Like 123, I use a water solution of 20 Mule Team Borax Laundry Booster and table sugar, both bought at the local grocery store. If you look at the ingredients on the Terro box, it is a 5.4% solution of borax by weight in water with enough sugar added to make it slightly syrupy. Since the borax weighs about as much as water, you can use a 1:20 ratio by volume too. The exact concentration is not critical but you want much more sugar than borax so that the ants lap up the solution readily.
Terro marks up the price of this solution by at least 100X.http://www.terro.com/products/liquid-ant-killer
Contains sodium tetraborate decahydrate (Borax) (5.40%) and other ingredients (94.60%)
I just put the solution in some old bottle caps from milk cartons in corners around the house. The carpenter ants eat it, return to the nest and regurgitate some of it to feed their young and queen. It kills all of them. You'll still get some ants until the last eggs in their nest have hatched - 6-7 days- but the main invasion is over in 2 days.
The solution gradually dries out over time as the water evaporates so you have to go around once every few weeks and add some water to the bottle caps.
Several years ago, carpenter ants apparently made a huge nest in our attic and invaded our kitchen. They drank at least a handful of this borax-sugar solution over two nights. It was surprisingly how much these little ants could cart away in just one night. Within 5 days, they were all gone for good.
Carpenter ants have a main nest and then put out ~10 satellite nests to expand their territory. If you keep the solution out, you'll get the whole complex and will not be bothered for a few years until a new horde moves into your neighborhood.