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 ConferenceJune 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

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

kdrrrfdt

dndefectsAugeriiik

31 )(

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

nnFRdt

dn),(0

Auger

defectsii

defectsAugerii

C

CFCFn

CnCFCnFR

)()(

)(),( 20

Steady State Solution(no carrier-carrier scattering)

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

nnFRdt

dn),(

defectsAugeriiii

defectsAugerii

CnCnFCFC

CnCnFCnFR

2

10

20

)()(

),(),(

Steady State Solutions(carrier-carrier scattering)

GaAs

drtrJtrL

tI

tIRtV

RVtV

np

pDqp

nDqn

qnpnApnnpnBgtp

qnpnApnnpnBgtn

trtrptrn

p

pp

nn

nii

nii

)),(),((J1

)(

:currentcarrier Total

)()(V=tV(t)/

: resistance and tagesupply volpower a of in terms )( tageswitch vol for theequation line Load

)(q

-=

:field electric for theequation sPoisson'

)(

)(

:currents hole andelectron for equationsCurrent

/1)())((/

/1)())((/

:densitiescarrier ),(n intrinsic and ),,( holes ),,( electronsfor equations Continuity

n

00

00

p

n

22

22

i

E

EvJ

EvJ

J

J

Continuum Calculations

Continuum Results

V0

(KV)

(sec)

VLO

(KV)

50 0 30

50 1x10-9 40

50 1x10-10 no lock-on

50 1x10-11 no lock-on

200 0 40

200 1x10-11 60

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.