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Fig. 1. Schematic of the proposed current reference circuit. Intelligent network systems for the future will require a great number of smart sensor LSIs that measure various physical data in surroundings. These LSIs have to operate with a low power, tens of microwatts or less, because they will probably be placed in non-ideal environments where energy for operation cannot be obtained sufficiently. To achieve such ultra-low power operation, CMOS circuits in sensor LSIs have to be operated in the subthreshold region of MOSFETs. For this purpose, we need to develop a circuit that provides a small reference current of less than tens of nanoamperes for the subthreshold CMOS circuits. We propose such a reference current circuit that is insensitive to temperature and power supply voltage. Current references for subthreshold CMOS circuits have been proposed by Oguey and Aebischer [1], and Hirose and others [2, 3]. The former current reference, however, shows a positive temperature coefficient of output current. The latter shows a zero temperature coefficient but consists of a complex circuit with many MOSFETs. Therefore, they are unsuitable for our purpose. The current reference circuit we propose will solve these problems and can be used for lowpower subthreshold MOS LSIs. The following describes the details of our current reference circuit.