Semiconductor Switch Materials Research
For the Semiconductor Switch Device Research page, click Here.

    My primary area of interest is the simulation of high electric field transport in photoconductive semiconductor switch (PCSS) materials such as GaAs. This is where my funding (Sandia National Laboratories & the Air Force Office of Scientific Research, AFOSR) comes from. A schematic of a PCSS is shown here:


    High field transport simulations in PCSS materials were the dissertation topics of my last two PhD students, Samsoo (Sam) Kang (PhD, 1998) & Kenneth (Ken) Kambour (PhD, 2003). At Sandia, my collaborator is Harry Hjalmarson, who served as co-Advisor to Ken Kambour. Ken is now a post-doc at Sandia, working on some extensions of his dissertation work. There, he can interact directly with Harry Hjalmarson, as well as with our experimental collaborators (Fred Zutavern & co-workers).
    Sam Kang's dissertation was a "proof in principle" that the collective impact ionization theory of
Hjalmarson, et al. is correct. The basic idea of this theory is that, at high electric fields AND high carrier densities, carrier-carrier (cc) scattering enhances the impact ionization rate, thus leading to electrical breakdown at fields much lower than the usual breakdown field. Using an Ensemble Monte Carlo (EMC) approach to high field transport, Sam successfully applied this theory to GaAs.
    In Ken Kambour's dissertation, a collective impact ionization approach was used to develop a generalized theory of electrical breakdown in insulators, which includes the dependence of impact ionization on both the electric field and the carrier density. This theory was applied to
PCSS materials and was used to explain the lock-on effect, an optically-triggered breakdown that occurs in GaAs and InP PCSS's.
This generalized breakdown theory uses a rate equation approach to find the carrier densities which, at a given electric field, result in a steady state. In this approach, the competition between carrier generation (by impact ionization) and carrier recombination (by Auger and defect recombination) governs whether or not electrical breakdown occurs. This leads to a definition of the bulk breakdown field as the lowest field for which the injection of an infinitesimally small carrier density will result in a steady state with a large carrier density. It also leads to the definition of the lock-on field as the lowest field for which a stable, non-zero steady state carrier density is possible.
    To implement this theory, the
EM method was used to calculate the carrier distribution function, including the effects of carrier-carrier scattering. Since the EMC calculations are computationally intense, this implementation also used both low and high density approximations for the distribution function. The low density limit was obtained using the EMC method without the inclusion of cc-scattering. The high density limit was obtained by approximating the distribution function as a Maxwellian. Using this theory, predictions were made for both the lock-on field and the bulk breakdown field in GaAs, InP, Si, and GaP.
    In this theory, the lock-on effect is a type of carrier-density dependent electrical breakdown which occurs in all insulating materials. Further, it is the difference between the predicted lock-on and breakdown fields which determines whether or not the lock-on effect will be observable as a phenomenon distinct from ordinary breakdown.
    Typical results are shown in the figure below, which plots the predicted steady state carrier density as a function of electric field for GaAs. The hollow circles are the
EMC results without carrier-carrier scattering. The solid circles are the EMC results with cc-scattering. The squares are Maxwellian results. The curves are included to guide the eye. The predicted breakdown field is the field (~177 kV/cm) at which the dashed curve intersects the horizontal axis. The predicted lock-on field is the minimum field (~90 kV/cm) on the solid curve.
Predicted Steady State Carrier Density vs. Electric Field for GaAs


    Recent publications which resulted from this project are #2,3,13,17 & 18 on the "Recent Publications" link below. This project has also resulted in MANY conference presentations. See #1,2,7,16,19,22,25,28,32,34 & 35 on the "Recent Conference Presentations" link below. Ken Kambour's dissertation: Abstract (Word); Document (PDF, 2.84 MB, 102 pages). PhD Graduation Photos.
    Recently, through collaboration and consultation on the MURI Compact Pulsed Power Program, a small effort on semiconductor switch device simulation has been started. More details on this topic may be found Here.

Abstract for a recent colloquium on semiconductor switches. (Word, 89.5 kB)
Slide summarizing recent work on the Sandia project. (Power Point, 187 kB).

                Semiconductor Switch Device Research

                Clathrate Semiconductor Research

                Recent Publications

                Recent Conference Presentations

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