Are the games by Valve all made by the same people in the company?

So I know that Portal 2 and Left 4 Dead were made by the same company: Valve. But are there different "teams" within Valve that each make different games? In other words, are the people who made Portal the SAME EXACT people who made left 4 dead? I was wondering this because the two games seem completely different from each other (one about sophisticated science and the other about gruesome zombies), and seemed like they were created by totally different people.