JP7522399B2 - Schottky Diode - Google Patents

Description

The present invention relates to a Schottky diode.

Conventionally, a diode called a junction barrier Schottky (JBS) diode, which has a structure combining a pn junction and a Schottky junction, has been known (see, for example, Patent Document 1). In a JBS diode, a surge current can be passed through the pn diode portion, and therefore the JBS diode has superior resistance to surge current compared to a Schottky barrier diode that does not have a pn junction.

The JBS diode described in Patent Document 1 has an n ^- type drift layer made of SiC and a p-type layer obtained by ion-implanting p-type impurities into the n ^- type drift layer, and the n ^- type drift layer and the p-type layer form a pn junction.

Also, a trench MOS type _{Ga2O3 -} based Schottky diode in which a gate electrode is embedded in a semiconductor layer has been known (see, for example, Patent Document 2). _{Ga2O3 -} based semiconductor devices are known to have high breakdown voltage and low loss due to the physical properties represented by the wide band gap of _{Ga2O3 .} Furthermore, the Schottky diode described in Patent Document 1 uses a trench MOS structure, so it can obtain a higher breakdown voltage without increasing the resistance of the semiconductor layer.

JP 2008-282973 A JP 2018-142577 A

However, when trying to improve the surge current resistance of a _{Ga2O3 -} based Schottky diode as shown in Patent Document 2, it is very difficult to obtain p-type _{Ga2O3 ,} so that it is not possible to form a pn junction region by adding p-type impurities to _{the Ga2O3 layer} .

An object of the present invention is to provide a _Ga2O3 -based _Schottky diode which has high breakdown voltage, low loss, and excellent resistance to surge current.

In order to achieve the above object, one aspect of the present invention provides the following Schottky diodes [1] to [3].

[1] A Schottky diode comprising: an n-type semiconductor layer made of _Ga2O3 -based _single crystal and having a plurality of trenches opening on one surface; an anode electrode connected to a mesa-shaped region between adjacent trenches of the n-type semiconductor layer; a trench anode electrode embedded in each of the plurality of trenches while being covered with an insulating film and electrically connected to the anode electrode; a cathode electrode connected directly or indirectly to the side of the n-type semiconductor layer opposite the anode electrode; and a p-type semiconductor member made of NiZnMgO connected to a part of the mesa-shaped region and to the anode electrode.
[2] A Schottky diode comprising: an n-type semiconductor layer made of _Ga2O3 -based _single crystal and having a plurality of trenches opening on one surface; a p-type semiconductor member made of NiZnMgO embedded in the trenches; a mesa-shaped region between adjacent trenches in the n-type semiconductor layer, an anode electrode connected to the p-type semiconductor member; and a cathode electrode directly or indirectly connected to the side of the n-type semiconductor layer opposite to the anode electrode.
[3] The Schottky diode according to the above [ ₁ ] or [2], wherein the energy of the upper end of the valence band of the p-type semiconductor member is equal to or lower than the energy of the upper end of the valence band of the Ga 2 O ₃ -based single crystal.

According to the present invention, it is possible to provide a Ga ₂ O ₃ based Schottky diode which has high breakdown voltage, low loss and excellent resistance to surge current.

FIG. 1 is a vertical cross-sectional view of a Schottky diode according to a first embodiment. FIG. 2 is a graph showing a schematic diagram of current-voltage characteristics when an inrush current flows in a Schottky diode when (A) the diode includes a p-type semiconductor member and when (B) the diode does not include a p-type semiconductor member. FIG. 3 is a diagram showing a band structure of a p-type semiconductor member and an n-type semiconductor layer made of a Ga ₂ O ₃ -based single crystal, and a schematic diagram showing the flow of holes. FIG. 4 shows a band diagram of Ga ₂ O ₃ , which is a typical material for an n-type semiconductor layer, and NiO, MgO, and ZnO which constitute NiZnMgO, which is a material for a p-type semiconductor member. FIG. 5 is a vertical cross-sectional view of a modified example of the Schottky diode according to the first embodiment. FIG. 6 is a vertical cross-sectional view of a Schottky diode according to the second embodiment.

First Embodiment
(Schottky diode configuration)
1 is a vertical cross-sectional view of a Schottky diode 1 according to a first embodiment. The Schottky diode 1 is a vertical Schottky diode having a trench MOS structure.

The Schottky diode 1 includes an n-type semiconductor substrate 10, an n-type semiconductor layer 11 that is a layer stacked on the n-type semiconductor substrate 10 and has a plurality of trenches 12 that open to a surface 18 opposite the n-type semiconductor substrate 10, an anode electrode 13 connected to a mesa-shaped region 110 between adjacent trenches 12 of the n-type semiconductor layer 11, a cathode electrode 14 formed on the surface of the n-type semiconductor substrate 10 opposite the n-type semiconductor layer 11, a trench insulating film 15 that covers the inner surface of the trench 12 of the n-type semiconductor layer 11, an anode electrode 16 that is embedded in the trench 12 of the n-type semiconductor layer 11 so as to be covered by the trench insulating film 15 and is electrically connected to the anode electrode 13, and a p-type semiconductor member that is connected to a part of the mesa-shaped region 110 and the anode electrode 13.

