Insulated Gate Bipolar Transistor Having High Breakdown Voltage

Field of the Invention

The present invention relates to a power semiconductor device and, more particularly, to an IGBT (Insulated Gate Bipolar Transistor) having a high breakdown voltage of 1,200 V or more.

Description of the Related Art

A conventional IGBT has a sectional structure as shown in FIG. 6, and its basic operation is described in Published Unexamined Japanese Patent Application No. 57-120369 in detail.

An N-channel IGBT formed in a silicon substrate will be described with reference to FIG. 6.

A semiconductor substrate 1 consists of a P.sup.+ -type anode layer 11 having a thickness of about 150 .mu.m and an impurity concentration of about 10.sup.20 /cm.sup.3, and an N.sup.- -type drain layer 12 formed on a first major surface of the anode layer 11. The N.sup.- -type drain layer 12 includes a pair of P-type base regions 13 in which an annular N.sup.+ -type source region 14 is provided. A polysilicon gate electrode 16 is provided on the substrate surface through a thin oxide film 15. A source electrode 17 is provided to short-circuit the source and base regions 14 and 13. A gate electrode 18 is provided on the polysilicon gate electrode 16, and an anode electrode 19 is formed on the lower surface of the P.sup.+ -type anode layer 11.

In the IGBT having this structure, when the gate electrode 16 is kept at a negative potential while grounding the source electrode 17 and applying a positive voltage to the anode electrode 19, the IGBT assumes an off-state. When a positive potential is applied to the gate electrode 16, an inversion channel layer is produced in the surface layer of the P-type base region 13, as in other conventional IGBTs. Therefore, electrons flow from the source region 14 into the surface layer of the drain layer 12 through the inversion channel layer to provide an electron accumulation layer. The electrons are further moved toward the anode electrode 19 through the drain layer 12 by the voltage applied between the source and the anode, thereby providing a forward-biased condition between the P.sup.+ -type anode layer 11 and the N.sup.- -type drain layer 12. Holes are then injected from the P.sup.+ -type anode layer 11 to the N.sup.- -type drain layer 12 to modulate the conductivity of the N.sup.- -type drain layer 12 and to turn on the IGBT. In this state, when the gate electrode 18 is returned to zero or negative potential, the channel is closed and the off-state is again obtained.

In the IGBT, however, a part of the minority carriers (holes) injected from the anode layer 11 to the drain layer 12 are undesirably accumulated in the drain layer 12 as excessive minority carriers. Accordingly, in order to turn off the IGBT, even if the gate voltage is set to zero to close the channel, thus stopping the flow of the electrons, the IGBT is not turned off until the accumulated minority carrier are removed from the drain layer 12. Furthermore, in this IGBT, when the electrons present in the drain layer 12 pass through the anode layer 11 during the turn-off operation, a new hole injection is induced from the anode layer 12 to increase the turn-off time. As a result, the current flowing through the IGBT is increased by about 10 times that of a conventional vertical MOSFET. However, the turn-off time is increased to about 10 times or more. In the case where such an IGBT is applied to switching devices, e.g., an inverter, its application is quite limited because a long turn-off time prevents an increase in the switching frequency.

For improving the turn-off time of the IGBT, it has been known to reduce the carrier life time. The carrier life time can be reduced by diffusing a heavy metal, e.g., Au, Pt, etc., into a desired semiconductor region, or by applying a radiation, e.g., a neutron beam, a gamma-ray, an electron beam, etc., thereto. In this case, however, although the turn-off time is improved, the degree of the conductivity modulation is degraded, so that the low on-resistance characteristic that is the greatest advantage of the IGBT is deteriorated. It may be also considered to decrease the impurity concentration of the P.sup.+ -type anode layer 11 in order to suppress the hole injection from the anode. When, however, the impurity concentration of the P.sup.+ -type anode layer 11 is decreased, the contact resistance with the metal electrode is increased to increase the on-resistance of the device. When the high breakdown voltage of 1,200 V or more is required for the IGBT, it is necessary that the N.sup.` -type drain layer has a very low impurity concentration of about 5.times.10.sup.13 /cm.sup.3 and a thickness of 100 .mu.m or more. However, it is difficult to stably provide such a drain layer according to present epitaxial growth techniques. Published Unexamined Japanese Patent Application No. 2-7569 discloses that a double diffusion type DMOS structure is provided in one surface of a lightly doped N.sup.- -type semiconductor substrate and that a P.sup.+ -type anode layer is formed in the other surface by ion implantation techniques. However, since the P.sup.+ -type anode layer has a very shallow junction depth of about 1 .mu.m, the structure can be easily influenced by the surface state, and stable characteristics cannot be obtained.

