Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2000-06-08
2002-07-23
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S328000, C438S372000, C257S109000, C257S471000
Reexamination Certificate
active
06423598
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Schottky diode having a structure for improving a reverse bias characteristic in particular, and more particularly, it relates to a Schottky diode employed in combination with an insulated gate semiconductor device. It also relates to a technique of protecting an insulated gate semiconductor device.
2. Discussion of the Background
FIG.
16
and
FIG. 17
are sectional views showing the structures of conventional Schottky diodes
100
and
101
respectively. In each of the Schottky diodes
100
and
101
, an oxide film
35
is selectively formed on an N
−
-type semiconductor substrate
11
, and a boundary layer
14
called a Schottky region is provided on a surface of the semiconductor substrate
11
not formed with the oxide film
35
. The boundary layer
14
can be formed by diffusing platinum into the surface of the semiconductor substrate
11
, for example. An anode electrode
51
is in contact with the boundary layer
14
and provided on its upper portion while covering part of the oxide film
35
.
In the Schottky diode
101
, a P-type impurity region
15
extending over the oxide film
35
and the boundary layer
14
is further provided in the surface of the semiconductor substrate
11
. This functions as a guard ring controlling the shape of a depletion layer and preventing an electric field from concentrating to the boundary layer
14
and reducing voltage resistance when a reverse bias is applied to the Schottky diode
101
. Hence the voltage resistance of the Schottky diode
101
becomes higher than the voltage resistance of the Schottky diode
100
.
In the case of applying a prescribed anode voltage V
AK
between the anode electrode
51
and the semiconductor substrate
11
as a forward bias, the Schottky diode
100
or
101
forwardly conducts when the anode voltage V
AK
exceeds a certain threshold. The threshold voltage at this time depends on the barrier height of the formed boundary layer
14
. In general, the threshold voltage of a Schottky diode is desirably low for reduction of power consumption, and set to about 0.3 V, for example. In the Schottky diode
101
, therefore, the threshold voltage of a P-N junction, which becomes about 0.6 V, formed by the impurity region
15
and the semiconductor substrate
11
does not inhibit the Schottky diode
101
from turning ON.
When applying a voltage becoming a reverse bias between the anode electrode
51
and the semiconductor substrate
11
, on the other hand, no current flows in the Schottky diode
100
or
101
up to a breakdown voltage of a junction formed by the boundary region
14
and the semiconductor substrate
11
except a leakage current.
In order to reduce the cost, it is also possible not to form the boundary region
14
in the Schottky diode
100
but form the anode electrode
51
by an aluminum alloy, for example, and employ silicon as the semiconductor substrate
11
for structuring a diode. In this case, aluminum contained in the anode electrode
51
diffuses into the surface of the semiconductor substrate
11
, whereby the conductivity type of a region in the semiconductor substrate
11
being in contact with the anode electrode
51
becomes a P
−
type. The barrier height is low as compared with a general diode comprising a P-N junction, whereby the threshold is also small and a diode having characteristics approximate to a Schottky diode can be obtained. A diode of such a structure is tentatively referred to as “pseudo Schottky diode” in this specification.
FIG. 18
is a circuit diagram showing the structure of a circuit
400
sensing, when an overcurrent flows in an insulated gate transistor such as an IGBT
21
, for example, the current and protecting the IGBT
21
. The gate and the collector of a current detection IGBT
22
are connected to the gate and the collector of the IGBT
21
respectively. A power source
23
applying a voltage becoming a forward bias is provided between the collector and the emitter of the IGBT
21
. An end of a resistor
24
is connected to the gates of the IGBTs
21
and
22
, and the IGBTs
21
and
22
are driven under the control of a driving circuit (not shown) connected to the other end of the resistor
24
.
A current detection part
25
is connected between the gate and the emitter of the IGBT
22
. The current detection part
25
is formed by a resistor
26
, a Schottky diode
27
and a MOSFET
28
. The resistor
26
is connected between the emitter of the IGBT
22
and the emitter of the IGBT
21
, while the anode of the Schottky diode
27
is connected to the gates of the IGBTs
21
and
22
and the cathode is connected to the drain of the MOSFET
28
respectively. The source of the MOSFET
28
is connected to the emitter of the IGBT
21
and the gate is connected to the emitter of the IGBT
22
respectively.
Regarding the IGBT
21
as a body and the IGBT
22
as that for current detection in general, the two are generally structured in a combined manner. The structure of the circuit
400
in such a case is disclosed in Japanese Patent Laying-Open Gazette No. 8-148675, for example.
Depending on a current flowing in the IGBT
21
, a current flows also in the IGBT
22
, and the latter current develops a voltage drop in the resistor
26
. When this voltage drop exceeds the threshold of the gate of the MOSFET
28
, the MOSFET
28
turns on and a current flows from the resistor
24
through the Schottky diode
27
and the MOSFET
28
. Hence the gate potential of the IGBT
21
lowers, and it follows that the current flowing therein is suppressed.
Study is now made as to what kind of characteristics to have as the Schottky diode
27
.
FIG. 19
is a graph showing the relations between anode voltages V
AK
and logarithmic values log I of currents I flowing in diodes as to a plurality of types of diodes. A graph
91
shows the characteristic of a diode (hereinafter tentatively referred to as “P-N junction diode”) formed by a P-N junction, a graph
92
shows the characteristic of the Schottky diode
100
and the graph
93
shows the characteristic of the pseudo Schottky diode respectively.
The characteristic of the Schottky diode
101
is shown by synthesis of the graph
92
in a region where the anode voltage V
AK
is lower than branching of a curve
90
shown by a broken line, the curve
90
and the graph
91
in a region where the anode voltage V
AK
is higher than joining of the curve
90
. The reason why the characteristic of the Schottky diode
101
is shown by such a synthesized graph is that the P-N junction formed by the impurity region
15
and the semiconductor substrate
11
does not conduct but is substantially equal to the characteristic of the Schottky diode
100
in a region where the anode voltage V
AK
is relatively small while this P-N junction forwardly conducts and the characteristic of the diode formed by the P-N junction in which a large current flows becomes dominant in the region where the anode voltage V
AK
is relatively small.
FIG. 20
is a graph showing a current flowing in the IGBT
21
and a voltage generated between its collector and emitter in the case of employing the Schottky diode
100
as the Schottky diode
27
of the circuit
400
shown in
FIG. 18
, and units are arbitrary as to both of the current and the voltage. There is shown that a clamp operation as to the current flowing in the IGBT
21
is performed and protection against an overcurrent is normally performed. Both of FIG.
21
and
FIG. 22
show operation characteristics of the circuit
400
in the case of employing a pseudo Schottky diode as the Schottky diode
27
and the case of employing the Schottky diode
101
as the Schottky diode
27
, and correspond to FIG.
20
. There is shown that an oscillation phenomenon takes place although a clamp operation is performed as to the current in each case.
The pseudo Schottky diode and the Schottky diode
101
comprise P-N junctions dissimilarly to the Schottky diode
100
. Therefore, it is conceivable that, when a voltage of at
Takahashi Hideki
Tominaga Shuuichi
Lindsay Jr. Walter L.
Niebling John F.
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