Simulation of Current Filaments in Photoconductive Semiconductor Switches K. Kambour, H. P. Hjalmarson, F. J. Zutavern and A. Mar Sandia National Laboratories* Charles W. Myles** Texas Tech University 15 th International IEEE Pulsed Power Conference June 16, 2005 * Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the United States Department of Energy under contract DE-AC04-94AL ** Supported in part by an AFOSR MURI Contract
Outline Photoconductive Semiconductor Switches (PCSS's) Lock-on Collective Impact Ionization Theory Monte Carlo Calculations Continuum Calculations Conclusions
A PCSS
Lock-on Characterized by a persistent or 'locked-on' electric field (~5 kV/cm) after laser turn off. High conductivity state Always accompanied by the formation of current filaments. The lock-on field is much lower than the bulk breakdown field for GaAs.
Current Filaments
Bistable Switch
Carrier Distribution Function
Collective Impact Ionization Theory Inside (high carrier density): the carrier-carrier scattering increases the efficiency of impact ionization for the hot carriers. Outside (low carrier density): the electric field is too low to create carriers by impact ionization. Explains highly conductive filaments sustained by a lock-on field lower than the breakdown field.
Monte Carlo Calculations Determining the distribution function Ensemble Monte Carlo Maxwellian Calculating the rate of change of particle number
Evolution to a Steady State Solution (no carrier-carrier scattering )
Steady State Solution (no carrier-carrier scattering)
Evolution to Steady State Solutions (carrier-carrier scattering included)
Steady State Solutions (carrier-carrier scattering)
GaAs
Continuum Calculations
Continuum Results
V 0 (KV) (sec) V LO (KV) x x no lock-on 501x no lock-on x Continuum Results
Conclusions Collective Impact ionization Theory (CIIT) predicts that lock-on will occur in GaAs at a field much less than the intrinsic breakdown field in GaAs, in qualitative agreement with experiment. CIIT also predicts that the lock-on field will be independent of rise time and that the lock-on current will flow in stable current filaments in agreement with experiment.