Abstract:
The present invention relates to a solid state switch that may be used as in optically-triggered switch in a variety of applications. In particular, the switch may allow for the reduction of gigawatt systems to approximately shoebox-size dimension. The optically-triggered switches may be included in laser triggered systems or antenna systems.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent application Ser. No. 61/683,188, filed Aug. 14, 2012, entitled “Optically-triggered Linear or Avalanche Solid State Switch for High Power Applications,” which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to optically-triggered switches used in radio frequency (RF) gigawatt (GW) class high power systems, including high power radar systems and ground penetrating radar systems. In particular, the present invention relates to bulk avalanche semiconductor switches useful for producing high power pulses in RF GW class systems that also operate in the gigahertz frequency range. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the field of high power semiconductor systems, many existing systems tend to be very large, heavy, and cumbersome. One such system is the original GEM System produced by Power Spectra Inc. The GEM system is a gigawatt class, high power microwave system that uses laser triggered switches built with gallium arsenide (GaAs) semiconductors. The GEM system uses an array of bulk avalanche semiconductor switching (BASS) modules that each produced 1 to 2 MW of peak power to achieve a total power output of 1 GW in a 500 MHz-1.5 GHz wideband output scheme. The GEM system is very large and bulky and produces a limited power output relative to many modern high powered systems and technologies. Therefore, a need exists for more compact and robust systems and devices for advanced high power applications. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a switch that is optically-triggered avalanche switch based on aluminum gallium nitride (AlGaN) or gallium nitride (GaN). The optically-triggered switch can be used for any high voltage application. A specific example includes use in the generation of RF signals for high power applications. In one embodiment, the optically-triggered switch includes a GaN or AlGaN semiconductor material and a set of conductive contacts deposited on the semiconductor material. One or more grounding electrodes may also be deposited on the semiconductor material. A portion of the semiconductor material separates the one or more grounding electrodes from the conductive set of contacts. The switch may be a horizontally switched structure or a vertically switched structure that incorporates a mesa structure for voltage hold off. A laser beam may be used to illuminate the switching region from a lateral side of the switch or the laser beam may illuminate the switch in a direction perpendicular to a conductive region of the switch. Once the switch is illuminated, the semiconductor material undergoes an avalanche breakdown leading to a highly conductive region that transfers the energy through the switch. The switch may be configured to operate when exposed to a voltage potential in a range from approximately 0.01 V to approximately 10 MV. In particular, the switch may be configured to operate when exposed to a voltage potential of approximately 0.01 MV to 10 MV. 
         [0005]    The optically-triggered switch is configured for use in a compact high power system to generate a power pulse on the order of kilowatts to gigawatts. In other embodiments, the switch may include AlGaN. Further, systems incorporating the GaN or AlGaN switches may be used as reconfigurable pulse width or frequency agile RF pulse sources. Alternatively, the switch may operate in either a linear mode or an avalanche mode. The switch may be used for accelerator systems or any system requiring a switch that can transition from a nonconductive region to a conductive region on the order of femtoseconds to nanoseconds. Alternatively, the AlGaN or GaN switches can be incorporated into an integrated circuit configured for protecting systems against cyber terrorism or incorporated into power semiconductor devices. 
     
    
     
       DESCRIPTION OF FIGURES 
         [0006]      FIG. 1  depicts an optically-triggered switch according to one embodiment. 
           [0007]      FIG. 2  depicts a semiconductor wafer with a number of optically-triggered switch conductive contacts and electrodes according to one embodiment. 
           [0008]      FIG. 3  depicts an optically-triggered vertical switch according to one embodiment. 
           [0009]      FIG. 4  illustrates a manufacturing process to produce a vertical optically-triggered switch according to one embodiment. 
           [0010]      FIG. 5  is a chart providing Baliga&#39;s Figure of Merit for various semiconductor materials according to one embodiment. 
           [0011]      FIGS. 6A-B  depict the optical absorption for various semiconductor materials according to one embodiment. 
           [0012]      FIG. 7  is a block diagram of a high power system that incorporates modular arrays of the optically-triggered switch according to one embodiment. 
           [0013]      FIG. 8  is a block diagram of a high power TSTL circuit geometry for monocycle generation that incorporates the optically-triggered switch according to one embodiment. 
           [0014]      FIG. 9A  depicts an embodiment of the optically-triggered switch that may be used in a TSTL pulse forming network. 
           [0015]      FIG. 9B  depicts an embodiment of the optically-triggered switch that may be used in high voltage insulation. 
