U.S. patent application number 14/656628 was filed with the patent office on 2015-07-02 for multi-sided optical waveguide-fed photoconductive switches.
The applicant listed for this patent is Scott D. Nelson, Hoang T. Nguyen, Stephen E. Sampayan. Invention is credited to Scott D. Nelson, Hoang T. Nguyen, Stephen E. Sampayan.
Application Number | 20150187470 14/656628 |
Document ID | / |
Family ID | 49714558 |
Filed Date | 2015-07-02 |
United States Patent
Application |
20150187470 |
Kind Code |
A1 |
Sampayan; Stephen E. ; et
al. |
July 2, 2015 |
MULTI-SIDED OPTICAL WAVEGUIDE-FED PHOTOCONDUCTIVE SWITCHES
Abstract
A photoconductive switch and optical transconductance varistor
having a photoconductive region e.g. a wide bandgap semiconductor
material substrate between opposing electrodes. An optical
waveguide is arranged to surround the photoconductive region for
directing conduction-inducing radiation into the photoconductive
region. And an optical diffusion element is arranged to
diffuse/disperse the radiation prior to entering the optical
waveguide and into the substrate, for uniformly illuminating the
substrate for conduction.
Inventors: |
Sampayan; Stephen E.;
(Manteca, CA) ; Nguyen; Hoang T.; (Livermore,
CA) ; Nelson; Scott D.; (Patterson, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sampayan; Stephen E.
Nguyen; Hoang T.
Nelson; Scott D. |
Manteca
Livermore
Patterson |
CA
CA
CA |
US
US
US |
|
|
Family ID: |
49714558 |
Appl. No.: |
14/656628 |
Filed: |
March 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13912162 |
Jun 6, 2013 |
|
|
|
14656628 |
|
|
|
|
61656467 |
Jun 6, 2012 |
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Current U.S.
Class: |
257/76 |
Current CPC
Class: |
H01L 31/0296 20130101;
H01C 7/1013 20130101; H01L 31/0312 20130101; H01L 31/08 20130101;
H01L 31/03044 20130101; H01L 31/02325 20130101; H01L 31/028
20130101; H01C 7/10 20130101; G02B 6/4295 20130101; H01L 31/02327
20130101; H01L 31/0224 20130101; H01L 31/09 20130101 |
International
Class: |
H01C 7/10 20060101
H01C007/10; H01L 31/0304 20060101 H01L031/0304; G02B 6/42 20060101
G02B006/42; H01L 31/028 20060101 H01L031/028; H01L 31/0296 20060101
H01L031/0296; H01L 31/0232 20060101 H01L031/0232; H01L 31/0312
20060101 H01L031/0312 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The United States Government has rights in this invention
pursuant to Contract No. DE-AC52-07NA27344 between the United
States Department of Energy and Lawrence Livermore National
Security, LLC for the operation of Lawrence Livermore National
Laboratory.
Claims
1. A photoconductive switch comprising: a wide bandgap
semiconductor material substrate; first and second electrodes in
contact with said substrate; an optical waveguide surrounding
exposed facets of the substrate for directing conduction-inducing
radiation into said exposed facets of the substrate; and an optical
diffusion element arranged to diffuse/disperse said radiation prior
to entering the optical waveguide and into the substrate.
2. The photoconductive switch of claim 1, wherein the optical
waveguide is coated with a reflective coating to reflect radiation
into the exposed facets of the substrate.
3. The photoconductive switch of claim 2, wherein the optical
diffusion element is a tapered light pipe connected to the optical
waveguide.
4. The photoconductive switch of claim 1, wherein the first and
second electrodes are in contact with said material so that a first
triple junction boundary region is formed between the substrate and
the first electrode and a second triple junction boundary region is
formed between the substrate and the second electrode, and the
substrate is located completely within a triple junction region
formed between the first and second triple junction boundary
regions.
