Device Characterization of Nanoscale Vertical-Channel Transistors Implemented with a Mesa-Shaped SiO₂ Spacer and an In–Ga–Zn–O Active Channel

A nanoscale vertical-channel thin film transistor (V-TFT) with a channel length shorter than 160 nm was fabricated and characterized, in which In–Ga–Zn–O (IGZO) and SiO₂ thin films were prepared by atomic layer deposition and plasma-enhanced chemical-vapor deposition as active and spacer layers, respectively. The prototype device showed sound transistor operation with an on/off ratio of 8.8 × 10³ and robust stabilities without any shift in transfer curves under positive/negative bias stresses at 1 MV/cm for 10⁴ s. It was also noteworthy that there was no anomalous increase in off-state current during the positive bias stress test, which is suggested to originate from a high-quality interface between the SiO₂ spacer and IGZO active layers on the back-channel region. Alternatively, high off-state current levels were found to result from the formation of conduction paths generated by carbon-related residues on the vertical back-channel region through the surface time-of-flight secondary ion mass spectrometer analysis. Improvements in device performance and analysis of operation failures will provide insight into the implementation of nanoscale oxide V-TFTs.