Conductivity-Modulating MOSFET

The present invention relates to a conductivity modulating MOSFET, wherein base current of bipolar junction transistors (BJT) is supplied by channel current of the MOSFET.

The conductivity-modulating MOSFET of the invention is also called an insulated gate-bipolar transistor (hereinafter abbreviated as IGBT). In the present power electronics field, the most attracted power devices are power MOSFET and the IGBT. The reason for this is that both provide high-speed switching performance and low-drive power consumption, which result in compactness and improved performance required by power electronics products. However, in the unlimited expansion of use and improved performance of power electronics products, the use of a larger current, higher voltage, and switching performance at a higher speed are required. In the BJT and IGBT module products used in inverter circuits for motor control, the module products with a withstand voltage of 1200 V and a current capacity of 400 to 800A have already been made available. The IGBT is by far superior to the BJT in terms of its high-speed switching performance. However, because of the high switching speed, in an inductance load generally used for switching a large power, IGBT generates an excessive spike voltage due to its large di/dt and when the voltage exceeds the breakdown voltage of a device, the device may be destroyed. Even if the voltage does not exceed the breakdown voltage, the electric field strength increased by the simultaneous application of high voltage and large current may exceed the breakdown field and generate avalanche multiplication, which results in destruction of a device. Particularly, under abnormal conditions, such as a load by short circuit, the power device is forcibly turned off in about 10 micro sec after an abnormal condition has been detected. At this time a very large short circuit current as well as a high withstand voltage are applied as shown in FIG. 2, and a further higher spike voltage is applied as shown by B because of di/dt by the turn-off of the device shown by A, which results in a breakdown or destruction due to the aforementioned mechanism.

In order to protect the device in the abnormal condition, it is very difficult to absorb energy in excess of the breakdown voltage in an IGBT of a high withstand voltage because of uneven electric field. Therefore, it is difficult to manufacture an IGBT with a high withstand voltage and a high avalanche withstand capacity with high yield and reliability.

Accordingly, it was proposed that as shown in FIG. 3, a high withstand voltage diode (32) with high avalanche withstand capacity of a withstand voltage BV.sub.R, which is slightly lower than the breakdown voltage BV.sub.CES of an IGBT (31), and a low withstand voltage diode (33) with a withstand voltage of about 20 to 30 V, which is slightly higher than the voltage between the gate and the emitter in the IGBT (31), are externally attached between the gate and the collector. In case overvoltage is generated in an abnormal condition, it exceeds the breakdown voltage BV.sub.R of the diode (32), which is lower than the overvoltage BV.sub.CES of the IGBT (31). Therefore, the current flows to a gate electrode (34) of the IGBT to charge the gate-to-emitter capacity of the IGBT (31). When its threshold voltage is exceeded, the IGBT (31) turns on to absorb the overvoltage energy evenly into the chip. As a result, it can withstand the larger energy.

However, the method of connecting the diodes (32) and (33) by means of an external installation as shown in FIG. 3 causes the following drawbacks.

(a) Diodes with a withstand voltage slightly lower than that of the IGBT (31) must be selected individually, which is not practical.

(b) In case of an external installation, an inductance content is added thereto because of long wiring.

As a practical method for obviating the deficiency of (a), diodes which have withstand voltages considerably lower than the minimum withstand voltage of the IGBT are selected and connected. However, considering the wide range of the withstand voltage in the diodes, a withstand voltage which is significantly lower than that of the IGBT has to be admitted. This requires that the withstand voltage of the IGBT be shifted preliminarily to the high side. However, this choice aggravates the relation between the on-voltage of IGBT and the switching time.

For instance, assuming that the IGBT has a withstand voltage of 1,250 V, diodes having a withstand voltage within a range of 1,050 V to 1,200 V are used while considering the variance of the withstand voltage. In this case, unless the voltage applied to switching at every cycle is lower than a maximum of 1,050 V, there is a good chance of a breakdown as a result of increased switching loss. This in turn means that even the IGBT with a withstand voltage of 1,250 V can only guarantee a spike voltage of 1,050 V or less, which is 200 V lower than the withstand voltage.

Accordingly, an object of the present invention is to obviate the above problems and to provide an IGBT which allows the withstand voltage of each and every transistor to be set at an approximately definite value of 50 V or 100 V less than the withstand voltage of protection diodes without selection.

