Thin-film transistors with polymorphous silicon active layer

Hydrogenated silicon thin films were obtained by plasma enhanced chemical vapour deposition under plasma conditions close to the formation of powder. By that means microcrystalline, polymorphous and amorphous silicon layers could be obtained depending on the process pressure. The films were deposited on patterned and thermally oxidised n-type silicon wafers to produce bottom-gate thin-film transistors (TFT). The electrical characteristics of the devices showed an improvement in the field-effect mobility with the process pressure. The best thin-film transistor incorporated a polymorphous silicon active layer which allowed a field-effect mobility of 0.80 ± 0.04 cm²/Vs with a threshold voltage of 9.0 ± 0.5 V. These results are comparable to the best ones reported for this kind of silicon TFT obtained at low substrate temperatures.