Performance of thin film Transistors on Unhydrogenated In-Situ Doped Polysilicon films Obtained by Solid Phase Crystallization

ABSTRACTLow Temperature Unhydrogenated in-situ doped polysilicon Thin Film Transistors (LTUTFT) are made through two types of four-mask aluminium gate process. Silicon layers are elaborated by a Low Pressure Chemical Vapor Deposition (LPCVD) method and crystallized by a thermal annealing. Source and drain regions are in-situ doped. An Atmospheric Pressure Chemical Vapor Deposition (APCVD) silicon dioxide ensures the gate insulation. Two structures A and B are fabricated, the difference is that for sample B the undoped/doped polysilicon layer interface is suppressed.The structure of the polysilicon films is studied using Transmission Electron Microscopy (TEM) and Current-Voltage characteristics of both types of TFTs indicate electrical quality of the polysilicon films.The best electrical properties are obtained with the B type TFTs: a low threshold voltage (VT=1.2V), a low subthreshold slope (0.7 V/dec), a high On/Off state current ratio (107) for a drain voltage VDS= 1V, and a very high field effect mobility (≥100 cm2 /Vs). It is worth to notice that these good results are obtained without hydrogenation.