Semiconductor device comprising an over-temperature detection element for detecting excessive temperature of amplifiers

A semiconductor device includes on a semiconductor substrate an output transistor which is composed of a collector region, a first base region and a first emitter region, and a temperature detection transistor composed of the collector region, a second base region and a second emitter region. The output transistor is provided at a center of the collector region of the semiconductor substrate. A vacant region is formed on a center of the output transistor, and the temperature detection transistor is provided in the vacant region.

BACKGROUND OF THE INVENTION 
This invention relates to a semiconductor device having a function of 
obviating thermal breakdown due to temperature rise of a semiconductor 
element for output and particularly relates to a high-functional 
semiconductor device having a semiconductor element for output which deals 
with large current. 
Conventionally, as a semiconductor device having a function of protecting a 
semiconductor element for output which is formed on a semiconductor 
substrate from thermal breakdown, there is a known technique that a 
temperature sensor composed of a thermistor is provided near a 
semiconductor element for output at an external part of the semiconductor 
substrate and the temperature sensor detects the temperature of the 
semiconductor element for output so that current is intercepted when the 
detected temperature rises to a prescribed temperature. 
In the above semiconductor device, however, since the temperature sensor is 
provided outside of the semiconductor substrate, a time lag and a 
dispersion of temperature difference are caused between the temperature of 
the semiconductor element for output and the detected temperature by the 
temperature sensor. 
With either the time lag or the dispersion of temperature difference 
between the temperature of the output semiconductor element and the 
detected temperature by the temperature sensor, even when the output 
semiconductor element reaches the danger temperature which may involve a 
thermal breakdown, the temperature sensor cannot detect the danger 
temperature which the output semiconductor element reaches, thus the 
output semiconductor element shall be broken down. 
Proposed in Japanese Patent Application Laying Open Gazette No.3-276636 is, 
as shown in FIGS. 8(a), (b), a semiconductor device in which an output 
transistor 61 is formed on one side of a semiconductor substrate 60 and a 
temperature detection resistor 62 for detecting the temperature of the 
output transistor 61 is provided on the other side thereof. In FIGS. 8(a), 
(b), reference numeral 63 is a collector region of the output transistor 
61, 64 is a base region thereof, 65 is an emitter region thereof, and 66 
is a resistor electrode of the temperature detection resistor 62. 
Also, a semiconductor integrated circuit device having a temperature 
detection element at a main heat generation part on a semiconductor 
substrate is proposed in Japanese Patent Application Laying Open Gazette 
No.1-290249. 
In the above two semiconductor devices, the problem of the time lag between 
the temperature of the output semiconductor element and the detected 
temperature by the temperature detection element is almost overcome 
because the temperature detection element is provided on the semiconductor 
substrate. 
As explained below, however, the problem that the output semiconductor 
element may be break down without the danger temperature of the output 
semiconductor element detected by the temperature sensor though the output 
semiconductor element reaches the danger temperature still remains in the 
above two semiconductor devices. 
In the former semiconductor device, since the output transistor is provided 
on one side of the semiconductor substrate and the temperature detection 
resistor is provided on the other side thereof, the gap between the 
temperature of the output transistor and the detected temperature by the 
temperature detection resistor is unavoidable. Even with the gap between 
the temperature of the output semiconductor element and the detected 
temperature, that the output semiconductor element reaches the danger 
temperature is detected anyhow by previously obtaining a correlativity 
therebetween. However, since the part at which the output semiconductor 
element on the semiconductor substrate is provided is far from the part at 
which the temperature detection element on the semiconductor substrate 
detects the temperature, the gap between the actual temperature of the 
output semiconductor element and the detected temperature by the 
temperature detection element results. As far as the gap are present 
between the temperature of the output semiconductor element and the 
detected temperature by the temperature detection element, the dispersion 
of the temperature gap is unavoidable. Therefore, the former semiconductor 
device has the problem that the temperature detection element does not 
detect the danger temperature of the output semiconductor element even 
when the output semiconductor element reaches the danger temperature. 
