JPH027473A - Conductivity modulation type mosfet - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) The present invention relates to a horizontal conductivity modulation type MOSFET in which a drain, a source, and a gate are formed on one side of a semiconductor wafer.
, regarding T.

(Conventional technology) Figure 3 shows a conventional horizontal conductive modulation type The main structure of MOSFET is shown. semiconductor wafers.

A p-type silicon substrate 1 on which a p-type layer 2 is epitaxially grown is used. A p-type base layer 7 is formed as a first base layer on the surface of this semiconductor wafer, and an n+-type source layer 9 is selectively formed on the surface portion. Also, adjacent to the p-type base layer 7, an n-
A type high resistance base layer (drift layer) 4 and an n type low resistance base layer (base layer) 3 are formed. n-type layer (,,
A p-type drain layer is selectively formed on the surface of the phase layer 3. A gate electrode 6 is formed on the surface of the semiconductor wafer in a region sandwiched between the n-type source 9 and the n--type drift layer 4 with a gate insulating film 5 interposed therebetween.

The source electrode 10 is arranged so as to contact the source layer 9 and the p-type base layer 7 at the same time, and the drain electrode 11 is arranged so as to contact the source layer 9 and the p-type base layer 7 at the same time.
It is in contact with the medium-sized drain layer 8.

In this conductivity modulation type MO3FET, when a positive bias is applied to the gate electrode 6 with respect to the source electrode 10, the surfaces of the p-type base layer 7 and the p'''' type layer 2 under the gate electrode 6 are reversed, and the source layer 7 Electrons are injected into the n-type drift layer 4 from there. When this electron current enters the drain layer 8 via the n-type buffer layer 3, the pn junction is forward biased.

Holes from the drain layer 8 pass through the n-type buffer layer 3 to become n-
) is injected into the M drift layer 4. In this way, electrons and holes are accumulated in the n-type drift layer 4, causing conductivity modulation. Due to the effect of this conductivity modulation, even if the n-type drift layer 4 is made to have a high resistance to achieve a high breakdown voltage, the resistance can be made substantially sufficiently small when it is on, and a low on-voltage can be obtained. By biasing the gate electrode 6 negative or zero with respect to the source electrode 10, the inversion layer of the channel disappears and is turned off.

In such a lateral conductivity modulation type MOSFET, in order to increase the switching speed at turn-off, n
It is necessary to quickly sweep out the carriers accumulated in the - type drift layer 4. If the electrons in the n-type drift layer 4 do not quickly escape to the drain layer 8 side, the p-medium type drain layer 8
-N-type layer sofa layer 3 and n'''' type drift layer 4-
The pnp transistor consisting of the p-type base layer 7 operates, and a large tail current flows. One way to speed up the turn-off operation is to shorten the carrier lifetime in the n-type drift layer 4. But this method.

Although this improves the turn-off characteristics, it has the drawback of increasing the on-voltage of the device.

In order to solve this problem, it has been proposed to provide a so-called anode short structure by not providing a p-type layer in the center of the p-medium drain layer 8, but instead providing an n-type layer here.
For example, IEDM85°pp74G-743). However, in the case of an anode short structure, the electron current supplied from the source side is short-circuited to the drain electrode at turn-on, so in order to put the pn junction of the drain in a forward bias state and cause conduction modulation, it is necessary to It is necessary to increase the lateral resistance of the n-type layer below the p-type drain layer. To increase this lateral resistance, ■
Increase the width of the p-type drain layer up to the short circuit. ■n
-Reducing the impurity concentration of the mold base layer. (2) A possible solution is to reduce the thickness of the base layer under the p+ type doridrain layer. However, method (2) causes an increase in the device area. Furthermore, methods (1) and (2) have limitations in ensuring that the element has sufficient breakdown voltage.

Further, when a conductivity modulation type MOSFET is used in an inverter circuit for driving a motor, etc., diodes are generally connected in a reverse row as shown in FIG. This is to regenerate the energy stored in the inductance component of the motor.

However, having to connect diodes in this way causes the device to become larger and cost to increase.

(Problems to be Solved by the Invention) As described above, the conventional horizontal conduction modulation type MO3FET has a trade-off relationship between turn-off characteristics and on-characteristics, and especially when applied to motor drive inverter circuits, etc. There is a problem in that it is difficult to obtain a compact and high-voltage device because it requires the addition of a diode.

