An insulated gate-type bipolar transistor with an overcurrent limiting function that is capable of keeping the ratio of a main current to a detection current constant even under different operating conditions, and capable of suppressing the voltage dependence of the limited-current value to perform stable overcurrent protection. P-wells are formed so that they are incorporated between main cell IGBTs as sensing cells for current detection on part of the semiconductor substrate on which a large number of main cells are formed integratedly, and current-detecting emitter electrodes connected to the P-wells are connected to an overcurrent-protection circuit and separated from the main emitter electrodes connected to the main IGBT cells. Given such a configuration, the overcurrent flowing into the main cells during a load short circuit in an inverter device is detected as a hole current from the P-wells with a high accuracy of keeping the current ratio to the current in the main cells constant, and moreover, stable overcurrent protection is performed keeping the limited current values suppressed below the short-circuit withstand capability without dependence on the power supply voltage.

FIELD OF THE INVENTION 
The present invention relates to an insulated gate-type bipolar transistor 
(IGBT) acting as a power-switching device that is applied to inverters and 
other similar devices. 
BACKGROUND OF THE INVENTION 
An insulated gate-type bipolar transistor (hereinafter referred to as IGBT) 
is a voltage-driven semiconductor switching device capable of a high-speed 
turning-off operation with relatively low voltage application, which is 
used widely in the power-electronics field in for example, inverters and 
other similar devices. 
An IGBT output-type inverter device may have an overcurrent flow into the 
IGBT if there is an inrush current when a motor is actuated, and failures 
such as a load short circuit and arm short circuit occur. Hence, superior 
electrical characteristics are required for the IGBT to protect against a 
high voltage and large current input. One important electric 
characteristic is the capacity to withstand breakdown known as 
short-circuit withstand capability. 
In accordance with this need for short-circuit protection, the inverter 
device incorporates a protection circuit to detect short-circuit failures 
when they occur resulting in turning the power supply off. However, this 
protection circuit requires 10 to 20 .mu.sec to detect the overcurrent and 
engage its protective function. The IGBT must not break down due to the 
overcurrent during this period. 
Therefore, many recent high-performance IGBT modules adopt an 
overcurrent-protection system disposed independently of the protection 
circuit in the inverter device, which can detect at a high speed an 
overcurrent flowing into the IGBT when a short-circuit failure occurs, and 
which can limit the current flowing in the IGBT to be within the 
short-circuit withstand capability of the IGBT. The overcurrent protection 
system operates by means of a gate control operated based on an 
overcurrent detection signal before the power supply is turned off by the 
first protection circuit. 
FIG. 6 shows a prior art IGBT independent overcurrent-protection circuit 
according to the over-current protection system. The circuit includes a 
main element 1 (IGBT), a current-detection sub-element 2 (different IGBT 
from the main element 1) connected in parallel to the main element 1, a 
current-detection resistance 3 connected in series to the sub-element 2, 
and a switching element (MOSFET) 4 connected to the gate-driving circuits. 
The main element 1 and the sub-element 2 perform on-off operations 
according to the voltage generated across the current-detection resistance 
3. 
Given such a configuration, when an overcurrent due to load short-circuit 
failure or the like flows into the main element 1 and the sub-element 2 
and causes the voltage drop generated between both ends of the 
current-detection resistance 3 to exceed the threshold voltage of the 
switching element 4, the switching element 4 turns on to reduce the gate 
voltage to both the main element 1 and the current-detection sub-element 2 
thus limiting the main current flowing in the IGBT main element 1. Thus, 
the main current flowing in the IGBT main element 1 can be suppressed to 
be within the short-circuit withstand capability of the IGBT element 1 by 
means of adequately setting the resistance of the current-detection 
resistance 3 and the threshold voltage of the switching element 4. 
When an overcurrent-protection circuit that includes the IGBT sub-element 2 
for current detection is constructed as an external, independent circuit 
to protect the IGBT main element 1 as described above in connection with 
the second protection circuit, it is technically difficult to maintain the 
operational characteristics of the main element 1 proportional to those of 
the sub-element 2. In other words, since the short-circuit phenomena in an 
inverter includes various modes such as an arm short circuit, series short 
circuit, output short circuit and ground fault, and it is anticipated that 
the collector-to-emitter voltage VCE applied to the IGBT element 1 to be 
protected will vary according to the short-circuit mode. This may cause 
the current ratio between the main element 1 and the sub-element 2 to 
vary, and may therefore vary the limited-current value if the 
collector-to-emitter voltage VCE varies as described above, making a 
stable overcurrent-protection operation difficult. 
