JP2007294740A - Semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve cutoff characteristics while suppressing a leakage current during a reverse operation. <P>SOLUTION: A semiconductor device is composed so that a distance of a current pathway flowing between a hetero junction 5 and a contact 8 through a hetero semiconductor region 4 is longer than at least a film thickness of the hetero semiconductor region 4 due to an insulating region 3. By this, a crystal grain boundary linearly connecting the contact 8 with the hetero junction 5 is not formed even when the hetero semiconductor region 4 is formed by polysilicon. Consequently, it is possible to limit an amount of electrons supplied from a first electrode 7 to the hetero junction 5 through the crystal grain boundary. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a semiconductor device, and more particularly to a technique for suppressing a leakage current during reverse operation and improving a cutoff characteristic.

Conventionally, a P ₊ -type polycrystalline silicon region is formed in contact with one main surface of a semiconductor substrate in which an N _-- type epitaxial region is formed on an N ₊ -type silicon carbide substrate. A semiconductor device in which a heterojunction is formed between regions is known (see Patent Document 1). In such a semiconductor device, a surface metal electrode is formed on the polycrystalline silicon region, and a back metal electrode is formed on the back surface of the silicon carbide substrate. Then, by applying a voltage between the electrodes using the front surface metal electrode and the back surface metal electrode as an anode and a cathode, respectively, a rectifying action is produced at the heterojunction interface, and diode characteristics can be obtained.

Specifically, when a positive potential is applied to the anode with the cathode grounded, a conduction characteristic corresponding to the forward characteristic of the diode is obtained, and conversely, when a negative potential is applied to the anode, the reverse of the diode is obtained. A blocking characteristic corresponding to the directional characteristic is obtained. These forward and reverse characteristics can be adjusted arbitrarily as well as exhibiting characteristics such as a Schottky junction composed of a metal electrode and a semiconductor material, compared with a diode using a Schottky junction. This has the advantage that it can be adjusted to the optimum pressure resistance system as required. Furthermore, by adjusting the impurity density and conductivity type of the polycrystalline silicon region to predetermined conditions, a very small leakage current characteristic can be obtained by an operation mechanism that is essentially different from that of the Schottky junction.
JP 2005-259797 A

  However, according to the structure of the semiconductor device described above, since a large leakage current is generated during reverse operation due to the influence of crystal grain boundaries existing in the polycrystalline silicon region, it is difficult to realize a theoretical cutoff characteristic. It was.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a semiconductor device capable of suppressing a leakage current during reverse operation and improving a cutoff characteristic.

  In order to solve the above-described problems, a semiconductor device according to the present invention is characterized by a first-conductivity-type semiconductor substrate and a band gap width that is in contact with one main surface of the semiconductor substrate and different from the band gap width of the semiconductor substrate. A semiconductor device comprising: a hetero semiconductor region having a first electrode connected to the hetero semiconductor region; and a second electrode connected to the semiconductor substrate, wherein the hetero junction between the semiconductor substrate and the hetero semiconductor region And a barrier region in which the distance of the current path flowing between the first electrode and the contact portion where the hetero semiconductor region is in contact is longer than the thickness of the hetero semiconductor region.

  According to the semiconductor device of the present invention, the distance of the current path flowing between the heterojunction portion and the contact portion through the hetero semiconductor region is at least longer than the thickness of the hetero semiconductor region. Is formed of polycrystalline silicon, a crystal grain boundary that linearly connects the contact portion and the heterojunction portion is not formed, and is supplied from the first electrode to the heterojunction portion via the crystal grain boundary. The amount of electrons can be limited. Therefore, according to the semiconductor device of the present invention, it is possible to reduce the leakage current during the reverse operation and improve the cutoff characteristic.

  Hereinafter, a configuration of a semiconductor device according to an embodiment of the present invention will be described in detail with reference to the drawings.

[Example 1]
First, the configuration of the semiconductor device according to the first embodiment of the present invention will be described with reference to FIG.