In the latter semiconductor integrated circuit device, the part where the 
main heat generation part on the semiconductor substrate is provided is 
not always a part where the temperature rises the highest in the 
semiconductor device. The part of the main heat generation part is the 
part where the temperature rises the highest when it is provided on a 
center part of the substrate, but when the main heat generation part is 
formed on a side of the semiconductor substrate as disclosed in Japanese 
Patent Application Laying Open Gazette No.1-290249, heat is likely to emit 
from the main heat generation part on the substrate and is hard to emit 
from the center part thereof. Therefore, when the output semiconductor 
element is formed on the center part of the substrate or the near part 
thereof, the temperature detection element does not detect that the output 
semiconductor element reaches the danger temperature even when it does so. 
SUMMARY OF THE INVENTION 
The present invention has its object of providing a semiconductor device 
whose temperature detection element is capable of detecting rapidly, 
accurately that the output semiconductor element reaches the danger 
temperature. 
A first semiconductor device according to the present invention comprises: 
a semiconductor substrate; 
an output semiconductor element formed on said semiconductor substrate and 
has on a center part thereof a vacant region; and 
a temperature detection element, provided in said vacant region, for 
detecting a temperature of said output semiconductor element. 
With the above construction, the temperature detection element is 
surrounded by the output semiconductor element, so that the temperature 
detection element detects a mean value of the temperature at each part of 
the output semiconductor element. Thereby, the gap between the temperature 
of the output semiconductor element and the detected temperature by the 
temperature detection element is made extremely small, so that the 
dispersion of the temperature gap is made small. Hence, the temperature 
detection element can detect accurately that the output semiconductor 
element reaches the danger temperature. 
A second semiconductor device according to the present invention comprises: 
a semiconductor substrate; 
an output semiconductor element formed on said semiconductor substrate; and 
a temperature detection element, provided on a center part of said 
semiconductor substrate, for detecting a temperature of said output 
semiconductor element. 
Consequently, the temperature detection element can detect the temperature 
of the center part of the semiconductor substrate which increases in 
temperature owing to heat storage, so that the temperature detection 
element can accurately detect that the output semiconductor element 
reaches the danger temperature. 
Preferably, the temperature detection elements in first and second 
semiconductor devices are each a temperature detection transistor. 
As is generally known, a forward voltage V.sub.BE at a PN junction part 
between a base region and an emitter region of a transistor varies 
linearly in accordance with temperature variation of the PN junction part, 
and a ratio between the forward voltage V.sub.BE and the temperature of 
the PN junction part under a given condition is 2 mV/.degree.C. For 
example, the temperature of the PN junction part of the transistor is 
100.degree. C. when the forward voltage V.sub.BE at the PN junction part 
is 200 mV, and the temperature thereof is 150.degree. C. when the forward 
voltage V.sub.BE thereat is 300 mV. Therefore, the transistor can serve as 
the temperature detection element surely. 
A pair of transistor in Darlington-connection is preferably used as the 
temperature detection transistor. 
Since the Darlington transistor has two PN junction parts, the ratio 
between the forward voltage V.sub.BE of the Darlington transistor and the 
temperature of the PN junction parts are 4 mV/.degree.C. Accordingly, the 
forward voltage of the Darlington transistor with respect to the 
temperature variation doubles that of a single transistor, thus enhancing 
an accuracy of the temperature detection of the output semiconductor 
element. 
In first or second semiconductor devices, the temperature detection element 
may be a temperature detection diode. 
As is generally known, the forward voltage V.sub.F at a PN junction part of 
a diode varies linearly in accordance with temperature variation of the PN 
junction part, and a ratio between the forward voltage V.sub.F and the 
temperature of the PN junction part under a given condition is 2 
mV/.degree.C. For example, the temperature of the PN junction part of the 
diode is 150.degree. C. when the forward voltage V.sub.F at the PN 
junction part is 300 mV, and the temperature thereof is 200.degree. C. 
when the forward voltage thereat V.sub.F is 400 mV. Therefore, the diode 
can serve as the temperature detection element surely. 
In first or second semiconductor devices, the temperature detection element 
may be a temperature detection resistor. 
As is generally known, a resistance of a resistor varies linearly in 
accordance with temperature variation of the resistor. Therefore, the 
resistors can serve as the temperature detection element surely. 
In first or second semiconductor devices, the temperature detection element 
is preferably formed in the same process as that for the output 
semiconductor element. 