5. An optical transconductance varistor comprising: a wide bandgap
semiconductor material substrate, whose conduction response to
changes in amplitude of incident radiation that is substantially
linear throughout a non-saturation region thereof, whereby the
material is operable in non-avalanche mode as a variable resistor;
first and second electrodes in contact with said substrate; an
optical waveguide surrounding exposed facets of the substrate for
directing conduction-inducing radiation into said exposed facets of
the substrate; and an optical diffusion element arranged to
diffuse/disperse said radiation prior to entering the optical
waveguide and into the substrate.
6. The optical transconductance varistor of claim 5, wherein the
optical waveguide is coated with a reflective coating to reflect
radiation into the exposed facets of the substrate.
7. The optical transconductance varistor of claim 6, wherein the
optical diffusion element is a tapered light pipe connected to the
optical waveguide.
8. The optical transconductance varistor of claim 5, wherein the
first and second electrodes are in contact with said material so
that a first triple junction boundary region is formed between the
substrate and the first electrode and a second triple junction
boundary region is formed between the substrate and the second
electrode, and the substrate is located completely within a triple
junction region formed between the first and second triple junction
boundary regions.
9. A photoconductive switch comprising: a wide bandgap
semiconductor material substrate; first and second electrodes in
contact with said substrate and defining a triple junction region
therebetween, with all remaining surfaces of the substrate having a
reflective coating to internally reflect conduction-inducing
radiation; and an optical diffusion element arranged to
diffuse/disperse said conduction-inducing radiation into the
substrate so that the triple junction region may be uniformly
illumated and made conductive.
10. The photoconductive switch of claim 9, wherein the optical
diffusion element is a tapered light pipe connected to a face of
the substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of prior application Ser.
No. 13/912,162, filed Jun. 6, 2013, which claims the benefit and
priorities of U.S. Provisional Application No. 61/656,467, filed on
Jun. 6, 2012, U.S. Provisional Application No. 61/656,470, filed on
Jun. 6, 2012, and U.S. Provisional Application No. 61/801,483,
filed on Mar. 15, 2013, all of which are hereby incorporated by
reference herein.
TECHNICAL FIELD
[0003] This patent document relates to photoconductive switches,
and in particular to a multi-sided optical waveguide-fed
photoconductive switch having an optical diffusion structure for
smoothing and homogenizing light to uniform illuminate the switch
material for conduction.
BACKGROUND
[0004] Photoconductive switches and switch packages typically
consist of a wide bandgap photoconductive material (such as GaN,
ZnO, diamond, AlN, SiC, BN, etc.), a source for energetic photons
(e.g. a laser), a method to couple the laser into the switch, and a
method for high voltage to enter and leave the switch package such
as via electrodes positioned on opposite sides of the substrate.
Arranged as such, the photoconductive switch package may be
characterized as a three terminal device similar to transistors;
with one of the terminals being a laser input or the voltage input
to the laser system. When the photoconductive switch material is
illuminated such as by a laser, the laser photons change the
conductivity of the photoconductive material and make it viable as
an optically controlled switch. Various package configurations and
methods are known for feeding the high voltage into the switch
package (while maintaining low capacitance and inductance),
reducing detrimental electric field effects, optical coupling
methods, and extracting high voltage and high current from the
switch package.
[0005] Traditional photoconductive switch (PCS) illumination
techniques, such as bulk free-space optics or fiber optic
assemblies, rely on the intrinsic laser beam shape for uniform
illumination of the PCS. However, these approaches may not
adequately illuminate the entire switch volume. In order to
illuminate the entire switch volume, the laser beam shape would
need to be enlarged to bigger than the switch dimensions, which in
turn results in inefficiencies due to unused laser light.
SUMMARY
[0006] In one example implementation, the present invention
includes a photoconductive switch comprising: a wide bandgap
semiconductor material substrate; first and second electrodes in
contact with said substrate; an optical waveguide surrounding
exposed facets of the substrate for directing conduction-inducing
radiation into said exposed facets of the substrate; and an optical
diffusion element arranged to diffuse/disperse said radiation prior
to entering the optical waveguide and into the substrate.
