Graphene transistors will possibly replace silicon transistors in the next 20 years. The transistors made of graphene will be less then 10 nanometers wide. Intel Corporation is currently constructing chips with 90 nanometer transistors however by 2008 they will retool their factory to have 45 nanometer transistors at a cost of over 1.5 billion dollars.

Graphene

Problems with graphene: “Etching graphene leaves edges with dangling carbon bonds, he says, which can cause background electrons to scatter. This makes the transistor less efficient, although the full effect of the scattering isn’t yet known. But the vast knowledge of carbon chemistry should hold the answer to how to overcome this kind of problem, he predicts, by chemically altering those edges.”
The following link gives more details on Graphene.

There is still room for improvement for silicon transistors but once they are unable to produce chips smaller and faster they will probably turn to grapheme to take over.