Silicon carbide semiconductor device having high breakdown voltage and low on resistance

A silicon carbide substrate is provided with a first surface and a second surface opposite the first surface. The silicon carbide substrate includes an n-type region connecting the first surface and the second surface, and a p-type region being in contact with the first surface and connecting the first surface and the second surface. A first anode electrode is Schottky-joined, on the first surface, to the n-type region. A first cathode electrode is ohmically joined, on the second surface, to the n-type region. A second anode electrode is ohmically joined, on the first surface, to the p-type region. A second cathode electrode is Schottky-joined, on the second surface, to the p-type region.

TECHNICAL FIELD

The present invention relates to silicon carbide semiconductor devices, and particularly, to a silicon carbide Schottky junction semiconductor device.

BACKGROUND ART

A pn junction diode made of silicon (Si) has been widely used. This diode relatively easily achieves a high withstand voltage and a low forward voltage. The diode, unfortunately, has a low switching-speed. Accordingly, as a diode having a higher switching-speed, a Schottky barrier diode (SBD) using silicon carbide as a semiconductor material has begun to be used. A typical SBD has a relatively simple main part as disclosed in, for instance, Japanese Patent Application Laid-Open No. 2002-261295 (Patent Document 1). To be specific, the main part of the SBD includes an substrate, an n-type buffer layer, an n-type drift layer, a Schottky electrode, and an ohmic electrode. The Schottky electrode is disposed on the n-type drift layer as an anode electrode. The ohmic electrode is disposed on the n+substrate as a cathode electrode.

PRIOR ART DOCUMENT

Patent Document

SUMMARY

Problem to Be Solved by the Invention

For semiconductor devices, and particularly, for power semiconductor devices, a reduction in power loss is an important problem. In particular, a reduction in forward voltage is the key to a reduction in power loss of the SBD. The SBD of the above document, which has a relatively simple main part, has a limited typical method for regulating its forward voltage. Specifically, such a typical method is increasing the carrier concentration of the n-type drift layer or reducing the thickness of the n-type drift layer. Unfortunately, the reduction in forward voltage involves a reduction in withstand voltage of the SBD in either way.

The present invention has been made to solve the aforementioned problem. It is an object of the present invention to provide a silicon carbide semiconductor device that achieves a more reduction in its forward voltage while having a sufficient withstand voltage.

Means to Solve the Problem

A silicon carbide semiconductor device according to the present invention includes a silicon carbide substrate, a first anode electrode, a first cathode electrode, a second anode electrode, and a second cathode electrode. The silicon carbide substrate is provided with a first surface and a second surface opposite the first surface. The silicon carbide substrate includes an n-type region connecting the first surface and the second surface, and a p-type region being in contact with the first surface and connecting the first surface and the second surface. The first anode electrode is Schottky-joined, on the first surface, to the n-type region. The first cathode electrode is ohmically joined, on the second surface, to the n-type region. The second anode electrode is ohmically joined, on the first surface, to the p-type region. The second cathode electrode is Schottky-joined, on the second surface, to the p-type region.

Effects of the Invention

According to the present invention, the silicon carbide semiconductor device achieves a more reduction in its forward voltage while having a sufficient withstand voltage.

DESCRIPTION OF EMBODIMENT(S)

The following describes the embodiments of the present invention with reference to the accompanying drawings.

FIG. 1is a schematic diagram illustrating a configuration of an equivalent circuit of a diode91(silicon carbide semiconductor device) according to a first embodiment. The equivalent circuit of the diode91includes an anode terminal AD, a cathode terminal CD, a Schottky barrier diode SBp, and a Schottky barrier diode SBn. The Schottky barrier diode SBp and the Schottky barrier diode SBn each have an anode side connected to the anode terminal AD. The Schottky barrier diode SBp and the Schottky barrier diode SBn each have a cathode side connected to the cathode terminal AD. In other words, the Schottky barrier diode SBp and the Schottky barrier diode SBn are connected in parallel to each other in the same forward direction.

FIG. 2is a schematic cross-sectional view of a configuration of the diode91. The diode91includes a silicon carbide substrate50, a first anode electrode32, a first cathode electrode31, a second anode electrode41, a second cathode electrode42, and a common anode electrode60. The silicon carbide substrate50is provided with a first surface S1, and a second surface S2opposite the first surface S1. The first surface S1and the second surface S2are substantially parallel to each other. The silicon carbide substrate50includes an n-type region10and a p-type region20.

The n-type region10connects the first surface S1and the second surface S2. The n-type region10includes an n−region11and an n+region12. The n+region12has a higher impurity concentration than the n−region11. The n−region11is disposed on the first surface S1. The n+region12is disposed on the second surface S2.

The p-type region20connects the first surface S1and the second surface S2. The p-type region20includes a p−region21and a p+region22. The p+region22has a higher impurity concentration than the p−region21. The p−region21is disposed on the second surface S2. The p+region22is disposed on the first surface S1.

