Transversely-illuminated high current photoconductive switches with geometry-constrained conductivity path
A photoconductive switch having a wide bandgap semiconductor material substrate between opposing electrodes, with one of the electrodes having an aperture or apertures at an electrode-substrate interface for transversely directing radiation therethrough from a radiation source into a triple junction region of the substrate, so as to geometrically constrain the conductivity path to within the triple junction region.
This patent document 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
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTThe 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.
TECHNICAL FIELDThis patent document relates to photoconductive switches, and in particular to a transversely-illuminated high current photoconductive switch or optical transconductance varistor with geometry-constrained conductivity path.
BACKGROUNDPhotoconductive 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, capable of operating in the linear mode. 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.
SUMMARYIn one example implementation, the present invention includes a photoconductive switch comprising: a wide bandgap semiconductor material substrate; and electrodes in contact with said substrate, wherein one of said electrodes has an aperture at an electrode-substrate interface for transversely directing radiation therethrough from a radiation source into a triple junction region of the substrate.
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; and electrodes in contact with said material, wherein one of said electrodes has an aperture at an electrode-substrate interface for transversely directing radiation therethrough from a radiation source into a triple junction region of the substrate.
These and other implementations and various features and operations are described in greater detail in the drawings, the description and the claims.
The present invention is generally directed to a transversely illuminated/fed optically controlled high power switch configuration which optically confines the illuminating light (optical energy) to the center region or triple junction region 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 areas of the substrate outside the triple junction region are not illuminated and rendered conductive. As such the induced conductivity of the substrate is geometrically constrained/confined to the triple junction region to mitigate photon loss as the laser propagates through the switch (due to absorption of photons) and increases the efficiency of use of laser photons.
In particular, the approach of the present invention illuminates the photoconductive material from a transverse direction with respect to the planar material, and in an axial direction with respect to the switch electrodes. As such, reference to both transverse illumination and axial illumination are intended to characterize the same directionalized illumination into the substrate via an aperture or apertures through the electrodes. This is different from most photoconductive switches which are typically 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. However, one principal disadvantage in this method of illumination is the amount of loss of the laser energy as the laser photons propagate through the photoconductive switch material in the longitudinal direction, which then makes illumination of a triple-junction region between opposing electrodes inefficient. As shown in
Variations of the present invention may include, for example: using a suitably small aperture on axis to prevent enhancement of the electric field at the optical aperture; using a diffuser so that the light coming through the aperture is divergent so as to fill the substrate volume with light; and treating (e.g. with reflective coating) and shaping the opposite electrode(s) in such a manner so as to reflect the light multiple bounces and/or to go multiple bounces in a directed fashion to keep it within the substrate following a multi-path photon path to allow substantially total capture of available photons. Furthermore, as an added safeguard, the upper and lower surfaces of the substrate could also be rendered conductive, such as by or by employing conductive liners on the entire substrate surface, or on a portion of the substrate surface, such as for example described in U.S. Pat. Publication 2011/0101376, incorporated by reference herein.
Some of the advantages of the present invention include high-current electrode capability, use of reflective (non-transparent) electrodes in order to constrain the photon path, allows for complete absorption of the photons (as they pass through volumes of material), the photon path is centered on the region under the electrodes, and the conductivity profile (during photon illumination) falls off towards the edges of the electrodes. Additionally by optically confining the incident optically energy this can greatly increase the efficiency of the use of optical energy in the system and to normalize. Optical confined may be enhanced by edge treatment of the switch (using a reflective coating) to prevent loss of optical energy out of the switch. By inserting optical energy into the switch from the transverse (axial) direction and then prevent it from leaving via the longitudinal (edge) direction, the optical energy may be substantially completely consumed.
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,
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.”
Turning now to the drawings,
A lens (shown as a hemispheric lens) is also shown provided to expand the laser beam and illuminate the triple point region of the switch. The laser beam may either be guided/transported in a fiber or through the air, for example. Beams from small-core fibers naturally diverge so a lens isn't typically required, however a lens may be used for other-sized fibers and through-air beams. And various types of lens may be used, such as for example spherical lens, hemispherical lens, a hollow sphere with reflective inner surface, or other reflective/refractive structure that expands/diffuses/disperses radiation through the aperture and into the triple junction region. In this regard, a lens, or other refractive surfaces, elements, or media may be used to expand the laser beam to illuminate the switch. The refractive surfaces, elements, or media can stand-alone or be integrated on or within the switch. Examples of refractive surfaces or media are lens, axicons, high refractive index polymers/coatings, etc. Or in the alternative, the use of diffractive surfaces, elements, or media may be more optimal in producing a uniform illumination distribution. As with the refractive surfaces, elements, or media, the diffractive surfaces, elements, or media can stand-alone or be integrated on or within the switch as well. Examples of diffractive surfaces or media: any obstructing object provides multiple, closely spaced openings, such as diffraction gratings, micro lens arrays, Fresnel zones, prism, diffusive features, etc.
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In
For the embodiments shown in
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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.
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.”
Claims
1. A photoconductive switch comprising:
- a wide bandgap semiconductor material substrate; and
- electrodes in contact with said substrate, wherein one of said electrodes has an aperture at an electrode-substrate interface for transversely directing radiation therethrough from a radiation source into a triple junction region of the substrate.
2. The photoconductive switch of claim 1,
- wherein the electrode having the aperture includes a diffuser for dispersing said radiation into the triple junction region.
3. The photoconductive switch of claim 1,
- wherein the electrode not having the aperture is reflective at the electrode-substrate interface.
4. The photoconductive switch of claim 1,
- wherein the substrate has reflective surfaces to keep said radiation in the substrate.
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 substrate is operable in non-avalanche mode as a variable resistor; and
- electrodes in contact with said material, wherein one of said electrodes has an aperture at an electrode-substrate interface for transversely directing radiation therethrough from a radiation source into a triple junction region of the substrate.
6. The optical transconductance varistor of claim 5,
- wherein the electrode having the aperture has a diffuser for dispersing said radiation into the triple junction region.
7. The optical transconductance varistor of claim 5,
- wherein the electrode not having the aperture is reflective at the electrode-substrate interface.
8. The optical transconductance varistor of claim 5,
- wherein the substrate has reflective surfaces to keep said radiation in the substrate.
Type: Application
Filed: Jun 6, 2013
Publication Date: Dec 12, 2013
Inventor: Scott D. Nelson (Patterson, CA)
Application Number: 13/912,162
International Classification: H01L 31/0224 (20060101);