It is, therefore, an object of the present invention to provide an IGBT having low on-resistance and high-speed switching characteristics.

It is another object of the present invention to provide an IGBT capable of freely setting an impurity concentration and a thickness of a drain layer.

It is still another object of the present invention to provide an IGBT having a high breakdown voltage.

According to an aspect of the present invention, there is provided an IGBT, which comprises a semiconductor substrate including a drain layer of one conductivity type and a first anode layer of an opposite conductivity type having a low impurity concentration, a pair of base regions of an opposite conductivity type provided in the drain layer, the pair of base regions being spaced apart from each other, a source region of one conductivity type provided in each of the base regions, a gate electrode provided over a surface of the drain layer through a gate insulating film so as to overlap the source region, and a plurality of second anode regions of an opposite conductivity type having a high impurity concentration and provided in the first anode layer.

The novel and distinctive features of the invention are set forth in the claims appended to the present application. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description and accompanying drawings in which:

FIG. 1 is a sectional view showing an IGBT according to a first embodiment of the present invention;

FIG. 2 is a sectional view showing an IGBT according to a second embodiment of the present invention;

FIG. 3 is a schematic sectional view showing an IGBT according to a third embodiment of the present invention;

FIG. 4 is a graph showing a relationship between the on-voltage and the turn-off time of the IGBT according to the present invention;

FIG. 5 is a graph showing a relationship between the on-voltage and the turn-off time of another IGBT according to the present invention; and

FIG. 6 is a sectional view showing a conventional IGBT.

The preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 shows an N-channel IGBT according to the first embodiment of the present invention. A semiconductor substrate 1 includes an N.sup.- -type drain layer 12 and a P-type anode layer 11 made by epitaxial growth, A thickness of the semiconductor substrate 1 is 250 to 300 .mu.m. The P-type anode layer 11 has a thickness of 10 to 50 .mu.m and an impurity concentration of 1.times.10.sup.16 to 1.times.10.sup.17 /cm.sup.3. The N.sup.- -type drain layer 12 has an impurity concentration of 1.times.10.sup.13 to 1.times.10.sup.14 /cm.sup.3. A pair of P-type base regions 13 are formed in the N.sup.- -type drain layer 12 to be spaced apart from each other by a predetermined distance. An N.sup.+ -type source region 14 is formed in each P-type base region 13. A polysilicon gate electrode 16 is provided on the surface of the semiconductor substrate 1 through a thin oxide film 15. The gate electrode 16 is disposed so as to overlap the pair of source regions 14 and the base region 13. A source electrode 17 is provided to short-circuit the source and base regions 14 and 13. A plurality of heavily doped P.sup.+ -type anode regions 21 are formed in the P-type anode layer 11. The impurity concentration of each anode region 21 is generally selected in the range of 1.times.10.sup.18 to 1.times.10.sup.20 /cm.sup.3, and the appropriate thickness thereof is 2 to 5 .mu.m. A gate electrode 18 is provided on the polysilicon gate electrode 16, and an anode electrode 19 is provided so as to cover the P-type anode layer 11 and the P.sup.+ -type anode regions 21. The gate oxide film 15 is covered with an insulating film 22 of, e.g., SiO.sub.2.

According to the present invention, since the impurity concentration of the P-type anode layer 11 is made low and the heavily doped anode regions 21 are provided in the P-type anode layer 11, a good ohmic contact can be provided. In addition, even if the carrier life time of the N.sup.- -type drain layer 12 is kept long, the high-speed switching characteristic can be obtained because the injection amount of holes is controlled. Further, since the carrier life time of the N.sup.- -type drain layer 12 is long and the recombination centers are reduced, holes injected from the P-type anode layer 11 contribute effectively to the conductivity modulation to decrease the on-voltage, thus providing an IGBT having low on-voltage and high-speed switching characteristics.

The second embodiment of the present invention will be described with reference to FIG. 2.

FIG. 2 shows an IGBT having an N.sup.+ -type buffer layer 20 between drain and anode layers 12 and 11. The N.sup.+ -type buffer layer 20 and the P-type anode layer 11 are sequentially grown on the N.sup.- -type drain layer 12 by, e.g., epitaxial growth. Because of the presence of the buffer layer 20, the turn-off time of the IGBT is improved and the high-speed characteristic can be obtained. Even if the impurity concentration of the P-type anode layer 11 is increased more or less, the characteristics of the IGBT are not particularly affected.

