Wide Bandgap Extrinsic Photoconductive Switches Page: 21 of 245
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resistance can become low enough to allow unacceptably high leakage currents from energy
storage components in the circuit. Silicon also exhibits thermal runaway that limits the time
that a Silicon PCSS can hold off high electric fields to a few microseconds. This requires that
the Si PCSS be operated in the pulsed bias mode. Energy storage components in the electrical
circuit (capacitors, pulse forming lines) must be pulse charged to permit use of a Si PCSS as
the switching element. In addition, the long recombination time in Silicon (> 10 ps) prevents
rapid opening of the PCSS. Nevertheless, linear, intrinsic, lateral, Silicon PCSS devices have
successfully operated at pulsed electric fields of 82 kV/cm and peak switch currents of 2
kiloamps (not simultaneously) [2.3].
GaAs is available in wafer form with resistivity greater than 108 S2-cm, which should limit
leakage current through a GaAs PCSS device. High purity GaAs has a critical electric field
strength of 350 - 400 kV/cm. These physical properties suggest that linear, GaAs PCSS
devices with very high voltage hold off should be realizable. This is not the case. The dc dark
breakdown field for GaAs is 8-20 kV/cm [2.3]. This low dc holdoff is attributed to carrier
injection at deep traps in the bulk GaAs. The low dark dc breakdown field of GaAs PCSS
requires that these devices be used in pulsed circuits where the high electric field appears
across the PCSS for a few microseconds. In addition, GaAs PCSS can collapse into the
nonlinear "lock-on" mode of operation if sufficient energy is left in the circuit when the
optical pulse terminates. Control of the GaAs PCSS is lost and current continues to flow until
the external circuit can no longer maintain the "lock-on" field in the PCSS. Linear, GaAs
PCSS have not achieved the high switch current and field operating parameters demonstrated
by linear Si and nonlinear GaAs PCSS. The DWA switch application requires simultaneous19
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Sullivan, J S. Wide Bandgap Extrinsic Photoconductive Switches, thesis or dissertation, January 17, 2012; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc835750/m1/21/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.