In order to achieve the above objects, the present invention uses an IGBT, in which a layer formed of second conductivity type is disposed on one side of a collector layer formed of first conductivity type, the layer of the second conductivity type including a high resistivity layer at least on a side away from the collector layer, and a base region formed of the first conductivity type is formed selectively in a surface layer of the high resistivity layer of the second conductivity type. Also, an emitter region formed of the second conductivity type is formed selectively in the surface layer of the base region. A portion sandwiched by the aforementioned high resistivity layer of the second conductivity type in the base region and the emitter region is used as a channel region, on which a gate electrode via an insulation film, an emitter electrode commonly contacting the base region and the emitter region, and a collector electrode contacting the collector layer are disposed respectively. In this IGBT, an annex region formed of the first conductivity type is selectively formed in the surface layer of the high resistivity layer of the second conductivity type to be separated from the base region. The width or thickness of the high resistivity layer formed of the second conductivity type on the collector layer side of the annex region is smaller than the width or thickness of the high resistivity layer formed of the second conductivity type on the collector layer side of the base region. A diode is disposed between an annex electrode contacting the annex region and the gate electrode such that a layer formed of the first conductivity type is positioned on the annex electrode side and a layer formed of the second conductivity type is positioned on the gate electrode side. Furthermore, the high resistivity layer formed of the second conductivity type may have a substantially even thickness, wherein the depth or thickness at the annex region is greater than that at the base region, or the annex region may have substantially the same depth or thickness as the base region, wherein a low resistivity layer formed of the second conductivity type is interposed between the high resistivity of the second conductivity type and the collector layer, and the depth or thickness of the low resistivity layer at a part facing the annex region is made thicker than that at a part facing the base region. In addition, it is effective that the diode connected between the annex electrode and the gate electrode is formed of semiconductor layers having a region made of the first conductivity type and a region made of the second conductivity type which are disposed via an insulation film on the surface where the base region of the high resistivity layer made of the second conductivity type and the annex region are arranged.

An equivalent circuit of the IGBT with a structure as described above takes a structure as shown in FIG. 4 in case the first conductivity type is a p-type, and the second conductivity type is an n-type. A main body of the IGBT has a p-n-p transistor (21) formed of a collector layer, a second conductivity type layer and a base region, an n-p-n transistor (22) formed of a second conductivity type layer, a base region and an emitter region, and a MOSFET (23) using the base region sandwiched between the emitter region and the second conductivity type layer as a channel region, and having a gate electrode thereupon via an insulation film. In the base region beneath the emitter region, there exists a base resistance R.sub.b, and there is a capacity CqE between the gate and the emitter. The annex region formed by the present invention forms the p-n-p transistor (24) together with the second conductivity type layer and the collector layer, and between the collector and the gate, a diode (25) corresponding to the diode (33) in FIG. 3 is connected. The p-n-p transistor (24) and the p-n-p transistor (21) share a common base, and there is a resistance R' between them. The width or thickness of the high resistivity layer of the second conductivity type of the p-n-p transistor (24), that is the open base width, is narrower than the open base width of the p-n-p transistor (21) of the IGBT main body, so that it punches through or passes easily. In other words, it breaks down at a lower voltage.

For instance, when the IGBT turns off under a large current and a high voltage, a high di/dt is generated because of the high switching speed inherent to that IGBT, and further, the IGBT is exposed to a high spike voltage as a result of the inductance content. However, because the collector-to-emitter breakdown voltage BV.sub.CEO of the p-n-p transistor (24) is lower than BV.sub.CEO of the p-n-p transistor (21), the p-n-p transistor (24) breaks down first, and the breakdown current goes through the diode (25) and charges the gate-emitter capacity C.sub.gE. When the voltage between the gate G and the emitter E finally reaches the threshold value of the MOSFET (23), the IGBT turns on. After it turns on, the current in the semiconductor element can be absorbed more evenly than in an avalanche breakdown condition, so that greater energy can be absorbed. In addition, when the inductance energy is absorbed while it is on, no more spike voltage is applied, and it is protected by a relatively low clamp voltage. These effects similarly occur in a p-channel IGBT, in which the conductivity type is reversed.

FIG. 1 is a cross-sectional view of an n-channel IGBT of one embodiment of the present invention;

FIG. 2 is an equivalent circuit diagram of an IGBT disposed with the conventional protection means;

FIG. 3 is an current and voltage waveform chart for an IGBT at a load short circuit;

FIG. 4 is an equivalent circuit diagram of an IGBT according to the present invention; and

FIG. 5 is a cross-sectional view of an n-channel IGBT representing another embodiment of the present invention.

