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This paper proposes an accurate four-transistor temperature sensor designed, and developed, for thermal testing and monitoring circuits in deep submicron technologies. A previous three-transistor temperature sensor, which utilizes the temperature characteristic of the threshold voltage, shows highly linear characteristics at a power supply voltage of 1.8 V or more; however, the supply voltage is reduced to 1 V in a 90-nm CMOS process. Since the temperature coefficient of the operating point's current at a 1-V supply voltage is steeper than the coefficient at a 1.8-V supply voltage, the operating point's current at high temperature becomes quite small and the output voltage goes into the subthreshold region or the cutoff region. Therefore, the operating condition of the conventional temperature sensor cannot be satisfied at 1-V supply and this causes degradation of linearity. To improve linearity at a 1-V supply voltage, one transistor is added to the conventional sensor. This additional transistor, which works in the saturation region, changes the temperature coefficient gradient of the operating point's current and moves the operating points at each temperature to appropriate positions within the targeted temperature range. The sensor features an extremely small area of 11.6times4.1 mum2 and low power consumption of about 25 muW. The performance of the sensor is highly linear and the predicted temperature error is merely -1.0 to +0.8degC using a two-point calibration within the range of 50degC to 125degC. The sensor has been implemented in the ASPLA CMOS 90-nm 1P7M process and has been tested successfully with a supply voltage of 1 V.