Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-12-03
2004-11-16
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S359000
Reexamination Certificate
active
06818954
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2001-369980, filed Dec. 4, 2001 in the Japanese Patent Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lateral MOSFET, and more particularly to a lateral high breakdown voltage MOSFET with a breakdown voltage between a source and a drain of equal to or more than several tens of volts, or to a semiconductor device including the lateral high breakdown voltage MOSFET.
2. Description of the Related Art
FIG. 9
is a cross sectional view of a p-type lateral high breakdown voltage MOSFET
600
as a first example of a conventional one. The lateral high breakdown voltage MOSFET
600
has a p-type drain diffused layer
614
with a depth of about 1 mm formed by diffusion in a desired region of an n-type semiconductor substrate
601
, and an n-well layer
605
formed by diffusion in a similar way on each surface side of the substrate
601
so as to surround an outer periphery of the drain diffused layer
614
. In the n-well layer
605
, a p-type source diffused layer
609
is formed from on a surface of the substrate
601
in a region apart by a specified distance from the above-described drain diffused layer
614
. In the drain diffused layer
614
, a p-type drain contact layer
610
is formed on the surface of the substrate
601
in a central region at an approximately equal distance from the boundary with the above-described n-well layer
605
.
Moreover, on the surface of the n-well layer
605
, a gate oxide film
607
is formed from a top end of the source diffused layer
609
over a top part of the drain diffused layer
614
. On a surface of the drain diffused layer
614
, in a region without the drain contact layer
610
and the gate oxide film
607
being formed, there is formed a field oxide film
606
.
A gate electrode
608
is formed above the gate oxide film
607
so as to project a part of the field oxide film
606
. On the source diffused layer
609
and on the drain contact layer
610
, a source electrode
612
and a drain electrode
613
are formed, respectively. The reference numeral
611
denotes an n+ contact layer on the n-well layer
605
.
FIG. 10
is a view showing equipotential lines (20V interval) in a reverse-biased state in which a voltage of 100V is applied to the source electrode
612
and the gate electrode
608
, and 0V to the drain electrode
613
, with the lateral high breakdown voltage MOSFET
600
being turned off. A depletion layer expands on both sides from pn junctions between the p-type drain diffused layer
614
and the n-type semiconductor substrate
601
, and the p-type drain diffused layer
614
and the n-well layer
605
. In
FIG. 10
, the equipotential lines of 0V and 100V are approximately equal to respective ends of the depletion layer.
Optimization in the lateral high breakdown voltage MOSFET is to find a structure where a breakdown voltage of an element becomes maximum. Optimization using a RESURF (Reduced Surface Field) structure is known by the reference “High Voltage Thin Layer Device” (IEDM Proceedings, 1979, pp. 238-241).
In the first example of a conventional MOSFET shown in
FIG. 9
, there is formed on the n-type semiconductor substrate
601
the drain diffused layer
614
, which corresponds to a drift region. Therefore, in order to cancel charges of n-type impurities in the n-type semiconductor substrate
601
, a total amount of p-type impurities in the drain diffused layer
614
is established as being about 1×10
12
/cm
2
which is made optimum in the above-described reference. Here, a total amount of the above-described impurities can be obtained by integrating a profile of the concentration (cm−3) in the drain diffused layer
614
with respect to a depth of the drain diffused layer
614
. Thus, the depletion layer at the reverse-biased state is to extend mainly toward the drain diffused layer
614
. Moreover, the gate electrode
608
, being formed so as to project onto the field oxide film, provides a structure in which a field plate effect is obtained to make the depletion layer to easily extend into the drain diffused layer
614
for lessening an electric field near the surface thereof.
The lateral high breakdown voltage MOSFET
600
shown in
FIG. 9
as the first example of conventional one, has a breakdown voltage of about 110V. To ensure the breakdown voltage, a projection (a distance indicated by “a” in
FIG. 9
) of the drain diffused layer
614
toward the n-well layer
605
and a channel length (a distance indicated by “b” in
FIG. 9
) determined by a distance from an end of the source diffused layer
609
to the drain diffused layer
614
are established as being in the order of 6 mm and 3 mm, respectively.
FIG. 11
is a cross sectional view of a second conventional p-type lateral high breakdown voltage MOSFET
700
. The lateral high breakdown voltage MOSFET
700
has an n-well layer
705
deeply formed by diffusion in a desired region of a p-type semiconductor substrate
701
on an order of 10 mm from a surface of the semiconductor substrate
701
, and a p-type drain diffused layer
714
with a depth of about 1 mm formed by diffusion on the surface side in the n-well layer
705
. In the n-well layer
705
, a p-type source diffused layer
709
is formed on a side of the substrate
701
surface in a region apart by a specified distance from a boundary of the drain diffused layer
714
. In the drain diffused layer
714
, a p-type drain contact layer
710
is formed on a side of the substrate surface
701
in a central region at an approximately equal distance from the boundary of the above-described n-well layer
705
.
Moreover, on a surface of the n-well layer
705
, a gate oxide film
707
is formed on an end of the source diffused layer
709
over a part of the drain diffused layer
714
. On a surface of the drain diffused layer
714
, in a region without the drain contact layer
710
and the gate oxide film
707
being formed, a field oxide film
706
is formed.
A gate electrode
708
is formed from above the gate oxide film
707
so as to project a part of the field oxide film
706
. On the source diffused layer
709
and on the drain contact layer
710
, a source electrode
712
and a drain electrode
713
are formed, respectively. The reference numeral
711
denotes an n+ contact layer of the n-well layer
705
.
FIG. 12
is a view showing equipotential lines (20V interval) in a reverse-biased state in which a voltage of 100V is applied to the source electrode
712
and the gate
708
, and 0V to the drain electrode
713
, with the lateral high breakdown voltage MOSFET
700
being turned off. A depletion layer expands on both sides from a pn junction between the p-type drain diffused layer
714
and the n-well layer
705
. In
FIG. 12
, the equipotential lines of 0V and 100V are approximately equal to respective ends of the depletion layer. Moreover, when the p-type semiconductor substrate
701
is set at 0V, the p-type semiconductor substrate
701
and the n-well layer
705
are reverse-biased as shown in
FIG. 12
to extend the depletion layer also to the pn junction.
In the second example of a conventional MOSFET shown in
FIG. 11
, the drain diffused layer
714
, which corresponds to a drift region, is formed in the n-well layer
705
. Therefore, in order to cancel charges of n-type impurities in the n-well layer
705
, a total amount of p-type impurities in the drain diffused layer
714
is established as being about 1×10
12
/cm
2
which is made optimum in the above-described reference. Thus, the depletion layer at the reverse-biased state extends mainly toward the drain diffused layer
714
.
Moreover, the gate electrode
708
, being formed so as to project onto the field oxide film
706
, provides a structure in which a field plate effect is obtained to make the depletion layer to easily extend into the drain diffused layer
714
to lessen an electric field near a surface.
Th
Saito Masaru
Tada Gen
Fuji Electric & Co., Ltd.
Nguyen Thinh T
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