JP2004140302A - SiC TRANSISTOR AND POWER CONVERTER - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an SiC transistor having a high manufacturing yield and is low in cost, and to provide a power converter. <P>SOLUTION: The SiC transistor has a first conductivity type SiC substrate 2, using as its raw material a low-quality SiC wafer containing unavoidable micropipe defects 3, a second conductivity-type base layer 4 formed in a laminated way on the SiC substrate, a first conductivity-type emitter layer 5 formed in a laminated way on the base layer, each base electrode 6 jointed in an ohmic manner to each missed portion generated, by missing a portion of the emitter layer or to each inverted-conductive portion, generated by inverting the conductive portion of the emitter layer, with each emitter electrode 7 jointed in the ohmic way to the emitter layer, and a collector electrode 8 jointed in the ohmic manner to the rear-surface side of the SiC substrate and requisite for the SiC substrate functioning as a collector. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a low-voltage SiC transistor using SiC as a substrate, and a power conversion device using the same, such as a DC-DC power supply, an inverter, a converter, and a charger.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, semiconductor devices such as bipolar transistors, IGBTs (Insulated Gate Bipolar Transistors), and FETs (Field Effect Transistors) have been used in power conversion devices such as inverters, DC-DC power supplies, and converters. Since these semiconductor elements are mainly composed of silicon (Si), the operating limit temperature inside the element is as low as 150 ° C. or less, and when applied to a power conversion device by switching operation or the like, the temperature rise inside the element due to loss inside the element is reduced. In order to reduce the power consumption, it is necessary to mount a large-sized cooler outside the element or to use a large-sized element having a larger heat radiation area.
[0003]
Incidentally, silicon carbide (SiC) is not only stable at high temperatures and chemically stable, but also has a large forbidden band width, an electron mobility comparable to that of silicon (Si), and a dielectric breakdown electric field strength. Since it is one order of magnitude larger than Si and has a saturation drift speed approximately twice that of Si, it is a material that has been attracting attention as a semiconductor device material. As described above, since SiC has many excellent characteristics as a semiconductor device material, low-loss, high-power SiC that operates stably in a severe environment (high temperature, high radiation irradiation) than Si devices has excellent durability. Consumers are expected to develop devices.
[0004]
The use limit temperature inside the element of a SiC transistor is theoretically as high as about 1500 ° C., and various trial manufactures have been made so far. For example, Patent Document 1 discloses a SiC transistor that can be used in a high temperature range up to about 1000 ° C.
[0005]
[Patent Document 1]
JP-T-2000-507394 (page 2, claim 1, FIG. 4 (A), FIG. 4 (B))
[0006]
[Problems to be solved by the invention]
However, in a conventional SiC transistor, a structural defect called a micropipe generated inside the SiC wafer during a wafer manufacturing process has a significant effect on performance, that is, in the presence of a micropipe defect, a leakage between a base and a collector occurs. There is a fatal problem that electric current flows and leads to dielectric breakdown. At present, the cause of such micropipe defects has not been identified, and no fundamental countermeasures have been established.
[0007]
For this reason, it is necessary to detect a portion where a micropipe defect exists in the wafer inspection, and to cut out the transistor substrate from the SiC wafer so as to avoid the detected portion. However, in this case, only a small amount is used in the product and a large part is discarded in order to manufacture a transistor having a predetermined chip area, so that the yield of materials is very low, and the final product becomes extremely expensive. By the way, in the case of a 2-inch diameter SiC wafer, the generation density of micropipe defects is said to be quite high at several tens or more / cm ² (data at the time of wafer production).
[0008]
The present invention has been made to solve the above problems, and has as its object to provide a high-yield, low-cost SiC transistor and a power conversion device.
[0009]
[Means for Solving the Problems]
Leakage current due to micropipe defects is a fatal problem in transistors for high voltage applications, which is being developed as a main target for SiC application. At present, the cause of micropipe defects cannot be identified, No fundamental countermeasures have been found. However, the present inventor has conducted extensive studies and found that a leak current generated by a micropipe defect hardly affects a normal operation of a transistor in a low voltage region of 300 V or less. The present invention has been made based on such knowledge, and the details thereof will be described below.
