U.S. patent application number 11/436745 was filed with the patent office on 2006-09-28 for semiconductor devices having a hybrid channel layer, current aperture transistors and methods of fabricating same.
Invention is credited to Adam William Saxler.
Application Number | 20060214196 11/436745 |
Document ID | / |
Family ID | 35374372 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060214196 |
Kind Code |
A1 |
Saxler; Adam William |
September 28, 2006 |
Semiconductor devices having a hybrid channel layer, current
aperture transistors and methods of fabricating same
Abstract
Transistors and/or methods of fabricating transistors that
include a source contact, drain contact and gate contact are
provided. In some embodiments, a channel region is provided between
the source and drain contacts and at least a portion of the channel
regions includes a hybrid layer comprising semiconductor material.
In particular embodiments of the present invention, the transistor
is a current aperture transistor. The channel region may include
pendeo-epitaxial layers or epitaxial laterally overgrown layers.
Transistors and methods of fabricating current aperture transistors
that include a trench that extends through the channel and barrier
layers and includes semiconductor material therein are also
provided.
Inventors: |
Saxler; Adam William;
(Durham, NC) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
35374372 |
Appl. No.: |
11/436745 |
Filed: |
May 18, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10849589 |
May 20, 2004 |
7084441 |
|
|
11436745 |
May 18, 2006 |
|
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Current U.S.
Class: |
257/243 ;
257/E21.407; 257/E29.05; 257/E29.253 |
Current CPC
Class: |
H01L 29/0657 20130101;
H01L 29/7787 20130101; H01L 29/0649 20130101; H01L 29/7783
20130101; H01L 29/1029 20130101; H01L 29/66462 20130101; H01L
29/2003 20130101; H01L 29/7788 20130101 |
Class at
Publication: |
257/243 |
International
Class: |
H01L 29/768 20060101
H01L029/768 |
Claims
1. A transistor comprising: a mask region on a substrate; a first
epitaxial laterally overgrown layer comprising semiconductor
material of a first conductivity type on the substrate and at least
a portion of the mask region; a second epitaxial laterally
overgrown layer comprising semiconductor material of a second
conductivity type and/or insulating on the first epitaxial
laterally overgrown layer comprising semiconductor material and at
least a portion of the mask region and that includes spaced apart
portions that extend from end surfaces of the portions of the first
epitaxial laterally overgrown layer on the mask region that are the
first conductivity type; a third epitaxial laterally overgrown
layer comprising unintentionally doped semiconductor material on
the second epitaxial laterally overgrown layer and that includes
portions that extend from the spaced apart portions and coalesce
and are the first conductivity type; a channel layer comprising
semiconductor material on the third epitaxial laterally overgrown
layer; a barrier layer on the channel layer; a source contact on
the barrier layer; a gate contact on the barrier layer; and a drain
contact electrically connected to the first layer comprising
semiconductor material.
2. The transistor of claim 1, further comprising a first layer
comprising semiconductor material comprising the first conductivity
type on the substrate and wherein the mask region is on the first
layer and the first epitaxial laterally overgrown layer is provided
on the first layer.
3. The transistor of claim 1, wherein the first conductivity type
is n-type and the second conductivity type is p-type.
4. The transistor of claim 1, wherein the semiconductor material
comprises a nitride-based semiconductor material.
5. The transistor of claim 4, wherein the substrate comprises
gallium nitride.
6. The transistor of claim 4, wherein the substrate comprises
silicon carbide.
7. The transistor of claim 6, wherein the silicon carbide substrate
is the first conductivity type and wherein the drain contact is
provided on the silicon carbide substrate.
8. The transistor of claim 4, wherein the nitride-based
semiconductor material comprises a GaN based semiconductor
material.
9. A current aperture transistor comprising: a first layer
comprising semiconductor material of a second conductivity type or
insulating on a substrate; a channel layer comprising semiconductor
material on the second layer; a barrier layer on the channel layer;
a trench extending through the barrier layer, the channel layer and
the first layer, the trench including semiconductor material of a
first conductivity type therein; a gate contact on the barrier
layer; a source contact on the barrier layer and opposite the
trench from the gate contact; and a drain contact electrically
connected to the first layer.
10. The transistor of claim 9, further comprising a second layer
comprising semiconductor material of the first conductivity type on
the substrate between the substrate and the first layer; and
wherein the trench extends to the second layer.
11. The transistor of claim 9, wherein the first conductivity type
is n-type and the second conductivity type is p-type.
12. The transistor of claim 9, wherein the semiconductor material
comprises a nitride-based semiconductor material.
13. The transistor of claim 12, wherein the substrate comprises
gallium nitride.
14. The transistor of claim 12, wherein the substrate comprises
silicon carbide.
15. The transistor of claim 14, wherein the silicon carbide
substrate is the first conductivity type and wherein the drain
contact is provided on the silicon carbide substrate.
16. The transistor of claim 9, wherein the nitride-based
semiconductor material comprises a GaN based semiconductor
material.
17. The transistor of claim 9, further comprising an insulating
layer between the gate contact and the barrier layer.
18. The transistor of claim 9, further comprising contact regions
of semiconductor material of the first conductivity type between
the source contact and the barrier layer.
19. The transistor of claim 10, wherein the trench extends into the
first layer.
20. The transistor of claim 10, further comprising a third layer
comprising semiconductor material of the first conductivity type
disposed between the substrate and the second layer comprising
semiconductor material.
21. A method of fabricating a transistor comprising: forming a mask
region on a substrate; forming a first epitaxial laterally
overgrown layer comprising semiconductor material of a first
conductivity type by epitaxial lateral overgrowth on the first
layer comprising semiconductor material and at least a portion of
the mask region; forming a second epitaxial laterally overgrown
layer comprising semiconductor material of a second conductivity
type and/or insulating by epitaxial lateral overgrowth on the first
epitaxial laterally overgrown layer comprising semiconductor
material and at least a portion of the mask region and that
includes spaced apart portions that extend from end surfaces of the
portions of the first epitaxial laterally overgrown layer on the
mask region that are the first conductivity type; forming a third
epitaxial laterally overgrown layer comprising unintentionally
doped semiconductor material by epitaxial lateral overgrowth on the
second epitaxial laterally overgrown layer and that includes
portions that extend from the spaced apart portions and coalesce
and are the first conductivity type; forming a channel layer
comprising semiconductor material on the third epitaxial laterally
overgrown layer; forming a barrier layer on the channel layer;
forming a source contact on the barrier layer; forming a gate
contact on the barrier layer; and forming a drain contact
electrically connected to the first layer comprising semiconductor
material.
22. The method of claim 21, further comprising forming a first
layer comprising semiconductor material of a first conductivity
type on the substrate, wherein forming a mask region comprises
forming a mask region on the first layer and wherein forming a
first epitaxial laterally overgrown layer comprises forming a first
epitaxial laterally overgrown layer on the first layer.
23. The method of claim 21, wherein the first conductivity type is
n-type and the second conductivity type is p-type.
24. The method of claim 21, wherein the semiconductor material
comprises a nitride-based semiconductor material.
25. The method of claim 24, wherein the substrate comprises gallium
nitride.
26. The method of claim 24, wherein the substrate comprises silicon
carbide.
27. The method of claim 26, wherein the silicon carbide substrate
is the first conductivity type and wherein forming the drain
contact comprises forming the drain contact on the silicon carbide
substrate.
28. The method of claim 24, wherein the nitride-based semiconductor
material comprises a GaN based semiconductor material.
29. A method of fabricating a current aperture transistor
comprising: forming a first layer comprising semiconductor material
of a second conductivity type or insulating on a substrate; forming
a channel layer comprising semiconductor material on the second
layer; forming a barrier layer on the channel layer; forming a
trench extending through the barrier layer, the channel layer and
the first layer, forming a region of semiconductor material of the
first conductivity type in the trench; forming a gate contact on
the barrier layer; forming a source contact on the barrier layer
and opposite the trench from the gate contact; and forming a drain
contact electrically connected to the first layer.
