string type .NET vs. char array

I've been working with some programs here at work for about a month now that have a lot of string parsing and such going on. I've been advised to use a char array for this stuff as opposed to a string because the char array is faster. I understand why a char array is fast, but what is it about the string type that makes it slower? What data structure is it implementing and is there any way to make it as fast as a char array?