In the Schottky diode 1, by applying a forward voltage (positive potential on the anode electrode 13 side) between the anode electrode 13 and the cathode electrode 14, the energy barrier at the interface between the anode electrode 13 and the n-type semiconductor layer 11 as viewed from the n-type semiconductor layer 11 is lowered, and a current flows from the anode electrode 13 to the cathode electrode 14.

On the other hand, when a reverse voltage (anode electrode 13 side is negative potential) is applied between the anode electrode 13 and the cathode electrode 14, no current flows due to the Schottky barrier. Also, when a reverse voltage is applied between the anode electrode 13 and the cathode electrode 14, a depletion layer spreads from the interface between the anode electrode 13 and the n-type semiconductor layer 11 and the interface between the trench insulating film 15 and the n-type semiconductor layer 11.

Generally, the upper limit of reverse leakage current for a Schottky diode is 1 μA. In this embodiment, the reverse voltage when a leakage current of 1 μA flows is defined as the breakdown voltage.

For example, according to data on the dependence of reverse leakage current on the Schottky interface electric field strength in a Schottky diode having a SiC semiconductor layer, which is described in "Hiroyuki Matsunami, Noboru Otani, Tsunenobu Kimoto, and Takashi Nakamura, "Semiconductor SiC Technology and Applications," Second Edition, Nikkan Kogyo Shimbun, September 30, 2011, p. 355," the electric field strength directly below the Schottky electrode when the current density of the reverse leakage current is 0.0001 A/ ^cm2 is approximately 0.8 MV/cm. Here, 0.0001 A/ ^cm2 is the current density directly below the Schottky electrode when a current of 1 μA flows through a Schottky electrode having a size of 1 mm×1 mm.

Therefore, even if the dielectric breakdown field strength of the semiconductor material itself is several MV/cm, if the electric field strength directly below the Schottky electrode exceeds 0.8 MV/cm, a leakage current exceeding 1 μA will flow.

For example, in order to obtain a breakdown voltage of 1200 V in a conventional Schottky diode that does not have a special structure for suppressing the electric field strength directly below the Schottky electrode, it is necessary to reduce the donor concentration in the semiconductor layer to the order of 10 ¹⁵ cm ^-3 and make the semiconductor layer very thick in order to suppress the electric field strength directly below the Schottky electrode to 0.8 MV/cm or less, which results in very large conduction losses and makes it difficult to fabricate a Schottky barrier diode with high breakdown voltage and low losses.

The Schottky diode 1 according to this embodiment has a trench MOS structure, so it is possible to obtain a high breakdown voltage without increasing the resistance of the semiconductor layer. In other words, the Schottky diode 1 is a Schottky diode with high breakdown voltage and low loss.

The n-type semiconductor substrate 10 is made of n-type Ga ₂ O ₃ based single crystal containing a Group IV element such as Si or Sn as a donor. The donor concentration of the n-type semiconductor substrate 10 is, for example, 1.0×10 ¹⁸ cm ^-3 or more and 1.0×10 ²⁰ cm ^-3 or less. The thickness of the n-type semiconductor substrate 10 is, for example, 10 to 600 μm. The n-type semiconductor substrate 10 is, for example, a Ga ₂ O ₃ based single crystal substrate.

Here, the _{Ga2O3 single} crystal is a _{Ga2O3 single} crystal or a Ga2O3 single crystal doped with one or both of Al and In, and is a _single crystal of a _Ga2O3 -based semiconductor having a composition expressed as ( _GaxAlyIn ( ₁ -x- _{y )} ) _2O3 (0<x≦1, 0≦y<1, 0<x+y≦1) _{. When Al is added to Ga2O3 , the} band gap becomes wider, and when In is added, the band gap becomes narrower. The above _{Ga2O3 single} crystal has, for example, _a β-type crystal structure.

The n-type semiconductor layer 11 is made of n-type _{Ga2O3 -} based single crystal containing a Group IV element such as Si or Sn as a donor. The donor concentration of the n-type semiconductor layer 11 is lower than the donor concentration of the n-type semiconductor substrate 10. The n-type semiconductor layer 11 is, for example, an epitaxial layer epitaxially grown on the n-type semiconductor substrate 10, which is a _{Ga2O3 -} based single crystal substrate.

