Charge transport and persistent conduction in high gain photoconductive semiconductor switches used in pulsed power applications

Summary form only given. High gain photoconductivity semiconductor switches (PCSS´s) used in pulsed power applications usually have deep level impurities in high concentration that are normally utilized in the compensation processes that make the devices semi-insulating (SI). The overall resistivity of these switches is thus improved and the devices have higher hold-off voltages. Depending on the bias applied, the conduction mechanisms in these devices also vary. Some of these devices continue to conduct even after the elimination of the optical source. In this presentation we analyze the behavior of optically triggered high gain GaAs PCSS, and discuss the nature of conduction at high and very high bias conditions, presenting a model for the persistent conduction of the switches after the withdrawal of the trigger source. The analysis takes into account GaAs specific inter-valley charge transfer properties, negative resistivity, field-assisted charge trapping and de-trapping, and impact ionization to give a model that can explain the physical processes during conduction. Experimental and modeling results of PCSS´s used in generating ultra wideband (UWB) high power microwaves (HPM) are used in our analysis. The devices analyzed consist of 1.2/spl times/1 cm opposed contact PCSS´s that were fabricated from GaAs that has a high concentration of EL2 traps at the mid-gap level. The device I-V characteristics were generated at the AFRL Laboratories, while the simulation studies were carried out at the University of New Mexico using the SILVACO semiconductor simulation code. The GaAs PCSS used in the simulation was generated through an interface with the BLAZE module that simulates III-V semiconductor devices. Our study shows that the characteristics of the PCSS can be affected by the impurity concentration, specifically those at the deep level where the EL2 donor (>10/sup 16//cm/sup 3/) sites dominate.