The p-type region20is in contact with the n-type region10. To be specific, the p−region21is in contact with the n−region11. This establishes a pn junction between the n-type region10and the p-type region20. This pn junction extends in a direction in which the pn junction intersects each of the first surface S1and the second surface S2(i.e., an up-and-down direction of the pn junction inFIG. 2). Accordingly, the diode91has a super-junction structure,

The first anode electrode32is a Schottky electrode, and is Schottky-joined, on the first surface S1, to the n−region11of the n-type region10. The first anode electrode32is a conductor layer containing a first metal element such as titanium (Ti). An example of the first anode electrode32is a Ti layer. The second anode electrode41is an ohmic electrode, and is ohmically joined, on the first surface S1, to the p+region22of the p-type region20. The second anode electrode41is preferably separated from the n-type region10. The second anode electrode41is preferably silicided on the first surface S1for a good ohmic-junction. The anode electrode41may contain a second metal element different from the first metal element, and may contain platinum (Pt) for instance. An example of the second anode electrode41is a Pt layer.

The common anode electrode60is in contact with the first anode electrode32and the second anode electrode41. Accordingly, the common anode electrode60functions as the anode terminal AD (FIG. 1). The first anode electrode32and the second anode electrode41may be in contact with each other. In this case, each of the first anode electrode32and the second anode electrode41functions as the anode terminal AD. Accordingly, the common anode electrode60can be omitted.

The first cathode electrode31is an ohmic electrode, and is ohmically joined, on the second surface S2, to the n+region12of the n-type region10. The first cathode electrode31is preferably separated from the p-type region20. The first cathode electrode31is preferably silicided on the second surface S2for a good ohmic-junction. The second cathode electrode42is a Schottky electrode, and is Schottky-joined, on the second surface S2, to the p-type region20. The first cathode electrode31and the second cathode electrode42may contain a common metal element. For instance, the first cathode electrode31may be a nickel (Ni) layer silicided on the second surface S2, and the second cathode electrode42may be a Ni layer. The first cathode electrode31and the second cathode electrode42may be connected to each other.

The aforementioned Schottky electrode can be formed through the following processes: forming a layer to be a Schottky electrode; and heating the layer for sintering. Further, the aforementioned ohmic electrode can be formed through the following processes: forming a layer to be an ohmic electrode; and heating the layer for siliciding. The heating process for sintering is performed at a lower temperature than the heating process for siliciding. For instance, the former is performed at about 400° C.; and the latter, at about 1100° C. The process of heating the layer to be a Schottky electrode for sintering may be performed with respect to both the layer to be a Schottky electrode and the layer to be an ohmic electrode. On the other hand, the process of heating the layer to be an ohmic electrode for siliciding is performed only with respect to the layer to be an ohmic electrode, and is not performed with respect to the layer to be a Schottky electrode.

According to the present embodiment, the pn junction between the n-type region10and the p-type region20establishes the super-junction structure. This enables a depletion layer to extend also in a transverse direction (i.e., a direction orthogonal to a direction of the thickness of the silicon carbide substrate50) when a reverse voltage is applied across the diode91. Consequently, the diode91has a sufficient withstand voltage even if the n-type region10as a drift layer (i.e., the n−region11) and the p-type region20as a drift layer (i.e., the p−region21) are set to have high impurity concentrations to a certain degree. Such high impurity concentrations reduce the resistances (differential resistances) of the drift layers with respect to a forward current. Further, the n-type region10and the p-type region20(F1G.2) constitute the Schottky barrier diode SBn and the Schottky barrier diode SBp (FIG. 1), respectively. Consequently, the diode91has a wide area for its practical function, i.e., a wide effective area when compared to a diode in which one of an n-type region and a p-type region forms a Schottky barrier diode. This enables a reduction in forward voltage. As a result, the diode91in the present embodiment achieves a reduction in forward voltage while securing a withstand voltage.

The second anode electrode41may contain the second metal element different from the first metal element contained in the first anode electrode32. In this case, the physical properties of a material of the first anode electrode that needs to be Schottky-joined with an n-type semiconductor are more optimized; so are the physical properties of a material of the second anode electrode that needs to be ohmically joined with the n-type semiconductor.

The first cathode electrode31and the second cathode electrode42may contain the common metal element. In this case, processing steps for forming the first cathode electrode31and the second cathode electrode42are simplified. To be specific, depositing steps for forming the first cathode electrode31and the second cathode electrode42are performed in a collective manner.

The following second to fifth embodiments achieve effects similar to those described above. Accordingly, the similar effects will not be elaborated upon in the other embodiments.

The diode91in the present embodiment is configured such that the first anode electrode32and the second anode electrode41are short-circuited to each other, and that the first cathode electrode31and the second cathode electrode42are short-circuited to each other. Nevertheless, these short-circuits are required to be established when the diode91is used. The first anode electrode32and the second anode electrode41, which are disposed on the same first surface S1of the same silicon carbide substrate50, are easy to be short-circuited. Moreover, the first cathode electrode31and the second cathode electrode42, which are disposed on the same second surface S2of the same silicon carbide substrate50, are easy to be short-circuited. For instance, these electrodes separated from each other can be short-circuited to each other when disposed on a common conductor. Alternatively, these electrodes are short-circuited to each other when electrically bonded to a common conductor. Therefore, as a modification of the diode91, the first anode electrode32and the second anode electrode41that are separated from each other may be provided without the common anode electrode60. Instead or at the same time, the first cathode electrode31and the second cathode electrode42that are separated from each other may be provided. The same holds true for the other embodiments.

