Methods of forming thin film devices with different electrical characteristics on a substrate comprising a driver circuit region and a pixel region. A first and a second polysilicon pattern layers are formed on the driving circuit region and the pixel region of the substrate, respectively. A first ion implantation is performed on the second polysilicon pattern layer using a masking layer covering the first polysilicon pattern layer as an implant mask, such that the first polysilicon pattern layer has an impurity concentration different from the second polysilicon pattern layer. After removal of the masking layer, a gate dielectric layer and a gate are successively formed on each of the first and second polysilicon pattern layers and a source/drain region is subsequently formed in each of the first and second polysilicon pattern layers to define a channel region therein.