[0007] In another example implementation, the present invention
includes an optical transconductance varistor comprising: a wide
bandgap semiconductor material substrate, whose conduction response
to changes in amplitude of incident radiation that is substantially
linear throughout a non-saturation region thereof, whereby the
substrate is operable in non-avalanche mode as a variable resistor;
first and second electrodes in contact with said substrate; an
optical waveguide surrounding exposed facets of the substrate for
directing conduction-inducing radiation into said exposed facets of
the substrate; and an optical diffusion element arranged to
diffuse/disperse said radiation prior to entering the optical
waveguide and into the substrate.
[0008] In another example implementation, the present invention
includes a photoconductive switch comprising: a wide bandgap
semiconductor material substrate; first and second electrodes in
contact with said substrate and defining a triple junction region
therebetween, with all remaining surfaces of the substrate having a
reflective coating to internally reflect conduction-inducing
radiation; and an optical diffusion element arranged to
diffuse/disperse said conduction-inducing radiation into the
substrate so that the triple junction region may be uniformly
illuminated and made conductive.
[0009] Various other aspects of the present invention may include
one or more of the following aspects to the above example
implementations, including wherein the optical waveguide is coated
with a reflective coating to reflect radiation into the exposed
facets of the substrate, and wherein the optical diffusion element
is a tapered light pipe connected to the optical waveguide, wherein
the first and second electrodes are in contact with said material
so that a first triple junction boundary region is formed between
the substrate and the first electrode and a second triple junction
boundary region is formed between the substrate and the second
electrode, and the substrate is located completely within a triple
junction region formed between the first and second triple junction
boundary regions.
[0010] These and other implementations and various features and
operations are described in greater detail in the drawings, the
description and the claims.
[0011] The present invention is generally directed to a multi-sided
optical waveguide fed photoconductive switch configuration which
distributes radiation to all sides of a material substrate e.g. a
photoconductive, semi-insulating, or semi-conducting material,
hereinafter collectively referred to as photoconductive material
for convenience, located between opposing electrodes so that the
substrate is rendered conductive. As such the induced conductivity
of the substrate may be uniformly produced in the substrate
material. A diffusive or diffractive element or a mask at one or
more facets of the substrate may also be used to control light
distribution along the height of the switch substrate. In
particular, it may be used to keep light intensity highest along a
center region (at or near the center plane) of the substrate and
keep it high to reach the electrode faces, while light intensity is
tapered down towards the top and bottom boundaries of the
substrate, so as to taper the resistivity around the triple
junction and thereby minimize field enhancement at the triple
junction.
[0012] Photoconductive switches have typically been illuminated
(fed) from the side or edge of the photoconductive material, i.e.
along a longitudinal direction of a planar switch, with electrodes
transversely positioned on either side of the switch material. As
shown in FIGS. 1 and 2, it can be seen that the region of the
semiconductor that does not have physical electrode attachment
cannot be excited by the radiation that renders the bulk of the
crystal conductive. When the switch is in the "off state" as shown
in FIG. 1, the normal field lines can terminate on the surface of
the substrate. Under these conditions, the field lines obey the
normal boundary conditions specified by Maxwell's equations. When
the substrate is rendered conductive by the radiation source, as
shown in FIG. 2, the condition of "Ohms Law" in general form must
be fulfilled. That is J=E, where J is the current density and E is
the applied electric field. In this case, an electric field line
that does terminate on an electrode supplies current to that
electrode. A field line that does not terminate on an electrode
will generate a surface charge at the boundary interface between
the substrate and the non-conductive surrounding media. That net
charge will distort the applied electric field as to enhance the
fields at the point where the electrode ends and cause catastrophic
breakdown. The present invention is intended to mitigate this issue
by transversely illuminating the triple junction region of the
photoconductive material (i.e. substrate) directly between opposing
electrodes so that no portion outside the triple junction region is
activated to conduct.