To solve the above problem, a configuration has already been proposed by 
the same applicant of the present invention in Japanese patent application 
No. 5-256197, wherein some of the cells formed integratedly on a 
semiconductor substrate are used as sensing cells to detect the current in 
the IGBT main element 1, and the emitter electrodes of the sensing cells 
are laid out separately from the emitter electrodes of the main cells 
formed on the same substrate and connected to a current-detection 
resistance in an overcurrent-protection circuit. 
In the construction that incorporates the main cells of the IGBT and the 
current detection sensing cells of the IGBT on the same semiconductor 
substrate as described above, the gate electric potential of the sensing 
cell may change with a voltage drop at the current-detection resistance of 
the overcurrent-protection circuit which is connected to the sensing 
cells, depending on the position of the sensing cells and the IGBT output 
characteristics. The change in the gate electric potential generates a 
difference in the current density between the main cells and the sensing 
cells causing the current ratio to vary. Experimental results have shown 
that the change thereof in the collector-to-emitter voltage VCE due to the 
current ratio variation causes the limited-current values to change. In 
particular the limited-current values increase in a low voltage region in 
which the collector-to-emitter voltage VCE is low. 
Moreover, if the voltage dependence of the limited-current values 
increases, trouble may occur in the overcurrent-protection operations if 
the IGBT is applied to an inverter device. Hence, it is necessary to 
suppress the voltage dependence of the limited-current values, keeping it 
as low as possible. 
SUMMARY OF THE INVENTION 
The present invention is intended to solve the above problems, and thereby 
provides an insulated gate-type bipolar transistor with an 
overcurrent-limiting function that is capable of keeping the ratio of the 
main current to the detection current constant even in different operating 
environments, while suppressing the voltage dependence of the 
limited-current values to ensure reliable overcurrent protection. 
According to the present invention, an insulated gate-type bipolar 
transistor has current-detection sensing cells formed in part of the 
semiconductor substrate on which a large number of main cells are formed 
integratedly, and the emitter electrodes of the sensing cells are 
separated from the emitter electrodes of the main cells and are connected 
to an overcurrent protection circuit, wherein the sensing cells are formed 
by means of P-wells, and are laid out in the vicinity of the main cell 
region. 
In the above configuration, the P-wells connected to the emitter electrodes 
of the sensing cells may be formed in such a way that they are 
incorporated between the main cells, preferably setting the distance 
between the P-wells and the adjoining main cells to 10 .mu.m or less. 
In the above configuration, the P-wells that form the sensing cells to 
detect the current are positioned in the active region of the main cells 
on the same semiconductor substrate, and a current proportional to the 
hole current flowing in the main cells is extracted via the P-wells during 
the IGBT voltage-on period, and is detected with high accuracy by a 
current-detection resistance in the overcurrent-protection circuit 
connected to the emitter electrodes of the sensing cells. In the case in 
which an overcurrent flows because of a load short circuit or other 
similar phenomenon, the gate voltage of the main cells is reduced by 
operation of the protection circuit to limit the main current to within 
the short-circuit withstand capability of the IGBT, thereby protecting the 
cells from a breakdown. 
Additionally, forming the sensing cells by means of the P-wells in this 
case eliminates the change in the current ratio between the main cells and 
the sensing cells, which occurs when an IGBT is used for the sensing 
cells, and the voltage dependence of the limited-current values caused by 
the change in this current ratio is well suppressed. Furthermore, by 
laying out the P-wells connected to the emitter electrodes of the sensing 
cells so that they are incorporated in narrow clearances between the main 
cells, the linearity of the current in the sensing cells to the hole 
current in the adjoining main cells can be ensured resulting to greatly 
improve accuracy in detecting the main current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1a and 1b are a plan view of an IGBT chip and a cross-sectional view 
of a cell, respectively, and FIG. 2 is an expanded view of the sensing 
cells in FIGS. 1a and 1b. Referring to FIG. 1a, the semiconductor 
substrate 5 has a large number of n-channel type IGBTs (part of the main 
cells) 6 which are parallel to each other in stripes, and the main emitter 
electrodes 7 and the gate electrodes 8 are formed for each IGBT 6 of the 
main cells. In addition, the current-detection sensing cells as described 
below are incorporated in the corners of the semiconductor substrate 5. 
Referring to FIG. 1b, each sensing cell consists of diffusion-formed 
P-wells 9 which are inserted like comb teeth between the stripes of p-base 
layers of the main cell IGBTs 6 and the emitter electrodes 10 for current 
detection connected to the P-wells 9, which are separated from the emitter 
electrodes 7 of the main cells as shown in FIG. 1b and FIG. 2. In this 
case, the IGBTs 6 of the main cells and the P-wells 9 running parallel 
therewith may be better arranged in proximity to each other with the 
distance between them set to 10 .mu.m or less, though this depends on the 
resistivity of the semiconductor substrate 5. In addition, the gate 
electrode 8 of the main cells is divided at the part where the emitter 
electrodes 10 are taken out from the P-wells 9, as shown in FIG. 1b. 