Consequently, fluctuation of performance of the output semiconductor 
element and that of the temperature detection element are made similar to 
each other, thus extremely reducing the dispersion of gap between the 
temperature of the output semiconductor element and the detected 
temperature by the temperature detection element. 
When the output semiconductor element is an output transistor, first or 
second semiconductor devices is remarkably effective. 
Because, the temperature of the output transistor which is likely to be 
broken owing to a liable temperature rise during operation is accurately 
detected. 
When the output semiconductor element is the output transistor, it is 
preferable that said temperature detection element is a temperature 
detection transistor, a collector of said output transistor and a 
collector of said temperature detection transistor are formed in a same 
region, a base of said output transistor and a base of said temperature 
detection transistor are respectively formed in different regions, and an 
emitter of said output transistor and an emitter of said temperature 
detection transistor are respectively formed in different regions. 
Accordingly, the output transistor and the temperature detection transistor 
can be formed easily, accurately.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Description is made below about a semiconductor device according to a first 
embodiment of the present invention. 
FIGS. 1(a) and (b) show a semiconductor device 1 according to the first 
embodiment, in which FIG. 1(a) is a plan view thereof and FIG. 1(b) is a 
section taken along I--I in FIG. 1(a). For the brevity's sake, electrodes 
are omitted in FIG. 1(a). 
As shown in FIGS. 1(a), (b), the semiconductor device 1 includes a 
collector region 10 which is an N-type silicon substrate, an annular first 
base region 11 formed on a center part of the collector region 10, a 
C-shaped first emitter formed on the first base region 11, a cubic second 
base region 13 formed in a vacant region within the first base region 11 
and the first emitter region 12 on the collector region 10, and a second 
emitter region 14 in the form of rectangular solid which is formed on the 
second base region 13. 
A collector electrode 15 is provided at a lower surface of the collector 
region 10. A first base electrode 16 is provided on the first base region 
11. A first emitter electrode 17 is provided on the first emitter region 
12. A second base electrode 18 is provided on the second base region 13. A 
second emitter electrode 19 is provided on the second emitter region 14. 
An output transistor 1A is, as an output semiconductor element, composed of 
the collector region 10, the first base region 11 and the first emitter 
region 12, and a temperature detection transistor 1B is, as a temperature 
detection element, composed of the collector region 10, the second base 
region 13 and the second emitter region 14. As described above, the 
temperature detection transistor 1B is provided at a center part of the 
N-type silicon substrate which composes the collector region 10, and the 
output transistor 1A is provided so as to surround the temperature 
detection transistor 1B. 
Discussed below is about a method of manufacturing the semiconductor device 
1 according to the first embodiment. 
First, boron is selectively diffused in an N-type silicon substrate (to be 
the collector region 10 of NPN transistor) in which phosphorus is diffused 
so as to concurrently form the first base region 11 and the second base 
region 13 which are separated from each other. 
Next, phosphorus is selectively diffused in first and second base regions 
11, 13 to form the first emitter region 12 and the second emitter region 
14 respectively. Thereafter, the collector electrode 15, the first base 
electrode 16, the first emitter electrode 17, the second base electrode 18 
and the second emitter electrode 19 are provided respectively at the lower 
surface of the collector region 10, on the first base region 11, on the 
first emitter region 12, on the second base region 13 and on the second 
emitter region 14. 
In this way, the output transistor 1A of NPN transistor and the temperature 
detection transistor 1B of NPN transistor are formed respectively on the 
N-type silicon substrate. 
Explained below is about a method for detecting temperature of the output 
transistor 1A by the temperature detection transistor 1B in the 
semiconductor device 1 according to the first embodiment. 
When about 1V voltage is applied between the second base electrode 18 and 
the second emitter electrode 19 of the temperature detection transistor 1B 
and the output transistor 1A is operated at 40W (20V voltage, 2A current, 
50% duty), a forward voltage V.sub.BE at a PN junction part between the 
second base region 13 and the second emitter region 14 of the temperature 
detection transistor 1B measures 340 mV. When the output transistor 1A is 
operated at 60W (20V voltage, 3A current, 50% duty), the forward voltage 
V.sub.BE at the PN junction part of the temperature detection transistor 
1B measures 460 mV. 
On the other hand, when temperature of the output transistor 1A operated in 
steady state is measured using a thermocouple, the temperature of the 
output transistor 1A is 170.degree. C. at 40W and 230.degree. C. at 60W. 