           [0016]      FIGS. 10A-B  are graphs illustrating that the waveshape and the number of cycles, respectively, can be tailored to the desired application according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The present invention relates to optically-triggered semiconductor-based switches and compact and easily transportable systems, including antennas that incorporate the optically-triggered switch, high power radar systems, and ground penetrating radar systems, among others. In various embodiments, the switches are built with Group III nitride semiconductors, including gallium nitride (GaN) or aluminum gallium nitride (AlGaN) solid state semiconductors. The switches may then be incorporated into megawatt to multiple terawatt class high power systems that are up to 100 to 200 times more powerful than existing power systems of the same size that rely on GaAs or silicon based switches and/or other photoconductive switches. Similarly, the switches and systems of the present invention may also produce comparable or greater levels of power within a reduced size. 
         [0018]    Referring now to  FIG. 1 , an embodiment of the optically-triggered switch  100  having a coplanar configuration is shown. The optically-triggered switch  100  includes a GaN wafer  102  consisting of an undoped, intrinsic GaN layer  202  having a thickness in a range between about 30 μm and about 2 cm grown on a substrate  200  having a thickness in a range between about 300 μm and about 1 mm, as shown in  FIG. 3 . In one embodiment, the GaN layer  202  is grown using any suitable method to a thickness of approximately 50 μm on an approximately 300 μm thick conductive gallium nitride (GaN) substrate  200 . Other substrate materials or additional base layers may also be incorporated into the wafer  102 . 
         [0019]    As shown in  FIG. 1 , the optically-triggered switch  100  also includes one or more ground plane electrodes  104 A-B. The optically-triggered switch  100  also includes a central conductor  106 . The central conductor  106  is a microstrip conductor having an impedance in a range between about 0.1Ω and about 1000Ω. Preferably, the conductor  106  has an impedance in the range between about 10Ω and 200Ω. 
         [0020]    In various embodiments, the central conductor  106  and the ground plane electrodes  104 A-B are spaced to provide a negligible coupling effect. For example, an analysis may be performed to verify the impedance of an input signal by varying the spacing “D” between the conductor  106  and the ground plane electrodes  104 A-B while recording the output voltage. In various embodiments, the spacing D may be in a range between about 1 mm and 1 cm. By way of example and not limitation, the spacing D between the conductive contact or conductor  106  and the ground plane electrodes  104 A-B may be carefully tailored to ensure minimal impedance variation 
         [0021]    In operation, an input signal  108  is applied to the conductive contact  106  while the ground plane electrodes  104 A-B are held at ground potential. When the optically-triggered switch  100  is illuminated by a laser pulse  110  in a laser illumination region  112 , the resistivity of the GaN or AlGaN decreases which forms a low-resistance electrical connection between the conductors and ground plan electrodes  104 A-B and  106 . As a result, a portion of the input signal  108  is reflected back to the input and a portion of the input signal is shunted to ground. The optically-triggered switch  100 , therefore provides dual, parallel current paths to ground (e.g. one to the left ground plane electrode  104 A and one to the right ground plane electrode  104 B), which decreases the effective channel resistance. In various embodiments, the channel resistance may be reduced to the order of milliohms or micro ohms. 
         [0022]      FIG. 2  depicts another embodiment of the optically-triggered switch  100 . This embodiment includes an AlGaN wafer  102  that includes multiple conductive contacts  106  and ground plane electrodes  104 A-B. The multiple arrangements  114  of conductors  106  and ground plane electrodes  104 A-B may be isolated or, alternately, in electrical communication with one another so that an input signal  108  at one conductor  16  may be shared across one or more other conductors. As such, an optical signal, including those generated by a laser or other suitable light source, at one of the arrangements  114  may cause the input signal  108  at another arrangement to be shunted to ground. Alternately, the optical signal may be used as a floating series electrode. 
         [0023]    While embodiments of the optically-triggered switch  100  may have a coplanar configuration as shown in  FIGS. 1-2 . A preferred and potentially more practical configuration is a vertical configuration as shown in  FIG. 3 , where the switch  100  shorts a conductor  106  on top of the switch through the switch material to a ground plane electrode  104 A-B located on a bottom surface of the switch. The vertical configuration of the switch  100  can be operated in the linear mode or in an avalanche mode. The vertically configured switch  100  may include a substrate and one or more semiconductor layers formed on the substrate. 
