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

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.