1. 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-AC04-94AL85000. ** Supported in part by an AFOSR MURI Contract

2. Outline  Photoconductive Semiconductor Switches (PCSS's)  Lock-on  Collective Impact Ionization Theory  Monte Carlo Calculations  Continuum Calculations  Conclusions

3. A PCSS

4. 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.

5. Current Filaments

6. Bistable Switch

7. Carrier Distribution Function

8. 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.

9. Monte Carlo Calculations Determining the distribution function Ensemble Monte Carlo Maxwellian Calculating the rate of change of particle number

10. Evolution to a Steady State Solution (no carrier-carrier scattering )

11. Steady State Solution (no carrier-carrier scattering)

12. Evolution to Steady State Solutions (carrier-carrier scattering included)

13. Steady State Solutions (carrier-carrier scattering)

14. GaAs

15. Continuum Calculations

16. Continuum Results

17. V 0 (KV)  (sec) V LO (KV) 50030 501x10 -9 40 501x10 -10 no lock-on 501x10 -11 no lock-on 200040 2001x10 -11 60 Continuum Results

18. 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.