Study On The Characteristics And Breakdown Mechanism Of High Power GaAs Photoconductive Semiconductor Switches

Due to the characteristics of high voltage, high current, low inductance and capacitance, ultrafast switching, and picosecond temporal resolution, photoconductive semiconductor switches (PCSS's) have been widely used in ultrahigh speed electronics, the field of high power microwave generation, pulse forming and THz radiation. Especially because of presence of the high-gain mode (also known as nonlinear mode or lock-on effect) of III-V compound semiconductors (such as GaAs, InP, et al), the using of PCSS's is more effective, convenient, and flexible. Until now, several models have been proposed to explain the high-gain mode. However, the mechanism of the high-gain mode has not well been understood to date due to the great complexity of the dynamics of carriers in GaAs under high electric fields. Moreover the premature breakdown is critical issue, whose mechanism is not fully understood and is an important area of study. In this paper, the characteristic of high field domain in GaAs PCSS, the mechanisms of high gain mode, and the high-power characteristic and breakdown mechanism of GaAs PCSS's are investigated systemically and deeply. The PCSS's with the current as high as 3.7 kA and the electric power higher than 100 MW has been developed through the electrode etching technique.Two important oscillation modes of photoactivated charge domain, namely quenched domain mode and delayed domain mode have been observed experimentally. It is show the formations of the two oscillations are due to the interaction of the circuit self-excitation and transferred-electron oscillation in the bulk of switch. During the transit of the domain, the bias electric feld (larger than Gunn threshold) across the switch is modulated by the alternating current (AC) electric field, when the instantaneous bias electric field is swinging below the sustaining field (the minimum electric field required to support the domain), and then the quenched-domain mode is obtained. If the instantaneous bias electric field is swinging above the sustaining field and below Gunn threshold electric field during the transit of the domain, when the domain is extinguished at the anode, new doamin can not be formed immediately, this mode of oscillation is delayed domain mode. Based on the characteristic of Gunn domain and the circuit of PCSS, the equivalent circuit of the photoactivated charge domain is presented. The frequency of quenched domain oscillation mode is calculated quantitatively by making use of the equivalent circuit. The theoretical calculation is well agreement with experimental result.A model for lock-on effect in the semi-insulating (SI) GaAs PCSS's is proposed for the first time, and the lock-on field of SI-GaAs PCSS's has been measured under different bias voltages. When PCSS's operate in nonlinear mode, the ultrahigh electric field of domain induced by photogenerated carriers leads to strong impact ionization accompanied by electron-hole recombination radiation in the switch. Therefore new avalanche domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation which causes the effective carrier velocities to be larger than the saturation velocity. Lock-on field resulted from the length proportional number of domains and steadfast electric fields inside and outside the domains, and the recovery of lock-on effect is caused by the domain quenching. Based on the mechanism of nonlinear mode of SI-GaAs PCSS's, a combined switch consisted of a traditional PCSS and a spark gap is developed. Using the combined switch the lock-on effect is suppressed effectively, and compeared with single PCSS higher output current is obtained with the combined switch. The analysis shows the suppression of lock-on effect is caused by the voltage transferring from the PCSS to the spark gap. Due to ture-on process of the spark gap, the electric field across the bulk PCSS is decreased below the sustaining electric field of Gunn domain, leading to the extinguishing of the domains, and then the PCSS will enter open state because of recombination of nonequilibrium carriers.Through electrode etching, current as high as 3.7 kA has been generated using a single photoconductive semiconductor switch exited by a laser pulse with the energy of～8 mJ and under a bias of 28 kV, and the highest withstand voltage is up to 32 kV. The PCSS with electrode gap of 14 mm was fabricated from semi-insulating GaAs. Under different bias voltages the "on" resistances of the PCSS were measured. The longevity of the PCSS reached 350 shots at 20 kV and 400 A.The breakdown mechanisms of PCSS's have been investigated. The theoretical analysis indicates different breakdown traces correspond different breakdown mechanisms. (1) For the filament breakdown trace across the bulk of the PCSS, the electron-trapping breakdown theory is an important mechanism for the breakdown of GaAs PCSS. Due to capture process of electron traps for electrons, as the number of trapped electrons attains a threshold, a chain of trapped electrons will reach two electrodes of the PCSS, and all of the trapped electrons move to the anode along the chain, thereby a conductive path formed by hole traps has been buildup. Electrons flow rapidly from cathode to anode through the path, and then a current path is formed, which makes the current of PCSS increase sharply. Since the capacitors highly charged produce a very high current density along the conductive path, which causes the PCSS breakdown instantaneously, and a melted deep groove will be formed by the high current density. (2) For the breakdown of serious damage for the anode, the breakdown of PCSS's fabricated from indirect band-gap semiconductors is mainly caused by trap filled limited conduction model, however, for PCSS's fabricated from materials that exhibit the transferred-electron effect, such as GaAs, breakdown of the PCSS's is mainly caused by negative resistance inducing electric field enhancement at the anode boundary. Based on the mutuality experiments and the breakdown mechanisms, the breakdown time and breakdown voltage are calculated respectively for two different types of breakdown, and the theoretical calculations are well agreement with experimental results.