Study Of The Tranport Of GaAs Photoconductive Semiconductor Switches

As a new class of switching devices, Photoconductive semiconductor switches (PCSS's) are a combination of ultra-fast pulsed laser and photoconductor. PCSS bears such advantages as high power, fast switching speed, small parasitic capacitance, high repetition frequency, and jitterfree, Therefore it possesses a broad application prospect in the fields of superspeed electronics, generation of high power electrical pulses, fine-synchronization control and terahertz technology.PCSS's output characteristic depended on the transportation of the photogenerated carriers and the transient electromagnetism process caused by the transportation of the photogenerated carriers. The transportation of the photogenerated carriers and the output characteristic under the linear mode and the nonlinear mode are studied individually, and the major points are summarized as below:1. A total current mode for linear mode of PCSS's is proposed.The transient electromagnetism process is studied under the condition of low biased field, the study shows that there are two types displacement current in the PCSS's generated by the transient electromagnetism process:1. Displacement current created by the built-in field in the photoconductor, which is formed by the transportation of the photogenerated carriers; 2. Displacement current created by the electric voltage shifting from the PCSS's to the load in the test circuit. Then, activation conditions of the two types of displacements are analyzed, and a Total Current mode is built based on the transportation of the photogenerated carriers and the transient electromagnetism process. In contrast with conductivity mode, the relationship of the transportation of the photogenerated carriers and the output current are described accurately by the new mode. 2. The current limiting effect of photoactivated charge domain is proposed.Under the condition of high bias electric field and nonuniform weak trigger laser, the nonlinear transportation process of photogenerated carriers in the linear GaAs PCSS's is studied by simulation method. The formation and transporting process of photoactivated charge domain (PACD) in linear mode are discussed. The current limiting effect of PACD is proposed, and the max output current is limited by the PACD.3. The secondary photo excited avalanche charge domain mode is proposed.First, the transient built-in field in the photoconductor of the nonlingear GaAs PCSS's is studied by simulation method. According to the GaAs optical characteristic, the transient optical characteristics are explored based on the transient built-in field and the transient photogenerated carries distribution. The result shows that there are not only an intense nonuniform electrical characteristic but also an intense nonuniform optical characteristic. Next, the redistribution law of photogenerated carriers through emitting and absorbing secondary photo is explored. The result shows that the PACD reached steady state can be excited once again by the secondary photo. Under adequate exciting by the secondary photo, the PACD grow into avalanche charge domain. Finally, the secondary photo excited avalanche charge domain mode is proposed for nonlinear mode of GaAs PCSS's, this mode is more comprehensive and more accurate.4. An experiment is performed to proof current limiting effect of PACD.Based on study on the electrode making,insulation protection,transmission line design, A series of GaAs PCSS's are made. A confirmatory experiment is successful performed and the results confirmed the current limiting effect of PACD. Then, an ultra-broadband electromagnetic wave is radiated by a horn antenna connected to the PCSS, and the spectrum covers from 650MHz to 2GHz.