A variable capacitor is formed from a pair of electrodes and a dielectric interposed between the electrodes over a substrate, and an external input is detected by changing capacitance of the variable capacitor by a physical or electrical force. Specifically, a variable capacitor and a sense amplifier are provided over the same substrate, and the sense amplifier reads the change of capacitance of the variable capacitor and transmits a signal in accordance with the input to a control circuit.