30. The method of claim 29, further comprising forming a second
layer comprising semiconductor material of a first conductivity
type on a substrate, wherein forming a first layer comprises
forming a first layer on the second layer and wherein forming a
trench comprises forming a trench extending through the barrier
layer, the channel layer and the first layer and to the second
layer.
31. The method of claim 29, wherein the first conductivity type is
n-type and the second conductivity type is p-type.
32. The method of claim 29, wherein the semiconductor material
comprises a nitride-based semiconductor material.
33. The method of claim 32, wherein the substrate comprises gallium
nitride.
34. The method of claim 32, wherein the substrate comprises silicon
carbide.
35. The method of claim 34, wherein the silicon carbide substrate
is the first conductivity type and wherein forming the drain
contact comprises forming the drain contact on the silicon carbide
substrate.
36. The method of claim 32, wherein the nitride-based semiconductor
material comprises a GaN based semiconductor material.
37. The method of claim 29, further comprising forming an
insulating layer between the gate contact and the barrier
layer.
38. The method of claim 29, further comprising forming contact
regions of semiconductor material of the first conductivity type
between the source contact and the barrier layer.
39. The method of claim 30, wherein the trench extends into the
second layer.
40. The method of claim 30, further comprising forming a third
layer comprising semiconductor material of the first conductivity
type disposed between the substrate and the second layer comprising
semiconductor material.
Description
CLAIM OF PRIORITY
[0001] The present application is a divisional of and claims
priority from co-pending U.S. application Ser. No. 10/849,589,
filed May 20, 2004, entitled "SEMICONDUCTOR DEVICES HAVING A HYBRID
CHANNEL LAYER, CURRENT APERTURE TRANSISTORS AND METHODS OF
FABRICATING SAME," which is assigned to the assignee of the present
application, the disclosure of which is hereby incorporated herein
by reference as if set forth fully.
FIELD OF THE INVENTION
[0002] The present invention relates to semiconductor devices and,
more particularly, to current aperture transistors.
BACKGROUND
[0003] In high voltage/high power electronic devices, a performance
characteristic of importance may be the breakdown characteristics
of the device. For example, the maximum reverse bias blocking
voltage sustainable by the device may limit the potential
applications for the device. In conventional transistors, the
structure of the device may result in a breakdown voltage that is
less than the breakdown voltage of the device. For example, in a
conventional U-metal oxide semiconductor field effect transistor
(UMOSFET), field crowding may result at a corner of the gate trench
such that breakdown occurs in the region of the field crowding
rather than in the bulk material. Various structures have been
proposed in different types of transistors to provide breakdown
voltages of the transistors that approach the bulk breakdown
voltage of the materials used in the transistors.
[0004] Current aperture transistors have been proposed as a
potential approach to achieve near bulk breakdown limits in
nitride-based devices. FIG. 1 illustrates a conventional current
aperture transistor 10. As seen in FIG. 1, the current aperture
transistor 10 includes a silicon doped GaN drain layer 12 with
spaced apart regions of insulating GaN 14 that form the current
aperture region 30. The current aperture region 30 is part of a
regrown unintentionally doped GaN layer 16 that has an AlGaN layer
18 formed thereon so that a two-dimensional electron gas (2DEG)
forms at the interface between the unintentionally doped GaN layer
16 and the AlGaN layer 18. A gate contact 24 and a source contact
22 are provided on the AlGaN layer 18 and a drain contact 26 is
provided on the silicon doped GaN layer 12.
[0005] In operation, electrons from the source contact 22 flow
along the 2DEG and through the current aperture 30 to the drain
contact 26. The Schottky gate contact 24 modulates the charge in
the 2DEG to control the flow of current through the aperture.
Because the pinched off region is located beneath the gate,
substantial charge does not accumulate at the gate edge. Thus, the
high field region may be buried in the bulk material. Further
details on current aperture transistors may be found in Ben-Yaacov
et al., "AlGaN/GaN current aperture vertical electron transistors
with regrown channels", Journal of Applied Physics, Vol. 95, No. 4,
15 Feb. 2004, pp. 2073-78.
SUMMARY OF THE INVENTION
[0006] Some embodiments of the present invention provide
transistors and/or methods of fabrication of transistors that
include a source contact, drain contact and gate contact. A channel
region is provided between the source and drain contacts and at
least a portion of the channel regions includes a hybrid layer of
semiconductor material. In particular embodiments of the present
invention, the transistor is a current aperture transistor.
[0007] In further embodiments of the present invention, the hybrid
layer of semiconductor material includes a Group III-nitride
semiconductor material. The hybrid layer of semiconductor material
may also include a region of p-type or insulating semiconductor
material and a region of n-type semiconductor material. A portion
of the channel region through the current aperture may include a
vertical portion and a horizontal portion. The hybrid layer may
include a pendeo-epitaxial layer having a higher doping level in
the laterally grown portions of the pendeo-epitaxial layer. The
hybrid layer may include a epitaxial laterally overgrown layer
having a higher doping level in the laterally grown portions of the
epitaxial laterally overgrown layer.
[0008] In additional embodiments of the present invention, the
transistor includes a first n-type nitride-based layer on a
substrate. The first n-type nitride-based layer has a first surface
opposite the substrate and an aperture having sidewalls. A
nitride-based layer is provided on the first n-type nitride-based
layer and extends onto the sidewalls of the aperture. A portion of
the nitride-based layer on the sidewalls of the aperture is n-type
and a portion of the nitride-based layer on the first surface of
the second n-type nitride-based layer is p-type and/or insulating.
An unintentionally doped nitride-based layer is provided on the
nitride based layer and extends to substantially fill the aperture.
The unintentionally doped nitride-based layer has portions of
n-type nitride-based semiconductor material on the n-type portions
of the nitride-based layer. First and second layers of
nitride-based semiconductor material are provided on the
unintentionally doped nitride-based layer and are configured to
provide a two-dimensional electron gas (2DEG) in a region of an
interface between the first and second semiconductor material
layers. The source contact and the gate contact are provided on the
second layer of nitride-based semiconductor material and the drain
contact is electrically connected to the first n-type nitride-based
layer.
[0009] In additional embodiments of the present invention, a second
n-type nitride-based layer is provided between the first n-type
nitride-based layer and the substrate.
[0010] In still further embodiments of the present invention, the
substrate has a trench formed therein and the first n-type
nitride-based layer, the nitride based layer and the
unintentionally doped nitride-based layer are cantilevered over the
trench.
[0011] In other embodiments of the present invention, a mask region
is provided on the substrate or the second n-type nitride-based
layer if present and a portion of the first n-type nitride-based
layer, the nitride based layer and the unintentionally doped
nitride-based layer extend onto the mask region.
[0012] Additional embodiments of the present invention include a
third n-type nitride-based layer between the second nitride-based
layer and the substrate. The substrate may be a silicon carbide
substrate and the drain contact may be provided on the substrate
opposite the first n-type nitride-based layer. An insulating layer
may also be provided between the gate contact and the second
layer.
[0013] In some embodiments of the present invention, the first
n-type nitride-based layer includes a GaN based layer, the second
n-type nitride-based layer includes a GaN based layer, the
nitride-based layer on the second n-type nitride-based layer
includes a GaN based layer, the unintentionally doped nitride-based
layer includes a GaN based layer, the first layer includes an
unintentionally doped GaN based layer and the second layer includes
an AlGaN and/or INAlN based layer. The substrate may also be a GaN
substrate.
