JP2002076371A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor device which improves the reverse recovery characteristics and avoid increasing the reverse leakage current. SOLUTION: The ratio Rsch of a region on a Schottky junction 7 to a region of an anode electrode 1, the cell pitch W, the diffusion depth Xj of a p+ anode layer 2 meet a relation W<K(Xj/Rsch) with K set to 5 or less. If K is set to 1.8 or less, it is more effective.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device, and more particularly, to a semiconductor device for power use.

[0002]

2. Description of the Related Art Although power semiconductor rectifiers (diodes) are used for various purposes, they have recently been used in high-frequency circuits for power and the like, and the switching speed has been reduced by shortening the reverse recovery operation time. There is a strong demand for faster. Conventional diodes are mainly PiN and have low reverse leakage current. However, PiN has a very low switching speed in the reverse recovery characteristic due to the accumulation effect of minority carriers. For this reason, a technique of controlling the lifetime of minority carriers by using heavy metal diffusion, electron beam irradiation, or the like is generally used.

[0003]

That is, PiN is:
Since the accumulation of the minority carriers is large, it takes time to sweep out the carriers, and the switching speed is low. Therefore, by arranging the Schottky contact portion in parallel with the PiN portion, accumulation of minority carriers can be reduced as compared with PiN, and a diode with soft recovery and low loss can be realized. However, such a diode has a disadvantage that the reverse leakage current increases due to the Schottky junction.

The present invention has been proposed to solve such a problem, and an object of the present invention is to provide a semiconductor device capable of improving reverse recovery characteristics and preventing an increase in reverse leakage current.

[0005]

To achieve the above object, a semiconductor device according to the present invention comprises a first conductive type cathode layer provided between a cathode electrode and an anode electrode and in contact with the cathode electrode. A first conductivity type drift layer provided between the cathode layer and the anode electrode and forming a Schottky junction with the anode electrode, the first conductivity type drift layer having a lower impurity concentration than the cathode layer; A semiconductor having a plurality of structural units comprising a second conductive type anode layer having a higher impurity concentration than the drift layer, wherein the structural unit is provided in contact with a layer and the anode electrode and forms an ohmic junction with the anode electrode. In the device, the pitch W2 of the Schottky junction among the pitches W in which the plurality of structural units are provided is determined by the K of the anode layer junction depth Xj Smaller (W2 <K · Xj), and the K is 5 or less, the specific resistance ρ of the drift layer is 6Ω
· Cm or more and 450Ω · cm or less. Further, it is more effective if K is 1.8 or less.

According to the above-described structure, the surface electric field intensity at the Schottky junction can be reduced, and the reverse recovery characteristic can be improved, and an increase in reverse leakage current can be prevented. If an intermediate layer having an impurity concentration intermediate between the drift layer and the cathode layer is provided between the drift layer and the cathode layer, the current flows smoothly between the respective layers. Further, in the present invention, the ratio of the Schottky junction region to the anode electrode region is defined by the length of the structural unit in the pitch direction. According to this, device design and manufacturing become easy.

In the present invention, the ratio R of the area of the Schottky junction to the area of the anode electrode
The sch is characterized by being larger than 40% and smaller than 90%. According to such a configuration, the reverse leakage current and the peak current value at the time of reverse recovery can be reduced. Further, in the present invention, the ohmic junction or the Schottky junction in the plurality of structural units is characterized by being arranged in a stripe shape or a dot cell shape, and various types according to use are provided. An arrangement shape can be adopted.

A first conductive type cathode layer provided between the cathode electrode and the anode electrode and in contact with the cathode electrode; and a first conductive type cathode layer provided between the cathode layer and the anode electrode and connected to the anode electrode. A first conductivity type drift layer having a lower impurity concentration than the cathode layer, forming a Schottky junction therebetween; and a low ratio formed at the bottom of the trench provided in the drift layer and filling the trench. In a semiconductor device having a plurality of structural units each including an ohmic junction with a resistor polysilicon and being in contact with the anode electrode through the polysilicon, a second conductive type anode layer, the plurality of structural units are provided. The pitch W to be formed, and the junction depth Xj0 of the anode layer,
It is characterized in that it is determined based on the depth Td of the trench and the ratio Rsch0 of the region of the Schottky junction to the region of the anode electrode.

