CMOS Voltage and Current Reference Circuits consisting of Subthreshold MOSFETs – Micropower Circuit Components for Power-Aware LSI Applications –

The development of ultra-low power LSIs is a promising area of research in microelectronics. Such LSIs would be suitable for use in power-aware LSI applications such as portable mobile devices, implantable medical devices, and smart sensor networks [1]. These devices have to operate with ultra-low power, i.e., a few microwatts or less, because they will probably be placed under conditions where they have to get the necessary energy from poor energy sources such as microbatteries or energy scavenging devices [2]. As a step toward such LSIs, we first need to develop voltage and current reference circuits that can operate with an ultra-low current, several tens of nanoamperes or less, i.e., sub-microwatt operation. To achieve such low-power operation, the circuits have to be operated in the subthreshold region, i.e., a region at which the gate-source voltage of MOSFETs is lower than the threshold voltage [3; 4]. Voltage and current reference circuits are important building blocks for analog, digital, and mixed-signal circuit systems in microelectronics, because the performance of these circuits is determined mainly by their bias voltages and currents. The circuits generate a constant reference voltage and current for various other components such as operational amplifiers, comparators, AD/DA converters, oscillators, and PLLs. For this purpose, bandgap reference circuits with CMOS-based vertical bipolar transistors are conventionally used in CMOS LSIs [5; 6]. However, they need resistors with a high resistance of several hundred megaohms to achieve low-current, subthreshold operation. Such a high resistance needs a large area to be implemented, and this makes conventional bandgap references unsuitable for use in ultra-low power LSIs. Therefore, modified voltage and current reference circuits for lowpower LSIs have been reported (see [7]-[12], [14]-[17]). However, these circuits have various problems. For example, their power dissipations are still large, their output voltages and currents are sensitive to supply voltage and temperature variations, and they have complex circuits with many MOSFETs; these problems are inconvenient for practical use in ultra-low power LSIs. Moreover, the effect of process variations on the reference signal has not been discussed in detail. To solve these problems, I and my colleagues reported new voltage and current reference circuits [13; 18] that can operate with sub-microwatt power dissipation and with low sensitivity to temperature and supply voltage. Our circuits consist of subthreshold MOSFET circuits and use no resistors.