We have fabricated single-walled carbon nanotube (SWNT) field effect transistors (FETs) with molybdenum source and drain contacts. Normally, these devices operate only as p-channel transistors, however, after polystyrene latex nanospheres were attached to the nanotubes close to the contacts, they changed to ambipolar operation. This strategy provides a simple method to modify the electrical behavior of unipolar SWNT-FETs by influencing the gate-channel electric field distribution and offset charge, so enabling complementary circuits to be fabricated.