A high donor concentration layer containing a high concentration of donors may be formed between the n-type semiconductor substrate 10 and the n-type semiconductor layer 11. This high donor concentration layer is used, for example, when the n-type semiconductor layer 11 is epitaxially grown on the n-type semiconductor substrate 10. In the initial stage of growth of the n-type semiconductor layer 11, the amount of dopant taken in may be unstable, or there may be diffusion of acceptor impurities from the n-type semiconductor substrate 10, which is the substrate. Therefore, if the n-type semiconductor layer 11 is directly grown on the n-type semiconductor substrate 10, the region of the n-type semiconductor layer 11 close to the interface with the n-type semiconductor substrate 10 may become highly resistive. To avoid such problems, a high donor concentration layer is used. The concentration of the high donor concentration layer is set to a higher concentration than that of the n-type semiconductor layer 11, for example, and more preferably, to a higher concentration than that of the n-type semiconductor substrate 10.

The more the donor concentration of the n-type semiconductor layer 11 increases, the more the electric field strength of each part of the Schottky diode 1 increases. In order to keep low the maximum electric field strength in the region directly under the anode electrode 13 in the n-type semiconductor layer 11, the maximum electric field strength in the n-type semiconductor layer 11, and the maximum electric field strength in the trench insulating film 15, the donor concentration of the n-type semiconductor layer 11 is preferably about 6.0×10 ¹⁶ cm ⁻³ or less. On the other hand, the lower the donor concentration, the higher the resistance of the n-type semiconductor layer 11 becomes, and the forward loss increases, so that in order to obtain a breakdown voltage of, for example, 1200 V or less, the donor concentration is preferably 3.0×10 ¹⁶ cm ⁻³ or more. In addition, in order to obtain a higher breakdown voltage, it is preferable to lower the donor concentration to, for example, about 1.0×10 ¹⁶ cm ⁻³ .

As the thickness T of the n-type semiconductor layer 11 increases, the maximum electric field strength in the n-type semiconductor layer 11 and the maximum electric field strength in the trench insulating film 15 decrease. By making the thickness T of the n-type semiconductor layer 11 approximately 6 μm or more, the maximum electric field strength in the n-type semiconductor layer 11 and the maximum electric field strength in the trench insulating film 15 can be effectively reduced. From the viewpoint of reducing these electric field strengths and miniaturizing the Schottky diode 1, it is preferable that the thickness T of the n-type semiconductor layer 11 be approximately 5.5 μm or more and 9 μm or less.

The electric field strength in each part of the Schottky diode 1 varies depending on the depth D of the trench 12. In order to keep low the maximum electric field strength in the region directly below the anode electrode 13 in the n-type semiconductor layer 11, the maximum electric field strength in the n-type semiconductor layer 11, and the maximum electric field strength in the trench insulating film 15, the depth D of the trench 12 is preferably approximately 2 μm or more and 6 μm or less, and more preferably approximately 3 μm or more and 4 μm or less.

The width _Wt of the trench 12 can be freely set because it is independent of the electric field strength in the region directly below the anode electrode 13. Since no current flows in the trench 12, it is preferable that the width _Wt is as narrow as possible. However, if the width _Wt is made too fine, it becomes difficult to embed the trench insulating film 15 and the anode electrode 16 in the trench 12, and the manufacturing yield deteriorates. On the other hand, the inventor's study has revealed that when the width _Wt is 3 μm or more, even if the width _Wt is increased, there is almost no effect on the embedding property of the anode electrode 16. In addition, when the width _Wt is less than 0.5 μm, a special exposure device is required to form the trench 12, and the manufacturing cost may increase. Therefore, the width _Wt is preferably 0.5 μm or more and 3 μm or less.

As the width _Wm of the mesa-shaped portion 110 between adjacent trenches 12 in the n-type semiconductor layer 11 decreases, the maximum electric field strength in the region immediately below the anode electrode 13 in the n-type semiconductor layer 11 decreases. In order to keep the maximum electric field strength in the region immediately below the anode electrode 13 in the n-type semiconductor layer 11 low, the width _Wm of the mesa-shaped portion 110 is preferably 2.5 μm or less. On the other hand, the smaller the width of the mesa-shaped portion 110, the more difficult it is to manufacture the trench 12, so the width _Wm of the mesa-shaped portion 110 is preferably 0.5 μm or more.

As the dielectric constant of the trench insulating film 15 increases, the maximum electric field strength in the trench insulating film 15 decreases, so it is preferable that the trench insulating film 15 be made of a material with a high dielectric constant. For example, Al ₂ O ₃ (relative dielectric constant of about 9.3) and HfO ₂ (relative dielectric constant of about 22) can be used as the material for the trench insulating film 15, and it is particularly preferable to use HfO ₂ with a high dielectric constant.

In addition, as the thickness of the trench insulating film 15 increases, the maximum electric field strength in the n-type semiconductor layer 11 decreases, but the maximum electric field strength in the trench insulating film 15 and the maximum electric field strength in the region directly below the anode electrode 13 increase. From the viewpoint of ease of manufacture, it is preferable that the thickness of the trench insulating film 15 is small, and more preferably 300 nm or less. However, it is necessary that the thickness is such that almost no current flows directly between the anode electrode 16 and the n-type semiconductor layer 11.

