【作者】 田立强；

【导师】 施卫；

【作者基本信息】 西安理工大学 ， 微电子学与固体电子学， 2009， 博士

【摘要】 光电导开关(Photoconductive Semiconductor Switches简称PCSS’s)具有耐压强度高、通流能力强、寄生电感电容小、开关速度快和皮秒时间精度等特性,使其在超高速电子学、大功率脉冲产生与整形技术领域(大功率亚纳秒脉冲源、超宽带射频发生器等)具有广泛的应用前景。特别是GaAs、InP等Ⅲ-Ⅴ族化合物半导体PCSS’s中存在的高倍增模式(亦称非线性模式、或lock-on效应),使光电导开关的应用更为有效、方便、灵活。本文针对光电导开关理论和应用研究中存在的问题,采用理论计算和实验相结合的方法,对半绝缘GaAs光电导开关中的高场畴特性、高倍增模式的产生机理、开关的高电压强电流特性和击穿机制进行了较为系统、深入的研究,具体研究内容和结论如下：从实验上发现了光激发电荷畴的两种振荡模式：猝灭畴模式和延迟畴模式。分析指出两种振荡模式是由于开关电路自激振荡形成的交流场对开关偏置电场周期性动态调制引起的。在适当的触发光能和偏置电场条件下,电荷畴在渡越过程中,当开关电场低于电荷畴维持电场(维持电荷畴生存需要的最小电场)时,电荷畴将在到达阳极前湮灭,形成光激发电荷畴的猝灭畴模式,交流场周期性的调制,便形成猝灭畴振荡；若在适当的触发光能和偏置电场条件下,电荷畴在渡越过程中,当开关电场低于电荷畴的阈值电场、而高于维持电场时,电荷畴到达阳极湮灭后,新的电荷畴不能及时形成,只有当开关电场再次上升到阈值电场以上时,电荷畴才可再次形成,即电荷畴的形成被延迟,以上过程的不断重复便形成延迟畴振荡模式。在理论分析的基础上,结合相关实验,给出了光激发电荷畴的等效电路模型。利用等效电路模型对开关的猝灭畴振荡作了定量的计算,理论计算与实验基本一致。提出半绝缘GaAs光电导开关非线性模式的多畴理论模型,并在不同偏置电压条件下测量了半绝缘GaAs光电导开关的lock-on电场。理论计算表明高浓度的光生载流子可使电荷畴内的电场达到材料的本征击穿强度,使畴内发生伴有复合辐射的强烈碰撞电离,新的电荷畴可以由复合辐射再吸收激发的载流子不断的形成,在开关体内形成一定数量的光激发电荷畴；理论分析指出开关的lock-on电场是由于开关体内固定的电荷畴数量和畴内外稳定的电场分布引起的,开关的快速导通是由于种子畴(传播在电离区最前面的电荷畴)的传播速度相当于以光速和电子的饱和漂移速度交替运动引起的。依据半绝缘GaAs非线性光电导开关的产生机理,设计了由光电导开关与火花隙构成的串联型组合开关。利用组合开关两部分先后导通过程中电压的转移,使光电导开关的分压小于畴的维持电压,通过畴的猝灭,迫使开关关断,有效的抑制了lock-on效应。同时提高了开关的输出电流。通过开关电极刻蚀,研制了耐压高达32kV、通流能力达3.7kA的高功率半绝缘GaAs光电导开关。测量了开关不同触发条件下的通态电阻,对开关进行了耐压实验和寿命实验,在20kV／400A工作条件下,开关的寿命达350次。分析了开关的击穿机理。理论分析表明不同类型的击穿痕迹是由于不同的击穿机理引起的。贯通电极的丝状击穿痕迹是由于电子俘获形成的陷阱链导电路径导致电流剧增使开关热击穿而形成；阳极严重损坏的击穿是由于陷阱填充和转移电子效应引起阳极电场剧增导致的。依据相关实验和击穿机理,对两种不同的击穿类型分别进行了理论计算,计算结果与实验基本吻合。更多还原

【Abstract】 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.更多还原