FIG. 1 shows an n-channel IGBT of one embodiment of the present invention. In the drawings, an n.sup.- layer (3) is laminated on a p.sup.+ substrate (1) through an n.sup.+ buffer layer (2) to function as a collector layer, and in the surface layer of this n.sup.- layer (3), p base regions (4) and a p.sup.+ base region (5) situated therebetween, and a p.sup.+ region (6) separated from these regions (4), (5) are selectively formed. The depth of the p.sup.+ region (6) is 12 micro meters, which is 4 micro meters deeper than the depth of the p.sup.+ base region (5) of 8 micro meters. In the surface layer of the p region 4 and the p.sup.+ region (5), n.sup.+ emitter regions (7) are formed selectively. Parts of the p region (4) sandwiched between the n.sup.+ region (7) and n.sup.- layer (3) are channel regions, where gate electrodes (8) making this region an inversion layer are disposed via gate oxide films (9) made of polycrystal silicon. Furthermore, a polycrystal silicon layer is deposited on thick insulation films (10) following the gate oxide films (9), where a diode is formed by means of a p.sup.+ region (11) and an n.sup.+ region (12). On the surface of the n.sup.- layer, formed are an emitter electrode (14), which is insulated from the gate electrode (8) by an insulation film (13) contacts both the p base regions (4) and the n.sup.+ source regions (7) and is connected to an emitter terminal E; a gate metal electrode (15), which contacts the gate electrode (8) and the n.sup.+ region (12) and is connected to a gate terminal G; and a connection electrode (16), which contacts the p.sup.+ region (6) and p.sup.+ region (11). Though not shown, a collector electrode contacts the surface of the p.sup.+ collector layer (1) and is connected to a collector terminal C.

In the structure shown in FIG. 1, a p-n-p transistor formed of the p.sup.+ layer (1), n.sup.+ layer (2), n.sup.- layer (3) and p.sup.+ region (6) corresponds to a clamping transistor (24) in FIG. 4, and a diode formed of tho p.sup.+ region (11) and n.sup.+ region (12) corresponds to a diode (25) in FIG. 4. While the p-n-p transistor formed of the p.sup.+ layer (1), n.sup.+ layer (2) and n.sup.- layer (3) and p.sup.+ base region (5) forms an active region of the IGBT, which corresponds to the transistor (21) in FIG. 4, wherein the collector-emitter breakdown voltage BV.sub.CEO of the p-n-p transistors (21), (24) can be controlled by the junction depth of its collector regions (5), (6). The resistivity and thickness may vary among the different silicon wafers, but the variance in one chip is extremely small. Therefore, the difference of BV.sub.CEO between the transistor (21) of the IGBT and the clamping transistor (24) can be easily manufactured at an approximately constant value such as 50 V or 100 V by controlling the depths or thickness of the p.sup.+ regions (4), (5).

FIG. 5 shows an IGBT of another embodiment of the present invention, in which the parts common to those in FIG. 1 are given the same numerals. In this case, the p.sup.+ region (6) for the clamping transistor is formed simultaneously with the p.sup.+ base region (5) at the same depth or thickness, and an n.sup.+ layer (17) which is thicker than a buffer layer (2) is disposed beneath the p.sup.+ region (6). Thus, a depletion layer in an n.sup.- layer (3) extending from the p.sup.+ region (6) can be punched through or pass to the n.sup.+ buffer layer (17), so that the BV.sub.CEO of the clamping p-n-p transistor (24) can be made lower than that of the p-n-p transistor (21) in the active region of the IGBT.

In the embodiments shown in FIGS. 1 and 5, the n.sup.+ buffer layer (2) is interposed between the p.sup.+ collector layer (1) and the n.sup.- layer (3), but the present invention can be applied to an IGBT without a buffer layer. In that case, for the system shown in FIG. 5, only the n.sup.+ buffer layer (17) is formed. The present invention can even be applied to a p-channel IGBT by inverting the conductivity type of all the layers and regions.

According to the present invention, a clamping bipolar transistor is formed on one semiconductor element, wherein the abnormal voltage applied across a collector and a gate of an IGBT is bypassed, and the IGBT is turned on. The BV.sub.CEO of this clamping transistor can be decreased to the intended difference of the BV.sub.CEO of that particular transistor by forming an open base width narrower than the open base width of the bipolar transistor which is the active region of the IGBT. Therefore, the energy of the abnormal voltage greater than the BV.sub.CEO of the clamping transistor is absorbed when the IGBT is turned on, so that its turn-off withstand capacity is increased, and its reliability is improved. In addition, because the variance in the difference between the BV.sub.CEO of the clamping transistor and the BV.sub.CEO of the bipolar transistor of the IGBT main body can be reduced, the difference of the BV.sub.CEO need not be formed large, and even in a high-withstand voltage IGBT, neither much margin nor a high withstand voltage setting is necessary. This effect greatly improves the relation between the on-voltage of the particular IGBT and the switching speed. Furthermore, the diode connected in series with the clamping transistor can be greatly reduced in terms of the inductance content in the wiring by forming it on the semiconductor elements, when comparing with the external installation.