This study presents a temperature sensor for biological medical devices using 40-nm CMOS technology to prevent the high operating temperature of devices from causing harm to human organs. Bipolar PNP transistors are employed to sense the temperature and generate a voltage proportional to the absolute temperature. To improve the resolution, the variation in output voltage per degree Celsius is amplified by the programmable gain amplifier (PGA) with adjustable gain. The structure of PGA is capacitor feedback and the input capacitor is controlled by switching to achieve different gains. The conversion of temperature to digital output is accomplished by a single-slope analog-to-digital converter (SS ADC). The SS ADC mainly consists of a comparator and a ramp generator. Considering the power, the ramp generator uses an integrated structure of switched capacitor, and a dynamic latch combined with a static comparator forms the comparator. The proposed temperature sensor can achieve a high resolution of 0.045 °C within the temperature range of 20–60 °C, with a conversion rate is 14 kHz.