[0013] As used herein and in the claims, the triple junction region
is that region of the substrate located between a first triple
junction boundary region defined between the substrate and a first
electrode, and a second triple junction boundary region defined
between the substrate and a second electrode. In particular, FIGS.
6-9 illustrate and define the triple junction region formed between
a first triple junction boundary region and a second triple
junction boundary region. The switch material may be
photoconductive, or semi-insulating or semi-conducting, and may be
induced to conduct by an electromagnetic radiation source, or a
particle radiation source.
[0014] It is also appreciated that by optically exciting wide
bandgap materials, the conductivity of bulk of the material is
modulated. In such a device, the entirety of the crystal
participates in the conduction process. For instance, a 100 micron
thick crystal will have the capability approaching 40 kV and would
replace ten equivalent junction devices. Thus, unlike junction
devices, the wide bandgap material can be made arbitrarily thick to
accommodate higher voltages in a single device. Furthermore, there
is both a linear region and a saturation region as is with a
typical transistor device. Thus, when the material is operated in
the linear region, amplification of an applied modulation to the
optical pulse will result in amplification of the applied signal.
Such a device may consist of the modulation input to the radiation
source (whether it be a laser, particle source, or x-ray source)
and the wide bandgap semiconductor material. The terminals would
then be the common electrode, the input to the modulation source,
and the output terminal. Because this device is similar to a
"transconductance varistor," or more commonly called a
"transistor," such as device may be we characterized as an optical
transconductance varistor, or "opticondistor."
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 shows a schematic side view of an example
photoconductive switch showing normal electric field lines during
an off state of the switch.
[0016] FIG. 2 shows a schematic side view of the example
photoconductive switch of FIG. 1 showing increased stress due to
increased conductivity during an on state of the switch due to
illumination.
[0017] FIG. 3 shows a schematic view of an example embodiment of
the photoconductive switch of the present invention.
[0018] FIG. 4 shows a cross-sectional view taken along line A-A of
FIG. 3.
[0019] FIG. 5 shows another example embodiment of the present
invention using a tapered light pipe for diffusing light prior to
entering the optical waveguide.
[0020] FIGS. 6-9 illustrate and define the triple junction region
of the present invention in terms of first and second triple
junction boundary regions.
DETAILED DESCRIPTION
[0021] Turning now to the drawings, FIG. 3 shows an example
embodiment of the multi-sided optical waveguide-fed photoconductive
switch, generally indicated at 10, with an optical diffusion or
diffraction structure (hereinafter referenced simply as optical
diffusion structure) shown at 11 for diffusing radiation into an
optical structure (shown together as an optical waveguide 13
surrounding a photoconductive region 12) to uniformly illuminate
the photoconductive region 12. The diffusive structure operates to
smooth and homogenize light sources while providing uniform light.
Hotspots and uneven light distribution are common problems with
fiber-optic and laser light sources. Homogenization provides a
uniform pattern of light. It is notable that the diffusive
structure does not need to be fabricated onto the input switch
fact, but can also be a separate optical component that resides in
front of the switch, with such configuration providing the same
effect. The circular central region of the optical structure is the
active photoconductive region 12 which may be a doped region of the
optical structure, or a separate photoconductive material such as a
wide bandgap semiconductor. Electrodes are provided on opposite
sides of the photoconductive region 12 (see for example 15 and 16
in FIG. 4). And the optical waveguide 13 surrounding the active
photoconductive region 11 operates to guide light centrally to the
active photoconductive region, and is of a type that cannot be
rendered conductive by the radiation. Materials such as for example
silicon dioxide would be a suitable material for the optical
waveguide. In FIG. 3, the diffusive diffraction structure 11 is
arranged at an aperture of the optical waveguide 13. And a high
reflective (HR) coating 14 is shown formed on the optical waveguide
(e.g. optical dielectric multi-layer thin films). Integrated
together as a photoconductive switch (PCS) package, this
configuration can enable illumination intensity uniformities and
electrical conductive efficiencies. These enhancements results in a
device illumination technique that utilizes minimal optical
elements, thus greatly simplifying PCS assemblies.