The third embodiment of the present invention will be described with reference to FIG. 3.

FIG. 3 shows a sectional view of the main part of an IGBT. Heavily doped P.sup.+ -type anode regions 21 are formed in the P-type anode layer 11, in the same manner as the first and second embodiments. A damaged crystal lattice layer 25 having a thickness of about 1 .mu.m is formed on the substrate surface. The damaged crystal lattice layer 25 can be formed by honing the substrate surface with grindstone particles to obtain a rough surface. Because of the presence of the damaged crystal lattice layer 25, an anode electrode 19 is in contact with the anode layer 11 and the heavily doped anode regions 21 to decrease the contact resistance, thereby stabilizing the IGBT characteristics.

As a material of the anode electrode 19, Ti, Al, Au, Pt, W, Mo, or the like can be used regardless of the presence of the damaged crystal lattice layer 25. When Al is used as the electrode material, the electrode is stabilized by a heat-treatment.

The effect of the impurity concentration of the lightly doped anode layer of the present invention will be described with reference to FIG. 4.

FIG. 4 is a graph showing the relationship between the on-voltage and the turn-off time when the impurity concentration of the lightly doped anode layer 11 is changed. The impurity concentration of the lightly doped anode layer 11 is changed in the range of 5.times.10.sup.15 to 5.times.10.sup.18 /cm.sup.3. When the impurity concentration of the anode layer 11 is decreased, the on-voltage is increased although the turn-off time is reduced. On the contrary, when the impurity concentration is increased, the turn-off time is increased although the on-voltage is decreased. An available frequency of an ordinary inverter is in the range of several kHz to several tens kHz. Therefore, it is necessary that the turn-off time is 1 .mu.s or less in order to suppress heat generation of the IGBT. Similarly, the on-voltage is preferably 4 V or less. Accordingly, the appropriate impurity concentration of the lightly doped anode layer 11 is in the range of 1.times.10.sup.16 to 1.times.10.sup.17 /cm.sup.3. When the impurity concentration is changed within the range, the on-voltage and the turn-off time can be controlled.

The effect of the impurity concentration of the heavily doped anode layer of the present invention will be described with reference to FIG. 5.

FIG. 5 is a graph showing the relationship between the on-voltage and the turn-off time when the area ratio of the heavily doped anode regions 21 to the lightly doped anode layer 11 is changed. In FIG. 5, a point A represents a characteristic obtained when no heavily doped anode regions 21 are provided; B a characteristic obtained when the area ratio of the regions 21 to the layer 11 is 1/1; and C, a characteristic obtained when no lightly doped anode region layer 11 is provided, respectively. At this time, the impurity concentration of the lightly doped anode layer 11 is defined by 5.times.10.sup.16 /cm.sup.3. when the area of the heavily doped anode regions 21 is small, the on-voltage becomes high, and the variation in characteristics becomes large. On the contrary, when the heavily doped anode regions 21 are formed on the entire surface of the P-type anode layer 11, the turn-off time is increased to about 0.4 .mu.s which cannot respond to a high frequency.

The above embodiments have described the N-channel IGBT. However, the present invention can be applied to a P-channel IGBT. In epitaxial growth, the wafer cost is increased in proportion to the thickness of the grown layer. However, according to the present invention, since it is not necessary to increase the thickness of the epitaxially grown layer considerably, the IGBT having the high breakdown voltage of 1,200 V or more required for the thickness of about 100 .mu.m can be realized in less than half the cost of the conventional IGBT.

According to the present invention, since the impurity concentration of the anode layer is made low and the heavily doped regions are provided in the anode layer, a good ohmic contact can be obtained. In addition, even if the carrier life time of the drain layer is kept long, the high-speed switching characteristic can be obtained because the injection amount of holes is controlled. Further, since the carrier life time of the drain layer is long and the recombination centers are reduced, the carrier injected from the anode layer contribute effectively to the conductivity modulation to decrease the on-voltage, thus providing an IGBT having low on-voltage and high-speed switching characteristics. More further, although an IGBT having a high breakdown voltage requires a thick drain layer having a low impurity concentration which is not manufactured by the present epitaxial growth techniques, the impurity concentration and thickness of the wafer can be freely set since the drain layer is formed of the wafer sliced from a monocrystalline silicon ingot.

It is further understood by those skilled in the art that the foregoing description is only of the preferred embodiments and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.