The material of the anode electrode 16 is not particularly limited as long as it is conductive. For example, highly doped polycrystalline Si or metals such as Ni and Au can be used.

As described above, the electric field strength in the Schottky diode 1 is affected by the width W _m of the mesa-shaped portion 110 between two adjacent trenches 12, the depth D of the trench 12, the thickness of the trench insulating film 15, etc., but is hardly affected by the planar pattern of the trench 12. For this reason, the planar pattern of the trench 12 in the n-type semiconductor layer 11 is not particularly limited.

The cathode electrode 14 is in ohmic contact with the n-type semiconductor substrate 10. The cathode electrode 14 is made of a metal such as Ti. The cathode electrode 14 may have a multilayer structure in which different metal films are stacked, for example, Ti/Au or Ti/Al. In order to ensure that the cathode electrode 14 and the n-type semiconductor substrate 10 are in ohmic contact, it is preferable that the layer of the cathode electrode 14 that is in contact with the n-type semiconductor substrate 10 is made of Ti. Note that when the Schottky diode 1 does not include the n-type semiconductor substrate 10, the cathode electrode 14 is connected to the side of the n-type semiconductor layer 11 opposite to the anode electrode 13 and is in ohmic contact with the n-type semiconductor layer 11.

The p-type semiconductor member 17 is a member used for surge protection and is made of p-type NiZnMgO. NiZnMgO is a mixed crystal oxide containing Ni, Mg, and Zn, obtained by mixing NiO, MgO, and ZnO. The p-type semiconductor member 17 made of NiZnMgO does not need to contain acceptor impurities, but it is preferable to contain acceptor impurities such as Li, as this improves electrical conductivity.

The NiZnMgO constituting the p-type semiconductor member 17 may be in any form, such as amorphous, polycrystalline, or single crystal. Usually, when the growth temperature of NiZnMgO is low, amorphous NiZnMgO is obtained, and when the growth temperature is high, polycrystalline NiZnMgO is obtained. Also, when the conditions such as the crystal structure and growth conditions of NiZnMgO, and the crystal orientation that serves as the base for epitaxial growth are met, single crystal NiZnMgO is obtained. The p-type semiconductor member 17 made of NiZnMgO is formed, for example, by radio frequency sputtering using a NiO target, an MgO target, and a ZnO target. Radio frequency sputtering is preferable to other manufacturing methods in terms of manufacturing costs.

The energy of the upper end of the valence band of the p-type semiconductor member 17 is equal to or less than the energy of the upper end of the valence band of the _{Ga2O3 -} based semiconductor constituting the n-type semiconductor layer 11 plus 2 eV. Therefore, by using the p-type semiconductor member 17, the resistance of the drift layer can be reduced when a surge current (also called an inrush current or starting current) occurs when the Schottky diode 1 is turned on, and heat generation and damage can be suppressed.

Normally, a pn diode has a higher on-voltage than a Schottky diode. For this reason, it is possible to design the pn diode portion, which is made up of the p-type semiconductor member 17 and the n-type semiconductor layer 11, so that it does not turn on at the voltage that turns on the Schottky diode 1. For example, the on-voltage of the Schottky diode 1 can be set to about 1 V, and the on-voltage of the pn diode portion can be set to about 2 V.

As a result, the pn diode portion does not turn on during normal operation of the Schottky diode 1, enabling the inherent high-speed operation of a Schottky diode. On the other hand, when an inrush current occurs, the voltage of the Schottky diode 1 rises and reaches a voltage at which the pn diode portion turns on, and holes are injected from the p-type semiconductor member 17 into the n-type semiconductor layer 11.

At that time, the same number of electrons as the injected holes are injected from the cathode electrode 14 into the n-type semiconductor layer 11, and the resistance of the drift layer is greatly reduced. As a result, a large current called an inrush current flows through the Schottky diode 1, but the voltage rise is suppressed, so the temperature rise is suppressed and damage to the Schottky diode 1 due to the inrush current can be prevented.

2 is a graph showing the current-voltage characteristics when an inrush current flows when the Schottky diode 1 includes (A) a p-type semiconductor member 17 and does not include (B). As shown in FIG. 2, when the p-type semiconductor member 17 is not included, the voltage continues to rise as the current increases, and the temperature of the Schottky diode 1 rises sharply and burns out. On the other hand, when the p-type semiconductor member 17 is included, the rate of voltage rise decreases when the voltage V _pn at which the pn diode portion turns on is reached. In this way, the current flowing through the Schottky diode 1 increases due to the inrush current, but the rise in voltage is suppressed, so that the temperature rise is suppressed and damage to the Schottky diode 1 is prevented.