The pitch W is defined as K0 which is the ratio of the sum of the junction depth Xj0 of the anode layer and the depth Td of the trench and the ratio Rsch0 of the Schottky junction region to the anode electrode region. (W2 <K0
× (Xj + Td)) and K0 is set to 5 or less. Further, it is more effective to set K0 to 1.8 or less.

The drift layer has a specific resistance of 6 Ωcm
As described above, the characteristic is set to 450 Ω · cm or less. The ratio of the Schottky junction region to the anode electrode region is defined by the length of the structural unit in the pitch direction.

The ratio Rsch 0 of the Schottky junction region to the anode electrode region is 40%
According to such a configuration, the reverse leakage current and the peak current value at the time of reverse recovery can be reduced.
Further, the ohmic junction or the Schottky connection in the plurality of structural units is characterized by being arranged in a stripe shape or a dot cell shape, and adopts various arrangement shapes according to use. be able to.

The drift layer has a specific resistance of 15 Ω ·
cm or more and 350 Ω · cm or less is more effective than the range of 6 Ω · cm or more and 450 Ω · cm or less.

[0013]

FIG. 1 is a sectional view showing a semiconductor rectifier as a semiconductor device according to an embodiment of the present invention. This figure shows a half of the unit cell. As shown in FIG. 1, this semiconductor rectifier element is formed by epitaxially growing an N-type N intermediate layer 4 on an N ⁺ cathode layer 5 and further epitaxially growing the N intermediate layer 4 to have a concentration slightly lower than that of the N intermediate layer 4. To obtain an N ^- drift layer 3. Here, the N intermediate layer 4 is provided for smoothing the flow of current from the N ⁺ cathode layer 5 to the N ⁻ drift layer 3, but the N intermediate layer 4 is not provided and the N ⁺ cathode The N ⁻ drift layer 3 may be provided directly on the layer 5. Then, a P ⁺ anode layer 2 is further formed thereon. An ohmic junction is formed at a width W1 where the P ⁺ anode layer 2 and the anode electrode 1 are in contact (here, a half width of the P ⁺ anode layer of the unit cell). ^- with a width W2 drift layer 3 are in contact to form a Schottky junction. In FIG. 1, 6 is a cathode electrode, 7 is a Schottky junction,
Denotes a cell pitch (in this case, a length corresponding to half of a unit cell, 2 × W is a unit cell pitch), and Xj denotes a junction depth of the P ⁺ anode layer 2.

That is, in the structure shown in FIG. 1, the cathode electrode 6 and the anode electrode 1 are formed on both surfaces of the semiconductor substrate, and the cathode electrode 6 and the first conductivity type (N-type) cathode layer (N ⁺ cathode) are formed. The first conductive type cathode layer 5 is in contact with the first conductive type cathode layer 5 and the first conductive type drift layer (N ⁻ drift layer) 3 having a lower impurity concentration than the cathode layer. A structure in which the Schottky junction is formed between the first conductive type and the anode electrode 1 is referred to as a first structure. Further, in the first structure, the second conductive type (P-type) having a higher impurity concentration than the drift layer in the first conductive type drift layer 3. ) Is formed as a second structure by forming an anode layer (P ⁺ anode layer) 2 and contacting the anode electrode 1 so as to form an ohmic junction, and the first structure and the second structure are one structural unit Distributed in parallel as It is. In this specification, the first conductivity type is N-type and the second conductivity type is P-type.

In the prototype of this embodiment, the surface concentration of the P ⁺ anode layer 2 is 7 × 10 ¹⁷ / cm ³ , the diffusion depth is 3.3 μm, and the concentration of the N ⁻ drift layer 3 is 1.68 × 10
¹⁴ / cm ³ , the concentration of the N intermediate layer 4 is 2.94 × 10 ¹⁴ / c
m 3, the concentration of the N ⁺ cathode layer 5 is 1.6 × 10 ¹⁸ / cm
^3, and the Schottky barrier height is designed to be 0.6 to 0.8 ev. Also, W2 /
Let W be the Schottky ratio Rsch.

