| Picosecond photoconductive semiconductor switches(PCSS)which possess the dual features of high power and high repetition frequency have gained considerable attention recently in the fields of ultrafast optoelectronics terahertz technology and high-power pulses,including terahertz wave radiation and detection,high-power microwave sources,particle accelerators,directed energy systems,and so on.Some of the advantages these switches offer compared to conventional switches include picosecond response,megawatt power density,small parasitic inductance and capacitance,and in some cases,lower laser energy requirement for initiating the switching.Unilluminated bulk resistivity of the PCSS has a very important effect on the withstand voltage of the switch.As the most mainstream switch chip material,currently,the resistivity of semi-insulating GaAs(SI-GaAs)at ambient temperature can reach 108?·cm,and the intrinsic breakdown field is as high as 250 kV/cm.So far,Since the technical means to improve the resistivity of chip materials through the production process and post-processing process have reached the limit,finding a new way to improve the resistivity of chip materials inevitably becomes an important research topic in the fields of terahertz functional devices and pulse power technology.In this paper,based on the in-depth study of carrier scattering and transport mechanisms,combined with the characteristics of SI-GaAs material,the MATLAB simulation software was used to study the temperature characteristic of dark-state resistivity for GaAs PCSS in the 5?500 K temperature range.The theoretical results are in very good agreement with the experimental results.Our results provide quantitative understanding of charge relaxation in GaAs-based electronic and photonic devices performance in different temperature ranges.The main contents of the thesis are as follows:(1)Starting from the Fermi golden rule in Quantum Mechanics and Boltzmann's transport equation,the scattering rates of elastic scattering,inelastic scattering,isotropic scattering and anisotropic scattering are approximately solved by using momentum relaxation rate.The Four main scattering mechanisms including ionized impurity scattering,acoustic phonon deformation potential scattering,acoustic phonon piezoelectric scattering and polar optical phonon scattering were analyzed and given according to the special energy band and crystal structure of GaAs materials.This research can provide a theoretical basis for quantitative study of the scattering mechanism and transport characteristics of carrier for GaAs PCSS.(2)Based on the three-level compensation mechanism of SI-GaAs,the variation rule of carrier concentration with the temperature and impurities concentration is calculated by using the statistical distribution of carriers in semiconductors.Combined with the physical model of the scattering mechanism of GaAs materials,the mobility under different scattering mechanisms has been theoretically analyzed.The temperature characteristics of electron mobility in SI-GaAs were rigorously solved by using the variational method.MATLAB calculation results show that the electron mobility of SI-GaAs is sequentially dominated by three scattering mechanisms in different temperature ranges,namely,ionized impurities scattering at lower temperatures(5?17 K),acoustic phonon piezoelectric scattering at moderate temperatures(17?72 K)and polar optical phonon scattering in the high temperature range(greater than 72 K).In the whole temperature range,the contribution of acoustic phonon deformation potential scattering to mobility will not exceed 6%and can be ignored.The peak mobility decreases sharply and the temperature corresponding to the peak mobility approaches the high temperature direction with the increase of the shallow donor concentration.The difference in EL2 concentration has no effect on the electron mobility of SI-GaAs.(3)The theoretical calculations of dark-state resistivity of GaAs PCSS over the temperature range of 5?500 K were simulated by MATLAB simulation software.It is shown that temperature characteristic curve of the resistivity exhibits unimodality,and the peak resistivity is approaches the value of about 1.29×1012 ?·cm,which is five orders of magnitude larger than the resistivity at 300 K(the simulation value of the resistivity is 1.13 x 107 ?·cm at 300 K,which is consistent with experimental values of 107?108 ?·cm).The effects of shallow donor impurity concentration and deep donor defect concentration on the peak resistivity of GaAs PCSS were studied.The results shown that the peak resistivity of the switch decreases with the increase of the impurity concentration,and the concentration of the deep donor defect EL2 has less influence on the peak resistivity of the switch than the shallow donor impurity concentration.The peak resistivity tends to the high temperature direction with the increase of concentration of shallow donor impurity.Otherwise,with the increase of concentration of EL2,the temperature of peak resistivity shows a tendency to move to low temperature.It was thus concluded that the dark-state resistivity of GaAs photoconductive semiconductor switches can be improved by appropriately reducing the concentration of shallow donor impurity in some ways,and thus realize the improvement of switching voltage withstand and high-power output performance. |
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