[0022] FIG. 4 shows a cross sectional view taken along line A-A of
FIG. 3 showing how the optical diffusion structure 11 may be
configured to shape the radiation so that it has a constant
intensity near a center plane of the substrate, with intensity
tapered lower near the opposing substrate surfaces. Optionally,
edge masks may also be used to direct laser light along near the
center plane of the substrate to target the triple point region.
FIG. 4 also shows electrodes 15 and 16 abutting on opposite sides
of the photoconductive region 12 of the optical structure, with the
optical waveguide 14 surrounding the photoconductive region.
[0023] In FIG. 5, another embodiment is shown, generally indicated
at 20, using a tapered light pipe 21 as the optical diffusion
structure for diffusing radiation uniformly into the optical
waveguide 14 and the photoconductive region 12. Here too, a high
reflective coating 14 (e.g. optical dielectric multi-layer thin
films) is shown used to redirect leaked light back into the
photoconductive region, and integrated onto a photoconductive
switch (PCS) to improve illumination intensity uniformities and
electrical conductive efficiencies.
[0024] Though not shown in the drawings, another embodiment of the
photoconductive switch includes a wide bandgap semiconductor
material substrate, and first and second electrodes are in contact
with said substrate to define a triple junction region
therebetween, with all remaining surfaces of the substrate having a
reflective coating to internally reflect conduction-inducing
radiation. An optical diffusion element, as described above, is
then arranged to diffuse/disperse said conduction-inducing
radiation into the substrate so that the triple junction region may
be uniformly illuminated and made conductive. Here too, the optical
diffusion element may be a tapered light pipe connected to a face
of the substrate.
[0025] Although the description above contains many details and
specifics, these should not be construed as limiting the scope of
the invention but as merely providing illustrations of some of the
presently preferred embodiments of this invention. Other
implementations, enhancements and variations can be made based on
what is described and illustrated in this patent document. The
features of the embodiments described herein may be combined in all
possible combinations of methods, apparatus, modules, systems, and
computer program products. Certain features that are described in
this patent document in the context of separate embodiments can
also be implemented in combination in a single embodiment.
Conversely, various features that are described in the context of a
single embodiment can also be implemented in multiple embodiments
separately or in any suitable subcombination. Moreover, although
features may be described above as acting in certain combinations
and even initially claimed as such; one or more features from a
claimed combination can in some cases be excised from the
combination, and the claimed combination may be directed to a
subcombination or variation of a subcombination. Similarly, while
operations are depicted in the drawings in a particular order, this
should not be understood as requiring that such operations be
performed in the particular order shown or in sequential order, or
that all illustrated operations be performed, to achieve desirable
results. Moreover, the separation of various system components in
the embodiments described above should not be understood as
requiring such separation in all embodiments.
[0026] Therefore, it will be appreciated that the scope of the
present invention fully encompasses other embodiments which may
become obvious to those skilled in the art. In the claims,
reference to an element in the singular is not intended to mean
"one and only one" unless explicitly so stated, but rather "one or
more." All structural and functional equivalents to the elements of
the above-described preferred embodiment that are known to those of
ordinary skill in the art are expressly incorporated herein by
reference and are intended to be encompassed by the present claims.
Moreover, it is not necessary for a device to address each and
every problem sought to be solved by the present invention, for it
to be encompassed by the present claims. Furthermore, no element or
component in the present disclosure is intended to be dedicated to
the public regardless of whether the element or component is
explicitly recited in the claims. No claim element herein is to be
construed under the provisions of 35 U.S.C. 112, sixth paragraph,
unless the element is expressly recited using the phrase "means
for."
* * * * *