A novel implantless MOS thin-film transistor with simple processing, excellent performance and ambipolar operation capability

A novel thin-film transistor (TFT) device that requires no implant step and is capable of ambipolar operation is proposed and successfully demonstrated. The new structure features an undoped Si active channel, a tap metal field-plate (i.e., the sub-gate), and Schottky source/drain. The equivalent circuit of the device is given. During device operation, a high fixed voltage is applied to the sub-gate to form a field-induced source/drain layer under the sub-gate region. Depending on the polarity of the sub-gate bias, the device can be set for n-channel operation with positive sub-gate bias, and p-channel operation with negative sub-gate bias. The new device is similar to conventional Schottky barrier (SB) MOSFET devices, with the exception of a field-induced source/drain region between the channel and Schottky source/drain. The existence of the field-induced source/drain region serves to supply abundant channel carriers during on-state, while reducing the notorious off-state leakage that has plagued all previous SB MOSFETs. The new device is also similar to MOSFETs with field-induced drain (FID), except that the heavily-doped source/drain region is replaced by Schottky source/drain. While retaining all the advantages of FID such as low off-state leakage and low junction leakage, the use of Schottky source/drain not only reduces processing steps (i.e., implant and annealing), but also allows ambipolar operation, thus greatly simplify processing steps especially for CMOS process integration.