U.S. patent application number 10/016748 was filed with the patent office on 2003-05-01 for lateral power mosfet for high switching speeds.
This patent application is currently assigned to Power Integrations, Inc.. Invention is credited to Disney, Donald Ray, Grabowski, Wayne Bryan.
Application Number | 20030080388 10/016748 |
Document ID | / |
Family ID | 21778744 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030080388 |
Kind Code |
A1 |
Disney, Donald Ray ; et
al. |
May 1, 2003 |
LATERAL POWER MOSFET FOR HIGH SWITCHING SPEEDS
Abstract
A lateral power metal-oxide-semiconductor field effect
transistor (MOSFET) having a gate structure in which the insulated
gate is coupled to the gate electrode through contacts at a
plurality of locations. The source electrode includes first and
second segments. The first segment is interposed between the drain
electrode and the gate electrode and acts as a field plate.
Inventors: |
Disney, Donald Ray;
(Cupertino, CA) ; Grabowski, Wayne Bryan; (Los
Altos, CA) |
Correspondence
Address: |
BURGESS & BEREZNAK LLP
800 WEST EL CAMINO REAL
SUITE 180
MOUNTAIN VIEW
CA
94040
US
|
Assignee: |
Power Integrations, Inc.
5245 Hellyer Avenue
San Jose
CA
95138
|
Family ID: |
21778744 |
Appl. No.: |
10/016748 |
Filed: |
October 29, 2001 |
Current U.S.
Class: |
257/401 ;
257/E29.116; 257/E29.12; 257/E29.122; 257/E29.133; 257/E29.134;
257/E29.136; 257/E29.268 |
Current CPC
Class: |
H01L 29/41725 20130101;
H01L 29/42372 20130101; H01L 29/7835 20130101; H01L 29/402
20130101; H01L 29/4238 20130101; H01L 29/41775 20130101; H01L
29/41758 20130101; H01L 29/42368 20130101 |
Class at
Publication: |
257/401 |
International
Class: |
H01L 031/0328 |
Claims
I claim:
1. A field-effect transistor comprising: elongated source and drain
regions separated by a channel region; an insulated gate disposed
over the channel region; a drain electrode coupled to the drain
region; a gate electrode coupled to the insulated gate through
first and second contacts located at respective first and second
contact regions of the insulated gate, the first and second regions
being separated by a contact-free region; and a source electrode
coupled to the source region, the source electrode including first
and second segments, the first segment being interposed between the
drain electrode and the gate electrode.
2. The field-effect transistor according to claim 1 wherein the
second segment of the source electrode is disposed substantially
over the source region.
3. The field-effect transistor according to claim 2 wherein the
drain electrode is disposed substantially over the drain
region.
4. The field-effect transistor according to claim 3 wherein the
source and drain electrodes each include a field plate portion.
5. The field-effect transistor according to claim 1 wherein the
first and second segments are disposed on opposite sides of the
gate electrode.
6. The field-effect transistor according to claim 1 wherein the
first and second segments surround the gate electrode.
7. The field-effect transistor according to claim 1 wherein the
second segment is wider than the first segment.
8. The field-effect transistor according to claim 1 wherein the
source and drain regions are interdigitated.
9. The field-effect transistor according to claim 1 wherein the
insulated gate has a first end and second end opposite the first
end, the first contact region being located at the first end and
the second contact region being located at the second end.
10. The field-effect transistor according to claim 1 wherein the
insulated gate has a first end and second end opposite the first
end, the first contact region being located at the first end and
the second contact region being located at an intermediate point
between the first and second ends, and wherein the gate electrode
is further coupled to the insulated gate through a third contact
located at a third contact region of the insulated gate, the third
contact region being located at the second end.
11. A lateral field-effect transistor comprising: elongated source
and drain regions separated by a channel region; an insulated gate
disposed over the channel region, the insulated gate having a first
end region and a second end region opposite the first end region; a
drain electrode coupled to the drain region; a gate electrode
coupled to the insulated gate through first and second contacts
located at the first and second end regions of the insulated gate,
respectively; and a source electrode coupled to the source region,
the source electrode including first and second segments, the first
segment being interposed between the drain electrode and the gate
electrode.