         [0024]    As shown in  FIG. 3 , one embodiment of the optically-triggered switch  100  includes a substrate  200  and multiple semiconductor layers  202 - 204  formed on the substrate. The substrate  200  is an n-type doped GaN substrate. In one embodiment, the substrate  200  is a Si-doped conductive GaN wafer approximately 0.3 mm thick and approximately 2 inches in diameter. In other embodiments, the GaN substrate  200  may be doped by other n-type dopants including, but not limited to, germanium. On top of the substrate  200 , an intrinsic or undoped layer  202  of GaN or AlGaN is grown. The top layer of the switch  100  is a p-type doped GaN layer  204 . In one embodiment, the top layer  204  is doped with magnesium, while in other embodiments, the dopant may be another p-type dopant, including but not limited to zinc. A mesa structure, as indicated by  206 , is formed in the switch  100  by any suitable process including etching. 
         [0025]    The optically-triggered switch  100 , as shown in  FIGS. 1-3  may be manufactured using a variety of methods including, but not limited to hydride vapor phase epitaxy (HVPE) growth techniques and chemical vapor deposition (CVD). The switch  100  may also be produced by depositing electrodes on a grown semiconductor crystal. 
         [0026]      FIG. 4  illustrates one embodiment of a manufacturing process  300  to produce the optically-triggered switch  100 . The process  300  begins by providing a substrate  200  at  302 . In one embodiment, the substrate  200  is a SiGaN wafer approximately 1 mm thick and approximately 2 inches in diameter. At  304 , an epitaxial layer  202  of GaN is grown on the substrate  200 . The epitaxial layer  202  may be grown by an HVPE process and extrinsically doped to provide an n+ doped layer approximately 50 μm thick. At  306 , the epitaxial layer  202  is etched to provide a mesa structure and conductive contacts or electrodes  106  and  104 A-B are bonded to the remaining epitaxial layer at  308 . A passivation layer of silicone oxide (SiO 2 )  208  is bonded to the switch  100  at  310  and at  312 , another contact  210  is bonded to the substrate  202  opposite the epitaxial layer  204 . The substrate  200  is then bonded to an AlN base layer  212  at  314 . At  316 , the switch  100  is exposed to a laser  214  which is used to define a switch channel  216 , as shown at  318 . 
         [0027]    In various embodiments, the optically-triggered switch  100  is configured for operation in horizontal and/or vertical bulk avalanche modes. In one embodiment, the optically-triggered switch  100  is approximately 1 mm thick and configured as a 10-25Ω Blumlein pulser. The optically-triggered switch  100  may be triggered by any suitable optical source including, but not limited to, a fiber-coupled Laser diode. Any laser or laser diode with a wavelength compatible with the semiconductor band gap can be used to illuminate the optical switch. 
         [0028]    The optically-triggered switch  100  has a projected output of approximately 2.5 to 5 GW which can be derated by a factor of approximately 4 such that each switch may have a derated output of approximately 1 GW. In various other embodiments, switches that can switch power systems on the order of watts to multiple terawatts can also be fabricated using GaN or AlGaN semiconductors. The rise time for various embodiments of the switch  100  is on the order of picoseconds, while the pulse width for the output of the switch  100  may be varied based on the geometry of the switch. Sub picosecond rise times may also be achievable. An impedance mismatch with the switch  100  may cause ring down with the switch. In one embodiment, the switch  100  may have approximately 4-5 ring down cycles. Alternatively, the laser triggered switches can be used in active amplifier circuits such as class E or Class D amplifiers to generate high power RF signals. 
         [0029]    In some embodiments, GaN is selected for the epitaxial layer  202  as GaN is highly chemically inert and does not require hermetic packaging. In addition, GaN has low thermal impedance that helps to prevent heat buildup and a thermal resistance approximately two to three times that of copper, which in turn allows for faster cooling of the switch. In some embodiments, the epitaxial layer  202  of the switch  100  may include AlGaN alone or in combination with GaN as both materials show superior optical properties and provide a superior Baliga&#39;s figure of merit (BFoM) as shown in  FIG. 5 . The BFoM is a geometry-independent comparison of materials for power devices. As shown, GaN has a voltage break down that is over ten times greater than that of GaAs or silicon. As such, the GaN-based switches  100  may achieve a peak power 100 to 200 times higher. 
         [0030]    Moreover, commonly used silicon or silicon-carbide based semiconductor materials are indirect band gap semiconductors and have limited optical amplification. Therefore, high power systems using silicon-based semiconductor switches typically require large lasers. Conversely, as shown in  FIGS. 6A-B , GaN, and alternatively AlGaN, have significantly higher optical amplification. Due to its direct band gap, GaN is suited for avalanche mode switching using reduced laser energies and/or sizes. Laser diodes or any other optical source can be used for triggering which is compatible with the band gap of the AlGaN or GaN material. 
