1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a circuit block constituted of transistors having a low threshold value, to which supply of a power supply voltage is interrupted while it is not in operation.
2. Description of the Related Art
As element structure of a semiconductor becomes finer, a power supply voltage supplied to a semiconductor integrated circuit becomes lower every year. When the power supply voltage becomes lower and a difference between the power supply voltage and a threshold voltage of a transistor becomes smaller, difficulty in turning on the transistor increases and its operating speed decreases. In order to prevent the above problems, the threshold voltage of the transistor is showing a tendency to lower together with the power supply voltage.
Moreover, accompanying microfabrication of the transistor, a leakage current (subthreshold current) of the transistor while it is not in operation has a tendency to increase. The increase in the subthreshold current allows power consumption of the semiconductor integrated circuit to increase during its standby period. Therefore, the increase in the power consumption is a significant problem in portable equipment using batteries.
Recently, the technology called MTCMOS (Multi-Threshold voltage CMOS) has been developed in order to reduce the subthreshold current. This kind of technology is disclosed in, for example, Japanese Patent Application Publication No. Hei 5-210976, Japanese Patent Application Publication No. Hei 7-212217, and so on. According to the MTCMOS technology, the threshold voltage of transistors inside a circuit block requiring high speed operation is set low, and a power supply pin of the circuit block is connected to a power supply line via a switch transistor having a high threshold voltage. The switch transistor is turned on while the circuit block is in operation and turned off while the circuit block is not in operation, to thereby reduce the power consumption during the standby period.
However, when an integration degree of the semiconductor integrated circuit increases accompanying the microfabrication of the element structure of the semiconductor, the power consumption is not reduced enough due to the subthreshold current of the switch transistor, even though the MTCMOS technology is employed. Therefore, it is necessary to further suppress the subthreshold current of the switch transistor.