Comparison of the low-temperature ( ≤ 600 °C) polysilicon thin-film transistors (TFTs) with two kinds of gate dielectrics

Abstract

Low-temperature ( ≤ 600 °C) polysilicon thin film transistors (TFTs) were processed with two types of gate insulator: wet oxide/LPCVD Si 3N 4 double layer and APCVD SiO 2 thin film. Electrical performances were analyzed and compared for the two types of TFT. The thermal oxide with LPCVD Si 3N 4 thin film provides a better gate insulator/poly-Si interface than the gate APCVD SiO 2 insulator in terms of effective interface state density, which is approximately ten times lower ( N T = 9 − 10 × 10 11 cm −2 eV −1 versus 8 − 9 × 10 12 cm −2 eV −1), and in terms of field effect mobility, which is much higher (μ = 58 cm 2 V −1 s −1 versus 6 cm 2 V −1 s −1). However, the threshold voltage is higher ( V T = 6.5 V versus 5 V) because of a hot carrier trapping effect at the SiO 2 Si 3N 4 interface. In addition, gate insulation with oxide/nitride is not as good as with APCVD SiO 2.