When the P ⁺ anode layer 2 is formed by ion implantation, impurity atoms are implanted through openings in the mask, and the impurity atoms are thermally diffused to form the P ⁺ anode layer 2.
Is formed. Then impurity atoms also diffuse laterally, W1 in Figure 1, depending on the X _J, becomes about 0.8 × X _J larger than the size of the opening of the mask. FIG. 2 is a schematic diagram showing a state when the depletion layer pinches off.

Although the junction depth Xj and the Schottky ratio Rsch are equal in FIGS. 2A and 2B, if the cell pitch W can be expressed by W = K (Xj / Rsch), K in FIG. (B) has a structure in which K is large. In addition, since Rsch is W2 / W, the above equation becomes W2 = K.XJ. It can be seen from these figures that even when the Schottky ratio is the same, if the distance W2 of the Schottky junction 7 is short, the depletion layer pinches off at a small voltage, so that the surface electric field intensity at the Schottky junction 7 can be reduced. it can.

When a reverse bias is applied to the structure shown in FIG. 1, the depletion layer expands from the P ⁺ anode layer 2, and when a certain voltage is reached, the depletion layer pinches off. However, if the pinch-off voltage is large, the surface electric field intensity becomes large near the Schottky junction 7 and the reverse leakage current increases. However, as the distance W2 forming the Schottky junction becomes shorter, the pinch-off voltage becomes smaller, and the surface electric field intensity of the Schottky junction 7 can be reduced. Here, the distance W2 forming the Schottky junction has a relationship of W2 = WRsch, but by reducing the cell pitch W, the pinch-off voltage can be reduced and the surface electric field intensity of the Schottky junction 7 can be reduced.

It is desirable to make the cell pitch W as small as possible, but for the same Schottky ratio Rsch,
When the junction depth Xj is increased, the voltage at which the depletion layer pinches off decreases, the surface electric field intensity near the Schottky junction 7 is reduced, and the reverse leakage current decreases. As the reverse leakage current decreases, the cell pitch W can be increased. Thus, the cell pitch W and the junction depth X
It turns out that j is closely related. Further, when the Schottky ratio Rsch is reduced, the cell pitch W can be similarly increased. Then, W = K (Xj / Rsc
h). It is desirable that the cell pitch W be as small as possible. However, by setting the cell pitch W to be not more than K (Xj / Rsch), the surface electric field intensity at the Schottky junction can be sufficiently reduced.

FIG. 10 is a graph showing the dependence of the reverse recovery peak current on the resistivity of the N ⁻ drift layer. N ^- drift layer 3
The reverse recovery peak current (Irp) can be suppressed by setting the resistance of the resistor to 15Ω · cm or more. Further, when the resistance is less than 6Ω · cm, the reverse recovery peak current sharply increases.
Even if it is small, 6 Ω · cm or more is good. FIG. 11 is a diagram showing that the n ⁻ drift layer of the spike voltage (overvoltage) during reverse recovery also has a specific resistance dependence.

When the specific resistance increases in the reverse bias state, N
^- expansion of the depletion layer in the drift layer 3 is faster, resulting in a punch-through. In this case, since the carriers on the cathode side disappear instantaneously, oscillation occurs and the spike voltage as shown in FIG. 12 increases. In recent years, with the demand for low noise, it is important to suppress the spike voltage and reduce the oscillation phenomenon. As shown in FIG. 11, the spike voltage can be suppressed by setting the specific resistance of N ⁻ drift layer 3 to 350 Ω · cm or less. Further, when the voltage exceeds 450 Ω · cm, the spike voltage sharply increases.

