Thin-film transistors on a-Si:H

A comparison has been made of the characteristics of nitride and oxide MOSFET´s fabricated with thin films of amorphous silicon. Published data indicate that Si3N4-Si:H thin film devices are superior to the oxide devices. Accumulation-mode MOSFET´s were fabricated in which the drain current arises from electric-field induced accumulation of electrons (majority carriers) at the a-Si:H-insulator interface. Hydrogenated amorphous silicon layers were deposited at 230°C by glow-discharge plasma decomposition in silane. The deposition conditions were found to be critical, and in the present study the films were grown on an electrically grounded substrate with RF power of 1 W applied to the counter electrode. The a-Si:H was deposited onto silicon nitride and silicon dioxide layers of 100-500-nm thickness, and thin-film transistors were fabricated with the inverted MOS configuration. Devices were tested with on/off drain current ratios greater than 10⁴for a gate voltage swing of 0 to 12 V and drain-current saturation for source-drain voltages of less than 12 V. The properties of MOSFET´s on a-Si:H are discussed with a comparison of the silicon-nitride-a-Si: H and silicon-dioxide-a-Si:H interfaces and an evaluation of doped active layers. The transistors on silicon dioxide are as good as any reported to date on silicon nitride.