12. The lateral field-effect transistor according to claim 11
wherein the second segment of the source electrode is disposed
substantially over the source region.
13. The lateral field-effect transistor according to claim 12
wherein the drain electrode is disposed substantially over the
drain region.
14. The lateral field-effect transistor according to claim 11
wherein the source and drain electrodes each include a field plate
portion.
15. The lateral field-effect transistor according to claim 11
wherein the first and second segments are disposed on opposite
sides of the gate electrode.
16. The lateral field-effect transistor according to claim 11
wherein the first and second segments surround the gate
electrode.
17. The lateral field-effect transistor according to claim 11
wherein the second segment is wider than the first segment.
18. The lateral field-effect transistor according to claim 11
wherein the source and drain regions are interdigitated.
19. The lateral field-effect transistor according to claim 11
wherein the first segment of the source electrode is spaced
equidistant from the gate and drain electrodes.
20. The lateral field-effect transistor according to claim 11
wherein the insulated gate has an intermediate region between the
first and second end regions, and wherein the gate electrode is
further coupled to the insulated gate through a third contact
located at the intermediate region.
21. A lateral power MOSFET comprising: an elongated source region
interdigitated between a pair of drain regions, the source region
being separated from the pair of drain regions by a channel region,
the elongated source region having a first and second ends; an
insulated gate disposed over the channel region, the insulated gate
having a first portion that extends from adjacent the first end to
adjacent the second end along one side of the elongated source
region and a second portion that extends from adjacent the first
end to adjacent the second end along an opposite side of the
elongated source region; a drain electrode coupled to the drain
region; a gate electrode coupled to the insulated gate through
first and second contacts located at opposite ends of the first
portion of the insulated gate; and a source electrode coupled to
the elongated source region, the source electrode including first
and second elongated segments, the first elongated segment being
interposed between the drain electrode and the gate electrode.
22. The lateral power MOSFET according to claim 21 wherein the
second segment of the source electrode is disposed substantially
over the source region.
23. The lateral power MOSFET according to claim 21 wherein the
drain electrode is disposed substantially over the drain
region.
24. The lateral power MOSFET according to claim 21 wherein the
source and drain electrodes each include a field plate portion.
25. The lateral power MOSFET according to claim 21 wherein the
first and second elongated segments are disposed on opposite sides
of the gate electrode.
26. The lateral power MOSFET according to claim 21 wherein the
first and second elongated segments surround the gate
electrode.
27. The lateral power MOSFET according to claim 21 wherein the
second elongated segment is wider than the first elongated
segment.
28. The lateral power MOSFET according to claim 21 wherein each of
the pair of drain regions is elongated.
29. The lateral power MOSFET according to claim 21 wherein the
first segment of the source electrode is spaced equidistant from
the gate and drain electrodes.
30. The lateral power MOSFET according to claim 21 wherein the
first portion of the insulated gate has an intermediate region
between the opposite ends, and wherein the gate electrode is
further coupled to the insulated gate through a third contact
located at the intermediate region.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to semiconductor devices
fabricated in a silicon substrate. More specifically, the present
invention relates to a high voltage field effect transistor with an
improved gate design.
BACKGROUND OF THE INVENTION
[0002] A common type of integrated circuit device is a
metal-oxide-semiconductor field effect transistor (MOSFET). A
MOSFET is a field effect device that includes a source region, a
drain region, a channel region extending between the source and
drain regions, and a gate provided over the channel region. The
gate includes a conductive gate structure disposed over and
separated from the channel region with a thin oxide layer.
[0003] Lateral field-effect transistors are widely used for high
voltage circuit applications, e.g., greater than 200 volts.
Examples of traditional lateral MOSFET device structures for power
applications include U.S. Pat. Nos. 5,869,875, 5,821,144,
5,760,440, and 4,748,936. Each of these devices has a source region
and a drain region separated by an intermediate region. A gate
structure is disposed over a thin oxide layer over the
metal-oxide-semiconductor (MOS) channel of the device. In the on
state, a voltage is applied to the gate to cause a conduction
channel to form between the source and drain regions, thereby
allowing current to flow through the device. In the off state, the
voltage on the gate is sufficiently low such that no conduction
channel is formed in the substrate, and thus no current flow
occurs. In this condition, high voltage is supported between the
drain and source regions.