[0014] In still further embodiments of the present invention,
transistors and/or methods of fabricating transistors that include
a substrate having a trench therein and a first pendeo-epitaxial
layer of semiconductor material of the first conductivity type is
provided on the substrate is provided. The first pendeo-epitaxial
layer has spaced apart cantilevered portions that extend over the
trench. A second pendeo-epitaxial layer of semiconductor material
of a second conductivity type and/or insulating is provided on the
first pendeo-epitaxial layer of semiconductor material and includes
spaced apart portions that extend from end surfaces of the
cantilevered portions of the first pendeo-epitaxial layer that are
the first conductivity type. A third pendeo-epitaxial layer of
unintentionally doped semiconductor material is provided on the
second pendeo-epitaxial layer and includes portions that extend
from the spaced apart portions and coalesce and are the first
conductivity type. A channel layer of semiconductor material is
provided on the third pendeo-epitaxial layer and a barrier layer is
provided on the channel layer. A source contact and a gate contact
are provided on the barrier layer. A drain contact is electrically
connected to the first layer of conformal semiconductor
material.
[0015] In additional embodiments of the present invention, a first
layer of conformal semiconductor material of a first conductivity
type is provided on the substrate and the trench. The first
pendeo-epitaxial layer may be provided on the first layer.
[0016] In further embodiments of the present invention, the first
conductivity type is n-type and the second conductivity type is
p-type. The semiconductor material may include a nitride-based
semiconductor material. The substrate may be a GaN substrate. The
substrate may also be silicon carbide. The silicon carbide
substrate may be the first conductivity type and the drain contact
may be provided on the silicon carbide substrate. The nitride-based
semiconductor material may include a GaN based semiconductor
material.
[0017] In yet further embodiments of the present invention,
transistors and methods of fabricating transistors that include a
mask region on a substrate are provided. A first epitaxial
laterally overgrown layer of semiconductor material of the first
conductivity type is provided on the substrate and at least a
portion of the mask region. A second epitaxial laterally overgrown
layer of semiconductor material of a second conductivity type
and/or insulating is provided on the first epitaxial laterally
overgrown layer of semiconductor material and at least a portion of
the mask region and includes spaced apart portions that extend from
end surfaces of the portions of the first epitaxial laterally
overgrown layer on the mask region that are the first conductivity
type. A third epitaxial laterally overgrown layer of
unintentionally doped semiconductor material is provided on the
second epitaxial laterally overgrown layer and includes portions
that extend from the spaced apart portions and coalesce and are the
first conductivity type. A channel layer of semiconductor material
is provided on the third epitaxial laterally overgrown layer. A
barrier layer is provided on the channel layer. A source contact
and a gate contact are provided on the barrier layer. A drain
contact is electrically connected to the first layer of
semiconductor material.
[0018] In further embodiments of the present invention, a first
layer of semiconductor material of a first conductivity type is
provided between the substrate and the first epitaxial laterally
overgrown layer. The mask region may be provided on the first
layer.
[0019] In additional embodiments of the present invention, the
first conductivity type is n-type and the second conductivity type
is p-type. The semiconductor material may be a nitride-based
semiconductor material. The substrate may be silicon carbide and/or
GaN. The silicon carbide substrate may be the first conductivity
type and the drain contact may be provided on the silicon carbide
substrate. The nitride-based semiconductor material may be a GaN
based semiconductor material.
[0020] Still further embodiments of the present invention provided
current aperture transistors and methods of fabricating current
aperture transistors that include a first layer of semiconductor
material of a second conductivity type or insulating on a
substrate. A channel layer of semiconductor material is provided on
the second layer. A barrier layer is provided on the channel layer.
A trench extends through the barrier layer, the channel layer and
the first layer. The trench includes semiconductor material of the
first conductivity type therein. A gate contact and a source
contact are provided on the barrier layer. The source contact is on
the barrier layer and opposite the trench from the gate contact. A
drain contact is electrically connected to the first layer.
[0021] In additional embodiments of the present invention, a second
layer of semiconductor material of a first conductivity type is
provided between the substrate and the first layer of semiconductor
material. In such embodiments, the trench may extend through the
first layer to and/or into the second layer.
[0022] In some embodiments of the present invention, the first
conductivity type is n-type and the second conductivity type is
p-type. The semiconductor material may be a nitride-based
semiconductor material. The substrate may be GaN. The substrate may
also be silicon carbide. The silicon carbide substrate may be the
first conductivity type and the drain contact may be provided on
the silicon carbide substrate opposite the first layer of
semiconductor material. The nitride-based semiconductor material
may be a GaN based semiconductor material. An insulating layer may
be provided between the gate contact and the barrier layer. Contact
regions of semiconductor material of the first conductivity type
may be provided between the source contact and the barrier layer.
The trench may extend into the first layer. A third layer of
semiconductor material of the first conductivity type may be
disposed between the substrate and the first layer of semiconductor
material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic drawing of a conventional current
aperture transistor.
[0024] FIG. 2 is a schematic drawing of current aperture
transistors according to some embodiments of the present
invention.
[0025] FIGS. 3A-3E are schematic drawings illustrating fabrication
of current aperture transistors as illustrated in FIG. 2.
[0026] FIG. 4 is a schematic drawing of current aperture
transistors according to further embodiments of the present
invention.
[0027] FIGS. 5A-5E are schematic drawings illustrating fabrication
of current aperture transistors as illustrated in FIG. 4.
[0028] FIG. 6 is a schematic drawing illustrating a current
aperture transistor according to further embodiments of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0029] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. In the drawings, the size
and relative sizes of layers and regions may be exaggerated for
clarity. It will be understood that when an element or layer is
referred to as being "on", "connected to" or "coupled to" another
element or layer, it can be directly on, connected or coupled to
the other element or layer or intervening elements or layers may be
present. In contrast, when an element is referred to as being
"directly on," "directly connected to" or "directly coupled to"
another element or layer, there are no intervening elements or
layers present. Like numbers refer to like elements throughout. As
used herein, the term "and/or" includes any and all combinations of
one or more of the associated listed items.
[0030] It will be understood that, although the terms first,
second, etc. may be used herein to describe various elements,
components, regions, layers and/or sections, these elements,
components, regions, layers and/or sections should not be limited
by these terms. These terms are only used to distinguish one
element, component, region, layer or section from another region,
layer or section. Thus, a first element, component, region, layer
or section discussed below could be termed a second element,
component, region, layer or section without departing from the
teachings of the present invention.
[0031] Furthermore, relative terms, such as "lower" or "bottom" and
"upper" or "top," may be used herein to describe one element's
relationship to another element as illustrated in the Figures. It
will be understood that relative terms are intended to encompass
different orientations of the device in addition to the orientation
depicted in the Figures. For example, if the device in the Figures
is turned over, elements described as being on the "lower" side of
other elements would then be oriented on "upper" sides of the other
elements. The exemplary term "lower", can therefore, encompass both
an orientation of "lower" and "upper," depending of the particular
orientation of the figure. Similarly, if the device in one of the
figures is turned over, elements described as "below" or "beneath"
other elements would then be oriented "above" the other elements.
The exemplary terms "below" or "beneath" can, therefore, encompass
both an orientation of above and below.
[0032] Embodiments of the present invention are described herein
with reference to cross-section illustrations that are schematic
illustrations of idealized embodiments of the present invention. As
such, variations from the shapes of the illustrations as a result,
for example, of manufacturing techniques and/or tolerances, are to
be expected. Thus, embodiments of the present invention should not
be construed as limited to the particular shapes of regions
illustrated herein but are to include deviations in shapes that
result, for example, from manufacturing. For example, a grown or
deposited region illustrated as a rectangle will, typically, have
rounded or curved features and/or a gradient of concentrations at
its edges with another region rather than a binary change from a
first region to a second region of different composition. Thus, the
regions illustrated in the figures are schematic in nature and
their shapes are not intended to illustrate the precise shape of a
region of a device and are not intended to limit the scope of the
present invention.
