1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device.
2. Description of the Related Art
In recent years, semiconductor devices have been developed which are capable of supplying (also referred to as feeding) power (also referred to as a power supply voltage) through wireless communication, and further capable of transmitting and receiving data (also referred to as data communication) through wireless communication. For example, if a feeding function through wireless communication can be added to a portable information terminal (e.g., a cellular phone) which is an example of the semiconductor devices, the portable information terminal does not need to be connected to an external power feeding portion and can be fed more easily, for example, in any environment.
An individual identification technology utilizing a radio frequency identification (RFID) tag is known as an example of semiconductor devices capable of data transmission, data reception, data storing, data erasing, and the like through wireless communication. The RFID tag is also referred to as an RF tag, a wireless tag, an electronic tag, or a wireless chip. The RFID tag is also referred to as an IC tag, an IC chip, or an IC card because it includes a functional circuit such as an integrated circuit (IC) for executing authentication or other processing. Data communication with the semiconductor device is performed by using a wireless communication device (such as a reader/writer, which is capable of transmitting and receiving a data signal through wireless communication). The individual identification technology using the semiconductor device is used for the production, management, or the like of an individual object and has been expected to be applied to personal authentication.
In the semiconductor device, the resonant inductance is set by a coil (antenna) or the like and the resonant capacitance is set by a capacitor or the like to set the resonance frequency, whereby a value of a power supply voltage to be supplied is set. However, an actual resonance frequency differs from a desired resonance frequency due to variations in a manufacturing process, in some cases. In that case, the resonance frequency needs to be readjusted after the process is finished.
As a measure against the above problem, a semiconductor device in which the resonance frequency is set at an optimal level by a resonance frequency adjustment circuit has been known (e.g., Patent Document 1).
In the semiconductor device disclosed in Patent Document 1, whether a capacitor is connected to an antenna in parallel or not is controlled by setting of a gate voltage of a control transistor. For example, when the control transistor is turned on and the capacitor is connected to the antenna in parallel, the resonance frequency changes.