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
15th 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
Explains highly conductive filaments sustained by a lock-on field lower than the breakdown field.
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.

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

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. Continuum Results V0 (KV) t (sec) VLO 50 30 1x10-9 40 1x10-1030
1x10-9 40
1x10-10
no lock-on
1x10-11
200 60

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.