Hardware is required to be further miniaturized aiming at advancement of the Internet of things and artificial intelligence. Widely used Si transistors, which have achieved miniaturization on the order of 10 nm, are apparently difficult to further miniaturize, and stacking techniques have been developed as a breakthrough. Our IGZO FETs have a gate length of 6.8 nm or less owing to the wide band gap of IGZO and an optimized transistor structure, and can be highly integrated by a contact formation technique.