3 is a diagram showing the band structure of the p-type semiconductor member 17 and the n-type semiconductor layer 11 made of Ga ₂ O ₃ -based single crystal, and the flow of holes. As shown in FIG. 3, if the energy E _V2 at the top of the valence band of the p-type semiconductor member 17 is equal to or less than the energy E _V1 at the top of the valence band of the Ga ₂ O ₃ -based single crystal plus 2 eV, holes can be injected from the p-type semiconductor member 17 to the n-type semiconductor layer 11. The magnitude of the energy E _C2 at the bottom of the conduction band of the p-type semiconductor member 17 is not limited to a specific range because it does not affect the injection of holes from the p-type semiconductor member 17 to the n-type semiconductor layer 11.

Furthermore, when the energy E _V2 of the upper end of the valence band of the p-type semiconductor member 17 is equal to or lower than the energy E _V1 of the upper end of the valence band of the _{Ga2O3 -} based single crystal constituting the n-type semiconductor layer 11, holes can easily move from the p-type semiconductor member 17 to the n-type semiconductor layer 11, thereby making it possible to further improve resistance to inrush current.

4 shows a band lineup including the band structures of Ga ₂ O ₃ , which is a typical material of the n-type semiconductor layer 11, and NiO, MgO, and ZnO, which constitute NiZnMgO, which is a material of the p-type semiconductor member 17. The rectangles in FIG. 4 show the band gaps of each material, the numbers inside the band gaps show the size of the band gaps [eV], the numbers above the band gaps show the band offset (energy difference at the bottom of the conduction band) [eV] of the conduction band with Ga ₂ O ₃ , and the numbers below the band gaps show the band offset [eV] of the valence band with Ga ₂ O ₃ .

4 also shows the band gaps of _AlO2 and _{In2O3 .} In the _{Ga2O3 -} based single crystal, which is the material of the n-type semiconductor layer 11, the band structure approaches _Al2O3 as the Al concentration increases, and the band structure approaches _In2O3 as _the In concentration increases. As shown in FIG _. 4, the amount of energy modulation at the top of the valence band due to changes in the Al concentration and In concentration in the _{Ga2O3 -} based single crystal is not large, and the band offset of the valence band _{with Ga2O3} is about 0.4 eV at most.

By changing the mixing ratio of NiO, MgO, and ZnO in NiZnMgO, the energy of the upper end of the valence band of the p-type semiconductor member 17 made of NiZnMgO can be adjusted to be equal to or lower than the energy of the upper end of the valence band of a _{Ga2O3 -} based semiconductor plus 2 eV.

For example, when the composition ratio of Ni, Zn, and Mg in NiZnMgO is Ni:Zn:Mg 90:5:5, the band offset of the valence band _{with Ga2O3} is 1.57 eV, when Ni:Zn:Mg is 80:10:10, the band offset of the valence band with _Ga2O3 is 1.38 eV, when Ni: _Zn : _Mg is 70:15:15, the band offset of the valence band with _Ga2O3 is 1.19 eV, and when Ni:Zn: _Mg is 60:20:20, the band offset of the valence band with _Ga2O3 is 1.00 eV. In addition, by making the Ni composition ratio closer to zero, the band offset of the valence band _{with Ga2O3} can be made negative, that is, the energy of the upper end of the valence band of the p-type semiconductor member 17 can be made lower than the energy of the upper end of the valence band of the _{Ga2O3 -} based single crystal constituting the n-type semiconductor layer 11.

The size, number, and arrangement of the p-type semiconductor member 17 are not particularly limited. The larger the contact area between the p-type semiconductor member 17 and the n-type semiconductor layer 11, the more efficiently the surge current can be released, but the more difficult it is for the current to flow during normal operation. For this reason, it is preferable that the total contact area between the p-type semiconductor member 17 and the n-type semiconductor layer 11 is 10% or more and 50% or less of the total contact area between the anode electrode 13 and the n-type semiconductor layer 11.

Figure 5 is a vertical cross-sectional view of Schottky diode 3, which is a modified version of Schottky diode 1 according to the first embodiment. Schottky diode 3 differs from Schottky diode 1 in that it does not have a trench structure, i.e., it does not have trench 12, trench insulating film 15, or anode electrode 16 in n-type semiconductor layer 11. Schottky diode 3 is inferior in voltage resistance compared to Schottky diode 1 because it does not have a trench structure, but has the same surge current resistance as Schottky diode 1 due to p-type semiconductor member 17.

Second Embodiment
(Schottky diode configuration)
6 is a vertical cross-sectional view of a Schottky diode 2 according to a second embodiment. The Schottky diode 2 is a vertical junction barrier Schottky (JBS) diode having a trench structure.

The Schottky diode 2 includes an n-type semiconductor substrate 20, an n-type semiconductor layer 21 formed on the n-type semiconductor substrate 20 and having a plurality of trenches 25 opening on a surface 26 opposite the n-type semiconductor substrate 20, a p-type semiconductor member 22 embedded in the trenches 25 of the n-type semiconductor layer 21, an anode electrode 23 provided on the surface 26 of the n-type semiconductor layer 21 and connected to the mesa-shaped region 210 between the adjacent trenches 25 of the n-type semiconductor layer 21 and the p-type semiconductor member 22, and a cathode electrode 24 formed on the surface of the n-type semiconductor substrate 20 opposite the n-type semiconductor layer 21.