FIG. 3 is a plan view for explaining the structure of the semiconductor device according to the embodiment of the present invention. FIG.
(A) shows a structure in which the anode electrode 1 shown in FIG. 1 and the first conductivity type drift layer (N ⁻ drift layer) 3 are Schottky-joined (Schottky junction 7) and the anode electrode 1
And an anode layer (P ⁺ anode layer) 2 of the second conductivity type and an ohmic junction (P
iN portion) are distributed in a stripe shape. The cell pitch W in this figure is the same as the cell pitch W in FIG. In addition, reference numeral 8 denotes a terminal portion.

FIG. 3 (b) shows the Schottky in FIG. 3 (a).
-The joint 7 is replaced with the PiN. Also,
FIG. 3C shows the anode electrode 1 and the drift layer of the first conductivity type.
(N ^-Drift layer) 3 with Schottky junction
(Schottky junction 7), anode electrode 1, and second conductive material
Type anode layer (P⁺Anode layer) 2 is ohmic junction
Are distributed in the form of dot cells.
You. FIG. 3D shows the Schottky junction in FIG. 3C.
7 and the PiN section are interchanged. In the following description
Proceed centering on the stripe structure.

Next, FIG.
The dependence of the reverse leakage current at 25 ° C. is shown. Here, the junction depth Xj was 3.3 μm and 6.6 μm, and the Schottky ratio Rsch was 50% and 75%. From this figure,
It can be seen that the reverse leakage current can be suppressed when the cell pitch W is 5 times or less of Xj / Rsch. Further, it can be seen that the same tendency exists even when the junction depth Xj is increased or the Schottky ratio Rsch is decreased.

FIG. 8 shows the relationship between the reverse leakage current density and K when the value of K in FIG. 4 is further reduced in a device in which Rsch is 65% and XJ is 6.6 μm. It is understood that the reverse leakage current density can be further suppressed by setting K to 1.8 or less. FIG. 5 shows an electric field intensity distribution in a cross section when the Schottky junction 7 side in the structure in FIG. 1 is cut in the vertical direction.

In FIG. 5, the cell pitch W is 20 μm and 50 μm. Also, the Schottky ratio Rsch is set to 50
%. It can be seen that when the cell pitch W is reduced, the Schottky surface electric field intensity is reduced. This is because the depletion layer is pinched off at a low voltage when the cell pitch is small. From this, the cell pitch W
When the value of is reduced, the surface electric field intensity can be relaxed and the reverse leakage current can be prevented from increasing.

FIG. 6 shows the Schottky ratio Rsch
4 shows the cell pitch dependency of the peak current value (Irp) at the time of the reverse recovery with respect to FIG. From this figure, the Schottky ratio R
Low Irp is obtained when sch is 40% or more and 90%. In this figure, the important characteristic is that the smaller the W2, the lower the Irp
Is shown. This can be considered as follows using FIG.

In FIG. 7, the spread of the space charge region when the anode voltage becomes Vo during the reverse recovery is represented by a depth Y
o, Xo in the horizontal direction (Xo <Yo). W2 is enough X
o, that is, K is small (FIG. 7)
(A)), the space charge region is pinched off, and carriers below the space charge region are swept out. However, when the cell pitch is sufficiently larger than Xo, that is, when K is large (FIG. 7)
(B), the space charge region does not pinch off, and undischarged carriers remain below the Schottky. Therefore, the charge during reverse recovery increases, and Irp also increases. Since the Schottky ratio is the same, Vf (forward voltage) is substantially the same even between different cell pitches. Therefore, the Irp-Vf trade-off is K
The smaller is the better.

As described above, according to the embodiment of the present invention, W2 is smaller than K times the junction depth Xj, and K is 5 or less, more preferably 1.8 or less. Is set to 40 to 90%, the reverse recovery characteristic can be improved, and an increase in reverse leakage current can be prevented. In this embodiment, the ratio Rsch of the Schottky junction region to the anode electrode region is set.
Is represented by the ratio of the length in the pitch direction, but it is needless to say that the ratio may be defined by the area.

