Abstract: An AC voltage generated by an AC power source 1 is rectified by a full-wave rectifying circuit 2, which generates a rectified voltage. An internal regulator 33 performs waveform shaping of the rectified voltage. A comparator 42 compares the rectified voltage output from the internal regulator 33 with a reference voltage V1 and detects a period in which the rectified voltage exceeds the reference voltage V1. According to an output signal of the comparator 42, a determination signal generation circuit 50 determines the power source voltage supplied form the AC power source 1 and generates a determination signal. Accordingly, there is no need of a capacitor, etc. for detecting the peak value of the rectified voltage, and it is possible to reduce the size and cost of an AC voltage detection circuit.

Abstract: The present invention provides a method for manufacturing a semiconductor device, by which a transistor including an active layer, a gate insulating film in contact with the active layer, and a gate electrode overlapping the active layer with the gate insulating film therebetween is provided; an impurity is added to a part of a first region overlapped with the gate electrode with the gate insulating film therebetween in the active layer and a second region but the first region in the active layer by adding the impurity to the active layer from one oblique direction; and the second region is situated in the one direction relative to the first region.