The n-type semiconductor layer 21 and the anode electrode 23 form a Schottky junction, and the Schottky diode 2 utilizes the rectification of this Schottky junction. Moreover, the Schottky diode 2 uses a p-type semiconductor member 22 made of NiZnMgO instead of p-type Ga ₂ O ₃ , which is difficult to form.

In the Schottky diode 2, by applying a forward voltage (positive potential on the anode electrode 23 side) between the anode electrode 23 and the cathode electrode 24, the potential barrier at the interface between the anode electrode 23 and the n-type semiconductor layer 21 as viewed from the n-type semiconductor layer 21 is lowered, and a current flows from the anode electrode 23 to the cathode electrode 24.

On the other hand, when a reverse voltage (anode electrode 23 side is negative potential) is applied between the anode electrode 23 and the cathode electrode 24, no current flows due to the Schottky barrier. At this time, a depletion layer spreads from the p-type semiconductor member 22, and the channel between adjacent p-type semiconductor members 22 is closed, so that the leakage current is effectively suppressed.

The Schottky diode 2 according to this embodiment has a trench-type JBS structure, and therefore, like the Schottky diode 1 according to the first embodiment, can obtain a high breakdown voltage without increasing the resistance of the semiconductor layer. In other words, the Schottky diode 2 is a Schottky barrier diode with high breakdown voltage and low loss.

The n-type semiconductor substrate 20 is made of n-type Ga ₂ O ₃ based single crystal containing a Group IV element such as Si or Sn as a donor. The donor concentration of the n-type semiconductor substrate 20 is, for example, 1.0×10 ¹⁸ cm ^-3 or more and 1.0×10 ²⁰ cm ^-3 or less. The thickness of the n-type semiconductor substrate 20 is, for example, 10 to 600 μm. The n-type semiconductor substrate 20 is, for example, a Ga ₂ O ₃ based single crystal substrate.

The n-type semiconductor layer 21 is made of n-type _{Ga2O3 -} based single crystal containing a Group IV element such as Si or Sn as a donor. The donor concentration of the n-type semiconductor layer 21 is lower than the donor concentration of the n-type semiconductor substrate 20. The n-type semiconductor layer 21 is, for example, an epitaxial layer epitaxially grown on the n-type semiconductor substrate 10, which is a _{Ga2O3 -} based single crystal substrate.

A high donor concentration layer containing a high concentration of donors may be formed between the n-type semiconductor substrate 20 and the n-type semiconductor layer 21. This high donor concentration layer is used, for example, when the n-type semiconductor layer 21 is epitaxially grown on the n-type semiconductor substrate 20. In the initial stage of growth of the n-type semiconductor layer 21, the amount of dopant taken in may be unstable, or there may be diffusion of acceptor impurities from the n-type semiconductor substrate 20. Therefore, if the n-type semiconductor layer 21 is directly grown on the n-type semiconductor substrate 20, the region of the n-type semiconductor layer 21 close to the interface with the n-type semiconductor substrate 20 may become highly resistive. To avoid such problems, a high donor concentration layer is used. The concentration of the high donor concentration layer is set to a higher concentration than that of the n-type semiconductor layer 21, for example, and more preferably, to a higher concentration than that of the n-type semiconductor substrate 20.

The higher the donor concentration of the n-type semiconductor layer 21, the higher the electric field strength of each part of the Schottky diode 2. In order to keep low the maximum electric field strength in the region directly under the anode electrode 23 in the n-type semiconductor layer 21 and the maximum electric field strength in the n-type semiconductor layer 21, the donor concentration of the n-type semiconductor layer 21 is preferably about 2.0×10 ¹⁷ cm ⁻³ or less. On the other hand, the lower the donor concentration, the higher the resistance of the n-type semiconductor layer 21 becomes, and the greater the forward loss becomes. Therefore, in order to ensure a breakdown voltage of, for example, 1200 V or less, the donor concentration is preferably 3.0×10 ¹⁶ cm ⁻³ or more. In addition, in order to obtain a higher breakdown voltage, the donor concentration may be lowered to, for example, about 1.0×10 ¹⁶ cm ⁻³ .

The greater the thickness T of the n-type semiconductor layer 21, the smaller the maximum electric field strength in the n-type semiconductor layer 21. By making the thickness of the n-type semiconductor layer 21 approximately 3 μm or more, the maximum electric field strength in the n-type semiconductor layer 21 can be effectively reduced. From the viewpoint of reducing the electric field strength and miniaturizing the Schottky diode 2, it is preferable that the thickness of the n-type semiconductor layer 21 be approximately 3 μm or more and 9 μm or less.