The present invention is a horizontal conductive modulation type M that solves these problems.
The purpose is to provide O5FET.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a horizontal conductivity modulation type MO3FET in which a plurality of openings are provided in the drain layer, and the drain electrode is brought into contact with the second base layer through the openings. It is characterized by:

(Function) With such a structure, an anode short structure is obtained, and carriers accumulated in the second base layer can be directly discharged to the drain electrode at turn-off. Therefore, high-speed turn-off is possible without reducing the lifetime of carriers in the base layer, in other words, without increasing the on-voltage. Furthermore, since the drain electrode is in direct contact with the second base layer in the opening provided in the drain layer, an anti-parallel diode is equivalently inserted between the drain and source, making it suitable for applications such as motor drive inverters. In this case, there is no need to prepare a separate diode. In particular, in the present invention, since the short-circuit portion is formed in a plurality of dispersed portions, the electron current flowing under the p-type drain layer to the drain electrode of the short-circuit portion at turn-on becomes high-density.

This improvement in current density makes it possible to obtain an effect equivalent to increasing the lateral resistance under the p-type drain layer, that is, to easily obtain the lateral potential difference necessary to forward bias the pn junction on the drain side. Therefore, a device with high breakdown voltage, small size, and low on-state voltage can be obtained.

(Example) Two aspects of the present invention will be described in detail below.

FIGS. 1(a) and 1(b) show an example of conductivity modulation type MO3F.
3 is a plan view showing the main structure of the ET and a sectional view taken along line A-A" thereof. Portions corresponding to those in FIG. 3, which is a conventional example, are given the same reference numerals as in FIG. 3, and detailed explanation thereof will be omitted. In this embodiment, a plurality of openings 1 are formed in the drain layer 8 below the drain electrode 11.
2, in which the n-type buffer layer 3 is exposed on the surface of the semiconductor wafer and is in direct contact with the drain electrode 11. In the opening 12, an n-type layer 13 having a high impurity concentration is diffused and formed in order to establish a low resistance ohmic contact between the drain electrode 11 and the n-type buffer layer 3.

The basic operation of this conductivity modulation type MOSFET is the same as the conventional one. Turn-on applies a positive bias to the gate electrode 6 with respect to the source electrode 10.

This is performed by injecting electrons from the source layer 10 into the n-type buffer layer 3 via the n-type drift layer. At this time, the electron current causes a lateral potential difference in the n-type buffer layer 3, and when this reaches a value that forward biases the p-n junction, holes are injected from the p-type drain layer 8 into the n-type buffer layer 3. In this device, since the short portions are distributed in a plurality of pieces, the electron current flowing laterally through the n-type buffer layer 3 has a high density near each short portion.

This effectively results in a large lateral potential difference.

As a result, conductivity modulation occurs in the n-type drift layer 4, resulting in a low on-voltage. The turn-off operation is performed by applying a negative bias or zero bias to the gate electrode 6. As a result, the channel under the gate electrode 6 disappears, electron injection from the source layer 9 disappears, and the device eventually turns off. During this turn-off, the n-type drift layer 4
Among the accumulated carriers in the p'' type layer 2 and p
It is discharged through the mold base layer 7 to the source electrode 10.

Further, electrons are discharged to the drain electrode 11 through the opening 12 provided in the p-type drain layer 8.

Thus, according to this embodiment, by employing the anode short structure, accumulated electrons are quickly swept away, and a high-speed turn-off operation is performed. In addition, by distributing the short-circuit portions into a plurality of pieces, the on-characteristics can be improved without increasing the size of the device and without impairing the high breakdown voltage characteristics. Since high-speed turn-off is possible without shortening carrier life, low on-voltage characteristics can be maintained. Furthermore, since the n-type buffer layer 3 is connected to the drain electrode 11 through the opening 12, the n-type buffer layer 3 is connected to the drain electrode 11 between the source electrode 10 and the drain electrode 11.

p-type base layer 7 and p-type layer 2-Q-type drift layer 4
．． A pn junction diode is constituted by an n-type buffer layer 3 and an n-type upper layer 13. This diode is equivalently placed anti-parallel to the device as shown in FIG. Therefore, according to this embodiment, even when this element is applied to an inverter circuit for driving a motor, there is no need to connect a diode externally. Furthermore, in the structure of this embodiment, n of the short circuit part
The + type layer 13 and the p + type drain layer 8 partially overlap, and if we pay attention to this part, we can see that the 9 layers 13-1 layer 8-
This becomes a pnpn thyristor of n layer 3.4 and p layer 7, and this is inserted between the source and drain. So if this latches up.

