In　this　letter,　the　triggering　efficiency　of　1-mm-gap,　opposed-contact　GaAs　Photoconductive　Semiconductor　Switch　(PCSS)　is　studied　in　avalanche　mode.　Compared　with　the　anode-triggered　PCSSs,　it　is　found　that　the　cathode-triggered　devices　are　with　shorter　delay　time　and　lower　on-state　resistance　under　the　same　operation　conditions.　The　delay　time　jitter　of　45.6　ps　is　also　achieved　at　the　power　level　of　$\sim　4$　MW　by　cathode　triggering.　Then　the　theory　of　multiple　avalanche　domains　is　introduced　to　interpret　the　influence　of　laser　spot　location　on　triggering　efficiency　by　a　two-dimensional　(2D)　device　simulation.　The　delay　time　and　on-state　resistance　are　essentially　attributed　to　the　formation　time　and　the　density　of　avalanche　domains　which　are　affected　by　the　triggering　position.　©　1980-2012　IEEE.