         [0031]    One or more of the optically-triggered switches  100  may be arranged into one or more modular arrays configured to provide as much power as desired. For example, the array(s) may be configured to provide 0.5 to 2.0 GW of power or more.  FIG. 7  is a block diagram of one embodiment of a compact laser-switched system  400  that includes multiple array modules  402  of the switch  100 . The system  400  also includes an energy storage system  404 . The energy storage system  404 , which may be a generator, a battery, an electrochemical double layer capacitor (an ultracapacitor) or other energy storage device. In one embodiment, operating the system  400  in a burst mode may reduce the average power of the system by storing energy in the energy storage system  404 . The energy storage system  404  is in communication with a power supply  406  and a processing device  408  having an interface  410  with which a user may activate, control, or otherwise use the system  400 . The processing device  408  is also in communication with the power supply  406 ; both of which are also in communication with the switch array modules  402 . The switch array modules  402  are in further communication with an antenna array  412  having one or more antennas. 
         [0032]    The use of multiple switch array modules  402  allows a user of the system  400  to select the frequency generated at the antenna array  412 . For example, as shown, each array module  402  may correspond to a particular frequency in a range between approximately 500 MHz and 4 GHz. Modules for frequencies above and below this range may also be used. The modular arrangement of the switch arrays  402  permits each array to be compact in size. In various embodiments, each modular array  402  may be approximately the same size as a shoebox. 
         [0033]    In another embodiment, one or more optically-triggered switches  100  may be used in systems employing a translationally symmetric transmission line (TSTL) circuit geometry for monocycle generation. For example, the switches  100  may be used in a limiter circuit  500  (e.g. a large area radar limiter circuit), as shown in  FIG. 8 . The circuit  500  may include a power conditioning system  502  in communication with two or more optically-triggered switches  100  through symmetric transmission lines  504 . 
         [0034]    As shown in  FIGS. 9A-B , another embodiment of the optically-triggered switch  100  may be integrated into a TSTL pulse forming network. As shown in  FIG. 9A , the switch  100  may have dual switch channels  600  that extend though the epitaxial layer  202  to the base layer  208 . A path  602  between the channels may be formed by AlN ceramic glue. By way of example, the switch  100  may be integrated into high voltage insulation  604  as shown in  FIG. 9B . In another example, the switch  100  may be incorporated into ruggedized package that is triggered by a high power laser diode, a tripled yttrium alexandrite garnet laser, or other laser device. 
         [0035]    In a wide variety of solid state laser triggered switched systems that can incorporate the switch  100 , including those shown in  FIGS. 5-9B , the peak power output increased and may be in a range between approximately 1 GW and 100 GW with an average power in a range between approximately 1 KW and 1 GW. The systems may output power at a frequency between about 10 KHz and 20 KHz; however the output may be rep-rate dependent as a burst mode of operation may reduce the average power output. The systems may generate a signal having an effective radiated power or equivalent radiated power (ERP) between about 100 GW and 10 TW or may boost the gain for antenna by approximately 20 to 30 dB or higher. 
         [0036]    Regarding the output signal, the systems may be frequency agile such that the frequency may be adjustable and in a range between approximately 500 MHz and 3 GHz. In various embodiments, the frequency of the output signal may be modified or tailored to various desired targets and concept of operations (CONOP) scenarios. As shown in  FIGS. 10A-B , the waveshape and the number of cycles can be tailored to the desired application. As such, a narrowband frequency signal may be generated. 
         [0037]    As previous described, the switch  100  may permit reductions in size of various high power systems. For example, the systems may be compact and housed in rugged and robust housings approximately the size of a shoebox. The solid state systems may be configured to be waterproof or water resistant, drop or shock resistant, and suitable for operational temperatures up to, but not limited to, approximately 200° C. 
         [0038]    Similarly, the weight of the systems may be reduced. In various aspects, the weight of the systems may depend on the desired rep-rate of the system. For example, the systems may be less than approximately 200 pounds, and may even weigh 20 pounds or less. Low rep-rate systems may be very compact and portable by one or two people. The size and weight of the systems may also be may be modified or tailored to various desired targets and CONOP scenarios. 
         [0039]    In various embodiments, the frequency, power, and rep-rate of the output signal as well as the weight of the system or device may be configured and varied with gains in one parameter being made for losses in another. For example, one system may be modified to generate a first output signal at a frequency of 500 MHz, for 5 cycles at a power of 1 GW and 1 KHz with an average power of 20 kW. The same system may be easily and dynamically modified to generate an output signal for 5 cycles at a power of 1 GW and 100 Hz with an average power of 2 kW. 
         [0040]    It will be appreciated that the device and method of the present invention are capable of being incorporated in the form of a variety of embodiments, only a few of which have been illustrated and described above. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.