The silicon integrated electronics on glass or plastic substrates attracts wide interests. The design, however, depends critically on the switching performance of transistors, which is limited by the quality of silicon films due to the materials and substrate process constraints. Here, the ultrathin channel device structure is proposed to address this problem. In a previous work, the ultrathin channel transistor was demonstrated as an excellent candidate for ultralow power memory design. In this letter, theoretical analysis shows that, for an ultrathin channel transistor, as the channel becomes thinner, stronger quantum confinement can induce a marked reduction of OFF-state leakage current (I-OFF), and the subthreshold swing (S) is also decreased due to stronger control of channel from the gate. Experimental results based on the fabricated nanocrystalline silicon thin-film transistors prove the theoretical analysis. For the 2.0-nm-thick channel devices, I-ON/I-OFF ratio of more than 10(11) can be achieved, which can never be obtained for normal thick channel transistors in disordered silicon.