface of the second conductivity type anode layer is stacked. An insulating film is formed, the anode electrode is in contact with the surface of the second conductivity type anode layer where the insulating film is not formed, and a cross-sectional shape of the insulating film and the anode electrode sandwiches the insulating film and the insulating film. It is assumed that a unit portion composed of the anode electrode formed as described above is repeatedly formed . In the following description, the first conductivity type is all n-type and the second conductivity type is p-type, but this can be reversed.
[0009]
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 to the first conductivity type drift layer, while from the first conductivity type cathode layer to the first electrode. Electrons are injected into the second conductivity type anode layer through the conductivity type drift layer. When the forward bias is increased to increase the current density, the carrier density of the first conductivity type drift layer increases due to conductivity modulation. In the second conductivity 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 to be due to the following reason. In the portion where the insulating film is formed, there is an effect that minority carriers in the semiconductor (electrons in the second conductivity type anode layer) are blocked by the insulating film. For this reason, the higher the injection state, the higher the electron concentration in the second conductivity type anode layer near the insulating film. Furthermore, 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 rate of increase of the forward voltage increases as the forward current increases as compared with the case where the anode electrode is in contact with no normal insulating film. Get smaller.
[0010]
A plurality of insulating films in a stripe shape or a dot shape may be arranged uniformly. By doing so, a large number of highly injected points are generated and the current is evenly distributed. In addition, a second conductivity type source region having a higher concentration than the second conductivity type anode layer is formed in a part of the second conductivity type anode layer, and an insulating film is formed on a part of the surface of the second conductivity type source region. The anode electrode is in contact with the surface of the second conductivity type source region and the second conductivity type anode layer that are formed and the insulating film is not formed, and the cross-sectional shape of the second conductivity type source region, the insulating film, and the anode electrode However, the second conductive type source region, the insulating film formed on a part of the surface of the second conductive type source region, and the unit portion composed of the anode electrode formed so as to sandwich the insulating film are repeatedly formed. It is good also as what was prepared .
[0011]
By doing so, since the second conductivity type anode layer has a low concentration, at the normal rated current density (˜100 A / cm ² ), the injection of hole is suppressed, and the reverse recovery is higher than that of a normal pin diode. Improved characteristics. 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 insulating film formation portion. On the contrary, a large number of electrons are injected from the first conductivity type drift layer into the second conductivity type anode layer, and the carrier concentration increases.
[0012]
In particular, the diffusion depth of the second conductivity type anode layer is greater than that of the second conductivity type source region.
By doing so, there is no bent portion at the pn junction between the second conductivity type anode layer and the first conductivity type drift layer, which is suitable for a high voltage device.
Further, the diffusion depth of the second conductivity type anode layer may be smaller than that of the second conductivity type source region.
[0013]
By doing so, the total amount of impurities in the second conductivity type anode layer is small, the number of carriers injected into the first conductivity type drift layer is reduced, and the reverse recovery characteristic is improved.
It is assumed that a plurality of source layers and insulating films are evenly arranged. Further, these are assumed to be dot-like and stripe-like.
By doing so, the current distribution is uniform, there is no current concentration, and the characteristics are stabilized.
[0014]
It is assumed that the insulating film is an oxide film.
An oxide film can be easily obtained in the element formation process.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, power diodes according to embodiments of the present invention will be described with reference to the drawings. In the following description, layers, regions, and the like bearing n and p mean layers, regions, and the like having electrons and holes as majority carriers, respectively. In addition, all of the first conductivity type is n-type and the second conductivity type is p-type, but this can be reversed.
[0016]
[Example 1]
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 has many unit parts as shown in the figure, and a guard ring structure is formed in the peripheral part of the chip as a withstand voltage part, which relates to the essence of the present invention. Since it is not a part, it is omitted.
[0017]
A p anode layer 4 is formed by diffusion on one side of an n drift layer (also referred to as i layer) 5 having a high resistivity, and an insulating film 3 is formed on a part of the surface on the p anode layer 4 side. The anode electrode 1 is provided in contact with the surface of the p anode layer 4 on which the insulating film 3 is not formed. An n ⁺ cathode layer 6 is formed by diffusion on the other side of the n drift layer 5, and a cathode electrode 2 is provided on the surface of the n ⁺ cathode layer 6 opposite to the n drift layer 5. Yes. 4 is different from the conventional pin diode of FIG. 4 in that a p ⁺ source region 7 having a higher concentration than the p anode layer 4 is formed on the surface of the p anode layer 4, and the surface of the p ⁺ source region 7. The insulating film 3 is formed on the surface. The p ⁺ source region 7 and the insulating film 3 are striped in the plan view. In the drawing, the anode electrode 1 is extended on the insulating film 3, but it does not necessarily have to be extended in this way.