A more reliable short circuit can be achieved than in the case of a simple reverse conduction diode.

FIGS. 2(a), 2(b), and 2(c) show the structure of a main part of a conductivity modulation type MO9FET according to another embodiment of the present invention in correspondence with the previous embodiment. FIG. 2(C) shows the B-B- in FIG. 2(a).
It is a cross section. The difference from the embodiment shown in FIG.
This means that it is continuously formed over two parts. Except for this point, this embodiment is the same as the embodiment shown in FIG.

This embodiment also provides the same effects as the previous embodiment. Further, in the case of this embodiment, there is no opening 12 and n
The source electrode 1 is connected to the portion where the ten-shaped layer 13 is provided.
As is clear from comparing the structure between 0 and the drain electrode 11 with FIG. It has a structure in which a pnpn thyristor comprising a p-type base layer 7 and a p-type base layer 7 has a large area. Like the diode in the previous embodiment, this silice is connected in parallel to the element, so it exhibits an excellent reverse conduction function.

The present invention is not limited to the above embodiments.

For example, the conductivity type of each part of the element can be reversed from that in the embodiment. Furthermore, it is effective to perform lifetime control using electron beam irradiation to increase speed.

[Effects of the Invention] As described above, according to the present invention, the horizontal conductive modulation type M
In the O5FET, by providing a plurality of openings in the drain layer and bringing the drain electrode into contact with the second base layer exposed at the surface, the turn-off operation can be sped up without increasing the on-voltage. Further, due to its structure, a reverse conduction diode or a reverse conduction thyristor is automatically incorporated inside, thereby making it possible to reduce the size and cost of the ishiverter for driving a motor.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) show a conductive modulation type M according to an embodiment of the present invention.
A plan view showing the main part structure of O3FET and its AA cross-sectional view, FIG. Its A-A
- and BB sectional views, Figure 3 is a sectional view showing the main structure of a conventional horizontal conduction modulation type MOSFET, and Figure 4 shows reverse conduction when a conduction modulation type MO8FET is applied to an inverter circuit for driving a motor. FIG. 3 is an equivalent circuit diagram showing how diode connections are made. 1-p-type silicon substrate, 2...p-type layer. 3... N-type buffer layer (low resistance second base layer). 4...n'''' type drift layer (high resistance second base layer). 5... Gate insulating film, 6... Gate electrode, 7...
P type base layer (first base layer), 8...p+ type drain layer, 9...n+ type source layer, 10... source electrode. 11...Drain electrode. 12...Opening, 13...N-type layer. Applicant's agent Patent attorney Takehiko Suzue Figure 2 Figure 3 Figure J$4

Claims (3)

[Claims] (1) a semiconductor wafer; a first base layer of a first conductivity type selectively formed on the surface of the semiconductor wafer;
a second conductive type source layer selectively formed on the surface of the base layer; and a second conductive type second base layer selectively formed on the surface of the semiconductor wafer adjacent to the first base layer. , a drain layer of a first conductivity type selectively formed on the surface of the second base layer, and a gate electrode formed in a region sandwiched between the source layer and the second base layer with a gate insulating film interposed therebetween; A conductivity modulation type MOSFET having a source electrode that contacts the source layer and the first base layer at the same time, and a drain electrode that contacts the drain layer,
The conductivity modulation type MOSFET, wherein the drain layer has a plurality of openings through which the second base layer is exposed to the surface, and the drain electrode contacts the second base layer through the openings. . (2) The conductivity modulation type M according to claim 1, wherein a second conductivity type high impurity concentration layer is formed in each of the plurality of openings.
OSFET. (3) The conductivity modulation type MOSFET according to claim 1, wherein a second conductivity type high impurity concentration layer is continuously formed across the plurality of openings.