Abstract
The origin of the long trailing shoulders observed from photoconductive switches fabricated on low-temperature-grown GaAs at high bias voltage was studied. By a comparative study of the photoconductive switches fabricated on LT-GaAs on different substrates and with different gapwidths, we found that the dependence of the trailing shoulders on the power dissipation is weak. Comparing the calculated electric fields at various depths in photoconductive switches with different gapwidths, we proposed that the long trailing tails should result mainly from the photo-carriers generated deep in the substrate; these carriers, though blocked by the AlAs layer, can modify the electric field distribution in the LT-GaAs layer, leading to a displacement current component. Our model was corroborated by the great reduction of the long trailing tails with the elimination of the SI GaAs substrate underneath the photoconductive switch.