[0018]
The power rectifying device of the present invention has a three-layer structure comprising a p anode layer 4, an n drift layer 5, and an n ⁺ cathode layer 6 by a manufacturing method similar to that of a normal pin diode, and a surface layer of the p anode layer 4 After selectively forming the p ⁺ source region 7, the insulating film 3 such as a thermal oxide film or a CVD oxide film obtained by a normal process is partially left by photolithography, and on 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. Lifetime control is performed by electron beam irradiation. In some cases, the insulating film 3 may be formed before the three-layer structure is formed.
[0019]
In this example, for example, the boron ion implantation amount for forming the p anode layer 4 is reduced, and the surface concentration of the p anode layer 4 is made about an order of magnitude smaller than that of the conventional pin diode. However, it is preferable that the p ⁺ source region 7 is made the same level as the surface impurity concentration of the conventional pin diode or increased several times. The impurity concentration or surface impurity concentration, thickness, or junction depth of each layer has the following values, for example. n ⁺ cathode layer: 1 × 10 ²⁰ / cm ³ , 80 μm, n drift layer: 3 × 10 ¹³ / cm ³ , 320 μm, p anode layer: 1 × 10 ¹⁵ / cm ³ , 6 μm, p ⁺ source region 7: 7 × 10 ¹⁶ / cm ³ , 3.4 μm, width, 11.4 μm, oxide film width: 6 μm, film thickness: 0.5 μm, width of the region where no oxide film is formed: 9 μm.
[0020]
FIG. 5 is a characteristic diagram in the forward direction of the power diode (A) according to the first embodiment of the present invention. As a comparative example, the structure 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) was 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 each of the power diode (A), the comparison diode (B), and the pin diode (C) exceeds 3000V. When comparing the forward voltage at 100 A / cm ² that is normally used as the rated current, the forward voltage of the pin diode (C) is 2.9 V, compared to the power diode (A) and the comparison diode (B). Is not much different from 3.1-3.2V.
[0021]
On the other hand, in FIG. 5, when the forward voltage is compared at a surge current region of 1000 A / cm ² or more, the power diode (A) is compared to the comparison diode (B) 9.6 V and the pin diode (C) 9.1 V. Then, the on-voltage is reduced to 8.9V. Therefore, heat generation during surge current application is reduced, and an increase in intrinsic carrier concentration is suppressed, so that surge current resistance is improved.
[0022]
As described in the previous section, in the power diode (A) in which the oxide film is formed, the minority carriers (electrons in the p anode layer 4) of the semiconductor surface layer are dammed to the insulating film in the region where the insulating film 3 is formed. The higher the injection state, the higher the electron concentration in the p anode layer 4 near the insulating film, and the higher the hole concentration in the n drift layer 5 so as to satisfy the neutral condition. This is probably because of this. 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 comparison diode (B) and the pin diode (C) without the insulating film 3. The rate is small. Normally, there is no reverse recovery mode operation in the surge current region. Accordingly, it is not necessary to consider the influence on the reverse recovery characteristics due to the promotion of hole injection in such a current region.
[0023]
FIG. 6 is a characteristic diagram comparing the recovery characteristics of the power diode (A), the comparative diode (B), and the conventional pin diode (C) of the second embodiment. The horizontal axis is the forward voltage at 100 A / cm ² , and the vertical axis is the reverse recovery peak current. The conditions for the reverse recovery operation are I _F = 100 A / cm ² , V _R = 1300 V, and −dI _F / dt = 250 A / (cm ² · μs). As seen in this figure, the forward voltage and the reverse recovery peak current are in a trade-off relationship, and 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 the trade-off characteristic is superior to (C). That is, when compared with diodes having the same forward voltage, the power diode (A) of this embodiment has a small reverse recovery peak current, so that the switching speed is high and the switching loss is small.
[0024]
As described in the previous section, 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. At (˜100 A / cm ² ), current mainly flows from the p anode layer 4 in contact with the anode electrode 1. This is because by reducing the surface concentration of the p anode layer 4, the hole injection can be suppressed lower than that of the conventional pin diode, and the reverse recovery characteristic is improved.
[0025]
In other words, the power diode of this example has higher forward characteristics in the surge current region and reverse recovery characteristics in the rated current region than the conventional pin diode, and can withstand higher speed and severe operation. It is. It can be said that the influence of the formation of the insulating film 3 on the characteristics is negligible.
