*2.2. Design of the PTAT Sensor*

Bipolar junction transistors (BJTs) have become attractive temperature sensing elements because of their lower cost, better stability, higher temperature sensitivity, lower power consumption, and better process compatibility and predictability [32]. Besides, it was recognized that, if two BJTs with the same emitter area operated at different current densities or two BJTs with proportional emitter areas operated at the same current density, then their emitter–base difference voltage is proportional to the absolute temperature [33]. Therefore, PTAT sensors that convert the temperature increment into an output voltage variation proportionally constitute a type of circuit structure that relies on BJTs' temperature characteristics [34]. In order to obtain detectors with higher responsivity, there is an effective approach to adopt an optimized PTAT sensor made up of four modules with enhanced temperature sensitivity. Previous research has proposed a sensitive PTAT sensor with an increased temperature sensitivity of 10.31 mV/◦C at 25 ◦C, as shown in Figure 3 [27]. The starting circuit is used to ensure the sensor quickly enters the normal operation state at the moment of supplying power. The PTAT current *I*PTAT could be generated by the PTAT core circuit, and the complementary to absolute temperature (CTAT) current *I*CTAT could be obtained from the CTAT current generation circuit. *I*PTAT and *I*CTAT are differentiated in a proper proportion to obtain a PTAT current with an enhanced positive temperature coefficient in the output circuit and, finally, a PTAT voltage is generated.

**Figure 3.** Schematic diagram of the optimized PTAT sensor.