[Configuration of semiconductor device]
As shown in FIG. 1, the semiconductor device of the present embodiment includes an N ₋ type drift region (N ^— SiC) on a substrate region (N ⁺ SiC) 1 made of silicon carbide of N ₊ type whose polytype is 4H type. ) Comprising a first semiconductor region 100 made of a substrate material on which 2 is formed. As the substrate region 1, for example, a material having a resistivity of several to several tens [mΩcm] and a film thickness of several tens to several hundreds [μm] can be used. As the drift region 2, for example, a material having an N-type impurity density of 10 ¹⁵ to 10 ¹⁸ [cm ⁻³ ] and a film thickness of several to several tens [μm] can be used. In the present embodiment, the impurity density of the drift region 2 is 10 ¹⁶ [cm ⁻³ ] and the film thickness is 10 μm. In the present embodiment, the case where the first semiconductor region 100 is formed of a substrate material including the substrate region 1 and the drift region 2 will be described. However, the first semiconductor region 100 regardless of the resistivity. May be formed only by the substrate region 1.

In the semiconductor device of the present embodiment, an insulating region 3 such as an oxide film and a hetero semiconductor region 4 made of polycrystalline silicon having a band gap width smaller than that of silicon carbide are connected to the drift region 2 and the substrate region 1. It is formed in contact with the main surface opposite to the surface. That is, in this semiconductor device, a heterojunction diode (heterojunction portion 5) is formed at the junction between the drift region 2 and the hetero semiconductor region 4 because the band gap width of silicon carbide and the band gap width of polycrystalline silicon are different. There is an energy barrier at the junction interface. In the present embodiment, a P-type material having an impurity density of 10 ¹⁹ [cm ⁻³ ] and a film thickness of 0.5 [μm] is used as the hetero semiconductor region 4.

  In the semiconductor device of this embodiment, an interlayer insulating film 6 such as an oxide film and a first electrode 7 made of a metal material are formed so as to be in contact with the hetero semiconductor region 4. Further, the hetero semiconductor region 4 and the first electrode 7 are connected at the contact portion 8. The semiconductor device also includes a second electrode 9 made of a metal material so as to be in contact with the substrate region 1. In addition, the distance of the current path that flows between the heterojunction portion 5 and the contact portion 8 through the hetero semiconductor region 4 is at least longer than the film thickness of the hetero semiconductor region 4 due to the presence of the insulating region 3.

[Operation of semiconductor device]
Next, the operation of the semiconductor device in the case of operating as a vertical diode by using the first electrode 7 and the second electrode 9 as an anode and a cathode, respectively, will be described separately for a forward operation and a reverse operation.

[Forward operation]
First, the forward operation of the semiconductor device will be described.

  When the second electrode 9 is set to the ground potential and a positive potential is applied to the first electrode 7, the heterojunction diode exhibits a forward characteristic and a conduction characteristic like a Schottky junction diode. That is, in this case, a forward current flows due to a voltage drop determined by the sum of the built-in potentials spreading from the heterojunction 5 to the drift region 2 side and the hetero semiconductor region 4 side. For example, in this embodiment, the sum of the built-in potentials extending from the heterojunction portion 5 to the drift region 2 and the hetero semiconductor region 3 is about 1.3 [V], and the forward current is caused by a voltage drop corresponding to this sum. Flowing.

  In the semiconductor device of this embodiment, since the distance from the contact portion 8 to the heterojunction portion 5 is longer than that in the conventional semiconductor device, the resistance in the hetero semiconductor region 4 is increased. However, the resistance in the drift region in the conventional semiconductor device has a magnitude that hardly affects the resistance in the hetero semiconductor region. That is, when the impurity concentration and thickness of the drift region and the hetero semiconductor region in the conventional semiconductor device are the same as those in the semiconductor device of this embodiment, it is two digits or more due to the impurity concentration and one digit or more due to the thickness. This is because the total is about 3 to 4 digits higher. For this reason, in the semiconductor device of this embodiment, the resistance in the hetero semiconductor region 4 hardly affects the on-resistance of the entire semiconductor device.

[Reverse operation]
Next, the reverse operation of the semiconductor device will be described.

  When the first electrode 7 is set to the ground potential and a positive potential is applied to the second electrode 9, the heterojunction diode exhibits reverse characteristics and enters a cut-off state. In the semiconductor device of this embodiment, since the conductivity type of the hetero semiconductor region 4 is P type in the main region portion of the active region, the cutoff characteristic operates like a PN junction diode. This is because, in the configuration in which the conductivity type of the hetero semiconductor region 4 is P-type and the conductivity type of the drift region 2 is N-type, leakage current characteristics due to carriers generated under a predetermined electric field as seen in a PN junction diode are dominant. This is because the leakage current through the energy barrier of the heterojunction interface 5 can be greatly reduced. The reason will be described below.

