Comparison of thin-film transistors fabricated at low temperatures (≤600 °C) on as-deposited and amorphized-crystallized polycrystalline Si

Abstract

A comparison of the performances of thin‐film transistors fabricated on as‐deposited and amorphized‐crystallized (low‐pressure chemical‐vapor deposited) polycrystalline Si films is reported in this paper. The polycrystalline Si films, 3500 Å thick, were grown on SiO₂ via the pyrolysis of SiH₄ at 625 °C, and the entire amorphization (ion implantation), crystallization, and device fabrication processes that followed were carried out at temperatures ≤600 °C, all compatible with low‐cost glass substrates. Both n‐channel and p‐channel metal‐oxide‐semiconductor (NMOS and PMOS) transistors with three different geometries (W/L=200/20, 200/10, and 200/5 μm/μm) were fabricated and compared. The results showed that the amorphized‐crystallized film was in general a more superior thin‐film transistor material than the as‐deposited film, although the ‘‘aspects’’ of superiority differed between the NMOS and the PMOS transistors. The NMOS transistors fabricated on the amorphized‐crystallized film exhibited a lower threshold voltage (21–25 vs 23–25 V), a higher field‐effect mobility (15 vs 3.5 cm²/V s), but the same on–off current ratio (2×10⁴) as those on the as‐deposited film. The PMOS transistors fabricated on the amorphized‐crystallized film exhibited a higher field‐effect mobility (10 vs 3.4 cm²/V s), a higher on–off current ratio (6×10⁴ vs 3×10⁴), but the same threshold voltage (−25 V) as those on the as‐deposited film. When fabricated on the same material (as‐deposited or amorphized‐crystallized), NMOS outperforms PMOS in terms of a lower threshold voltage and a higher field‐effect mobility, but PMOS outperforms NMOS in terms of a higher on–off current ratio. All thin‐film transistors fabricated in this work qualify as the switching elements for large‐area liquid‐crystal displays.