[0010]
The SiC transistor according to the present invention includes a first conductivity type SiC substrate made of a low-grade SiC wafer unavoidably containing micropipe defects, and a second conductivity type base laminated on the SiC substrate. Layer, an emitter layer of the first conductivity type laminated and formed on the base layer, or a part of the emitter layer is missing, or the conductivity type of a part of the emitter layer is inverted, and the missing or A base electrode having an ohmic junction with the inverted portion, an emitter electrode having an ohmic junction with the emitter layer, and a collector electrode having an ohmic junction with the back side of the SiC substrate and having the SiC substrate function as a collector. It is characterized by doing.
[0011]
Since the SiC substrate is stable at high temperatures and chemically stable, it is stable even in harsh environments such as when exposed to high temperatures exceeding 150 ° C or when a large amount of radiation is irradiated. Can work. For this reason, a transistor using an SiC substrate has excellent durability, and is suitable as, for example, an element of a low-voltage power supply circuit of a hybrid electric vehicle (HEV) or the like.
[0012]
Further, the SiC substrate has a large forbidden band width, an electron mobility comparable to that of silicon (Si), a breakdown electric field strength one order of magnitude higher than that of Si, and a saturation drift speed about twice that of Si. Have lower loss and higher power than the Si transistor.
[0013]
The SiC wafer is manufactured using a sublimation recrystallization method, a chemical vapor deposition method (CVD), a line-of-sight evitaxy method (MBE), a photoexcited epitaxial growth method, or the like.
[0014]
As the SiC substrate, a low-grade SiC wafer inevitably containing micropipe defects, for example, a wafer cut from a SiC wafer having a micropipe defect density per unit area of 10 / cm ² or more (at the time of manufacture) is used. be able to.
[0015]
By incorporating a SiC transistor having such a SiC substrate having a micropipe defect as a collector into a low-voltage power supply circuit as a conversion element, a power conversion device that is less expensive than conventional products can be provided. When a SiC transistor is incorporated in a low-voltage power supply circuit as a DC-DC voltage conversion element, a DC / DC converter is provided. When a SiC transistor is incorporated in a low-voltage power supply circuit as an AC-DC voltage conversion element, an inverter is provided.
[0016]
When a SiC substrate having a micropipe defect is applied to a 50 W DC power supply having a rated power of 100 V to convert a rated AC input voltage of 100 V to a DC output voltage of 5 V, the loss due to existing leakage current (leakage current loss) has no effect on the conversion efficiency. 0.1%, which is slight.
[0017]
Condition: Input AC100V → DC5V, DC power efficiency drop of 50W <0.1%
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, various preferred embodiments of the present invention will be described with reference to the accompanying drawings.
[0019]
(Example 1)
First, a power conversion device according to a first embodiment will be described with reference to FIG.
[0020]
The power converter 20 according to the first embodiment is a step-down synchronous rectification DC / DC converter that converts an input voltage Vin into an output voltage Vout. The voltage conversion circuit of the DC / DC converter 20 includes a first SiC transistor Q1 (13), a first diode D1, and an inductor L. Among them, the first SiC transistor Q1 and the first diode D1 are connected in parallel on the input side. The first diode D1 functions as a protection element that prevents the first SiC transistor Q1 from being damaged when the output voltage becomes excessive. The inductor L is connected in series on the output side with respect to the first SiC transistor Q1 / first diode D1. Note that a commercially available general-purpose product was used for the diodes D1 and D2 in this example.
[0021]
A smoothing capacitor C is inserted between the inductor L and the output terminal. This smoothing capacitor C has a function of smoothing the DC output voltage of the voltage conversion circuit.