[0004] Lateral power transistors are generally designed with source
and drain regions that are elongated, or much longer than they are
wide, and interdigitated. Such a device structure is disclosed in
U.S. Pat. No. 6,084,277, which is assigned to the assignee of the
present application. The '277 patent teaches a lateral power MOSFET
or transistor having an improved gate design that provides a large
safe operating area (SOA) performance level and high current
capability with moderate gate speed to suppress switching noise.
This is achieved by providing a metal gate electrode in parallel
with the polysilicon gate structure along the length of the power
MOSFET finger. The metal and polysilicon of the gate electrode and
structure, respectively, are connected using metal contacts that
are spaced apart along the gate structure. In one embodiment, the
'277 patent teaches locating contacts at multiple locations between
the gate electrode and gate structure along the power MOSFET finger
to improve the propagation of the gate signal along the length of
the finger for high switching speeds.
[0005] One drawback associated with the lateral power transistor
structure taught by the '277 patent is high gate-to-drain
capacitance due to the proximate location of the gate and drain
electrodes. The drain electrode serves as a drain field plate and
the gate and/or source electrodes serve as source field plates to
improve the breakdown voltage of these devices. Therefore, the
extent and spacing of these electrodes is determined largely by
breakdown voltage requirements. For instance, the '277 patent
teaches an example device capable of sustaining 700 volts between
the source and drain in the off state. Accordingly, this device
includes a relatively large spacing between the drain and gate or
source metal lines.
[0006] But in the case where the device is designed for a much
lower voltage, the closer spacing between the drain electrode and
the gate electrode results in high gate-to-drain capacitance. A
MOSFET designed with a breakdown voltage of 200 volts, for example,
might have a spacing of less than 5 microns between the drain and
gate electrode. Because these electrodes are commonly very long
(e.g., 300-400 mm) the capacitance between the drain electrode and
the gate or source electrode can be very large. This large
capacitance degrades the high-speed switching performance of the
transistor. High gate-to-drain capacitance is especially
problematic because it is amplified by the gain of the
transistor.
[0007] Therefore, what is needed is a high voltage power transistor
structure that achieves fast switching at high current conduction
levels with good propagation of gate signal. Such a device should
minimize drain-to-gate capacitance without increasing overall
device size or cell pitch (i.e., silicon "footprint").
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The present invention is illustrated by way of example, and
not limitation, in the figures of the accompanying drawings,
wherein:
[0009] FIGS. 1A-1C are three different topological views of an
interdigitated lateral power MOSFET in accordance with one
embodiment of the present invention.
[0010] FIG. 2 is a cross-sectional side view taken through cut
lines A-A' in the embodiment of FIG. 1A.
DETAILED DESCRIPTION
[0011] A high-voltage lateral power MOSFET with reduced
gate-to-drain capacitance for high switching speeds is described.
In the following description, numerous specific details are set
forth, such as material types, dimensions, structural features,
etc., in order to provide a thorough understanding of the present
invention. Practitioners having ordinary skill in the semiconductor
arts will understand that the invention may be practiced without
many of these details. In other instances, well-known elements,
techniques, and processing steps have not been described in detail
to avoid obscuring the invention.
[0012] The field effect transistor of the present invention
includes a metal gate electrode coupled with an insulated gate
disposed over a channel region that separates elongated source and
drain regions of the power MOSFET. In one embodiment the gate
comprises polysilicon, and the gate electrode is coupled to the
polysilicon gate using metal contacts (also known as vias) that are
located at opposite ends of a portion of the insulated gate
adjacent one side of the elongated source region. The source
electrode includes two segments: a wide segment that carries most
of the current flowing when the device is in the on state, and a
narrow segment interposed between the drain electrode and the gate
electrode that greatly reduces the drain-to-gate capacitance of the
device.
