Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1998-08-31
2004-04-20
Nadav, Ori (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S329000, C257S449000
Reexamination Certificate
active
06724039
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to both discrete devices and integrated circuits, and more particularly to a merged semiconductor device having a Schottky diode and a method for forming the device. The semiconductor device uses the layout area more efficiently, and thus may be smaller, than prior-art merged semiconductor devices that include Schottky diodes.
BACKGROUND OF THE INVENTION
MOS-gated semiconductor devices, such as transistors, IGBTs, and MCTs, are used in many of today's electronic applications. For example,
FIGS. 1A and 1B
are respectively a cross-sectional and a schematic symbol of a vertical DMOS transistor
10
. The transistor
10
includes a drain contact
12
, which is disposed on a side of an N+ semiconductor substrate
14
. An N− epitaxial layer
16
is disposed on the other side of the substrate
14
such that the layer
16
acts as the drain and the substrate
14
acts as the drain contact region of the transistor
10
. P-body regions
18
are disposed in the layer
16
, and N+ source regions
20
are disposed in the body regions
18
. A gate
21
is disposed over the body regions
18
and is isolated therefrom by a gate insulator
22
. Source/body contacts
23
are disposed on the layer
16
in contact with both the body regions
18
and the source regions
20
. These contacts allow the body and source regions
18
and
20
to be biased to the same voltage as is desired in many applications. As a consequence, however, the body regions
18
form the anode and the layer
16
forms the cathode of a PN diode
24
. Fortunately, as discussed below, this “built-in” diode serves to protect the transistor
10
from damage if the source voltage exceeds the drain voltage. Furthermore, the gate
21
, contacts
23
, and the underlying body and source regions
18
and
20
may be cellular structures such as squares, hexagons, or octagons, may be a meshed structure, may be interdigitated or striped, or may be other well-known geometries.
During a typical period of operation, the voltage on drain contact
12
is more positive than the source voltage on the source/body contacts
23
, and the gate voltage on the gate
21
is greater than or equal to one threshold voltage above the source voltage. These conditions cause a channel region to form at the tops of the body regions
18
between the respective source regions
20
and the drain layer
16
so that a drain-to-source current flows from the drain contact
12
, through the substrate
14
, the layer
16
, the channel regions, and the source regions
20
, to contacts
23
.
Conversely, during a transient period, the source voltage of the transistor
10
may become greater than the drain voltage. With the diode
24
, however, if the source voltage would otherwise exceed the drain voltage by more than the forward voltage of the diode
24
(typically 0.7 V for a silicon diode), then the diode
24
conducts a current from the source contacts
23
, through the body regions
18
, layer
16
, and substrate
14
, to the drain contact
12
. Thus, the diode
24
limits the source-to-drain voltage to approximately one diode drop.
Unfortunately, the conduction of a current by the diode
24
during such a transient period may adversely affect the subsequent operation of the transistor
10
. More specifically, when the diode
24
conducts a current to limit the source-to-drain voltage of the transistor
10
, minority carriers, here “holes”, are injected from the P body regions
18
into the N− drain layer
16
. In some instances, the minority carriers in the drain layer
16
will continue to support a flow of current through the diode
24
even after the source voltage becomes less than the drain voltage. In some applications, this continuing current flow may hinder or prevent the desired operation of the transistor
10
.
To prevent the diode
24
from conducting current when the source voltage exceeds the drain voltage, a Schottky diode having a lower forward voltage can be added in parallel to the diode
24
. As discussed below, because of its lower forward voltage, the Schottky diode will both protect the transistor
10
and prevent the diode
24
from conducting a current. A Schottky diode also does not introduce minority carriers into the drain region
16
, preventing the problems that occur when minority carriers are introduced by a PN junction.
FIGS. 2A-2B
are respectively a cross-section and a schematic symbol of a vertical DMOS transistor
30
, which is similar to the transistor
10
of
FIGS. 1A-1B
except that it includes a built-in Schottky diode
32
. The Schottky diode
32
is shown in the exploded section of FIG.
2
A and in FIG.
2
B. For clarity, like reference numerals are used
FIGS. 2A-2B
for elements common to
FIGS. 1A-1B
.
Referring to
FIG. 2A
, the transistor
30
has outer source/body regions
34
, which include N+ source regions
36
and P body regions
38
, and also has inner source/body regions
40
, which include N+ source regions
42
and a P body regions
43
. A gate
44
is disposed over the P body regions
38
and
43
and is insulated therefrom by a gate insulator
45
. Outer source/body contacts
46
contact the source regions
36
and the body regions
38
, and a source/body/Schottky contact
48
contacts the source regions
42
and the body regions
43
as well as the drain layer
16
. During operation, the contacts
46
and
48
are electrically coupled together. The contact
48
includes a Schottky contact
50
, which contacts the drain layer
16
. Thus, the contact
50
forms the anode and the drain layer
16
forms the cathode of the Schottky diode
32
. The contact
50
also contacts the source and body regions
42
and
43
, and thus acts as an ohmic contact thereto. The contact
48
also includes a layer
51
of metal disposed on the Schottky contact
50
. A built-in PN junction diode
52
, which is similar to the diode
24
of
FIGS. 1A-1B
, is formed by parallel diodes
53
and
54
. The P body regions
38
and
43
form the anodes of the diodes
53
and
54
, respectively, and the N− drain layer
16
forms a common cathode for the diodes
53
and
54
. As discussed below, so that the diode
52
does not turn on during a transient period, the Schottky diode
32
is constructed to have a lower forward voltage than the PN junction diode
52
. For example, using conventional techniques, the Schottky diode
32
can be constructed to have a forward voltage of 0.3-0.5V, which is less than the 0.7V forward voltage of the diode
52
. The device shown in cross-section in
FIG. 2A
may have any of the surface geometries that the device of
FIG. 1A
has.
During a typical period of operation, the transistor
30
operates in a manner similar to that described above for the transistor
10
.
During a transient period, if the source voltage would otherwise exceed the drain voltage by more than the forward voltage of the Schottky diode
32
, then the diode
32
conducts a current from the metal
51
, through the Schottky contact
50
, layer
16
, and substrate
14
, to the drain contact
12
. Thus, the diode
32
protects the transistor
30
by limiting the source-to-drain voltage to the diode
32
forward voltage. Furthermore, because the forward voltage of the Schottky diode
32
is less than that of the diode
52
, the diode
52
does not turn on, and thus does not cause minority carriers to be injected into the layers
14
and
16
.
Unfortunately, the Schottky diode
32
occupies a relatively large layout area, and thus significantly increases the layout area of the transistor
30
as compared to the transistor
10
of
FIGS. 1A-1B
. Furthermore, the reverse breakdown voltage of the Schottky diode
32
—the maximum value by which the voltage on the drain layer
16
can exceed the voltage on the contact
50
without causing the diode
32
to break down—is often relatively low. Thus, Schottky diode
32
may lower the maximum drain-to-source voltage of the transistor
30
below that of the transistor
10
. Additionally, the processes a
Carlson David V.
Jorgenson Lisa K.
Nadav Ori
STMicroelectronics Inc.
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