The electric field strength in each part of the Schottky diode 2 varies depending on the depth D of the trench 25. In order to keep the maximum electric field strength in the region directly below the anode electrode 23 in the n-type semiconductor layer 21 and the maximum electric field strength in the n-type semiconductor layer 21 low, it is preferable that the depth D of the trench 25 is approximately 1.5 μm or more and 6 μm or less.

The narrower the width _Wt of the trench 25, the more the conduction loss can be reduced. However, the narrower the width Wt, the higher the difficulty of manufacturing, which in turn reduces the manufacturing yield. Therefore, the width Wt is preferably 0.3 μm or more and 5 μm or less.

As the width _Wm of the mesa-shaped portion 210 between adjacent trenches 25 in the n-type semiconductor layer 21 decreases, the maximum electric field strength in the region immediately below the anode electrode 23 in the n-type semiconductor layer 21 decreases. In order to keep the maximum electric field strength in the region immediately below the anode electrode 23 in the n-type semiconductor layer 21 low, the width _Wm of the mesa-shaped portion 210 is preferably 5 μm or less. On the other hand, since the manufacturing difficulty of the trench 25 increases as the width of the mesa-shaped portion 210 is smaller, the width _Wm of the mesa-shaped portion 210 is preferably 0.25 μm or more.

The anode electrode 23 is made of a material that makes Schottky contact with the n-type semiconductor layer 21 at the portion of the anode electrode 23 that is in contact with the n-type semiconductor layer 21. That is, when the anode electrode 23 has a single-layer structure, the entire anode electrode 23 is made of a material that makes Schottky contact with the n-type semiconductor layer 21, and when the anode electrode 23 has a multi-layer structure, at least the layer that makes Schottky contact with the n-type semiconductor layer 21 is made of a material that makes Schottky contact with the n-type semiconductor layer 21.

To reduce the turn-on voltage of the Schottky diode 2, it is preferable that the portion of the anode electrode 23 that contacts the n-type semiconductor layer 21 is made of Fe (iron), Cu (copper), Mo (molybdenum), or W (tungsten).

When the part of the anode electrode 23 in contact with the n-type semiconductor layer 21 is made of Mo or W, the turn-on voltage of the Schottky diode 2 is 0.4 V or more and 0.6 V or less. When the part of the anode electrode 23 in contact with the n-type semiconductor layer 21 is made of Fe, the turn-on voltage of the Schottky diode 2 is 0.4 V or more and 0.7 V or less. When the part of the anode electrode 23 in contact with the n-type semiconductor layer 21 is made of Cu, the turn-on voltage of the Schottky diode 2 is 0.6 V or more and 0.9 V or less.

In the Schottky diode 2, a potential barrier is formed in the mesa-shaped portion 210, so that the turn-on voltage depends on the width _Wm of the mesa-shaped portion 210, and becomes larger as the width _Wm becomes smaller.

As described above, the electric field strength in the Schottky diode 2 is affected by the width W _m of the mesa-shaped portion 210 between two adjacent trenches 25, the depth D of the trench 25, etc., but is hardly affected by the planar pattern of the trench 25 (the planar pattern of the p-type semiconductor member 22). For this reason, the planar pattern of the trench 25 in the n-type semiconductor layer 21 (the planar pattern of the p-type semiconductor member 22) is not particularly limited.

The cathode electrode 24 is in ohmic contact with the n-type semiconductor substrate 20. The cathode electrode 24 is made of a metal such as Ti. The cathode electrode 24 may have a multilayer structure in which different metal films are stacked, for example, Ti/Au or Ti/Al. In order to ensure that the cathode electrode 24 and the n-type semiconductor substrate 20 are in ohmic contact, it is preferable that the layer of the cathode electrode 24 that is in contact with the n-type semiconductor substrate 20 is made of Ti. Note that when the Schottky diode 2 does not include the n-type semiconductor substrate 20, the cathode electrode 24 is connected to the side of the n-type semiconductor layer 21 opposite to the anode electrode 23 and is in ohmic contact with the n-type semiconductor layer 21.

The p-type semiconductor member 22 is a member used for surge protection, similar to the p-type semiconductor member 17 according to the first embodiment, and is made of NiZnMgO. The p-type semiconductor member 22 made of NiZnMgO does not need to contain acceptor impurities, but it is preferable to contain acceptor impurities such as Li, as this improves electrical conductivity. The p-type semiconductor member 22 made of NiZnMgO is formed, for example, by high-frequency sputtering using a NiO target, an MgO target, and a ZnO target. High-frequency sputtering is preferable to other manufacturing methods in terms of manufacturing costs.

The energy of the upper end of the valence band of the p-type semiconductor member 22 is equal to or less than the energy of the upper end of the valence band of the _{Ga2O3 -} based semiconductor constituting the n-type semiconductor layer 21 plus 2 eV. Therefore, by using the p-type semiconductor member 22, the resistance of the drift layer can be reduced when a surge current (also called an inrush current or a starting current) occurs when the Schottky diode 2 is turned on, and heat generation and damage can be suppressed.

