Starters, or Bigas: freezing and defrosting?

Has anyone use a microwave to defrost frozen starter? Or, do you have do it on the countertop at room temperature. If the heat of the microwave kills the starter yeast, is that a problem? Is all the fermentation from the yeast that goes into the dough with starter and additional flour? Or, do you need whatever may be still in the starter. Has anyone tried this? Thanks for any ideas. Kent