The forward voltage V.sub.BE at the PN junction part of the temperature 
detection transistor 1B means a voltage margin between the second base 
electrode 18 and the second emitter electrode 19 of the temperature 
detection transistor 1B, and varies according to the temperature of the PN 
junction part of the temperature detection transistor 1B. 
By obtaining beforehand a correlativity between the temperature of the 
output transistor 1A and the forward voltage V.sub.BE of the temperature 
detection transistor 1B, the temperature of the output transistor 1A is 
made correspondent to the forward voltage V.sub.BE at the PN junction part 
of the temperature detection transistor 1B. Accordingly, the temperature 
of the output transistor 1A is easily detected by measuring the forward 
voltage V.sub.BE at the PN junction part of the second base region 13 and 
the second emitter region 14 of the temperature detection transistor 1B. 
In the semiconductor device 1 according to the first embodiment, since the 
temperature detection transistor 1B is formed within the output transistor 
1A, a dispersion of the detected temperature due to difference in location 
between the output transistor 1A and the temperature detection transistor 
1B is reduced, and the time lag between the temperature of the output 
transistor 1A and the detected temperature due to the distance between the 
output transistor 1A and the temperature detection transistor 1B is 
cleared. 
Consequently, with the semiconductor device 1 according to the first 
embodiment, accuracy and rapidity in detecting the temperature of the 
output transistor 1A is enhanced. 
Description is made below about a semiconductor device according to a 
second embodiment of the present invention. 
FIGS. 2(a) and (b) show a semiconductor device 2 according to the second 
embodiment, in which FIG. 2(a) is a plan view thereof and FIG. 2(b) is a 
section taken along II--II in FIG. 2(a). For the brevity's sake, 
electrodes are omitted in FIG. 2(a). 
As shown in FIGS. 2(a) and (b), the semiconductor device 2 includes a 
collector region 20 which is an N-type silicon substrate, an annular base 
region 21 formed on a center of the collector region 20, a C-shaped 
emitter region 22 formed on the base region 21, a cubic cathode region 28 
formed in a vacant region within the base region 21 and the emitter region 
22 on the collector region 20, and an anode region 24 in the form of 
rectangular solid which is formed on the cathode region 28. 
A collector electrode 25 is provided at a lower surface of the collector 
region 20. A base electrode 26 is provided on the base region 21. An 
emitter electrode 27 is provided on the emitter region 22. A cathode 
electrode 28 is provided on the cathode region 23. An anode electrode 29 
is provided on the anode region 24. 
An output transistor 2A is, as an output semiconductor element, composed of 
the collector region 20, the base region 21 and the emitter region 22, and 
a temperature detection diode 2B is, as a temperature detection element, 
composed of the cathode region 23 and the anode region 24. As described 
above, the temperature detection diode 2B is provided at a center part of 
the N-type silicon substrate which composes the collector region 20, and 
the output transistor 2A is provided so as to surround the temperature 
detection diode 2B. 
Discussed below is about a method of manufacturing the semiconductor device 
2 according to the second embodiment. 
First, boron is selectively diffused in an N-type silicon substrate (to be 
the collector region 20 of NPN transistor) in which phosphorus is diffused 
so as to form the base region 21 of the NPN transistor and the cathode 
region 23 of the diode at the same time. 
Next, phosphorus is selectively diffused in the base region 21 and the 
cathode region 23 to form the emitter region 22 of NPN transistor and the 
anode region 24 of the diode respectively. Thereafter, the collector 
electrode 25, the base electrode 26, the emitter electrode 27, the cathode 
electrode 28 and the anode electrode 29 are provided respectively at the 
lower surface of the collector region 20, on the base region 21, on the 
emitter region 22, on the cathode region 23 and on the anode region 24. 
In this way, the output transistor 2A of NPN transistor and the temperature 
detection diode 2B are formed respectively on the N-type silicon 
substrate. 
Explained below is about a method for detecting temperature of the output 
transistor 2A by the temperature detection diode 2B in the semiconductor 
device 2 according to the second embodiment. 
When about 1V voltage is applied between the cathode electrode 28 and the 
anode electrode 29 of the temperature detection diode 2B and the output 
transistor 2A is operated at 40W (20V voltage, 2A current, 50% duty), a 
forward voltage V.sub.F at a PN junction part of the cathode region 23 and 
the anode region 24 of the temperature detection diode 2B measures 340 mV. 
