1 A Ultra Low Power Temperature Sensor in CMOS 130nm Technology Dr. Mukul Sarkar, Vikas Aggarwal, and Pawan Kumar Abstract—A fully-integrated ultra low power temperature sen- sor has been designed. Temperature dependent and independent currents are being generated to get temperature dependent frequency. Magnitude of current has been reduced to nW level by large value of resistor. Many high gain feedback amplifiers are being used, consuming power just in the range of nW. Full sensor has been designed in UMC130nm technology. No external reference circuit is required. Index Terms—Fully integrated, subthreshold, temperature sen- sor, ultra-low power, UMC130nm. I. I NTRODUCTION U LTRA low power wireless systems are need of current market. These systems are equipped with many sensors. Among various sensors, temperature sensor is most important sensor and hence, the design of low power temperature sensor is very critical. A lot of challenges are faced for design of temperature sensor of wireless system due to a limited battery size and correspondingly small energy capacity. The large internal resistance of the battery also limits the maximum current that can be drawn from the battery at a time. Further, the sensor should be fully-integrated and self- contained since accurate external references are not readily available in highly integrated systems. Various types of tem- perature sensors have been designed in CMOS technology. Most conventional temperature sensors are based on bipolar junction transistors (BJTs). These sensors measure temperature by comparing a temperature-depen- dent voltage(PTAT) to a temperature-insensitive voltage(VREF).The ratio between the PTAT and reference voltages is fed to an analog-to-digital converter (ADC) to be digitized. These offer high resolution but power consumption is in order of uW. For low power wireless systems, MOSFET-based temperature sensors have been introduced. For low power operation, time-to-digital frequency-to-digital conversion is used instead of ADCs. Temperature can be calculated using a reference clock and a temperature-dependent frequency or pulse. These sensors consume less power than BJT-based sensors at the expense of resolution and accuracy. Power consumption is reduced to hundreds of nW, but an external clock is needed as a reference. The performance of these sensors highly depends on the accuracy of the reference clock, which is not typically available in a wireless system. Moreover, the reference clock itself can increase power consumption significantly. Recently, a temperature sensor based on dynamic threshold MOSTs (DTMOSTs) is introduced. The sensor achieves high reso- lution (0.063 C) and accuracy but with sub- uW of power consumption (excluding clock generation power). II. DESIGN OF VOLTAGE REFERENCE There are several approaches to design voltage references in CMOS technology. The most common method is a bandgap voltage reference using parasitic BJTs (bipolar junction tran- sistors). To generate a temperature insensitive output volt- age, bandgap references linearly combine two voltages with opposing temperature characteristics: a complementary-to- absolute-temperature (CTAT) voltage and a proportional-to- absolute-temperature (PTAT) voltage. [1] PTAT and CTAT currents can also combined, rather than voltages, to generate a temperature-independent output voltage. Voltage references is also designed by employing two devices of different threshold voltages, which are implemented by distinct gate doping or selective channel implantation. Another approach uses subthreshold-biased transistors to lower minimum functional supply voltage and power consumption. Voltage reference must consume low power. Very few design are there which consume nW of power. Here, we are designing 2T volatge reference. 2T voltage reference uses two different types of devices. M1 is native device with approx. zero threshold voltage. M2 is thick oxide device as shown in Fig.1 Mathematical calculation shows that VREF is function of both PTAT(thermal voltage) and CTAT(threshold voltage) so it is fairly constant with temperature. Fig. 1. 2T reference unit III. DESIGN OF TEMPERATURE SENSING UNIT Temperature sensing unit is also modified version of 2T reference unit. Here, instead of using two different types of devices both the devices are of same kind. It removes the CTAT component due to threshold voltage difference of two devices. Due to this output voltage is linear function of thermal