FIG. 3is a schematic diagram illustrating a configuration of a diode92(silicon carbide semiconductor device) according to a second embodiment. The diode92includes p-type wells14within the silicon carbide substrate50. The p-type wells14partly form the first surface S1on the n−region11of the n-type region10. Thus, the first anode electrode32is in contact with the p-type wells14as well as the n−region11. The p-type wells14each can be formed on the n−region11of the n-type region10through selective implantation of an impurity by the use of an implanting mask. Each p-type well14may have an impurity concentration profile in a depth direction similar to that of the p+region22. In this case, the p-type wells14and the p+region22can be formed in a collective manner.

The other configurations are almost the same as those described in the first embodiment. Thus, identical or corresponding components are denoted by identical reference symbols, and will not be elaborated upon here.

In the present embodiment, when a large forward current IFis supplied to the diode92, a current also flows not only via the Schottky junction between the first anode electrode32and the n−region11but also via the p-type wells14. Consequently, the diode92has a more reduced forward voltage VFwhen supplied with a large current than the diode91. This increases a capacity of I2t (inrush current capacity) determined by IF×VF.

FIG. 4is a schematic diagram illustrating a configuration of a diode93(silicon carbide semiconductor device) according to a third embodiment. The diode93includes n-type wells24within the silicon carbide substrate50. The n-type wells24each partly form the second surface S2on the p−region21of the p-type region20. Thus, the second cathode electrode42is in contact with the n-type wells24as well as the p−region21. The n-type wells24each can be formed on the p−region21of the p-type region20through selective implantation of an impurity by the use of an implanting mask. Each n-type well24may have an impurity concentration profile in a depth direction similar to that of the n+region12. In this case, the n-type wells24and the n+region12can be formed in a collective manner.

The other configurations are almost the same as those described in the first embodiment. Thus, identical or corresponding components are denoted by identical reference symbols, and will not be elaborated upon here.

In the present embodiment, when a large forward current IFis supplied to the diode93, a current also flows not only via the Schottky junction between the second cathode electrode42and the p−region21but also via the n-type wells24. Consequently, the diode93has a more reduced forward voltage VFwhen supplied with a large current than the diode91. This increases a capacity of I2t (inrush current capacity) determined by IF×VF.

The p-type wells14(FIG. 3: the second embodiment) may be provided in addition to the n-type wells24(FIG. 4). In this case, the aforementioned effect is further enhanced.

FIG. 5is a schematic diagram illustrating a configuration of a diode94(silicon carbide semiconductor device) according to a fourth embodiment. The diode94is configured such that the p-type region20has a smaller width than the n-type region10(the size in a transverse direction of the drawing, i.e., the size in a direction orthogonal to a direction of the thickness of the p-type region20). As a result of this configuration, the p-type region20can have a smaller area than the n-type region10in plan view. In other words, the p-type region20has a smaller effective area than the n-type region10.

Nevertheless, it is preferable that the width of the p-type region20be not small to an excessive degree. To be specific, the width of the p-type region20is preferably greater than a distance by which a depletion layer extends across the p−region21when the diode94is reverse-biased, and is more preferably about three times as large as this distance in view of variations.

The other configurations are almost the same as any of those described in the first to third embodiments. Thus, identical or corresponding components are denoted by identical reference symbols, and will not be elaborated upon here.

According to the present embodiment, the p-type region20has a smaller width than the n-type region10. Accordingly, as carriers for the operation of the diode94, electrons, which has a high mobility, make up a great proportion than holes, which has a low mobility. Consequently, the carries are rapidly removed during the recovery of the diode94. This reduces a loss of the recovery.

FIG. 6is a schematic diagram illustrating a configuration of a diode95(silicon carbide semiconductor device) according to a fifth embodiment. The diode95is configured such that the second anode electrode41has a larger area than the second cathode electrode42. The other configurations are almost the same as those described in any of the first to third embodiments. Thus, identical or corresponding components are denoted by identical reference symbols, and will not be elaborated upon here.

In the present embodiment, an ohmic-contact region between the p+region22of the p-type region20and the second anode electrode41is wider than a Schottky-contact region between the region21of the p-type region20and the second cathode electrode42. The wider ohmic-contact region enables carriers to be quickly removed during the recovery of the diode95. This reduces a loss of the recovery.

The second cathode electrode42may have a smaller area than the first anode electrode32. In this case, the loss of the recovery is reduced by an effect similar to that described in the fourth embodiment.

It is noted that in the present invention, the individual embodiments can be freely combined, or can be modified and omitted as appropriate, within the scope of the invention. While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.

EXPLANATION OF REFERENCE SIGNS