  The leakage current characteristic of the Schottky junction diode is determined almost uniquely by the height of the Schottky barrier formed by the difference between the electron affinity of the semiconductor material and the work function of the Schottky metal. However, since heterojunction diodes are composed of junctions of different semiconductor materials, their leakage current characteristics are mainly due to the supply of majority carriers that are the source of the energy barrier height and leakage current that occurs between different semiconductor materials. It depends on the size of the source. Among these, the height of the energy barrier is almost determined by the semiconductor materials of the drift region 2 made of silicon carbide and the hetero semiconductor region 4 made of silicon, and therefore has the same performance as a Schottky junction diode. In addition, the semiconductor device of the present embodiment is much smaller than the Schottky junction diode and the conventional semiconductor device in terms of the supply source of majority carriers that cause the leakage current. This is because in the semiconductor device of the present embodiment, the hetero semiconductor region 4 is configured as a P-type, and conduction electrons serving as majority carriers for the N-type drift region 2 are less likely to be generated in the hetero-semiconductor region 4. This is because it has a structure that suppresses the origin of the occurrence.

  However, in the conventional semiconductor device, conduction electrons serving as majority carriers are supplied from the first electrode to the heterojunction through the crystal grain boundary due to the presence of the crystal grain boundary in the polycrystalline silicon region, and during reverse operation. Since leakage current is generated, there is a limit to the improvement of withstand voltage. If electrons are supplied to the heterojunction portion, the original reverse characteristics of the heterojunction diode composed of the P-type hetero semiconductor region 4 and the N-type drift region 2 cannot be obtained. Leakage current characteristics. In particular, when a polycrystalline silicon layer is actually formed on a silicon carbide substrate by LPCVD, each silicon crystal grows in a columnar shape. As a result, the crystal grain boundary linearly connects the first electrode and the heterojunction portion. Thus, electrons are easily supplied from the first electrode to the heterojunction portion. Here, although described as a polycrystalline silicon region, even if single crystal silicon or another material is used instead of polycrystalline silicon, the same phenomenon is caused by the existence of crystal results and a predetermined heat treatment process. May occur.

  On the other hand, in the semiconductor device of this embodiment, the distance of the current path flowing between the heterojunction portion 5 and the contact portion 8 via the hetero semiconductor region 4 is at least a film of the hetero semiconductor region 4 due to the presence of the insulating region 3. Since it is longer than the thickness, even when the hetero semiconductor region 4 is formed of polycrystalline silicon, a crystal grain boundary that linearly connects the contact portion 8 and the hetero junction portion 5 is not formed, and the crystal The amount of electrons supplied from the first electrode 7 to the heterojunction portion 5 through the grain boundary can be limited. Therefore, according to the semiconductor device of the present embodiment, it is possible to reduce the leakage current during reverse operation.

  Note that the distance of the current path in the hetero semiconductor region 4 is desirably longer than the effective diffusion distance of electrons supplied from the first electrode 7 via the crystal grain boundary. According to such a configuration, it is possible to prevent electrons from being supplied to the heterojunction portion 5, thereby realizing the leakage current characteristics inherent in a heterojunction diode different from a Schottky junction diode.

  In the present embodiment, polycrystalline silicon is used as the material of the hetero semiconductor region 4. However, even if single crystal silicon or another material is used instead of polycrystalline silicon, the presence of crystal defects or predetermined Since electrons are supplied from the first electrode 7 by this heat treatment process, the insulating region 3 can suppress the diffusion of electrons to the heterojunction portion 5 and reduce the leakage current.

  In addition, as shown in FIG. 2, the insulating region 3 may be formed in the hetero semiconductor region 4 so that the insulating region 3 does not contact the drift region 2. According to such a configuration, since the effective area of the heterojunction portion 5 can be increased, it is possible to reduce the leakage current during the reverse operation and reduce the on-resistance caused by the junction during the forward operation. it can.

  In addition, as shown in FIG. 3, a plurality of insulating regions 3 are formed in each layer in the hetero semiconductor region 4 by forming the insulating regions 3 at positions not in contact with the drift region 2 in the hetero semiconductor region 4. You may make it form. According to such a configuration, the distance of the current path that flows between the heterojunction portion 5 and the contact portion 8 through the hetero semiconductor region 4 in the limited region can be further increased, so that it is higher. Heterojunction diodes can be formed at a high density, and the degree of integration can be improved.