[0022]
A control circuit is inserted between the first SiC transistor Q1 (first diode D1) and the inductor L. This control circuit includes a second SiC transistor Q2 (13) and a second diode D2 connected in parallel, and has a function of turning on or off the smoothing operation of the smoothing capacitor C.
[0023]
The first and second SiC transistors 13 are bipolar transistors having the structure shown in FIG. 2, and a pSi base layer 4 and an nSi emitter layer 5 are sequentially stacked on an n ⁺⁺ SiC substrate 2.
[0024]
The SiC substrate 2 is cut from a 4H-type silicon carbide wafer having a four-layer structure, and the density of the micropipe defects 3 is about 30 / cm ² (at the time of manufacture). The number and size of the micropipe defects 3 vary depending on the wafer manufacturing method. Extra-large micropipe defects 3 penetrating the wafer thickness from one side to the other side rarely occur. The present invention allows the existence of micropipe defects 3 in order to use a low-quality SiC substrate for a low-voltage power supply circuit. It is desirable to use as far as possible.
[0025]
The base electrode 6 of the SiC transistor 13 is grounded, the emitter electrode 7 is connected to the output terminal side, and the collector electrode 8 is connected to the input terminal side. The base electrode 6 is formed at a portion where the base layer 6 is exposed by pattern-etching the emitter layer 5.
[0026]
In this embodiment, the input voltage Vin is set to 300 volts or less.
[0027]
Next, an outline of the operation of the DC / DC converter 20 of the present embodiment will be described.
[0028]
The input voltage Vin is applied to a series circuit of an inductor L and a smoothing capacitor C via a first transistor Q1 as a switching element. When the first transistor Q1 is turned off and the second transistor Q2 is turned on, the energy stored in the inductor L further charges the smoothing capacitor C. The smoothing capacitor C smoothes the DC voltage and outputs it as an output voltage Vout. As described above, the on / off operation of the first and second transistors Q1 and Q2 is performed in synchronization with each other, so that the output voltage Vout obtained by stepping down the input voltage Vin is output as the output voltage.
[0029]
On the other hand, when the output voltage Vout is going to be higher than the constant voltage value, when the first transistor Q1 is turned off and the second transistor Q2 is turned on, the current flows through the smoothing capacitor C, the inductor L, the second transistor Q2, and the smoothing capacitor C2. , And discharges the smoothing capacitor C. When the first transistor Q1 is turned on and the second transistor Q2 is turned off, the energy stored in the inductor L is fed back to the input power via the first transistor Q1. Further, when the first and second transistors Q1 and Q2 are switched, a current flows through the diode D1 or D2 while both transistors Q1 and Q2 are off.
[0030]
As described above, the step-down synchronous rectification DC / DC converter 20 as the power converter of the first embodiment operates. However, since the input voltage Vin is a low-voltage power supply circuit of 300 V or less, the first and second SiC transistors are used. Q1 and Q2 perform a switching operation without any trouble.
[0031]
As described above, in the present embodiment, a low-quality SiC substrate that has been conventionally discarded can be effectively used, and the manufacturing cost is greatly reduced as compared with the conventional product. Such a low-grade SiC transistor is suitably used in a low-voltage power supply circuit such as a hybrid electric vehicle (HEV).
[0032]
(Example 2)
Next, a power converter according to a second embodiment will be described with reference to FIG.
[0033]
The power converter 30 according to the second embodiment is a self-excited voltage-type three-phase inverter that converts a DC input voltage into an AC output voltage. The three-phase inverter 30 includes six SiC transistors Q1 to Q6 and six diodes D1 to D6. The corresponding diodes D1 to D6 are connected in parallel to the respective SiC transistors Q1 to Q6. The diodes D1 to D6 are protection elements for protecting the SiC transistors Q1 to Q6 and for preventing backflow so that current does not flow from a normal power supply to a failed power supply.
[0034]
Two of these six sets of SiC transistors Q1 to Q6 and diodes D1 to D6 are combined to form three series circuits. Both sides of the three series circuits are connected in parallel to input terminals, respectively. The input terminal is connected to a DC power supply (not shown) via a large-capacity capacitor (not shown).