When the output transistor 2A is operated at 60W (20V voltage, 3A current, 
50% duty), the forward voltage V.sub.F at the PN junction part of the 
temperature detection diode 2B measures 460 mV. 
On the other hand, when a temperature of the output transistor 2A operated 
in steady state is measured using a thermocouple, the temperature of the 
output transistor 2A is 170.degree. C. at 40W and 230.degree. C. at 60W. 
The forward voltage V.sub.F at the PN junction part of the temperature 
detection diode 2B means a voltage difference between the cathode 
electrode 28 and the anode electrode 29 of the temperature detection diode 
2B. and varies according to the temperature of the PN junction part of the 
temperature detection diode 2B. 
By obtaining beforehand a correlativity between the temperature of the 
output transistor 2A and the forward voltage V.sub.F of the temperature 
detection diode 2B, the temperature of the output transistor 2A is made 
correspondent to the forward voltage V.sub.F at the PN junction part of 
the temperature detection diode 2B. Accordingly, the temperature of the 
output transistor 2A is easily detected by measuring the forward voltage 
V.sub.F at the PN junction part between the cathode region 23 and the 
anode region 24 of the temperature detection diode 2B. 
In the semiconductor device 2 according to the second embodiment, since the 
temperature detection diode 2B is formed within the output transistor 2A, 
a dispersion of the detected temperature due to difference in location 
between the output transistor 2A and the temperature detection diode 2B is 
reduced, and the time lag between the temperature of the output transistor 
2A and the detected temperature due to the distance between the output 
transistor 2A and the temperature detection diode 2B is cleared. 
Consequently, with the semiconductor device 2 according to the second 
embodiment, accuracy and rapidity in detecting the temperature of the 
output transistor 2A is enhanced. 
Discussed next is about a semiconductor device according to a third 
embodiment of the present invention. 
FIGS. 3(a) and (b) show a semiconductor device 3 according to the third 
embodiment, in which FIG. 3(a) is a plan view thereof and FIG. 3(b) is a 
section taken along III--III in FIG. 3(a). For the brevity's sake, 
electrodes are omitted in FIG. 3(a). 
As shown in FIGS. 3(a) and (b), the semiconductor device 3 includes a 
collector region 30 which is an N-type silicon substrate, an annular base 
region 31 formed on a center part of the collector region 30, a C-shaped 
emitter region 32 formed on the base region 31, and an H-shaped resistor 
3B for temperature detection which is formed, as a temperature detection 
element, at a vacant region within the base region 31 and the emitter 
region 32 on the collector region 30. 
A collector electrode 33 is provided at a lower surface of the collector 
region 30. A base electrode 34 is provided on the base region 31. An 
emitter electrode 35 is provided on the emitter region 32. Temperature 
detection electrodes 36, 37 are provided on the temperature detection 
resistor 3B. 
A transistor 3A for output is, as a semiconductor element for output, 
composed of the collector region 30, the base region 31 and the emitter 
region 32. As described above, the temperature detection resistor 3B is 
formed on the center of the N-type silicon substrate which composes the 
collector region 30, and the output transistor 3A is provided so as to 
surround the temperature detection resistor 3B. 
Discussed below is about a method of manufacturing the semiconductor device 
3 according to the third embodiment. 
First, boron is selectively diffused in an N-type silicon substrate (to be 
the collector region 30 of NPN transistor) in which phosphorus is diffused 
so as to form the base region 31 and the temperature detection resistor 3B 
of the NPN transistor at the same time. 
Next, phosphorus is selectively diffused in the base region 31 to form the 
emitter region 32 of the NPN transistor. Thereafter, the collector 
electrode 33, the base electrode 34, the emitter electrode 35 and the 
temperature detection electrodes 36, 37 are provided respectively at the 
lower surface of the collector region 30, on the base region 31, on the 
emitter region 32, on the temperature detection resistor 3B. 
In this way, the output transistor 3A and the temperature detection 
resistor 3B of NPN transistor are formed respectively on the N-type 
silicon substrate. 
Explained below is about a method for detecting temperature of the output 
transistor 3A by the temperature detection resistor 3B in the 
semiconductor device 3 according to the third embodiment. 