FIG. 9 is a diagram showing another embodiment of the present invention. This structure is a case where a trench 22 is formed in the N ⁻ drift layer 3 and a P ⁻ anode layer 21 is formed at the bottom thereof. In this semiconductor rectifying device, an N-type N intermediate layer 4 is formed on the N ⁺ cathode layer 5 by epitaxial growth, and the N ⁻ drift layer 3 is further epitaxially grown to be slightly lower than the concentration of the N intermediate layer 4. obtain. A trench 22 is formed in the N ⁻ drift layer 3,
An oxide film 24 is formed on the side wall and the bottom, and the oxide film on the bottom is removed. Polysilicon 23 is filled in trench 22, and a P-type impurity is ion-implanted through polysilicon 22 and thermally diffused to form P ⁻ anode layer 21. The polysilicon 23 and the anode electrode 1 make ohmic contact. Further, the Schottky junction 7 is formed at a position where the trench 22 is not formed. The planar pattern of the trench 22 and the P ⁻ anode layer 21 is a stripe shape or a dot shape.

W10 is the width of the trench, Td is the depth of the trench, XJ0 is the diffusion depth of the P ⁺ anode layer 5, W2 is the width of the Schottky junction, and W is the cell pitch. When the trench 22 has a stripe shape, W = W10 + W2 and the Schottky ratio Rsch0 = W2 / W. Td + XJ0 in FIG.
Is substituted for X _j described in FIG. 2, and Rsc described in FIG.
Assuming that h is Rsch0 and K is K0, the same effect can be expected by setting the range of K0 to the range of K described with reference to FIGS.

It should be noted that Td = 3 μm
Assuming that m, W10 = 0.5 μm, W2 = 4 μm, and X _JO = 1 μm, K0 = 1 can be achieved, and the leakage current of the element can be greatly reduced.

[0033]

As is apparent from the above description, the present invention provides a cathode layer of the first conductivity type provided between a cathode electrode and an anode electrode and in contact with the cathode electrode; A first conductivity type drift layer having a lower impurity concentration than the cathode layer, wherein the drift layer forms a Schottky junction with the anode electrode, and the anode electrode in the drift layer. A semiconductor device having a plurality of structural units formed in contact with and forming an ohmic junction with the anode electrode, the second conductive type anode layer having a higher concentration than the drift layer; The pitch W at which the unit is provided is determined by the junction depth Xj and the ratio R of the area of the Schottky junction to the area of the anode electrode.
Since the ratio is determined based on the ratio Xj / Rsch with respect to sch, a semiconductor device capable of improving reverse recovery characteristics and preventing an increase in reverse leakage current can be obtained.

Also, a trench is formed in the drift layer,
When an anode layer is formed at the bottom of the trench, the pitch W at which a plurality of structural units are provided is determined by a junction depth Xj0, a depth Td of the trench, and the anode in a region of the Schottky junction. If the ratio is determined based on the ratio (Xj0 + Td) / Rsch0 to the ratio Rsch0 of the electrode to the region, a semiconductor device can be obtained which has improved reverse recovery characteristics and can prevent an increase in reverse leakage current.

Specifically, W = K × Xj / Rsch, W =
If K0 × (Xj0 + Td) / Rsch0, K or K
When the value of 0 is set to 5 or less, the above-described effect is produced.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a semiconductor rectifier as a semiconductor device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram two-dimensionally showing a state when a depletion layer pinches off.

FIG. 3 is a plan view of the semiconductor device according to the embodiment of the present invention;

FIG. 4 is an explanatory diagram showing the dependence of the reverse leakage current on K in the embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an electric field intensity distribution in a cross section when a reverse bias is applied in the embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a cell pitch dependency of Irp with respect to a Schottky ratio in the embodiment of the present invention.

FIG. 7 is an explanatory diagram showing expansion of a space charge region during reverse recovery according to the embodiment of the present invention.

8 is a diagram showing a reverse leakage current when the value of K in FIG. 4 is reduced.

FIG. 9 is a cross-sectional view showing a semiconductor rectifier as a semiconductor device according to another embodiment of the present invention.

FIG. 10 is a diagram showing the specific resistance dependence of the N ⁻ drift layer of the reverse recovery peak current.

FIG. 11 is a diagram showing the dependency of the spike voltage (overvoltage) during the reverse recovery on the specific resistance of the n ⁻ drift layer.