As long as the insulating film 3 and the p ⁺ source region 7 have a structure in which currents are not concentrated, such as stripes or dots, stable characteristics can be obtained. The thickness and width of the insulating film 3 and the surface concentration, width and junction depth of the p anode layer 4 and p ⁺ source region 7 can be controlled by existing process technology, and can be optimized in accordance with the device structure and the like. In this example, the insulating film 3 is the oxide film that is the easiest to form, but an insulating material other than the oxide film, such as a nitride film or polyimide, may be used.
[0026]
[Example 2]
FIG. 2 is a partial cross-sectional view of the power diode according to the second embodiment of the present invention. This example shows a case where the diffusion depth of the p anode layer 4 is smaller than that of the p ⁺ source region 7 as compared with the embodiment 1 of FIG. Insulating film 3 is provided in contact with the surface of p ⁺ source region 7. By doing in this way, the effect | action which suppresses further the injection | pouring of the hole from the p anode layer 4 to the n drift layer 5 is acquired, and a reverse recovery characteristic can be improved.
[0027]
[Example 3]
FIG. 3 is a partial sectional view of a unit portion near the center of the chip of the power diode according to the 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. The insulating film 3 is provided in contact with the surface of the p anode layer 4. However, in this example, the boron ion implantation amount for forming the p anode layer 4 is increased, and the surface concentration of the p anode layer 4 is about one order of magnitude higher than that in the case of Example 1, that is, the p anode layer 4 The surface impurity concentration and the diffusion depth are 3 × 10 ¹⁶ / cm ³ and 3.4 μm, respectively.
[0028]
Therefore, the power diode of the third embodiment can reduce the man-hours by the amount that the p ⁺ source region 7 is not formed as compared with the first embodiment. Further, in the power diode of Example 3, a forward voltage lower than that of Example 1 was obtained in the surge current region.
[0029]
【The invention's effect】
As described above, according to the present invention, by providing an insulating film on a part of the surface of the second conductive type anode layer or the surface of the second conductive type source region, and contacting the anode electrode to a part without the insulating film, The on-voltage in the surge current region is reduced, heat generation is suppressed, and the surge current resistance is improved. In particular, if the impurity concentration of the second conductivity type anode layer is lowered, reverse recovery characteristics at the rated current can be improved, and a large surge current withstand capability and high-speed reverse recovery characteristics, which were difficult with conventional pin diodes, are possible. A power diode capable of coexisting with the above is provided.
[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. 4 is a partial cross-sectional view of a conventional power pin diode. FIG. 5 is a diagram of forward static characteristics of a power diode according to an embodiment of the present invention and a comparative example. Trade-off characteristics diagram of reverse recovery peak current and on-state voltage of power diode of example and comparative example [description of symbols]
DESCRIPTION OF SYMBOLS 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 (8)

The first conductivity type cathode layer, the first conductivity type drift layer having a lower impurity concentration than the first conductivity type cathode layer, and the second conductivity type anode layer are all stacked in this order, and the second conductivity type anode layer In the power diode having an anode electrode in contact with the surface of the first electrode 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, and the anode electrode is in contact with the surface of the second conductivity type anode layer where the insulation film is not formed ,
A power diode comprising a cross-sectional shape of the insulating film and the anode electrode in which a unit portion composed of the anode electrode formed so as to sandwich the insulating film and the insulating film is repeatedly formed . 2. The power diode according to claim 1 , wherein the insulating film has a stripe shape . 2. The power diode according to claim 1, wherein the insulating film has a dot shape . The first conductivity type cathode layer, the first conductivity type drift layer having a lower impurity concentration than the first conductivity type cathode layer, and the second conductivity type anode layer are all stacked in this order, and the second conductivity type anode layer In the power diode having an anode electrode in contact with the surface of the first electrode and a cathode electrode in contact with the back surface of the first conductivity type cathode layer,
A second conductivity type source region having a concentration higher than that of the second conductivity type anode layer is formed on a part of the second conductivity type anode layer, and an insulating film is formed on a part of the surface of the second conductivity type source region. The anode electrode is in contact with the surface of the second conductivity type source region and the second conductivity type anode layer where the insulating film is not formed,
A cross-sectional shape of the second conductive type source region, the insulating film, and the anode electrode, wherein the second conductive type source region and the insulating film formed on a part of the surface of the second conductive type source region; A power diode comprising: a unit portion formed of an anode electrode formed so as to sandwich the insulating film . 5. The power diode according to claim 4, wherein the diffusion depth of the second conductivity type anode layer is larger than that of the second conductivity type source region . 5. The power diode according to claim 4, wherein the diffusion depth of the second conductivity type anode layer is smaller than that of the second conductivity type source region . 5. The power diode according to claim 4, wherein the second conductivity type source region and the insulating film are striped . 5. The power diode according to claim 4, wherein the second conductivity type source region and the insulating film are in the form of dots .

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