Modeling optically activated high power semiconductor switches

A two-dimensional time dependent computer model of a GaAs PCSS has been developed to investigate nonuniformities in the electric field observed in the operation of a high power photoconductive semiconductor switch. The model solves the continuity equations for electrons, holes, and traps, the energy equation for the lattice, Poisson's equation, and a circuit equation. Physical effects in the model include band-to-band impact ionization, trap impact ionization, photoionization, recombination radiation transport, and negative differential resistance. The model has the ability to address different switch geometries and the consequences of nonuniformities in carrier injection, permittivity, and illumination of the sample on switch operation.Results from modeling bulk and coplanar GaAs (Si:Cu) switches show that the switching cycle is sensitive to the type of laser spatial distribution, the trap doping levels, the circuit parameters, and the geometry of the device. Delivering light directly under the contacts decreases the voltage closing level. Intrinsic impact ionization is essential to closing the GaAs (Si:Cu) switch. For a coplanar switch with an ungrounded base, field enhancement occurs near the anode and cathode as the switch begins to close. This enhancement shifts to the cathode during the on state. When opened, an electric field enhancement front begins to migrate back toward the anode. The mobile front is a consequence of negative differential resistance. The spatial shape of laser penetration affects the shape of this field enhancement. For the ungrounded base, field enhancement occurs only near the anode during closing. In the opening stage a high field region forms near the anode inhibiting opening. As a result, opening is less sensitive to the opening laser pulse when a grounded base coplanar switch geometry is used.Band-to-band recombination radiation transport plays an important role in carrier transport when the switch is closed with a spatially nonuniform laser pulse. Reabsorption of the radiation ultimately reduces the electric field at the contacts, which allows the switch to close. The model predicts that the switch is sensitive to the location of the activating laser pulse. Less laser fluence is required to close the switch near the cathode than near the anode.U of I OnlyETDs are only available to UIUC Users without author permissio