  Further, as shown in FIG. 4, the insulating region 3 is formed so that the film thickness in the vicinity of the hetero semiconductor region 4 in contact with the contact portion 8 is small and the film thickness in the vicinity of the hetero semiconductor region 4 in contact with the hetero junction portion 5 is large. May be formed obliquely. According to such a configuration, while suppressing an increase in resistance in the current path in the hetero semiconductor region 4, a crystal grain boundary where electrons diffuse around the hetero semiconductor region 4 to which electrons are supplied from the first electrode 7. The route can be reduced. That is, the leakage current can be further suppressed while maintaining a predetermined on-resistance. Further, in this case, as shown in FIG. 5, variations in manufacturing can be easily suppressed by forming a plurality of insulating regions 3 in layers.

  In the semiconductor device shown in FIGS. 1 to 5, the hetero semiconductor region 4 has a single conductivity type and impurity density. However, as shown in FIGS. A second hetero semiconductor region 10 having a different impurity density may be provided. In this case, the second hetero semiconductor region 10 may be either P-type or N-type, and the impurity density may be larger or smaller than the hetero semiconductor region 4.

In addition, although the semiconductor device illustrated in FIGS. 1 to 7 has a simple configuration in which a heterojunction diode is formed, a breakdown voltage structure may be formed in the outermost peripheral portion of the heterojunction diode. Specifically, the end of the hetero semiconductor region 4 may run on an interlayer insulating film made of SiO ₂ , or a mesa in which the electric field relaxation region, the guard ring, and the drift region 2 are dug in the drift region 2. You may have a structure.

[Example 2]
Next, the configuration of the semiconductor device according to the second embodiment of the present invention will be described with reference to FIG.

[Configuration of semiconductor device]
In the semiconductor device of the present embodiment, a P-type well region 11 is formed in the drift region 2 instead of the insulating region 3 in the semiconductor device according to the first embodiment.

[Operation of semiconductor device]
Next, the operation of the semiconductor device in the case of operating as a vertical diode by using the first electrode 7 and the second electrode 9 as an anode and a cathode, respectively, will be described separately for a forward operation and a reverse operation.

[Forward operation]
First, the forward operation of the semiconductor device will be described.

  When the second electrode 9 is set to the ground potential and a positive potential is applied to the first electrode 7, the heterojunction diode exhibits forward characteristics and a current flows with a low on-resistance. In this case, the PN junction diode formed between the well region 11 and the drift region 2 is also in a forward bias state. However, since the built-in potential is about 3 [V], the well region 11 and the drift region are This is because no current flows between the two.

[Reverse operation]
Next, the reverse operation of the semiconductor device will be described.

  When the first electrode 7 is set to the ground potential and a positive potential is applied to the second electrode 9, the heterojunction diode exhibits reverse characteristics and enters a cut-off state. This is because the PN junction diode formed between the well region 11 and the drift region 2 is also in a reverse bias state, and the leakage current is smaller than that of the heterojunction between the hetero semiconductor region 4 and the drift region 2. Further, when the positive potential of the second electrode 9 is increased, a depletion layer spreads between the PN junctions of the well region 11 and the drift region 2, and the drift region 2 sandwiched between the well regions 11 facing each other becomes the entire region. When depletion occurs, the electric field at the heterojunction portion 5 is shielded, and the generation of leakage current can be further reduced.

  In this embodiment, the well region 11 is used as the barrier region. However, the insulating region 3 and the well region 11 may be used in combination as shown in FIG. Further, when the insulating region 3 and the well region 11 are used in combination, the technical effect of the semiconductor device according to the present invention can be obtained without the interlayer insulating film 6 as shown in FIG. In any case, by providing the barrier region, the distance of the current path flowing between the heterojunction portion 5 and the contact portion 8 through the hetero semiconductor region 4 is at least longer than the film thickness of the hetero semiconductor region 4. It only has to be. In the present embodiment, the P-type well region 11 activated as a semiconductor has been described. However, the semiconductor may be an inactive high-resistance region or an electrically insulated region. .

