Optically triggered high-gain semiconductor switch achieves milestone
They weren’t popping champagne corks, but there was celebration nonetheless in Department 15333 earlier this month when researchers achieved a milestone with a High Gain PhotoConductive Semiconductor Switch (PCSS).
The optically triggered gallium arsenide (GaAs) electrical switch exceeded 100 million pulses — switching on and off without fail for that record number of times. At this point the test was terminated while the switch was still functioning properly.
Among those on hand to celebrate the accomplishment were team members Darwin Brown, Gary Denison, Wes Helgeson, Guillermo Loubriel, Alan Mar, Luis Molina, Martin O’Malley, and Fred Zutavern (all 15333); Albert Baca and Melissa Cavaliere (both 1711); and Harry Hjalmarson (9225). "We’ve been working toward this moment since 1987," says Guillermo, Manager of Directed Energy Special Applications Dept. 15333. "By developing a switch that pulses this many times without a glitch, we showed we have an extremely robust device."
Robust is important, because someday the technology may be used as a firing set switch for nuclear weapons, in high-peak-power impulse radars, in driving laser diode arrays for sensing, in triggering large lasers, in high-power microwaves, in particle beam accelerators, and in high-voltage pulsers.
The High Gain GaAs PCSS is of interest because it can be optically triggered with very low timing variations, uses small laser diodes, can carry high currents, and switches high voltages. It is these very aspects that damage standard switches — the high current flows in filaments between the metal contacts, and, as a result of the current confinement, damage occurs at the contacts. In fact, the average life of a standard switch is in the tens of thousands of pulses, compared to the new PCSS’s record to date of 100 million.
The large improvement in the PCSS was accomplished by modifying the design of the contacts. Instead of contacting directly to nonconductive GaAs, the new structure incorporates dopants into the GaAs surface where the metal contacts are placed, making the GaAs conductive in these contact regions.
The filaments terminate on the doped GaAs region instead of the contact metal itself, eliminating the contact metal damage. Various methods are available to fabricate the doped regions, including ion implantation, thermal diffusion, and epitaxial regrowth.
The new record was accomplished in collaboration with Xerox’s Palo Alto Research Center (PARC) under contract to Sandia using PARC’s thermal diffusion process, with the balance of the switch fabrication being done at Sandia. Work is nearing completion to replicate the new doped PCSS structure using Sandia’s epitaxial regrowth capabilities where the dopants are added to the GaAs base through a layering process.
"We are pleased with our success with the PCSS and are moving forward to come up with ways to make it even better," says Fred Zutavern, team member. "The epitaxial regrowth is our next step."