When the output transistor 3A is operated at 40W (20V voltage, 2A current, 
50% duty), the resistance thereof measures 5000 .OMEGA.. When the output 
transistor 3A is operated at 60W (20V voltage, 3A current, 50% duty), the 
resistance thereof measures 7000 .OMEGA.. 
On the other hand, when the temperature of the output transistor 3A 
operated in steady state is measured using the thermocouple, the 
temperature of the output transistor 3A is 170.degree. C. at 40W and 
230.degree. C. at 60W. 
Since the resistance of the temperature detection resistor 3B varies 
according to the temperature of the output transistor 3A, the temperature 
of the output transistor 3A is made correspondent to the resistance of the 
temperature detection resistor 3B. Consequently, by measuring the 
resistance of the temperature detection resistor 3B, the temperature of 
the output transistor 3A is easily detected. 
In the semiconductor device 3 according to the third embodiment, since the 
temperature detection transistor 3B is formed within the output transistor 
3A, a dispersion of the detected temperature due to difference in location 
between the output transistor 3A and the temperature detection transistor 
3B is reduced, and the time lag between the temperature of the output 
transistor 3A and the detected temperature due to the distance between the 
output transistor 3A and the temperature detection transistor 3B is 
cleared. 
Consequently, with the semiconductor device 3 according to the third 
embodiment, accuracy and rapidity in detecting the temperature of the 
output transistor 3A is enhanced. 
Described below is a semiconductor device 4 according to a fourth 
embodiment of the present invention. 
FIGS. 4(a) and (b) show a semiconductor device 4 according to the fourth 
embodiment, in which FIG. 4(a) is a plan view thereof and FIG. 4(b) is a 
section taken along IV--IV in FIG. 4(a). For the brevity's sake, 
electrodes are omitted in FIG. 3(a). 
As shown in FIGS. 4(a) and (b), the semiconductor device 4 includes a 
collector region 40 which is an N-type silicon substrate, an annular first 
base region 41 of P.sup.+ region which is formed on a center part of the 
collector region 40, a C-shaped first emitter region 42 of N.sup.+ region 
which is formed on the first base region 41, a second base region 43 and a 
third base region 44 of P.sup.+ region which are formed in a vacant 
region within the first base region 41 on the collector region 40, a 
second emitter region 45 of N.sup.+ region which is formed on the second 
base region 43 and a third emitter region 46 of N.sup.+ region which is 
formed on the third base region 44. In FIG. 4(b), reference numeral 47 
indicates a channel stopper region formed on the collector region 40, and 
48 indicates an insulating film. 
A collector electrode 49 is provided at a lower surface of the collector 
region 40. A first base electrode 50 is provided on the first base region 
41. A first emitter electrode 51 is provided on the first emitter region 
42. A second base electrode 52 is provided on the second base region 43. A 
third emitter electrode 53 is provided on the third emitter region 46. The 
second emitter region 45 and the third base region 44 are electrically 
connected by a Darlington-connection electrode 54. 
A transistor 4A for output is, as an output semiconductor element, composed 
of the collector region 40, the first base region 41 and the first emitter 
region 42. A first transistor 4B is composed of the collector region 40, 
the second base region 43 and the second emitter region 45, and a second 
transistor 4C is composed of the collector region 40, the third base 
region 44 and the third emitter region 46. A temperature detection 
transistor is composed of the first transistor 4B and the second 
transistor 4C which are Darlington-connected with each other. As described 
above, the Darlington-connected transistors 4B, 4C are provided at the 
center part of the N-type silicon substrate which composes the collector 
region 40, and the output transistor 4A is provided so as to surround the 
Darlington-connected transistors 4B, 4C. 
Discussed below is about a method of manufacturing the semiconductor device 
4 according to the fourth embodiment. 
First, boron is selectively diffused in an N-type silicon substrate (to be 
the collector region 40 of NPN transistor) in which phosphorus is diffused 
so as to concurrently form the first base region 41, the second base 
region 43 and the third base region 44 which are separated from one 
another. 
