Effect of composite spacer on suppression of source-drain leakage current and performance enhancement in ultra-small-scaled vertical thin-film transistors

This work establishes a leakage current model for ultra-thin spacer layers in ultra-small-scaled vertical thin-film transistors (VTFTs) under source-drain bias conditions at medium to high electric field strengths. It was found that high dielectric constant, high barrier height and deep trap energy level contribute to the suppression of leakage currents generated via Schottky emission (SE), Poole-Frenkel emission (PF), and Fowler-Nordheim tunneling (FN) in the ultra-thin spacer. Using this model, the leakage current densities of common spacer materials, namely Al₂O₃, SiO₂ and HfO₂, which are widely used in the semiconductor industry, as well as an Al₂O₃/HfO₂ composite film—were calculated to evaluate their potential as spacer layers in ultra-small-scaled VTFTs. Both calculated and experimental results demonstrate a significant reduction in leakage current density with the Al₂O₃/HfO₂ composite spacer. Under conditions including a source-drain overlap area of 100 μm², spacer thickness of 20 nm, channel length of 62.7 nm, channel width of 50 μm, and V_d = 1.0 V, the In-Sn-Zn-O (ITZO) VTFT exhibited an off-state current (I_off) as low as 1.57 × 10⁻¹³ A, achieving an on/off current ratio (I_on/I_off) of approximately 8.3 × 10⁸, a saturation mobility (μ_sat) of 60.82 cm²/V·s, a current drivability (C_dr) of 26.41 μA/μm, a subthreshold swing (S.S) of 0.082 V/dec, and a drain-induced barrier lowering (DIBL) coefficient of 0.211 V/V.