Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-12-09
2004-07-13
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S356000, C257S550000, C257S551000, C257S175000
Reexamination Certificate
active
06762461
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device including a protective circuit, and more particularly to a semiconductor device including a protective circuit for preventing breakdown of an insulated gate bipolar transistor which is caused by a stress due to flow of an overcurrent or application of an overvoltage, for example.
2. Description of the Background Art
There has conventionally been a protective circuit which is formed in the same semiconductor device that includes an IGBT (Insulated Gate bipolar Transistor) and functions to protect the IGBT from a stress due to flow of an overcurrent or application of an overvoltage.
FIGS. 11 through 13
illustrate such a conventional protective circuit.
FIG. 11
is a circuit diagram of the conventional protective circuit.
FIG. 12
is a sectional view of a portion of a semiconductor device in which the conventional protective circuit is formed.
FIG. 13
is a plan view of the portion of the semiconductor device illustrated in FIG.
12
. The sectional view of
FIG. 12
is taken along a line XII—XII of FIG.
13
.
In the conventional protective circuit illustrated in
FIG. 11
, a terminal P for establishing connection with an external power supply (which will be hereinafter referred to as an “external connection terminal”) is connected to one end of a resistor R
1
, the other end of which is connected to a cathode of a first zener diode D
1
and a semiconductor element such as an IGBT not shown. An anode of the first zener diode D
1
is connected to any kind of a constant potential node such as a ground.
Next, a structure of the semiconductor device in which the conventional protective circuit illustrated in
FIG. 11
is formed will be described with reference to the sectional view of the semiconductor device in FIG.
12
.
An n
+
-type semiconductor layer
2
is formed on a p-type semiconductor substrate
1
by using epitaxial growth. On the n
+
-type semiconductor layer
2
, an n
−
-type semiconductor layer
3
is formed by using epitaxial growth. An oxide film
4
is formed on the n
−
-type semiconductor layer
3
, and a polysilicon region is provided in a portion of the oxide film
4
. A p-type diffusion layer
5
and an n
+
-type diffusion layer
6
are formed in respective predetermined portions of the polysilicon region by diffusing impurities into the corresponding portions.
Further, an insulating film
7
is formed so as to cover respective top faces of the oxide film
4
, the p-type diffusion layer
5
and the n
+
-type diffusion layer
6
. Contact holes extending from a surface of the insulating film
7
and respectively reaching the diffusion layers
5
and
6
are formed in respective predetermined portions of the insulating film
7
. Each of interconnects
8
and the external connection terminal P is formed by filling each of the contact holes with a conductor such as a metal in a predetermined pattern.
Moreover, an electrode
10
used for an IGBT or the like is formed on a back face of the p-type semiconductor substrate
1
.
In the semiconductor device with the foregoing structure, a junction between the p-type diffusion layer
5
and the n
+
-type diffusion layer
6
forms the first zener diode D
1
, while the n
+
-type diffusion layer
6
which connects one of the interconnects
8
and the external connection terminal P forms the resistor R
1
.
The one of the interconnects
8
which is connected to the n
+
-type diffusion layer
6
is to be connected to the IGBT, while another one of the interconnects
8
which is connected to the p-type diffusion layer
5
is to be connected to a constant potential node.
FIG. 13
is a plan view of the foregoing configuration, in which the insulating film
7
covering the diffusion layers
5
and
6
are omitted for purposes of clarifying the formation of the first zener diode D
1
and the resistor R
1
.
In the protective circuit with the above-mentioned configuration, the first zener diode D
1
is formed in order to protect the IGBT from a stress due to application of an overvoltage which is induced by a surge such as static electricity externally supplied.
More specifically, upon application of an overvoltage which is induced by a surge such as static electricity via the external connection terminal P, a zener breakdown takes place in the first zener diode D
1
, to absorb the overvoltage induced by a surge as applied. This prevents a voltage equal to or higher than a breakdown voltage from being applied to the IGBT. Accordingly, it is possible to prevent the IGBT from being broken down under a stress due to application of an overvoltage which is induced by a surge.
On the other hand, the resistor R
1
is formed in order to protect the first zener diode D
1
from a stress due to flow of an overcurrent from an external power supply.
More specifically, the resistor R
1
is formed in order to provide for occurrence of an event where an external power supply Vd for driving the IGBT is improperly connected in a direction contrary to a normal direction as illustrated in FIG.
14
. In such an event, even if a direct current I continues to flow from the external power supply Vd for a predetermined time, a value (flow rate) of the direct current I is limited to a degree where the first zener diode D
1
is not broken, by the resistor R
1
.
As described above, according to the conventional protective circuit, the first zener diode D
1
is formed in order to protect a semiconductor element such as an IGBT from a stress due to application of an overvoltage which is induced by a surge such as static electricity, and the resistor R
1
is formed in order to protect the first zener diode D
1
from a stress due to flow of an overcurrent which may possibly be caused by improper connection of the external power supply Vd.
However, if an overvoltage is applied to the conventional protective circuit including the resistor R
1
and the first zener diode D
1
as illustrated in
FIG. 11
because of occurrence of a surge, a voltage difference which is equal to a difference between the applied overvoltage and a breakdown voltage is provided between opposite ends of the resistor R
1
.
As a result, an electric power is generated due to the voltage difference between the opposite ends of the resistor R
1
so that the resistor R
1
is excessively heated. Thus, if an overvoltage equal to or higher than a predetermined voltage is applied to the protective circuit because of occurrence of a surge, the resistor R
1
is likely to be burnt and be disconnected from the protective circuit. Thus, because of the formation of the resistor R
1
within the protective circuit, an entire voltage tolerance of the protective circuit to a stress due to application of an overvoltage induced by a surge is degraded.
In order to improve the voltage tolerance of the protective circuit including the resistor R
1
to a stress due to application of an overvoltage induced by a surge, minimization of a resistance of the resistor R
1
may be effective on one hand. However, on the other hand, minimization of a resistance of the resistor R
1
would reduce a current (flow rate) tolerance of the protective circuit for protecting the first zener diode D
1
from a stress due to flow of an overcurrent which is induced by improper connection of an external power supply. In view of this, there are limits to how much the voltage tolerance and the current tolerance can be improved, respectively.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor device including a protective circuit capable of improving a voltage tolerance of the protective circuit while maintaining a predetermined current tolerance of the protective circuit.
According to the present invention, a semiconductor device with a semiconductor element formed on a semiconductor substrate and a protective circuit for the semiconductor element includes a resistor, a first zener diode and a plurality of second zener diodes. The resistor has one en
Dickey Thomas L
Flynn Nathan J.
Mitsubishi Denki & Kabushiki Kaisha
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