Next, phosphorus is selectively diffused in first, second and third base 
regions 41, 43, 44 to form the first emitter region 42, the second emitter 
region 45 and the third emitter region 46 respectively. Thereafter, the 
collector electrode 49, the first base electrode 50, the first emitter 
electrode 51, the second base electrode 52 and the third emitter electrode 
53 are provided respectively at the lower surface of the collector region 
40, on the first base region 41, on the first emitter region 42, on the 
second base region 43 and on the third emitter region 46. Further, the 
Darlington-connection electrode 54 is provided over the third base region 
44 and the second emitter region 45. 
In this way, the output transistor 4A of NPN transistor and first and 
second transistors 4B, 4C of NPN transistor are formed respectively on the 
N-type silicon substrate. 
Discussed below is about a comparable test conducted for evaluating the 
semiconductor device 4 according to the fourth embodiment. 
When the output transistor in the semiconductor device 4 according to the 
fourth embodiment and an output transistor in a conventional semiconductor 
device shown in FIGS. 8(a), (b) are operated at 40W (20V voltage, 2A 
current, 50% duty) and at 60W (20V, 3A current, 50% duty), forward 
voltages V.sub.BE at PN junction parts of the respective temperature 
detection transistors are measured. Also, when the output transistors 
operated in steady state in the respective semiconductor devices are 
measured, using a thermocouple. The respective measured results are shown 
in Table 1. 
TABLE 1 
______________________________________ 
V.sub.BE and temperature of 
temperature 
temperature detection of output 
operation 
transistor transistor 
current 4th embodiment 
conventional (actual) 
______________________________________ 
40 W 680 mV (170.degree. C.) 
300 mV (150.degree. C.) 
170.degree. C. 
60 W 920 mV (230.degree. C.) 
400 mV (200.degree. C.) 
230.degree. C. 
______________________________________ 
As cleared from Table 1, the semiconductor device according to the fourth 
embodiment measures the temperature of the output transistor accurately, 
compared with the conventional semiconductor device. 
FIG. 5 shows a comparison of dispersion of the detected temperature in the 
semiconductor devices according to first and second embodiments with that 
of the detected temperature in a conventional semiconductor device having, 
as a temperature sensor, a thermistor at an external part of a 
semiconductor substrate. Wherein, the semiconductor devices according to 
first and second embodiments and the conventional semiconductor are 
employed to an audio system, and the temperature of the output transistor 
is set to be 210.degree. C. 
In FIG. 5, P indicates a distribution area of the detected temperature in 
the semiconductor devices according to first and second embodiments, and Q 
indicates a distribution area of the detected temperature in the 
conventional semiconductor device. As shown in FIG. 5, the dispersion of 
the detected temperature in first and second embodiments is remarkably 
smaller than that in the conventional semiconductor device. 
FIG. 6 shows a comparison of dispersion of the detected temperature in the 
semiconductor device according to the third embodiment with that in the 
conventional semiconductor device. Wherein, the semiconductor device 
according to the third embodiment and the conventional semiconductor 
device are employed to an audio system, and the temperature of the output 
transistor is set to 210.degree. C. 
As shown in FIG. 6, the dispersion area of the detected temperature in the 
semiconductor device according to the third embodiment is smaller than 
that in the conventional semiconductor device, but is not so small as that 
according to first and second embodiments. 
FIG. 7 shows a comparison of dispersion of the detected temperature in the 
semiconductor device according to the fourth embodiment with that in the 
conventional semiconductor device. Wherein, the semiconductor device 
according to the fourth embodiment and the conventional semiconductor 
device are employed to an audio system, and the temperature of the output 
transistor is set to 210.degree. C. 
As shown in FIG. 7, the dispersion of the detected temperature in the 
semiconductor device according to the fourth embodiment is remarkably 
smaller than that in the conventional semiconductor device, and smaller 
than that according to first and second embodiments. 
A high voltage is applied to the audio system generally even at a normal 
operation, so that the temperature of the semiconductor device built in 
the audio system is increased, compared with semiconductor devices built 
in other systems. This means that fault of the audio system often happens 
owing to the semiconductor device. When the semiconductor device according 
to the present invention is built in the audio system, the fault 
occurrence rate of the audio system is lowered. 
In first to fourth embodiments, a single transistor formed on the 
semiconductor substrate is used as the output transistor. However, the 
equivalent effect is obtained even when the Darlington transistor is 
formed thereon as the output transistor, instead of the single transistor.