The invention provides a semiconductor device having a current input type
pixel in which a signal write speed is increased and an effect of
variations between adjacent transistors is reduced. When a set operation
is performed (write a signal), a source drain voltage of one of two
transistors connected in series becomes quite low, thus the set operation
is performed to the other transistor. In an output operation, the two
transistors operate as a multi-gate transistor, therefore, a current
value in the output operation can be small. In other words, a current in
the set operation can be large. Therefore, an effect of intersection
capacitance and wiring resistance which are parasitic on a wiring and the
like do not affect much, thereby the set operation can be performed
rapidly. As one transistor is used in the set operation and the output
operation, an effect of variations between adjacent transistors is
lessened.