A semiconductor device includes a semiconductor substrate, and first and second transistors over the semiconductor substrate. Both the first and second transistors are p-type transistors or both the first and second transistors are n-type transistors. The first and second transistors have the same nominal operating voltage. The first transistor has a higher threshold voltage than the second transistor. The second transistor has at least one of a source region or a drain region with higher charge carrier mobility than at least one of a source region or a drain region of the first transistor.