Have you ever used a shock collar?

I've never used shock collars on my Yorkies but lately I have been considering it :-( It makes me sad to think about having to go this route but I cannot control their barking! They are quiet all through out the day but naturally as soon as someone knocks on the door or rings the door bell, their barking is SO loud and out of control. It doesn't even help to yell at them to be quiet because they can't even hear me over their extremely loud barking.