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A thin film transistor having a source region and a drain region having a low melting point region composed of a semiconductor with a melting point lower than that of the semiconductor of the channel region is provided. In the thin film transistor, the dopant concentrations of the low melting point

A thin film transistor having a source region and a drain region having a low melting point region composed of a semiconductor with a melting point lower than that of the semiconductor of the channel region is provided. In the thin film transistor, the dopant concentrations of the low melting point region of the source region adjacent to the channel region and the low melting point region of the drain region adjacent to the channel region are precisely controlled. Using the thin film transistor, a high performance thin film transistor array substrate is also provided, as well as a high display speed liquid crystal display device and a high display speed electroluminescent display device having a high aperture ratio or a high pixel resolution.

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1. A method of producing a thin film transistor comprising:forming a semiconductor thin film on an insulating substrate, the semiconductor thin film composed of a first semiconductor;patterning the semiconductor thin film by photolithography and etching to form a patterned semiconductor thin film;fo

1. A method of producing a thin film transistor comprising:forming a semiconductor thin film on an insulating substrate, the semiconductor thin film composed of a first semiconductor;patterning the semiconductor thin film by photolithography and etching to form a patterned semiconductor thin film;forming a first electrically insulating film so as to cover the patterned semiconductor thin film;subsequent to the step of forming a first electrically insulating film, forming a gate electrode on the patterned semiconductor thin film;implanting ions having group IVB atoms into a portion of the patterned semiconductor thin film to form a pair of low melting point regions on the patterned semiconductor thin film, the low melting point regions being spaced apart from each other and composed of a second semiconductor;doping the regions where the pair of the low melting point regions have been formed with a dopant to form a source region, a drain region, and a channel region, such that the source and drain regions are doped with the dopant and that the channel region is not doped with the dopant and is sandwiched between the source region and the drain region;forming a second electrically insulating film so as to cover the gate electrode;forming a source electrode and a drain electrode, the source electrode piercing through the first and second electrically insulating films on the source region and being electrically insulated from the gate electrode, the drain electrode piorcing through the first end second electrically insulating films on the drain region and being electrically insulated from the gate electrode and electrically connected to the drain electrode; andheating the source region and the drain region at a predetermined temperature to activate the dopant contained in the source region and the drain region. 2. A method of producing a thin film transistor according to claim 1, further comprising a step of crystallizing a predetermined region in the semiconductor thin film by heating the predetermined region. 3. A method of producing a thin film transistor according to claim 2, wherein the method further comprises, subsequent to the stop of doping, additionally doping a portion of the drain region with the dopant to form a low concentration-doped drain region and a high concentration-doped drain region, the law concentration-doped drain region having not been subjected to the additional doping and in contact with the channel region, the high concentration-doped drain region having been subjected to the additional doping and having a dopant concentration higher than the low concentration-doped drain region. 4. A method of producing a thin film transistor according to claim 2, wherein in the step of heating, the predetermined temperature is 600° C. or lower. 5. A method of producing a thin film transistor according to claim 2, wherein:in the step of forming a semiconductor thin film, the semiconductor thin film is an amorphous silicon thin film;in the step of implanting ions, the group IVB atoms are germanium atoms; andin the step of crystallizing, the channel region, the source region, and the drain region are subjected to the crystallizing.