Thin film transistors fabricated by in situ doped unhydrogenated polysilicon filmsobtained by solid phase crystallization

Abstract

High-mobility low-temperature (⩽600 °C) unhydrogenated in situ doped polysilicon thin film transistors (TFTs)are made. Polysilicon layers are grown by a low pressure chemicalvapour deposition (LPCVD) technique andcrystallized in a vacuum by thermal annealing. The source and drainregions are in situ doped. The gate insulator is made ofan atmospheric pressure chemical vapour deposition (APCVD)silicon dioxide. Hydrogen passivation is not performedon the transistors. One type of transistor is made of twopolysilicon layers, the other one is fabricated from a single polysilicon layer.The electrical properties are better for transistors made of asingle polysilicon layer: a low threshold voltage (1.2 V), asubthreshold slope S = 0.7 V/dec, a high field effect mobility(≈100 cm2 V-1 s-1) and an on/off-state current ratiohigher than 107 for a drain voltage Vds = 1 V.At low drain voltage, for both transistors, the off-statecurrent results from a pure thermal emission of trappedcarriers. However, at high drain voltage, the electricalbehaviour is different: in the case of single polysilicon TFTs,the current obeys the field-assisted (Poole-Frenkel) thermalemission model of trapped carriers while for TFTs made of two polysilicon layers, the higher off-state current results from afield-enhanced thermal emission.