Furthermore, FIG. 1b shows a main emitter terminal 11, a current-detection 
emitter terminal 12, a gate terminal 13, and a collector terminal 14. The 
current-detection emitter terminal 12 is connected to the 
current-detection resistance 3 in the overcurrent-protection circuit in 
the way shown in FIG. 6. The current-detection resistance 3 and the 
sub-element MOSFET 4 that constitute the overcurrent-detection circuit may 
be constructed as an external circuit separated from the IGBTs, or formed 
around the gate electrode on said semiconductor substrate 5. 
FIG. 3 shows an applied embodiment where the IGBTs 6 of the main cells are 
distributed like an archipelago. In this embodiment, the P-wells 9 of the 
sensing cells run zigzag between the main cells. 
The overcurrent-protection operation of the IGBT 6 according to the 
construction in FIG. 3 is nearly the same as what is illustrated in FIG. 
6. The overcurrent flowing in the main cell IGBT 6 is detected as a hole 
current by means of the P-wells 9 of the current-detection sensing cells 
formed on the same semiconductor substrate 5, and the detected current is 
fed back to the overcurrent-protection circuit which then reduces the gate 
voltage so that the overcurrent is restricted to within the short-circuit 
withstand capability. In addition, because the P-wells 9 for the sensing 
cells are arranged in close proximity to the active region of the main 
cell IGBT 6, the on-state voltage for this main cell is higher than that 
for the other main cells. Hence, no current concentration occurs in this 
area. Furthermore, since latch up phenomenon is prevented as a result of 
forming the sensing cells with P-wells, the turn-off failure due to 
current concentration in this area can be prevented. 
FIG. 4 shows the waveforms of the main current Ic and the voltage VCE 
observed in a short-circuit test using the overcurrent-protection circuit 
connected to the IGBT with the configuration described in FIG. 3 (with a 
withstand voltage of 600 V, and a rated current of 100 A) and using a 
power supply voltage of 400 V. As can be seen from this waveform graph, 
the main current in the IGBT is reduced in a period of a few microseconds 
so that the limited-current value relative to the rated current of 100 A 
is within the short-circuit withstand capability of 250 A. 
FIG. 5 is a characteristics graph that illustrates the limited-current 
values of an overcurrent when the power supply voltage applied across the 
collector and the emitter changes in order to compare the embodiment of 
the present invention with a conventional system (a system using an IGBT 
as a sensing cell). The tested IGBT shown in this graph is an element with 
a large gain. The prior art applied to this type of element shows a trend 
in which the limited-current value increases in a low voltage region as 
shown by characteristics line 20, while in the present invention the 
limited-current value changes very little and remains the same from a high 
voltage region to low voltage region as shown by characteristics line 21. 
This means that the voltage dependence of the limited-current value is 
greatly reduced. This effect stabilizes overcurrent protection when the 
IGBT is used in the inverter device regardless of the short-circuit mode 
that may take place. 
As described above, the insulated gate-type bipolar transistor according to 
the present invention, characterized in that sensing cells for current 
detection are formed on part of a semiconductor substrate with a large 
number of main cells that are formed integratedly, the emitter electrodes 
of said sensing cells are laid out separately from the emitter electrodes 
of the main cells and are connected to an overcurrent-protection circuit, 
an overcurrent that flows into the main cells due to a load short circuit 
in an inverter device is detected as a hole current with high accuracy 
from P-wells while the current ratio between the main cells and the 
sensing cells is maintained constant because of a construction in which 
said sensing cells are formed by means of the P-wells and laid out in the 
vicinity of the main cell region. As a result, the limited-current value 
is suppressed to within the short-circuit withstand capability without 
dependence on the power supply voltage because of said construction, which 
in turn makes stable overcurrent protection possible. 
Furthermore, by forming the P-wells connected to the emitter electrodes of 
the sensing cells by incorporating them between the main cells, and by 
setting the distance between the P-wells and the adjoining main cells to 
10 .mu.m or less, linearity of the sensing cell current to the hole 
current in the main cells is ensured, thereby further improving the 
current detection accuracy. 
The foregoing merely illustrates the principles of the invention. It will 
thus be appreciated that those skilled in the art will be able to devise 
numerous systems and methods which, although not explicitly shown or 
described herein, embody the principles of the invention and are thus 
within the spirit and scope of the invention.