[0013] FIGS. 1A-1C are a topological view of an interdigitated
lateral power MOSFET in accordance with one embodiment of the
present invention. FIG. 1A provides a view of the upper portion of
the device, FIG. 1B is a view of the middle portion of the device,
and FIG. 1C shows the lower portion of the same transistor device.
Each of these views illustrates a computer-aided design (CAD)
layout of the drain, gate and source electrodes, the location of
the polysilicon gate structure, and the placement of a select
number of contacts or vias. It should be understood that the drain
and source contacts are not shown and the underlying substrate
diffusion regions are also omitted from FIGS. 1A-1C in the interest
of clarity. (During the following description, FIGS. 1A-1C should
be viewed together with FIG. 2, which is a side view of the
transistor taken through cut lines A-A', for a more comprehensive
understanding.)
[0014] FIG. 1A illustrates the layout of the upper end of one
embodiment of the interdigitated lateral power device of the
present invention. The field-effect transistor of FIG. 1A includes
an N-type source region 44 disposed in a semiconductor substrate
and connected to a source electrode 21 by contacts (not shown in
FIG. 1). Source electrode 21 comprises source electrode segments
21A and 21B. Source region 44 is interposed in the semiconductor
substrate between two drain regions 42, each of which is connected
to a drain electrode 22 by contacts (see FIG. 2).
[0015] Both of the source electrode segments 21A & 21B are
located between drain electrode segments 22A and 22B. FIG. 1C shows
the segments 22A and 22B merging into a single piece of metal at
the bottom of the transistor. It is appreciated that the device
layout structure of FIGS. 1A-1C only shows a portion of the
complete transistor in this embodiment. The full and complete
device comprises a repeated pattern of the interdigitated
source/drain/gate structures illustrated in the Figures. It is
understood that since the drain and source structures are much
longer than they are wide, they are often referred to as drain and
source "fingers".
[0016] The polysilicon material 28 that comprises the gate of the
transistor is depicted in FIGS. 1A-1C by dashed lines. The gate
itself comprises two parallel members 28A and 28B disposed over
channel region 49, extending the length of the source finger. FIG.
1C shows the polysilicon gate 28 wrapping around the far end, or
fingertip, of the source region where the member 28A merges with
member 28B. Members 28A and 28B many also be connected in the
middle portion of the transistor, as shown in FIG. 1B.
[0017] At the upper end of the transistor, the polysilicon material
of the gate extends under source electrode 21 and connects to metal
line 29 through a plurality of contacts 30. Metal line 29 couples
with control or switching circuitry used to drive the
transistor.
[0018] As shown in FIG. 2, the two polysilicon gate members 28A
& 28B are insulated from the underlying semiconductor material
by a gate dielectric layer 48. Dielectric layer may comprise
silicon dioxide, but other insulating materials, such as silicon
nitride, may also be used. A portion of each member 28A and 28B
extends over a thicker field oxide layer 41 for field plating
purposes. Each of the polysilicon gate members 28A and 28B is
covered with an interlayer dielectric 40 that insulates the gate
from the overlying metal gate and source electrodes, as can be seen
in the cross-sectional side view of FIG. 2.
[0019] The gate electrode 25 contacts one end of the polysilicon
gate adjacent the base of the source finger through contact 31, as
shown in FIG. 1A. In the illustrated embodiment, gate electrode
runs lengthwise from one end of gate member 28A to the opposite end
adjacent the fingertip of the source region. FIG. 1C shows gate
electrode 25 being connected to the opposite end of gate member 28A
through a pair of contacts 34. Note that gate electrode 25 ends
adjacent the fingertip of the source and does not extend over gate
member 28B, which runs alongside the opposite side of source region
44.
[0020] It is appreciated that the connection of gate electrode 25
in parallel with insulated gate member 28A advantageously
distributes the applied gate signal along the length of each finger
for improved high switching speed performance. Depending on the
length of the source, drain, and gate fingers, one or more
additional contacts may be located at intermediate points or
regions of the insulated gate between the opposite ends of the
finger. For example, FIG. 1B illustrates an optional additional
contact 33 connecting gate electrode 25 to gate member 28A at an
intermediate point approximately halfway between the two ends of
gate member 28A. Gate member 28B may also be connected to gate
member 28A in proximity to contact 33 to provide the same advantage
in distribution of the gate signal along member 28B.