FIG. 12 is a voltage / current waveform diagram at the time of reverse recovery.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 anode electrode, 2 P ⁺ anode layer, 3 N ⁻ drift layer, 4 N intermediate layer, 5 N ⁺ cathode layer, 6 cathode electrode, 7 Schottky junction, 8 termination, 21
P ^- anode layer, 22 trench groove, 23 polysilicon, 24 oxide film.

Claims (13)

[Claims] A first conductive type cathode layer provided between the cathode electrode and the anode electrode and in contact with the cathode electrode; a first conductive type cathode layer provided between the cathode layer and the anode electrode; An ohmic junction between the first conductive type drift layer having a lower impurity concentration than the cathode layer and forming a Schottky junction between the drift layer and the anode electrode; A semiconductor device having a plurality of structural units each including a second conductive type anode layer having a higher impurity concentration than the drift layer, wherein the Schottky junction portion is included in a pitch W in which the plurality of structural units are provided. Is smaller than K times the junction depth Xj of the anode layer (W2 <K · Xj), and
K is 5 or less, and the resistivity ρ of the drift layer is 6Ω · c.
A semiconductor device characterized by being not less than m and not more than 450 Ω · cm. 2. The semiconductor device according to claim 1, wherein K is 1.8 or less. 3. The semiconductor device according to claim 1, wherein a ratio of a region of the Schottky junction to a region of the anode electrode is defined by a length of the structural unit in a pitch direction. A semiconductor device characterized by the above-mentioned. 4. The semiconductor device according to claim 1, wherein a ratio Rsch of a region of the Schottky junction to a region of the anode electrode is larger than 40% and smaller than 90%. Characteristic semiconductor device. 5. The semiconductor device according to claim 1, wherein the ohmic junction or the Schottky connection in the plurality of structural units is arranged in a stripe shape or a dot cell shape. A semiconductor device. 6. A cathode layer of a first conductivity type provided between a cathode electrode and an anode electrode and in contact with the cathode electrode, and a cathode layer provided between the cathode layer and the anode electrode, A first conductivity type drift layer having a lower impurity concentration than the cathode layer, forming a Schottky junction therebetween; and a low ratio formed at the bottom of the trench provided in the drift layer and filling the trench. In a semiconductor device having a plurality of structural units each including an ohmic junction with a resistor polysilicon and being in contact with the anode electrode through the polysilicon, and a second conductive type anode layer, the plurality of structural units are provided. The pitch W to be formed is determined by the junction depth Xj0 of the anode layer and the depth Td of the trench,
A semiconductor device characterized in that it is determined based on a ratio Rsch0 of the region of the Schottky junction to the region of the anode electrode. 7. The semiconductor device according to claim 6, wherein said W2 is a sum of a junction depth Xj0 of said anode layer and a depth Td of said trench, and said anode electrode in a region of said Schottky junction. A semiconductor device wherein the ratio of the ratio to the region Rsch0 is smaller than K0 times (W2 <K0 × (Xj + Td)) and K0 is 5 or less. 8. The semiconductor device according to claim 7, wherein said K0 is 1.8 or less. 9. The semiconductor device according to claim 7, wherein the drift layer has a specific resistance ρ of 6 Ω · cm or more and 450 Ω or more.
-A semiconductor device characterized by being not more than cm. 10. The semiconductor device according to claim 7, wherein a ratio of the Schottky junction region to the anode electrode region is defined by a length of the structural unit in a pitch direction. A semiconductor device characterized by being performed. 11. The semiconductor device according to claim 7, wherein a ratio Rsch0 of the Schottky junction region to the anode electrode region is larger than 40% and smaller than 90%. Characteristic semiconductor device. 12. The semiconductor device according to claim 7, wherein the ohmic junction or the Schottky connection in the plurality of structural units is arranged in a stripe shape or a dot cell shape. A semiconductor device. 13. The semiconductor device according to claim 1, wherein the drift layer has a specific resistance of 15 Ω · cm.
As described above, the semiconductor device has a resistance of 350 Ω · cm or less.