Normally, a pn diode has a higher on-voltage than a Schottky diode. For this reason, it is possible to design the pn diode portion, which is made up of the p-type semiconductor member 22 and the n-type semiconductor layer 21, so that it does not turn on at the voltage that turns on the Schottky diode 2. For example, the on-voltage of the Schottky diode 2 can be set to about 1 V, and the on-voltage of the pn diode portion can be set to about 2 V.

As a result, the pn diode portion does not turn on during normal operation of the Schottky diode 2, enabling the inherent high-speed operation of a Schottky diode. On the other hand, when an inrush current occurs, the voltage of the Schottky diode 2 rises and reaches a voltage at which the pn diode portion turns on, and holes are injected from the p-type semiconductor member 22 into the n-type semiconductor layer 21.

At that time, the same number of electrons as the injected holes are injected from the cathode electrode 24 into the n-type semiconductor layer 21, and the resistance of the drift layer is greatly reduced. As a result, a large current called an inrush current flows through the Schottky diode 2, but the voltage rise is suppressed, so the temperature rise is suppressed and damage to the Schottky diode 2 due to the inrush current can be prevented.

Furthermore, when the energy of the upper end of the valence band of the p-type semiconductor member 22 is equal to or lower than the energy of the upper end of the valence band of the _{Ga2O3 -} based single crystal constituting the n-type semiconductor layer 21, holes can easily move from the p-type semiconductor member 22 to the n-type semiconductor layer 21, thereby making it possible to further improve resistance to inrush current.

(Manufacturing method of Schottky diode)
An example of a method for manufacturing the Schottky diode 2 will be described below.

First, a Ga ₂ O ₃ based single crystal containing Si as a donor is epitaxially grown on an n-type semiconductor substrate 20 by the VPE method to form an n-type semiconductor layer 21 having a thickness of about 5 mm and a Si concentration of about 6×10 ¹⁵ cm ⁻³ .

Next, a trench 25 is formed on the surface 26 of the n-type semiconductor layer 21 opposite the n-type semiconductor substrate 20 using photolithography and dry etching.

Next, a cathode electrode 24 having a Ti/Au laminated structure or the like is formed on the bottom surface of the n-type semiconductor substrate 20 by electron beam evaporation. Then, a heat treatment is performed to reduce the contact resistance between the cathode electrode 24 and the n-type semiconductor substrate 20.

Next, a non-doped or Li-doped NiZnMgO film is deposited by radio frequency sputtering over the entire surface 26 of the n-type semiconductor layer 21 to a thickness sufficient to fill the trenches 25.

Next, the NiZnMgO film is etched by RIE until the mesa-shaped portion 210 between adjacent trenches 25 of the n-type semiconductor layer 21 is exposed.

Next, an anode electrode 23 having a Mo/Au laminated structure is formed on the surface 26 of the n-type semiconductor layer 21 by electron beam evaporation. The anode electrode 23 is patterned into a predetermined shape, such as a circle, by lift-off.

(Effects of the embodiment)
According to the above embodiment, it is possible to provide a Ga ₂ O ₃ based Schottky diode that has high breakdown voltage, low loss, and excellent resistance to surge current.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the gist of the invention. Furthermore, the embodiment described above does not limit the invention related to the claims. It should be noted that not all of the combinations of features described in the embodiment are necessarily essential to the means for solving the problem of the invention.

1, 2... Schottky diode, 10, 20... n-type semiconductor substrate, 11, 21... n-type semiconductor layer, 12, 25... trench, 13, 23... anode electrode, 14, 24... cathode electrode, 15... trench insulating film 16... anode electrode, 17, 22... p-type semiconductor member

Claims (3)

An n-type semiconductor layer made of _Ga2O3 -based _single crystal and having a plurality of trenches opening on one surface thereof;
an anode electrode connected to a mesa-shaped region between adjacent trenches of the n-type semiconductor layer;
a trench anode electrode that is embedded in each of the trenches while being covered with an insulating film and is electrically connected to the anode electrode;
a cathode electrode connected directly or indirectly to the n-type semiconductor layer on the opposite side to the anode electrode;
a p-type semiconductor member made of NiZnMgO connected to a part of the mesa-shaped region and to the anode electrode;
A Schottky diode having An n-type semiconductor layer made of _Ga2O3 -based _single crystal and having a plurality of trenches opening on one surface thereof;
A p-type semiconductor member made of NiZnMgO embedded in the trench;
an anode electrode connected to a mesa-shaped region between adjacent trenches of the n-type semiconductor layer and to the p-type semiconductor member;
a cathode electrode connected directly or indirectly to the n-type semiconductor layer on the opposite side to the anode electrode;
A Schottky diode having The energy of the upper end of the valence band of the p-type semiconductor member is equal to or lower than the energy of the upper end of the valence band of the Ga ₂ O ₃ -based single crystal;
3. The Schottky diode according to claim 1 or 2.

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