[0033] Some embodiments of the present invention provide vertical
transistors that have a current aperture that is at least partially
horizontal and methods of forming such transistors. In some
embodiments of the present invention, the current aperture is
laterally grown, for example, through lateral pendeo-epitaxial
growth and/or through epitaxial lateral overgrowth (ELO). In some
embodiments of the present invention, the use of lateral growth
allows for the formation of a layer of semiconductor material
having differing regions of conductivity types and/or differing
conductivities as a result of the preferential incorporation of
dopants in the laterally grown regions of the device. Such a single
layer having differing lateral regions that are formed as part of a
single growth or deposition process are referred to herein as a
"hybrid layer." Hybrid layers may be distinguished from layers that
have a substantially uniform conductivity type and/or conductivity
in a lateral direction or layers that are formed of a substantially
uniform conductivity type and/or conductivity that are subsequently
processed by, for example, ion implantation and/or diffusion to
alter the characteristics of a portion of the layer. Some
embodiments of the present invention provide a transistor having at
least a portion of the channel of the transistor provided by a
hybrid layer.
[0034] Embodiments of the present invention may be suited for use
in nitride-based vertical current aperture transistors, such as
Group III-nitride based devices. As used herein, the term "Group
III nitride" refers to those semiconducting compounds formed
between nitrogen and the elements in Group III of the periodic
table, usually aluminum (Al), gallium (Ga), and/or indium (In). The
term also refers to ternary and quaternary compounds such as AlGaN
and AlInGaN. As is well understood by those in this art, the Group
III elements can combine with nitrogen to form binary (e.g., GaN),
ternary (e.g., AlGaN, AlInN), and quaternary (e.g., AlInGaN)
compounds. These compounds all have empirical formulas in which one
mole of nitrogen is combined with a total of one mole of the Group
III elements. Accordingly, formulas such as Al.sub.xGa.sub.1-xN
where 0.ltoreq.x.ltoreq.1 are often used to describe them.
[0035] FIG. 2 is a schematic cross sectional illustration of a
current aperture transistor according to some embodiments of the
present invention. As illustrated in FIG. 2, a substrate 110, such
as a silicon carbide substrate, has a trench 150 formed therein. In
some embodiments of the present invention, the trench may be from
about 1 .mu.m to about 20 .mu.m deep and from about 0.5 .mu.m to
about 10 .mu.m wide. As discussed below, the substrate 110 may be
an n-type semiconductor material. The substrate 110 may be doped to
provide a carrier concentration of from about 10.sup.17 to about
10.sup.20 cm.sup.-3. Alternatively, the substrate 110 may be an
insulating or semi-insulating substrate. In such a case, a contact
may be provided to the n+ layer 112, for example, in a trench or at
a peripheral edge of the device as illustrated in FIG. 1.
[0036] As is further illustrated in FIG. 2, a first layer 112 of
n-type semiconductor material, such as a nitride-based
semiconductor material, is provided on a surface of the substrate
110 and, in some embodiments, in the trench 150. The first layer
112 may, for example, be AlGaN and may be doped with, for example,
Si, Ge and/or O to provide an n+ layer of AlGaN. In some
embodiments of the present invention, the first layer 112 has a
thickness of from about 0 (i.e. is an optional layer) to about 1
.mu.m. The first layer 112 may also be doped to provide a carrier
concentration of from about 10.sup.17 to about 10.sup.20 cm.sup.-3.
In some embodiments of the present invention where the substrate
110 is a GaN based substrate, such as a GaN substrate, the first
layer 112 may be omitted.
[0037] A second layer 114 of n-type semiconductor material is
provided on the first layer 112 (or the substrate if the first
layer is omitted) and is cantilevered over the trench 150. The
second layer 114 may, for example, be an n+ GaN layer and may be
formed as discussed below, by pendeo-epitaxial growth. The second
layer 114 is provided on a first surface of the first layer 112
that is opposite the substrate 110 and does not extend completely
across the trench 150 but includes spaced apart regions that have
opposing sidewalls. In some embodiments of the present invention,
the second layer 114 has a thickness of from about 0.1 .mu.m to
about 5 .mu.m. The second layer 114 also may be doped with an
n-type dopant, such as Si, Ge and/or O, to provide a carrier
concentration of from about 10.sup.17 to about 10.sup.20
cm.sup.-3.
[0038] A third layer 116 of p-type and/or insulating semiconductor
material is provided on the second layer 114 and has portions 117
of n-type semiconductor material that extend from the sidewalls of
the second layer 114. Thus, the third layer 116 may be provided as
a hybrid layer. The third layer 116 does not extend completely
across the trench 150 but includes spaced apart regions that have
opposing sidewalls. In some embodiments of the present invention,
the third layer 116 may be p-type or insulating GaN formed as
discussed below, by pendeo-epitaxial growth in the presence of an
n-type dopant, such as Si, Ge and/or O, such that the portions 117
that are laterally grown are n-type GaN. The insulating and/or
p-type regions of third layer 116 may substantially block current
flow by, for example, having a Fermi level positioned at greater
than about 0.5 eV from the conduction band. The portions 117 of the
third layer 116 may be doped to provide a carrier concentration of,
for example, from about 10.sup.14 to about 10.sup.19 cm.sup.-3. The
third layer 116 may have a thickness of from about 0.1 .mu.m to
about 10 .mu.m.
[0039] As is further illustrated in FIG. 2, a fourth layer 118 of
unintentionally doped semiconductor material is provided on the
third layer 116. As discussed below, the unintentionally doped
semiconductor material may be lightly doped. The fourth layer 118
has portions 119 of n-type semiconductor material that extend from
the sidewalls of the third layer 116. Thus, the fourth layer 118
may be provided as a hybrid layer. In some embodiments, the fourth
layer 118 extends completely across the trench 150 and may coalesce
where the portions 119 extending from the sidewalls of the third
layer 116 meet. Thus, a hole or opening 152 may be provided by the
trench beneath the fourth layer 118. In some embodiments of the
present invention, the fourth layer 118 may be primarily
unintentionally doped GaN formed as discussed below, by
pendeo-epitaxial growth in the presence of an n-type dopant, such
as Si, Ge and/or O such that the portions 119 that are laterally
grown are n-type GaN. The n-type dopants may be affirmatively
supplied and/or may be unintentional, for example, the n-type may
be provided as a by-product of a prior processing step and/or
contamination. The unintentionally doped regions of the fourth
layer 118 may be lightly n-type or have a low net concentration of
acceptors or deep levels. The portions 119 of the fourth layer 118
may be doped to provide a carrier concentration of, for example,
from about 10.sup.15 to about 10.sup.20 cm.sup.-3. The fourth layer
118 may have a thickness of from about 0.1 .mu.m to about 10
.mu.m.
[0040] A channel layer 120 and a barrier layer 122 may be provided
on the fourth layer 118. The channel layer 120 and the barrier
layer 122 are configured so as to form a 2DEG near the interface
between the channel layer 120 and the barrier layer 122. The
channel layer 120 and/or the barrier layer 122 may be provided by a
single or multiple layers. In some embodiments of the present
invention, the channel layer 120 is an unintentionally doped GaN
layer. The barrier layer 122 may be an AlGaN and/or AlN layer. In
some embodiments of the present invention, the channel layer 120 is
GaN having a thickness of from about 5 m to about 5000 nm and the
barrier layer 122 is AlGaN having a concentration of aluminum of
from about 5% to about 100% and having a thickness of from about 1
nm to about 100 nm.
[0041] A gate contact 130 is provided on the barrier layer 122 and,
in some embodiments, may be provided on an oxide (not shown) on the
barrier layer 122. In some embodiments of the present invention, at
least a portion of the gate contact 130 overlaps with the trench
150. In some embodiments of the present invention, the gate contact
130 is confined to a region above the trench 150. In still further
embodiments, the gate contact 130 may be provided without overlap
of the trench 150. The gate contact 130 may be formed of, for
example, Ni/Au.
