Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – Combined with field effect transistor
Reexamination Certificate
2000-04-13
2002-09-03
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
Combined with field effect transistor
C257S147000, C257S150000, C257S127000, C257S181000, C257S182000, C257S719000, C257S768000
Reexamination Certificate
active
06445013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gate commutated turn-off semiconductor device comprising a gate commutated turn-off (GCT) semiconductor switching element capable of commutating most of a main current flowing between an anode and a cathode at a turn-on into a gate side at a turn-off.
2. Description of the Background Art
In a prior-art GTO (Gate Turn-Off) thyristor, to give a signal to a gate electrode, a method of gate connection from one direction (see, for example, a technique disclosed in Japanese Patent Application Laid Open Gazette No. 56-125863 and the like) has been widely adopted. In such a structure, however, it is difficult to immediately stop a main current flowing between an anode and a cathode at a turn-off because of large inductance in a gate of an element.
For this reason, a GCT thyristor which allows reduction of gate inductance in an element has been developed. The GCT thyristor adopts a connection structure comprising a ring-shaped gate connection structure, a ring-shaped gate connection terminal formed on a gate drive substrate and a gate driver for controlling a current flowing in the gate (see, for example, techniques disclosed in Japanese Patent Application Laid Open Gazette Nos. 10-294406 and 8-330572 and the like), instead of the method of drawing a gate current from one direction. This makes it possible to reduce the inductance of a loop including the GCT thyristor, the gate drive substrate and the gate driver (referred to as inductance on the gate side) to about a hundredth of that of the GTO thyristor.
In the GCT thyristor, with the inductance value on the gate side remarkably reduced to be lower than that of the GTO thyristor, a gate reverse current rise rate (di
GQ
/dt) at a turn-off is raised up to a value about hundredth times as high as that of the GTO thyristor and almost all the main current can be thereby commutated into the gate side in a short time at the turn-off. In other words, it is possible to cut the time required to turn off and make the value of a turn-off gain almost one. Thus, the turn-off characteristics can be improved.
Further, with this, it is possible to suppress a breakdown due to local heat generation inside a semiconductor substrate and as a result, it also becomes possible to control a large current.
FIG. 11
is a plan view showing an exemplary constitution of a gate commutated turn-off semiconductor device including a GCT thyristor in the prior art. This gate commutated turn-off semiconductor device comprises a gate drive substrate
7
, a GCT thyristor
100
fixed to the gate drive substrate
7
and a gate driver
200
connected to the gate drive substrate
7
. Further, a case
13
is attached to the gate drive substrate
7
so as to cover a lower surface thereof. The case
13
also serves as a reinforcing member to prevent a bend of the gate drive substrate
7
due to a load of the gate driver
200
.
FIG. 12
is a cross section taken in the section line C—C of
FIG. 11
, and
FIG. 13
is a cross section showing an enlarged part of FIG.
12
. The GCT thyristor
100
comprises a disk-shaped semiconductor substrate (wafer)
24
having a pnpn structure and a gate region on its outer peripheral side, a cathode strain buffer plate
25
connected to a cathode region of the semiconductor substrate
24
and an anode strain buffer plate
26
connected to an anode region of the semiconductor substrate
24
, on its center portion. A cathode post electrode
2
is connected to the cathode strain buffer plate
25
and an anode post electrode
3
is connected to the anode strain buffer plate
26
. Further, a conductive cathode spacer
4
is connected to the cathode post electrode
2
and a cathode fin electrode
5
is connected to the cathode spacer
4
. An anode fin electrode
6
is connected to the anode post electrode
3
. The semiconductor substrate
24
, the cathode strain buffer plate
25
, the anode strain buffer plate
26
, the cathode post electrode
2
, the anode post electrode
3
and the cathode spacer
4
are sandwiched and pressed by the cathode fin electrode
5
and the anode fin electrode
6
.
The GCT thyristor
100
comprises a ring-shaped cathode flange
20
held by the cathode post electrode
2
penetrating therethrough and a ring-shaped anode flange
23
held by the anode post electrode
3
penetrating therethrough. An insulating tube
21
made of ceramics (e.g., alumina) is provided between the cathode flange
20
and the anode flange
23
. In
FIG. 12
, the semiconductor substrate
24
, the cathode strain buffer plate
25
, the anode strain buffer plate
26
, the cathode post electrode
2
and the anode post electrode
3
penetrate as a unit through the insulating tube
21
.
As shown in
FIG. 13
, a gate electrode
7
b
is formed on an upper surface of the gate drive substrate
7
to serve as a passage of a current between the gate driver
200
and a gate of the GCT thyristor
100
. On the other hand, a cathode electrode
7
a
is formed on a lower surface of the gate drive substrate
7
to serve as a passage of a current between the gate driver
200
and a cathode of the GCT thyristor
100
. Providing the cathode electrode
7
a
and the gate electrode
7
b
forms a loop between the gate and cathode of the GCT thyristor
100
and the gate driver
200
. With a gate current flowing into this loop at a commutation, the main current flowing between the cathode and anode of the GCT thyristor
100
is immediately stopped.
In the semiconductor substrate
24
of
FIG. 12
, the gate region is formed on a side of the cathode region and a ring-shaped gate electrode
29
is so formed as to be connected to the gate region. The gate electrode
29
is connected to an inner peripheral side of a ring-shaped gate flange
11
, and the gate flange
11
, being sandwiched by the insulating tube
21
, protrudes from a side surface of the insulating tube
21
and extends towards the outside of the insulating tube
21
. A portion of the gate flange
11
extendedly existing outside the insulating tube
21
is threaded into a conductive gate spacer
10
with a screw
12
. Further, the gate flange
11
is provided with a bend portion
11
a
to absorb oscillation and stress caused by a switching operation.
The gate spacer
10
is connected to the gate electrode
7
b
on the upper surface of the gate drive substrate
7
and threaded into the gate drive substrate
7
with a screw
9
. The cathode spacer
4
is connected to the cathode electrode
7
a
on the lower surface of the gate drive substrate
7
and threaded into the gate drive substrate
7
with the screw
9
.
Further, to prevent a short circuit between a pair of the cathode spacer
4
and the cathode electrode
7
a
and a pair of the gate spacer
10
and the gate electrode
7
b
due to presence of the screw
9
, a screw hole for the screw
9
is provided with an insulating bush
8
.
In the above-described gate commutated turn-off semiconductor device, the cathode spacer
4
has a function of holding a load of the gate driver
200
by fixing the gate drive substrate
7
and the case
13
to the GCT thyristor
100
. If only this function is needed, a case having a structure in which the case
13
and the cathode spacer
4
are formed as a unit may be used. The cathode spacer
4
, however, also has a function of achieving an excellent conductivity with both the cathode post electrode
2
and the cathode fin electrode
5
and a function of achieving an excellent conductivity with the cathode electrode
7
a
on the lower surface of the gate drive substrate
7
. To achieve such an excellent conductivity, it is necessary that the cathode spacer
4
should come into contact with respective surfaces of the cathode post electrode
2
, the cathode fin electrode
5
and the gate drive substrate
7
while keeping a highly precise flatness on its surface. For this reason, a conductive disk-like member having a thickness of 5 to 10 mm other than the case
13
is processed to be used as the cathode spacer
4
.
Further, as the gate spacer
10
, l
Jackson, Jr. Jerome
Mitsubishi Denki & Kabushiki Kaisha
Nguyen Joseph
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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