In recent years, an active matrix display device such as a liquid crystal display device and a light emitting device has been developed in response to the expanding demand for portable use and the like. In particular, technology for integrally forming a pixel and a driver circuit (referred to as an internal circuit, hereinafter) on an insulator by using a transistor formed by using a polycrystalline semiconductor (polysilicon) has been actively developed. An internal circuit includes a source signal line driver circuit and a gate signal line driver circuit, and controls pixels arranged in matrix.
Further, the internal circuit is connected to a controller IC and the like (referred to as an external circuit, hereinafter) through a flexible printed circuit (FPC) and the like to control its operation. An external circuit typically operates at a lower voltage than a power supply voltage of an internal circuit because an IC used for the external circuit is in single crystal form. At present, the external circuit normally operates at a power supply voltage of 3.3 V while the internal circuit operates at a power supply voltage of approximately 10 V. Thus it is necessary to amplify the CK signal to the voltage equivalent to the power supply voltage of the internal circuit with a level shifter and the like for operating a shift register of the internal circuit by a clock (referred to as CK, hereinafter) signal of the external circuit.
In the case of amplifying the CK signal by the external circuit, problems arise in that the number of components such as a level shifter IC and a power supply IC is increased, which consumes more power. In the internal circuit, by providing an input portion of an FPC with a level shifter for amplifying the CK signal to supply it to all the stages of the shift register, problems arise in that the size of layout area and the power consumption are increased and a high frequency operation becomes difficult.
In view of the aforementioned, a shift register which is operated at a CK signal of a low voltage has been proposed. By providing a data transfer portion as a differential amplifier, the shift register of the invention can be operated satisfactorily even at a low power supply voltage and a low voltage input signal (e.g. see Japanese Patent Application Laid-open No. Hei 11-184432).
The shift register provided with the data transfer portion as a differential amplifier may not operate accurately in the case where the characteristics of transistors configuring the differential amplifier are not the estimated ones. In a polysilicon TFT other than a single crystalline TFT, variation in characteristics is a considerable problem.
In view of the above problems, the invention provides a shift register which is hardly affected by variation in characteristics of transistors and operates with low power consumption.