*2.3. Co-Design of Antenna and PTAT Sensor*

As 0.91 THz waves are incident normally, under the action of electric field horizontally to the right along xoy plane, the right side of the outer octagonal ring would gather a large amount of positively induced charge, while the left side would gather equal amounts of negatively induced charge. By this means, the outer octagonal ring generates an induced electromotive force (EMF) and an induced electric field, which is opposite to the incident electric field of 0.91 THz waves. As the circumference of the outer octagonal ring is about a dielectric wavelength of 0.91 THz waves, accumulated anisotropic charges cause an induced current distributed in full wave among the outer octagonal ring, and the current finally flows to the resistor through feeding structures. Similarly, as 2.58 THz waves or 4.2 THz waves are incident normally, the middle octagonal ring or inner octagonal ring would generate an induced electric field and an induced current, which also flows to the resistor. It could be concluded that octagonal rings and the resistor constitute heat sources. Therefore, the temperature distribution of the antenna is obtained using the EM field frequency domain module and solid heat transfer module in COMSOL tools. The lumped port and boundary conditions are set to make sure that the antenna could frequency-selective receive THz waves, and incident power at the excitation port is set according to the receiving efficiency enabling the antenna to receive EM energy of 0.1 mW. Besides, the antenna and resistor are set as heat sources, and the initial temperature of the model is set to 25 ◦C. The temperature distribution of the antenna is obtained by frequency domain research and steady state research.

In order to clarify the layout of temperature-sensing elements and determine whether all temperature-sensing elements could be distributed within the same raised temperature distribution area, the raised temperature distribution of the antenna needs to be simulated. As the antenna receives 0.91 THz waves, the resistor at the termination of the antenna becomes the main heat source owing to ohmic loss, showing a strong temperature distribution in a certain area around the resistor. According to the antenna size, the certain area could be approximately considered as 30 μm × 30 μm, and this means that the resistor generated a strong, uniform, and raised temperature distribution in this area, as shown in Figure 4.

**Figure 4.** Temperature distribution of the proposed detector.

Then, 2.58 THz waves or 4.2 THz waves with the same EM energy of 0.1 mW were incident on the antenna, so the resistor that acts as the main heat source also generated a strong and uniform temperature in this area. Based on this, two BJTs of the PTAT sensor are the main temperature-sensing elements, and the transistor M11 working in the subthreshold region could also sense the raised temperature. Thus, two BJTs and M11 should be distributed within this area, which is centered around the resistor with an area of 30 μm × 30 μm. In this way, it not only benefits sensing the raised temperature effectively, but also could ensure that all temperature-sensing elements sense the same temperature. In addition, two BJTs are composed of a single transistor unit and seven parallel transistor units with an emitter area of 5 μm × 5 μm and an overall size of 10 μm × 10 μm. Besides, the overall size of M11 is 3 μm × 25 μm. It could be seen that, as two BJTs are closely arranged in a "square" shape, and there is an empty position of one transistor unit in the center to construct a polysilicon resistor, BJTs could be centrally arranged around the resistor. At the same time, when transistor M11 is distributed near two BJTs, the layout area formed by temperature-sensing elements is basically consistent with the temperature distribution generated by the antenna.