[0042] Optionally, a dielectric layer, such as a silicon nitride or
silicon oxide layer is provided on the barrier layer 122. In such
embodiments, the gate contact 130 may be formed in a recess in the
dielectric layer or, in some embodiments, may be formed on the
dielectric layer.
[0043] A source contact 132 is also provided on the barrier layer
122. Optionally, the source contact 132 may be provided on a recess
into the barrier layer 122 and/or on a contact region so as to
reduce resistance between the source contact 132 and the channel
layer 120. The source contact 132 may be formed of, for example,
Ti/Al/Ni/Au.
[0044] A drain contact 134 is provided on the substrate 110
opposite the first layer 112. Optionally, a backside implant or
epitaxial layer (not shown) may be provided so as to reduce
resistance between the drain contact 134 and the substrate 110. The
backside implant or epitaxial layer may, for example, be an n+
region formed in or on the substrate 110 on which the drain contact
134 is provided. Furthermore, the substrate 110 may be thinned
prior to formation of the drain contact 134. The drain contact may
be formed of, for example, nickel if a silicon carbide substrate is
used. As discussed above, the drain contact 134 may, optionally, be
formed on the first layer 112, similar to the configuration
illustrated in FIG. 1. In such embodiments of the present
invention, an insulating substrate or a semi-insulating substrate,
such as semi-insulating silicon carbide or sapphire, may be
utilized for the substrate 110.
[0045] Without wishing to be bound by any theory of operation,
electrons from the source contact 132 may flow along the 2DEG
formed near the interface of the barrier layer 122 and the channel
layer 120 and then vertically into the n-type portions 119 of the
fourth layer 118 and then laterally across the n-type portion 117
of the third layer 116 to the n+ layer 114, vertically to the n+
layer 112 and to the drain contact 134 through the substrate 110.
Thus, a current aperture may be provided by the p or i regions of
the third layer 116 with a current path provided by the n-type
regions incorporated in the laterally grown portions of the third
layer 116 and the fourth layer 118. The hole 152 may also define
the current aperture. Accordingly, a portion of the channel region
of the device may be provided by a hybrid layer. Furthermore, the
channel region through the current aperture may include a vertical
portion through the fourth layer 118 and a horizontal portion
through the third layer 116. The Schottky gate contact 130 (or MOS
gate if an insulating layer provided on the barrier layer 122) may
modulate the charge in the 2DEG to control the flow of current
through the aperture.
[0046] Fabrication of embodiments of the present invention as
illustrated in FIG. 2 is schematically illustrated in FIGS. 3A-3E.
As seen in FIG. 3A, a substrate 110 is provided on which nitride
based devices may be formed. In particular embodiments of the
present invention, the substrate 110 may be an n-type silicon
carbide (SiC) substrate that may be, for example, 4H polytype of
silicon carbide. Other silicon carbide candidate polytypes include
the 3C, 6H, and 15R polytypes. Appropriate SiC substrates are
manufactured by, for example, Cree, Inc., of Durham, N.C., the
assignee of the present invention, and methods for producing are
described, for example, in U.S. Pat. Nos. Re. 34,861; 4,946,547;
5,200,022; and 6,218,680, the contents of which are incorporated
herein by reference in their entirety. Similarly, techniques for
epitaxial growth of Group III nitrides have been described in, for
example, U.S. Pat. Nos. 5,210,051; 5,393,993; 5,523,589; and
5,292,501, the contents of which are also incorporated herein by
reference in their entirety.
[0047] Optional buffer, nucleation and/or transition layers (not
shown) may be provided on the substrate 110.
[0048] Although silicon carbide may be a preferred substrate
material, embodiments of the present invention may utilize any
suitable substrate, such as sapphire, aluminum nitride, aluminum
gallium nitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP
and the like. In some embodiments, an appropriate buffer layer also
may be formed. In devices having an insulating or semi-insulating
substrate, a second contact may be formed in a recess that extends
to a drain layer opposite the current aperture from the gate.
[0049] As is further illustrated in FIG. 3A, a trench 150 is formed
in the substrate 110, for example, by masking and etching the
substrate 110. A dry etch, wet etch or other etch technique known
to those of skill in the art may be utilized. The particular etch
technique may depend on the substrate material. For example, in
some embodiments of the present invention, a reactive ion etch may
be utilized for a silicon carbide substrate. Other techniques for
patterning the substrate to form a trench may also be utilized. For
example, the trench could be provided by a saw cut or laser removal
of material.
[0050] The first layer 112 is blanket formed on the substrate 110
including in the trench 150. The first layer 112 is formed
conformally so as to maintain the opening 152 formed by the trench
150. The first layer 112 may be formed, for example, by plasma
enhanced chemical vapor deposition (PECVD), metal organic chemical
vapor deposition (MOCVD), molecular beam epitaxy (MBE), low
pressure chemical vapor deposition (LPCVD) or other technique known
to those of skill in the art. The particular deposition technique
may depend on the composition of the first layer 112 and whether or
not buffer layers such as those described above are utilized.
[0051] In some embodiments of the present invention, the first
layer 112 may be omitted. For example, if the substrate 110 is a
GaN substrate, the first layer 112 may be omitted and the trench
may be formed in the GaN substrate.
[0052] FIG. 3B illustrates formation of the second layer 114. The
second layer 114 is formed by pendeo-epitaxial growth so as to be
cantilevered over the opening 152 of the trench 150 and the first
layer 112. Techniques for pendeo-epitaxial growth and/or epitaxial
lateral overgrowth are known to those of skill in the art and need
not be described further herein. However, examples of
pendeo-epitaxial growth and/or epitaxial lateral overgrowth are
described, for example, in U.S. Pat. Nos. 6,582,906, 6,706,114,
6,686,261, 6,621,148, 6,608,327, 6,602,764, 6,602,763, 6,586,778,
6,582,986, 6,570,192, 6,545,300, 6,521,514, 6,489,221, 6,486,042,
6,462,355, 6,380,108, 6,376,339, 6,261,929, 6,255,198, 6,177,688
and 6,051,849, the disclosures of which are incorporated herein by
reference as if set forth fully herein. The particular conditions
for formation of the second layer 114 may depend on the composition
of the second layer 114. Furthermore, the growth may be oriented
along different facets of the crystal structure so as to provide a
higher degree of impurity incoroporation. Such an orientation may
be provided, for example, by the orientation of the trench 150
and/or the mask 425 described below and/or the growth conditions
such as temperature, ratio of source materials (e.g. ratio of Group
III to Group V sources) and/or growth pressure.
[0053] The second layer 114 may be grown in the presence of an
n-type dopant, for example, for a GaN second layer 114, Si, Ge
and/or O may be incorporated in the second layer 114 so as to
provide an n+ second layer 114. During lateral growth of the second
layer 114, the laterally grown portion may be more susceptible to
incorporation of the dopant and, therefore, the laterally grown
portions (illustrated by the dotted line in FIG. 2) may have a
higher dopant concentration than the vertically grown portions of
the second layer 114. Thus, while the second layer 114 is
illustrated as an n+ layer, the dopant concentration and,
accordingly, the carrier concentration, may differ in different
regions of the second layer 114.
[0054] FIG. 3C illustrates the formation of the third layer 116.
The third layer 116 is also cantilevered over the opening 152 of
the trench 150. The third layer 116 may be formed by
pendeo-epitaxial growth as described above. The third layer 116 is
formed as insulating or p-type semiconductor material and may be
formed in the presence of a small amount of n-type dopant, such as
Si, by for example, including SiH.sub.4 in the reaction chamber
when the third layer 116 is grown. For a GaN based layer, a p-type
dopant, such as Mg, Zn or Be, may may be provided during formation
of the third layer 116. An insulating GaN layer may be formed in
the absence of a dopant other than that discussed herein with
regard to the n-type laterally grown region of the third layer 116
and/or in the presence of a compensating dopant, such as Fe. The
quantity of n-type dopant provided may depend on the composition of
the third layer 116. However, the quantity should be sufficiently
small so that the vertically grown portions of the third layer 116
are provided as p-type or insulating despite the presence of the
n-type dopant (e.g. the p-type dopant or compensating dopant swamps
the n-type dopant absent the preferential incorporation of n-type
dopant provided by lateral growth) but sufficiently large so that
the preferential incorporation of the n-type dopant in the
laterally grown portions of the third layer 116 provide n-type
regions.