[0035]
In this embodiment, the input voltage Vin is set to 300 volts or less.
[0036]
On the other hand, midpoints U, V, and W of the three series circuits are connected to output terminals on the motor side, respectively, whereby a three-phase half-bridge circuit is formed between the three terminals and the motor serving as a load.
[0037]
Further, an on / off control circuit (not shown) is connected to the SiC transistors Q1 to Q6 and the diodes D1 to D6, respectively. This on / off control circuit individually controls on / off operation of each of the SiC transistors Q1 to Q6 and the diodes D1 to D6.
[0038]
The substrate 2 used for the SiC transistors Q1 to Q6 of the present embodiment is made of a 6H silicon carbide wafer having a six-layer structure, and the density of the micropipe defects 3 is about 40 / cm ² (at the time of manufacturing). It is.
[0039]
In such an inverter 30, by turning on / off the six sets of SiC transistors Q1 to Q6 / diodes D1 to D6, the half bridge of each phase is turned on by 180 ° and switched, and six operations are performed in one cycle. Mode. When the reference point of the potential is set to the midpoint 0 _{DC of the} DC power supply, the potentials v _U , v _V , and v _W of the output terminals become Vd when the plus side transistors (Q1, Q3, Q5) are on, and become minus. It becomes zero when the transistor (Q2, Q4, Q6) on the side is on. Incidentally, the phase voltage is a square wave of 180 ° width having an amplitude of ± Ed / 2 with respect to the midpoint 0 _{DC of the} DC power supply. The line voltage is a square wave having an amplitude of ± Ed and a width of 120 °, and the load voltage is a step-like six-step waveform.
[0040]
【The invention's effect】
As described above, there is no obstacle in practical use, and a cooler can be eliminated or its use in a high-temperature environment that cannot be used conventionally can be realized by its high-temperature limit use performance. These can be realized using a low-quality, low-cost defective SiC wafer.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a power converter (DC / DC converter) according to a first embodiment of the present invention.
FIG. 2 is a schematic sectional view showing a bipolar transistor used in the power converter of the present invention.
FIG. 3 is a circuit diagram showing a power converter (inverter) according to a second embodiment of the present invention.
[Explanation of symbols]
2 ... SiC substrate (collector)
3: Micropipe defects,
4: Base layer,
5 ... emitter layer,
6 ... Base electrode,
7 ... Emitter electrode,
8 ... collector electrode,
10, 20, 30 ... power conversion device,
13, Q1 to Q6 ... SiC transistors,
D1 to D6: diodes.

Claims (5)

A SiC transistor used for a low-voltage power supply circuit,
A first-conductivity-type SiC substrate made of a low-grade SiC wafer inevitably containing micropipe defects,
A second conductivity type base layer laminated on the SiC substrate;
A first conductivity type emitter layer laminated on the base layer,
A part of the emitter layer is missing or the conductivity type of a part of the emitter layer is inverted, and a base electrode that is ohmic-joined to the missing or inverted part,
An emitter electrode ohmic-joined to the emitter layer;
A collector electrode that is ohmic-bonded to the back surface of the SiC substrate and causes the SiC substrate to function as a collector;
A SiC transistor comprising: The SiC transistor according to claim 1, wherein the SiC substrate has a micropipe defect density per unit area of 10 / cm ² or more. A power converter comprising: a low-voltage power supply circuit in which a SiC transistor having a SiC substrate having a micropipe defect as a collector is incorporated as a conversion element. The power converter according to claim 3, wherein the SiC transistor is incorporated in the low-voltage power supply circuit as a DC-DC voltage conversion element that converts a voltage between DC and DC. The power conversion device according to claim 3, wherein the SiC transistor is incorporated in the low-voltage power supply circuit as an AC-DC voltage conversion element that converts a voltage between AC and DC.

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