FinFET structures and methods of forming the same are disclosed. A device includes a semiconductor fin. A gate stack is on the semiconductor fin. The gate stack includes a gate dielectric on the semiconductor fin and a gate electrode on the gate dielectric. The gate electrode and the gate dielectric have top surfaces level with one another. A first inter-layer dielectric (ILD) is adjacent the gate stack over the semiconductor fin. The first ILD exerts a compressive strain on the gate stack.