Thin film transistor and method of fabricating the same

Abstract

A thin film transistor which can be used in an LCD display panel includes an insulator substrate, a gate electrode located on the insulator substrate, an insulator film provided on the insulator substrate and the gate electrode, and a polycrystalline silicon film located on the insulator film. A channel is defined in a first portion of the polycrystalline silicon film over the gate electrode, and a drain and a source are defined in second and third portions of the polycrystalline silicon film over the insulator substrate. Grain sizes of the drain and source are equal to or greater than a grain size of the channel.

What is claimed is:  
     
         1 . A method of fabricating a thin film transistor comprising the steps of: 
 forming a gate electrode on an insulator substrate;    forming a gate insulator film over the insulator substrate and the gate electrode;    forming an amorphous silicon film on the gate insulator film;    irradiating laser light on a surface of the amorphous silicon film to heat the amorphous silicon film, thereby forming a polycrystalline silicon film; and    setting energy of the laser light such that a grain size of a first portion of the polycrystalline silicon film over the insulator substrate becomes equal to or greater than a grain size of a second portion of the polycrystalline silicon film over the gate electrode.    
     
     
         2 . The method according to    claim 1   , further comprising a step of defining a source and a drain of the thin film transistor in the first portion and defining a channel of the thin film transistor in the second portion.  
     
     
         3 . The method according to    claim 2   , wherein the step of setting the energy of the laser light includes the steps of: 
 setting a maximum energy of the laser light such that grain sizes of the drain and the source become substantially equal to a grain size of the channel; and    setting a minimum energy of the laser light to obtain a grain size of the channel enough to provide a desired device characteristic of the thin film transistor.    
     
     
         4 . The method according to    claim 3   , wherein the grain size of the channel which is enough to provide the desired device characteristic of the thin film transistor lies in a range of about 500 Å to about 20000 Å.  
     
     
         5 . The method according to    claim 3   , wherein the grain size of the channel which is enough to provide the desired device characteristic of the thin film transistor lies in a range of about 1500 Å to about 20000 Å.  
     
     
         6 . The method according to    claim 3   , wherein the grain size of the channel which is enough to provide the desired device characteristic of the thin film transistor lies in a range of about 3000 Å to about 10000 Å.  
     
     
         7 . A thin film transistor comprising an active layer including a polycrystalline silicon film where a drain, a source and a channel are defined, grain sizes of the drain and source being equal to or greater than a grain size of the channel.  
     
     
         8 . A thin film transistor comprising: 
 an insulator substrate;    a gate electrode located on the insulator substrate;    an insulator film provided on the insulator substrate and the gate electrode; and    a polycrystalline silicon film located on the insulator film, a channel defined in a first portion of the polycrystalline silicon film over the gate electrode, a drain and a source defined in second and third portions of the polycrystalline silicon film over the insulator substrate, grain sizes of the drain and source being equal to or greater than a grain size of the channel.    
     
     
         9 . The thin film transistor according to    claim 8   , wherein the grain size of the channel is set large enough to provide a desired device characteristic of the thin film transistor.  
     
     
         10 . The thin film transistor according to    claim 9   , wherein the grain size of the channel lies in a range of about 500 Å to about 20000 Å.  
     
     
         11 . The thin film transistor according to    claim 9   , wherein the grain size of the channel lies in a range of about 1500 Å to about 20000 Å.  
     
     
         12 . The thin film transistor according to    claim 9   , wherein the grain size of the channel lies in a range of about 3000 Å to about 10000 Å.  
     
     
         13 . An active matrix type display apparatus comprising: 
 a plurality of pixels; and    a plurality of drive elements for respectively driving the plurality of pixels, each drive element including a thin film transistor, the thin film transistor having: 
 an insulator substrate;  
 a gate electrode located on the insulator substrate;  
 an insulator film provided on the insulator substrate and the gate electrode; and  
 a polycrystalline silicon film located on the insulator film, a channel defined in a first portion of the polycrystalline silicon film over the gate electrode, a drain and a source defined in second and third portions of the polycrystalline silicon film over the insulator substrate, grain sizes of the drain and source being equal to or greater than a grain size of the channel.  
   
     
     
         14 . The display apparatus according to    claim 13   , wherein the grain size of the channel is set large enough to provide a desired device characteristic of the thin film transistor.  
     
     
         15 . The display apparatus according to    claim 14   , wherein the grain size of the channel lies in a range of about 500 Å to about 20000 Å.  
     
     
         16 . The display apparatus according to    claim 14   , wherein the grain size of the channel lies in a range of about 1500 Å to about 20000 Å.  
     
     
         17 . The display apparatus according to    claim 14   , wherein the grain size of the channel lies in a range of about 3000 Å to about 10000 Å.