Using the first-principles pseudopotential method within a generalized gradient approximation of the density functional theory, the structural and electronic properties of SrSe and SrTe have been studied. The calculated lattice parameters are in excellent agreement with experimental values, whereas the error in the minimum gap value is as high as 43.25% in SrSe. To get reliable band gap values of SrSe and SrTe, we employ the GW (G is the Green's function and W is the screened Coulomb interaction) approximation method. The result in GW approximation improves the band gap value of the SrSe greatly and agrees with the value of experimental measurement.

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The density distribution characteristics of non-equilibrium carrier from spontaneous emission ahead of the tip of the current filaments in high gain semi-insulating GaAs photoconductive semiconductor switches are analyzed. The results show that the ratio of volume to area of the current filament and the average carrier density in the current filaments are two key factors that affect the photoionization effects of the current filaments. The reabsorption of wavelengths λ ≤ 876 nm radiations plays a dominant role in producing the maximum carrier density. The maximum density of photo-generated carrier is approximately 1~3 orders of magnitude lower than the average density of excess carrier inside the current filaments.

The electron avalanche domains (EAD) in high gain semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSS) are analyzed. The EAD are closely related to the growing domains associated with carrier injection. The EAD formation requires high carrier density and high field impact ionization. The avalanche carrier generation in the EAD can cause and complete the localized direct transform from theN-shaped current-voltage (I-V) characteristics toS-shapedI-Vcharacteristics. Then a transition from the EAD to current filament mechanisms can occur. The EAD ideas can explain the branch and the bend of the filaments during the formation and propagation of the filaments.