There is provided a method of easily forming thin film transistors having the same characteristics in fabricating a differential circuit or a current mirror circuit utilizing two thin film transistors made of a polycrystalline silicon semiconductor. Four each thin film transistors are used in a differential circuit and a current mirror circuit, respectively. The thin film transistors are arranged to be symmetric to each other about a symmetry center instead of using thin film transistors arranged adjacently on the substrate in the respective circuits.