[0055] FIG. 3D illustrates formation of the fourth layer 118. The
fourth layer is formed by pendeo-epitaxial growth as discussed
above and may be grown to cover the opening 152 of the trench 150.
Portions of the fourth layer 118 may be laterally grown to coalesce
over the trench 150. The fourth layer 118 is formed as an
unintentionally doped layer and may be formed in the presence of a
small amount of n-type dopant, such as Si, by for example,
including SiH.sub.4 in the reaction chamber when the fourth layer
118 is grown. The quantity of n-type dopant provided may depend on
the composition of the fourth layer 118. However, the quantity
should be sufficiently small so that the vertically grown portions
of the fourth layer 118 incorporate only an insubstantial amount of
the n-type dopant and, thus, may be considered unintentionally
doped. The quantity of n-type dopant should, however, be
sufficiently large so that the preferential incorporation of the
n-type dopant in the laterally grown portions of the fourth layer
118 provides n-type regions.
[0056] FIG. 3E illustrates formation of the channel layer 120 and
the barrier layer 122. Such layers may be formed by conventional
fabrication techniques based on the composition of the respective
layers. A drain contact 134, a source contact 132 and a gate
contact 130 may be formed on the barrier layer utilizing
conventional techniques as illustrated in FIG. 2. In some
embodiments of the present invention, the gate contact may be
formed on a dielectric layer (not shown) on the barrier layer
122.
[0057] FIG. 4 illustrates a transistor according to further
embodiments of the present invention. As seen in FIG. 4, a first
layer 412 of n-type semiconductor material is provided on a
substrate 410, such as an n-type semiconductor material substrate.
The substrate 410 may be a silicon carbide substrate as described
above with reference to the substrate 110. The substrate 410 may be
doped to provide a carrier concentration of from about 10.sup.17 to
about 10.sup.20 cm.sup.-3. Alternatively, the substrate 410 may be
an insulating or semi-insulating substrate. In such a case, a
contact may be provided to the n+ layer 414, for example, in a
trench or at a peripheral edge of the device as illustrated in FIG.
1.
[0058] As is further illustrated in FIG. 4, a first layer 412 of
n-type semiconductor material, such as a nitride-based
semiconductor material, is provided on a surface of the substrate
410. The first layer 412 may, for example, may be AlGaN and may be
doped with, for example, Si to provide an n-type layer of AlGaN. In
some embodiments of the present invention, the first layer 412 has
a thickness of from about 0 (i.e. is optional) to about 1 .mu.m.
The first layer 412 may also be doped to provide a carrier
concentration of from about 10.sup.17 to about 10.sup.20
cm.sup.-3.
[0059] A second layer 414 of n-type semiconductor material is
provided on the first layer 412. The second layer 414 may, for
example, be an n+ GaN layer. The second layer 414 is provided on a
first surface of the first layer 412 that is opposite the substrate
410. In some embodiments of the present invention, the second layer
414 has a thickness of from about 0.1 .mu.m to about 10 .mu.m. The
second layer 414 also may be doped to provide a carrier
concentration of from about 10.sup.17 to about 10.sup.20
cm.sup.-3.
[0060] Mask regions 425 are formed on the second layer 414. The
mask regions correspond to a region of the current aperture and, in
some embodiments, may be provided sufficiently thin such that
during the growth of the subsequent layers, the center is etched
away leaving an interior opening. The mask regions 425 may suppress
vertical growth from the second layer 414. The mask regions 425 may
be, for example, an oxide such as silicon dioxide.
[0061] In some embodiments of the present invention, the first
layer 412 and/or second layer 414 may be omitted. For example, if
the substrate 410 is a GaN substrate, the first layer 412 and/or
the second layer 414 may be omitted and the mask 425 may be
provided on the substrate 410. A third layer 416 of n+
semiconductor material is provided on the second layer 414 and
laterally overgrows at least a portion of the mask regions 425. The
third layer 416 does not completely cover the mask regions 425 and
has spaced apart sidewalls. In some embodiments of the present
invention, the third layer 416 may be n-type GaN formed as
discussed above using epitaxial lateral overgrowth. The third layer
416 may be doped to provide a carrier concentration of, for
example, from about 10.sup.17 to about 10.sup.20 cm.sup.-3. The
third layer 416 may have a thickness of from about 0.1 .mu.m to
about 10 .mu.m.
[0062] A fourth layer 418 of p-type or insulating semiconductor
material is provided on the third layer 416 and at least a portion
of the mask regions 425 and has portions 419 of n-type
semiconductor material that extend from the sidewalls of the third
layer 416. Thus, the fourth layer 418 may be provided as a hybrid
layer. The fourth layer 418 does not extend completely across the
mask regions 425 but includes spaced apart regions that have
opposing sidewalls. In some embodiments of the present invention,
the fourth layer 418 may be p-type or insulating GaN formed as
discussed above, by epitaxial lateral overgrowth in the presence of
an n-type dopant, such as Si, Ge or O, such that the portions 419
that are laterally grown are n-type GaN. The insulating and/or
p-type regions of fourth layer 418 may substantially block current
flow by, for example, having a Fermi level positioned at greater
than about 0.5 eV from the conduction band. The portions 419 of the
fourth layer 418 may be doped to provide a carrier concentration
of, for example, from about 10.sup.14 to about 10.sup.19 cm.sup.-3.
The fourth layer 418 may have a thickness of from about 0.1 .mu.m
to about 10 .mu.m.
[0063] As is further illustrated in FIG. 4, a fifth layer 420 of
unintentionally doped semiconductor material is provided on the
fourth layer 418. The fifth layer 420 has portions 421 of n-type
semiconductor material that extend from the sidewalls of the fourth
layer 418. Thus, the fifth layer 420 may be provided as a hybrid
layer. In some embodiments, the fifth layer 420 extends completely
between the sidewalls of the fourth layer 418 and may coalesce
where portions extending from the sidewalls of the fourth layer 418
meet. In some embodiments of the present invention, the fifth layer
420 may be primarily unintentionally doped GaN formed as discussed
elsewhere herein, by epitaxial lateral overgrowth in the presence
of an n-type dopant, such as Si such that the portions 421 that are
laterally grown are n-type GaN. The unintentionally doped regions
of the fifth layer 420 may be lightly n-type or have a low net
concentration of acceptors or deep levels. The portions 421 of the
fifth layer 420 may be doped to provide a carrier concentration of,
for example, from about 10.sup.15 to about 10.sup.20 cm.sup.-3. The
fifth layer 420 may have a thickness of from about 0.1 .mu.m to
about 10 .mu.m.
[0064] A channel layer 422 and a barrier layer 424 may be provided
on the fifth layer 420. The channel layer 422 and the barrier layer
424 are configured so as to form a 2DEG near the interface between
the channel layer 422 and the barrier layer 424. The channel layer
422 and/or the barrier layer 424 may be provided by a single or
multiple layers. In some embodiments of the present invention, the
channel layer 422 is an unintentionally doped GaN layer. The
barrier layer 424 may be an AlGaN and/or AlN layer. In some
embodiments of the present invention, the channel layer 422 is GaN
having a thickness of from about 5 mm to about 5000 nm and the
barrier layer 424 is AlGaN having a concentration of aluminum of
from about 5% to about 100% and having a thickness of from about 1
to about 10 nm.