  As mentioned above, although embodiment which applied the invention made by the present inventors was described, this invention is not limited by the description and drawing which make a part of indication of this invention by this embodiment. For example, in the above embodiment, the substrate material is silicon carbide, but the substrate material may be other semiconductor materials such as silicon, silicon germanium, gallium nitride, and diamond. In addition, although the polytype of silicon carbide is the 4H type, other polytypes such as 6H and 3C may be used. Further, the semiconductor device is a so-called vertical structure diode in which the second electrode 9 and the first electrode 7 are arranged so as to face each other with the drift region 2 interposed therebetween, and a current flowing between the two electrodes flows in the vertical direction. Alternatively, a transistor having a so-called lateral structure in which the second electrode 9 and the first electrode 7 are arranged on the same main surface and a current flows in the lateral direction may be used. Further, although polycrystalline silicon is used as the material used for the hetero semiconductor region 4 and the second hetero semiconductor region 10, other silicon such as single crystal silicon and amorphous silicon can be used as long as it is a material that forms a heterojunction with silicon carbide. The material may be other semiconductor materials such as germanium and silicon germanium, and other polytypes of silicon carbide such as 6H and 3C. Further, N-type silicon carbide is used as the drift region 2 and P-type polycrystalline silicon is used as the hetero semiconductor region 4, but N-type silicon carbide and P-type polycrystalline silicon, P-type silicon carbide and P-type polycrystalline silicon are used. Other combinations such as type polycrystalline silicon, P type silicon carbide and N type polycrystalline silicon may be used. As described above, it should be added that other embodiments, examples, operation techniques, and the like made by those skilled in the art based on this embodiment are all included in the scope of the present invention.

It is sectional drawing which shows the structure of the semiconductor device used as the 1st Embodiment of this invention. It is sectional drawing which shows the structure of the application example of the semiconductor device shown in FIG. It is sectional drawing which shows the structure of the application example of the semiconductor device shown in FIG. It is sectional drawing which shows the structure of the application example of the semiconductor device shown in FIG. It is sectional drawing which shows the structure of the application example of the semiconductor device shown in FIG. It is sectional drawing which shows the structure of the application example of the semiconductor device shown in FIG. It is sectional drawing which shows the structure of the application example of the semiconductor device shown in FIG. It is sectional drawing which shows the structure of the semiconductor device used as the 2nd Embodiment of this invention. FIG. 9 is a cross-sectional view illustrating a configuration of an application example of the semiconductor device illustrated in FIG. 8. FIG. 9 is a cross-sectional view illustrating a configuration of an application example of the semiconductor device illustrated in FIG. 8.

Explanation of symbols

1: substrate region 2: drift region 3: insulating region 4: hetero semiconductor region 5: heterojunction interface 6: interlayer insulating film 7: first electrode 8: contact portion 9: second electrode

Claims (8)

A semiconductor substrate of a first conductivity type, a hetero semiconductor region in contact with one main surface of the semiconductor substrate and having a band gap width different from the band gap width of the semiconductor substrate, and a first semiconductor connected to the hetero semiconductor region A semiconductor device having an electrode and a second electrode connected to the semiconductor substrate,
A barrier region in which a distance of a current path flowing between a heterojunction portion between the semiconductor substrate and the hetero semiconductor region and a contact portion where the first electrode and the hetero semiconductor region are in contact is longer than a thickness of the hetero semiconductor region; A semiconductor device comprising:   2. The semiconductor device according to claim 1, wherein a distance of the current path is longer than at least an effective diffusion distance of electrons supplied from the first electrode.   3. The semiconductor device according to claim 1, wherein at least a part of the region functioning as the barrier region is formed of an insulating film formed so as to be in contact with at least the hetero semiconductor region. Semiconductor device.   4. The semiconductor device according to claim 1, wherein the barrier region is disposed so as not to contact the heterojunction portion and the contact portion. 5. Semiconductor device.   5. The semiconductor device according to claim 1, wherein at least a part of a region functioning as the barrier region is a second conductivity type well region formed in the semiconductor substrate. 6. A semiconductor device comprising:   6. The semiconductor device according to claim 1, wherein at least a part of a conductivity type of the hetero semiconductor region is a second conductivity type.   7. The semiconductor device according to claim 1, wherein the semiconductor substrate is formed of any one of silicon carbide, gallium nitride, and diamond. Semiconductor device.   8. The semiconductor device according to claim 1, wherein the hetero semiconductor region is any one of single crystal silicon, polycrystalline silicon, amorphous silicon, germanium, and silicon germanium. A semiconductor device formed by the method described above.

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