Schottky barrier

Details Schottky barrier Schottky barrier

1998-06-02

2001-01-16

Chaudhuri, Olik (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Schottky barrier

C257S474000, C257S484000, C257S928000

Reexamination Certificate

active

06175143

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a pn junction diode having a pn junction, a Schottky diode having a Schottky junction, and a composite diode having both of the pn junction and Schottky junction, each of these diodes having a rectifying function.
BACKGROUND OF THE INVENTION
Diodes having a rectifying function are the most fundamental semiconductor elements or components, and various types of diodes are known which have different junction structures.
FIG. 37
is a cross-sectional view showing a pn junction diode
101
having a basic planar-type pn junction. To provide the diode
101
, a high-concentration n
+
cathode layer
1
is formed on one of opposite surfaces of a low-concentration n drift layer
2
, and a p anode region
3
is formed in a surface layer at the other surface of the n drift layer
2
. Cathode electrode
4
and anode electrode
5
are formed in contact with the surfaces of the n
+
cathode layer
1
and p anode region
3
, respectively. The diode
101
further includes an oxide film
6
that covers the surface of the pn junction, and a protective film
7
in the form of a nitride film. A p-type peripheral region
8
is formed in a peripheral portion of the pn junction diode
101
, and a peripheral electrode
11
is provided on the surface of the peripheral region
8
, to extend over a part of the oxide film
6
.
The n drift layer
2
is laminated by epitaxial growth on the n
+
cathode layer
1
as a substrate. For example, the impurity concentrations of the n
+
cathode layer
1
and n drift layer
2
are 1×10
19
cm
−3
, and 1×10
15
cm
−3
, respectively, and the thicknesses of these layers
1
,
2
are 450 &mgr;m and 10 &mgr;m, respectively. The p anode region
3
is formed by implanting p-type impurities, such as boron ions, using the oxide film
6
as a mask, and thermally diffusing the implanted ions. The p anode region
3
thus formed has a surface impurity concentration of 1×10
9
cm
−3
, and a diffusion depth of 3 &mgr;m.
The graph of
FIG. 38
shows a profile of the resistivity measured along a cross section of the pn junction diode
101
of FIG.
37
. In
FIG. 38
, the vertical axis indicates the thickness as measured from the surface of the semiconductor substrate including the n cathode layer
1
and n drift layer
2
, and the horizontal axis indicates the resistivity plotted on a logarithmic scale. As shown in the cross section, the diode
101
includes the p anode region
3
having a thickness of 3 &mgr;m as measured from the surface of the semiconductor substrate, n drift layer
2
having a thickness of about 60 &mgr;m, and the n cathode layer
1
having a low resistivity, which is formed under the n drift layer
2
. Generally, the resistivity of a portion of the surface of the p anode region
3
which has the lowest resistance is about 0.01 &OHgr;·m.
FIG. 39
is a cross-sectional view of a pn junction diode
102
which is a slightly modified example of the planar-type diode of FIG.
37
. As in the pn junction diode
101
of
FIG. 37
, a high concentration n
+
cathode layer
1
and a low-concentration n drift layer
2
constitute a semiconductor substrate, and a p anode region
3
is formed in a surface layer of the n drift layer
2
of the semiconductor substrate. The pn junction diode
102
is different from the diode
101
of
FIG. 37
in that a p ring region
12
having a ring-like shape and a large diffusion depth is formed at the outer periphery of the p anode region
3
. While breakdown of the pn junction diode of
FIG. 37
is likely to occur in the vicinity of the periphery of the p anode region
3
, the p ring region
12
having a larger diffusion depth than the p anode region
3
is formed in the diode of
FIG. 39
, so as to decrease the gradient of the impurity concentration, thereby to prevent occurrence of the breakdown at around the p anode region
3
. As a result, the breakdown occurs uniformly throughout the p anode region
3
.
FIG. 40
is a cross-sectional view of a pn junction diode
103
in which p high-concentration regions
13
having a high surface impurity concentration and a large diffusion depth are formed between p anode regions
3
having a low surface impurity concentration and a small diffusion depth, as disclosed in Shimizu et al., IEEE Trans. on Electron Devices ED-31, (1984) p. 1314). When rated current is applied to the diode, the current flows through the p anode regions
3
, and therefore the diode exhibits an excellent reverse recovery characteristic. In the reverse bias situation, a depletion layer spreads out from the p high-concentration regions
13
, and thus the diode shows a high breakdown voltage. The p high-concentration regions
13
may also serve as the p ring region
12
as described above.
FIG. 41
is a cross-sectional view of a Schottky diode
104
having a basic Schottky junction. To form the diode
104
, a Schottky electrode
15
made of a metal, such as molybdenum, which provides a high Schottky barrier, is formed on a surface of a low-concentration n drift layer
2
. A cathode electrode
4
is provided on the rear surface of a n
+
cathode layer
1
. A p ring region
12
is formed in a surface layer of the n drift layer
2
so as to surround a contact portion of the Schottky electrode
15
. With the p ring region
12
thus provided, an electric field is prevented from concentrating at the edge of the Schottky electrode
15
, and the breakdown voltage of the resulting diode can be increased.
The n drift layer
2
is laminated by epitaxial growth on the high-concentration n
+
cathode layer
1
serving as a substrate. For example, the n
+
cathode layer
1
has a resistivity of 0.004 &OHgr;·cm, and a thickness of 350 &mgr;m, and the n drift layer
2
has a resistivity of 0.90 &OHgr;·cm, and a thickness of 7 &mgr;m.
The graph of
FIG. 38
also shows a profile of the resistivity measured along a cross section of the Schottky diode
104
of FIG.
41
. The vertical axis indicates the depth as measured from the surface of the semiconductor substrate comprising the n
+
cathode layer and n drift layer
2
, and the vertical axis indicates the resistivity plotted on a logarithmic scale. In the case of a Schottky diode having a breakdown voltage of 60V, for example, the n drift layer
2
having a resistivity of 0.9 &OHgr;·cm extends from the surface of the semiconductor substrate to a depth of about 7 &mgr;m, and the n
+
cathode layer
1
having a resistivity of 0.004 &OHgr;·cm is formed under the n drift layer
2
.
FIG. 42
is a cross-sectional view showing a Schottky diode
105
as a slightly modified example of the Schottky diode
104
of FIG.
41
. In the diode
105
, trenches
16
are formed in a surface layer of the n drift layer
2
, and a Schottky electrode
15
made of molybdenum, for example, is formed on the surface of the n drift layer
2
and the inner walls of the trenches
16
. With the trenches
16
thus provided, a contact area of the Schottky electrode
15
is increased, thereby to increase the current capacitance.
FIG. 43
is a cross-sectional view of a composite diode
106
having a pn junction and a Schottky junction. In the composite diode
106
, a relatively wide p ring region
12
is formed in a surface layer of an n drift layer
2
so as to surround a contact portion of a Schottky electrode
15
, such that the Schottky electrode
15
is in contact with the surface of the p ring region
12
as well as the n drift layer
2
, as disclosed in Zettler, R. A. et al.: IEEE Trans. on Electron Devices ED-16, (1969) p. 58. In this case, the p ring region
12
provides a p anode region
3
of a pn junction diode. Thus, the composite diode, in which the pn junction and Schottky junction are combined, provides a low forward voltage when it is forward biased, a high breakdown voltage, and an effect of reducing noise.
FIG. 44
is a cross-sectional view of a composite diode
107
which is a modified example of the composite diode of FIG.
43
. In this example, not only the p ring re

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