[0065] A gate contact 130 is provided on the barrier layer 424 and,
in some embodiments, may be provided on an oxide (not shown) on the
barrier layer 424. In some embodiments of the present invention, at
least a portion of the gate contact 130 overlaps with the region
above the mask regions 425. In some embodiments of the present
invention, the gate contact 130 is confined to a region above the
mask 425 and circumscribed by the mask 425. In still further
embodiments, the gate contact 130 may be provided without overlap
of the mask 425. The gate contact 130 may be formed of, for
example, Ni/Au.
[0066] Optionally, a dielectric layer, such as a silicon nitride or
silicon oxide layer is provided on the barrier layer 424. In such
embodiments, the gate contact 130 may be formed in a recess in the
dielectric layer or, in some embodiments, may be formed on the
dielectric layer.
[0067] A source contact 132 is also provided on the barrier layer
424. Optionally, the source contact 132 may be provided on a recess
in to the barrier layer 424 and/or on a contact region so as to
reduce resistance between the source contact 132 and the channel
layer 120. The source contact 132 may be formed of, for example,
Ti/Al/Ni/Au.
[0068] A drain contact 134 is provided on the substrate 410
opposite the first layer 412. Optionally, a backside implant or
epitaxial layer (not shown) may be provided so as to reduce
resistance between the drain contact 134 and the substrate 410. The
backside implant or epitaxial layer may, for example, be an n+
region formed in or on the substrate 410 on which the drain contact
134 is provided. Furthermore, the substrate 410 may be thinned
prior to formation of the drain contact 134. The drain contact may
be formed of, for example, nickel, if a silicon carbide substrate
is used. As discussed above, the drain contact 134 may, optionally,
be formed on the first layer 412, similar to the configuration
illustrated in FIG. 1. In such embodiments of the present
invention, an insulating substrate or a semi-insulating substrate,
such as semi-insulating silicon carbide or sapphire, may be
utilized for the substrate 410.
[0069] Without wishing to be bound by any theory of operation,
electrons from the source contact 432 may flow along the 2DEG
formed near the interface of the barrier layer 424 and the channel
layer 422 and then vertically into the n-type portions 421 of the
fifth layer 420 and then laterally across the n-type portion 419 of
the fourth layer 418 to the n+ layer 416, vertically to the n+
layer 414 and the first layer 412 and to the drain contact 134
through the substrate 410. Thus, a current aperture may be provided
by the p or i regions of the fourth layer 418 with a current path
provided by the n-type regions incorporated in the laterally grown
portions of the fourth layer 418 and the fifth layer 410. The
current aperture may also be defined by the mask 425.
[0070] Accordingly, a portion of the channel region of the device
may be provided by a hybrid layer. Furthermore, the channel region
through the current aperture may include a vertical portion through
the fifth layer 420 and a horizontal portion through the fourth
layer 418. Current may also flow through the opening in the mask
425 is such opening is provided. The Schottky gate contact 130 (or
MOS gate if an insulating layer provided on the barrier layer 424)
may modulate the charge in the 2DEG to control the flow of current
through the aperture.
[0071] Fabrication of embodiments of the present invention as
illustrated in FIG. 4 is schematically illustrated in FIGS. 5A-5E.
As seen in FIG. 5A, a substrate 410 is provided on which nitride
based devices may be formed. Fabrication of the substrate 410 may
be provided as discussed above with reference to the substrate
110.
[0072] The first layer 412 and the second layer 414 are
sequentially formed on the substrate 410. The first layer 412
and/or second layer 414 may be formed, for example, by plasma
enhanced chemical vapor deposition (PECVD), metal organic chemical
vapor deposition (MOCVD), molecular beam epitaxy (MBE), low
pressure chemical vapor deposition (LPCVD) or other technique known
to those of skill in the art. The particular deposition technique
may depend on the composition of the first layer 412 and/or the
second layer 414.
[0073] FIG. 5A further illustrates formation of the mask region
425. The mask region 425 may, for example, be fabricated by
depositing an oxide layer onto the second layer 414 and then
selectively etching the deposited oxide layer, for example, using a
subsequent etch mask, to provide the mask region 425. Alternatively
or additionally, a mask could be formed on the second layer 414 and
the oxide layer selectively deposited in openings in the mask.
Variable thickness of the mask regions 425 may result from etching
of the mask during subsequent growth operations as a result of the
high growth temperatures. In some embodiments of the present
invention, the mask has a thickness such that an opening at the
interior of the mask results from this etching. In other
embodiments of the present invention, the thickness is such that a
portion of the mask remains and no opening is formed.
[0074] FIG. 5B illustrates formation of the third layer 416. The
third layer 416 is formed by epitaxial lateral overgrowth so as to
extend partly over the mask region 425 and on the second layer 414.
Techniques for epitaxial lateral overgrowth are known to those of
skill in the art and are described above. The particular conditions
for formation of the third layer 416 may depend on the composition
of the third layer 416. As is further seen in FIG. 5B, the mask
region 425 may be partially etched by the high temperature growth
of the third layer 416. Thus, the mask region 425 may have a
tapered structure as the portions of the mask region 425 where no
growth has occurred will be exposed and, therefore, etched for a
longer period of time. Accordingly, in some embodiments of the
present invention, the growth of the third layer 416 is carried out
at a temperature sufficient to etch the mask region 425.
[0075] The third layer 416 may be grown in the presence of an
n-type dopant, for example, for a GaN third layer 416, Si, Ge or O
may be incorporated in the third layer 416 so as to provide an n+
third layer 416. During lateral growth of the third layer 416, the
laterally grown portion may be more susceptible to incorporation of
dopant and, therefore, the laterally grown portions (illustrated by
the dotted line in FIG. 4) may have a higher dopant concentration
than the vertically grown portions of the third layer 416. Thus,
while the third layer 416 is illustrated as an n+ layer, the dopant
and accordingly the carrier concentration, may differ in different
regions of the second layer 416.
[0076] FIG. 5C illustrates the formation of the fourth layer 418.
The fourth layer 418 also extends over a portion of the mask region
425. The fourth layer 418 may be formed by epitaxial lateral
overgrowth as described above. The fourth layer 418 is formed as
insulating or p-type semiconductor material and may be formed in
the presence of a small amount of n-type dopant, such as Si, by for
example, including SiH.sub.4 in the reaction chamber when the
fourth layer 418 is grown. For a GaN based layer, a p-type dopant,
such as Mg, Zn or Be, may be provided during formation of the
fourth layer 418. An insulating GaN layer may be formed in the
absence of a dopant other than that discussed herein with regard to
the n-type laterally grown region of the fourth layer 418 and/or in
the presence of a compensating dopant, such as Fe. The quantity of
n-type dopant provided may depend on the composition of the fourth
layer 418. However, the quantity should be sufficiently small so
that the vertically grown portions of the fourth layer 418 are
provided as p-type or insulating despite the presence of the n-type
dopant (e.g. the p-type dopant or compensating dopant swamps the
n-type dopant absent the preferential incorporation of n-type
dopant provided by lateral growth) but sufficiently large so that
the preferential incorporation of the n-type dopant in the
laterally grown portions 419 of the fourth layer 418 results in
n-type regions.
[0077] As is further seen in FIG. 5C, the mask region 425 may be
further etched by the high temperature growth of the fourth layer
418. Thus, the mask region 425 may have a tapered structure as the
portions of the mask region 425 where no growth has occurred will
be exposed and, therefore, etched for a longer period of time.
Accordingly, in some embodiments of the present invention, the
growth of the fourth layer 418 is carried out at a temperature
sufficient to etch the mask region 425.