[0021] To accommodate the placement of contact 33 the width of the
gate electrode 25 metal is made slightly wider to adequately
surround contact 33. To maintain design rule separation between the
source and gate electrodes, source electrode segment 21B is made
slightly narrower by a corresponding dimension. Note that no change
in the width of source electrode segment 21A (or drain electrodes
22A & 22B) is needed to accommodate the placement of additional
contact 33.
[0022] The gate electrode 25 is completely surrounded by source
electrode 21. The wider, current-carrying portion of the source
electrode is denoted as source electrode segment 21B in FIGS.
1A-1C. Segment 21B carries the vast majority of the source current
to the source bond pads (not shown) located at the upper end of the
source finger. The source electrode segment 21B also includes a
laterally extended portion that overlaps gate member 28B. This
laterally extended portion is located on the side of the finger
opposite gate electrode 25 and functions as a field plate. The main
source electrode segment 21B is disposed substantially over and
contacts the substrate source region 44.
[0023] The narrower portion of the source electrode is denoted as
source electrode segment 21A in FIGS. 1A-1C. Source electrode
segment 21A is interposed between gate electrode 25 and drain
electrode 22A. In the embodiment of FIGS. 1A-1C, source electrode
segment 21A has no contacts to the underlying source diffusion, and
does not carry a significant amount of source current. It functions
as a source field plate and also reduces the drain-to-gate
capacitance of the transistor. It should be understood that in
other embodiments, source electrode segment 21A may include
contacts to the source diffusion region and/or may carry more
significant amounts of source current.
[0024] In addition, although each of the source, drain, and gate
electrodes are shown in FIGS. 1 & 2 as comprising a single
layer of metal, alternative embodiments may utilize multiple levels
of conductor material for each or all of the electrodes.
[0025] FIG. 1C illustrates source electrode segment 21A wrapped
around the source fingertip region of the transistor and connected
to source electrode segment 21B. It should be understood that the
connection between segments 21A and 21B at this end of the source
finger is optional. That is, these two segments need not connect at
this point. However, source electrode segment 21A should be
connected to segment 21B at some point in the layout so that both
remain at substantially the same potential during device operation.
This is consistent with the purpose of source segment 21A to serve
as a source field plate interposed between the gate electrode 25
and the drain electrode 22.
[0026] FIG. 2 shows a cross-sectional view of the lateral power
transistor of FIGS. 1A-1C taken through cut lines A-A'. This
diagram shows that drain electrode 22A extends down through
interlayer dielectric 40 to contact N+drain region 42. Drain
electrode 22A also includes a field plate portion that extends
laterally over interlayer dielectric 40 toward the source electrode
segment 21A. In the illustrated embodiment, the distance between
the drain electrode 22A and the source electrode segment 21A is the
same as the distance (e.g., 3 microns) between the gate electrode
25 and source electrode segment 21A. The same distance separates
the gate electrode 25 from the source electrode segment 21B, which
extends down to the surface of the substrate to contact N-type
source region 44 and P+diffusion region 45. This spacing is
consistent with minimum design rules for an exemplary lateral power
transistor having a 200V breakdown voltage.
[0027] Drain diffusion region 42 is disposed in N-well region 51,
which itself is a deep diffusion formed in P-substrate 60. Source
diffusion region 44 is disposed in a P-well 50 formed adjacent to
N-well 51 in substrate 60. The channel region 49 is defined between
the boundaries of the N-well region 51 and source region 44. The
polysilicon gate members 28A and 28B are formed over a thin gate
oxide 48 above channel region 49. Note that each of gate members
28A and 28B include field plating that extends over thicker field
oxide layer 41. The region between N+drain region 42 and channel 49
is commonly referred to as the extended drain region of the
device.
[0028] It is appreciated that the foregoing details of the various
regions formed in the semiconductor material beneath the top
surface of the substrate are specific to the embodiment shown and
are not deemed essential to the present invention. In other words,
the layout structure of the metal electrodes and gate members may
be utilized in transistors having different doped semiconductor
regions of the device.
* * * * *