[0078] FIG. 5D illustrates formation of the fifth layer 420. The
fifth layer 420 is formed by epitaxial lateral overgrowth as
discussed above and may be grown to cover the mask region 425 and
the region between the mask regions 425. Portions of the fifth
layer 420 may be laterally grown to coalesce. The fifth layer 420
is formed as an unintentionally doped layer and may be formed in
the presence of a small amount of n-type dopant, such as Si, by for
example, including SiH.sub.4 in the reaction chamber when the fifth
layer 420 is grown. The quantity of n-type dopant provided may
depend on the composition of the fifth layer 420. However, the
quantity should be sufficiently small so that the vertically grown
portions of the fifth layer 420 incorporate only an insubstantial
amount of the n-type dopant and, thus, may be considered
unintentionally doped. The quantity of n-type dopant should,
however, be sufficiently large so that the preferential
incorporation of the n-type dopant in the laterally grown portions
421 of the fifth layer 420 results in n-type regions.
[0079] As is further seen in FIG. 5D, the mask region 425 may be
partially etched by the high temperature growth of the fifth layer
420. In some embodiments, the mask region 425 may be completely
etched through in an interior portion. Thus, the mask region 425
may have a tapered structure as the portions of the mask region 425
where no growth has occurred will be exposed and, therefore, etched
for a longer period of time. Accordingly, in some embodiments of
the present invention, the growth of the fifth layer 420 is carried
out at a temperature sufficient to etch the mask region 425.
[0080] FIG. 5E illustrates formation of the channel layer 422 and
the barrier layer 424. Such layers may be formed by conventional
fabrication techniques based on the composition of the respective
layers. A drain contact 134, a source contact 132 and a gate
contact 130 may be formed utilizing conventional techniques as
illustrated in FIG. 4. In some embodiments of the present
invention, the gate contact may be formed on a dielectric layer
(not shown) on the barrier layer 424.
[0081] Depending on the particular composition of the
layers/regions and/or contacts illustrated in FIGS. 2, 3A-3E, 4
and/or 5A-5E, an anneal or multiple anneals may be carried out to
activate dopants and/or anneal contacts. Such annealing techniques
are known to those of skill in the art and, therefore, need not be
described further herein.
[0082] FIG. 6 illustrates a transistor according to further
embodiments of the present invention. In FIG. 6, the layer
structure of the transistor may be formed and a trench subsequently
formed through the layer structure and filled with n-type
semiconductor material to provide the current aperture. As seen in
FIG. 6, a first layer 612 of n-type semiconductor material is
provided on a substrate 610, such as an n-type semiconductor
material substrate. The substrate 610 may be a silicon carbide
substrate as described above with reference to the substrate 110.
The substrate 610 may be doped to provide a carrier concentration
of from about 10.sup.17 to about 10.sup.20 cm.sup.-3.
Alternatively, the substrate 610 may be an insulating or
semi-insulating substrate. In such a case, a contact may be
provided to the n+ layer 614, for example, in a trench or at a
peripheral edge of the device as illustrated in FIG. 1. In such
embodiments, the first layer 612 may be omitted or provided as a
buffer layer.
[0083] As is further illustrated in FIG. 6, a first layer 612 of
n-type semiconductor material, such as a nitride-based
semiconductor material, is provided on a surface of the substrate
610. The first layer 612 may, for example, may be AlGaN and may be
doped with, for example, Si to provide an n-type layer of AlGaN. In
some embodiments of the present invention, the first layer 612 has
a thickness of from about 0 (i.e. the first layer is optional) to
about 1 .mu.m. The first layer 612 may also be doped to provide a
carrier concentration of from about 10.sup.17 to about 10.sup.20
cm.sup.-3.
[0084] A second layer 614 of n-type semiconductor material is
provided on the first layer 612. The second layer 614 may, for
example, be an n+ GaN layer. The second layer 614 is provided on a
first surface of the first layer 612 that is opposite the substrate
610. In some embodiments of the present invention, the second layer
614 has a thickness of from about 0.1 .mu.m to about 10 .mu.m. The
second layer 614 also may be doped to provide a carrier
concentration of from about 10.sup.17 to about 10.sup.20
cm.sup.-3.
[0085] A third layer 616 of p-type or insulating semiconductor
material is provided on the second layer 614. In some embodiments
of the present invention, the third layer 616 may be p-type or
insulating GaN formed as discussed above. The insulating and/or
p-type regions of third layer 616 may substantially block current
flow by, for example, having a Fermi level positioned at greater
than about 0.5 eV from the conduction band. The third layer 616 may
have a thickness of from about 0.1 .mu.m to about 10 .mu.m.
[0086] A channel layer 618 and a barrier layer 620 may be provided
on the third layer 616. The channel layer 618 and the barrier layer
620 are configured so as to form a 2DEG near the interface between
the channel layer 618 and the barrier layer 620. The channel layer
618 and/or the barrier layer 620 may be provided by a single or
multiple layers. In some embodiments of the present invention, the
channel layer 618 is an unintentionally doped GaN layer. The
barrier layer 620 may be an AlGaN and/or AlN layer. In some
embodiments of the present invention, the channel layer 618 is GaN
having a thickness of from about 5 nm to about 5 .mu.m and the
barrier layer 620 is AlGaN having a concentration of aluminum of
from about 5% to about 100% and having a thickness of from about 1
nm to about 100 nm.
[0087] Optionally, a dielectric layer 626, such as a silicon
nitride or silicon oxide layer is provided on the barrier layer
620. A trench 622 is formed through the dielectric layer 626, the
barrier layer 620, the channel layer 618 and the third layer 616
and extends to, and in some embodiments, into the second layer 614.
A region of n-type semiconductor material 624, such as a
nitride-based semiconductor material, such as GaN, is provided in
the trench 622.
[0088] In some embodiments of the present invention, the first
layer 612 and/or second layer 614 may be omitted. For example, if
the substrate 610 is a GaN substrate, the first layer 612 and/or
the second layer 614 may be omitted and the trench 622 may be
provided to extend to and/or into the substrate 610.
[0089] A gate contact 630 is provided on the dielectric layer 626
and, in some embodiments, may be provided directly on the barrier
layer 620. The gate contact may be adjacent and spaced apart from
the trench 622. The gate contact 630 may be formed of, for example,
Ni/Au.
[0090] Source contact regions 628 may be provided of n-type
semiconductor material, such as a nitride-based semiconductor
material including, for example, GaN. The source contact regions
628 extend through the dielectric layer 626 to the barrier layer
620. A source contact 632 is also provided on the barrier layer
620. Optionally, the source contact 632 may be provided on a recess
in the barrier layer 620. The source contact 632 may be formed of,
for example, Ti/Al/Ni/Au.
[0091] A drain contact 634 is provided on the substrate 610
opposite the first layer 612. Optionally, a backside implant (not
shown) may be provided so as to reduce resistance between the drain
contact 634 and the substrate 610. The backside implant may, for
example, be an n+ region formed in the substrate 610 on which the
drain contact 634 is provided. Furthermore, the substrate 610 may
be thinned prior to formation of the drain contact 634. The drain
contact may be formed of, for example, nickel, for a silicon
carbide substrate. As discussed above, the drain contact 634 may,
optionally, be formed on the first layer 612 or the second layer
614, similar to the configuration illustrated in FIG. 1. In such
embodiments of the present invention, an insulating substrate or a
semi-insulating substrate, such as semi-insulating silicon carbide
or sapphire, may be utilized for the substrate 610.
[0092] Without wishing to be bound by any theory of operation,
electrons from the source contact 632 may flow from the n-type
contact regions 628 along the 2DEG formed near the interface of the
barrier layer 620 and the channel layer 618 and then into the
n-type region 624 to the n+ layer 614 and the first layer 612 and
to the drain contact 634 through the substrate 610. The gate
contact 630 (or Schottky gate if an insulating layer is not
provided on the barrier layer 620) may modulate the charge in the
2DEG to control the flow of current through the aperture.
[0093] In the drawings and specification, there have been disclosed
typical embodiments of the invention, and, although specific terms
have been employed, they have been used in a generic and
descriptive sense only and not for purposes of limitation, the
scope of the invention being set forth in the following claims.
* * * * *