*Article* **Concurrent-Mode CMOS Detector IC for Sub-Terahertz Imaging System**

**Moon-Jeong Lee 1, Ha-Neul Lee 1, Ga-Eun Lee 1, Seong-Tae Han 2 and Jong-Ryul Yang 1,\***


**Abstract:** A CMOS detector with a concurrent mode for high-quality images in the sub-terahertz region has been proposed. The detector improves output-signal coupling characteristics at the output node. A cross-coupling capacitor is added to isolate the DC bias between the drain and gate. The detector is designed to combine a 180◦ phase shift based on common source operation and an inphase output signal based on the drain input. The circuit layout and phase shift occurring in the cross-coupled capacitor during phase coupling are verified using an EM simulation. The detector is fabricated using the TSMC 0.25-μm mixed-signal 1-poly 5-metal layer CMOS process, where the size, including the pad, is 1.13 mm × 0.74 mm. The detector IC comprises a folded dipole antenna, the proposed detector, a preamplifier, and a voltage buffer. Measurement results using a 200-GHz gyrotron source demonstrate that the proposed detector voltage responsivity is 14.13 MV/W with a noise-equivalent power of 34.42 pW/√Hz. The high detection performance helps resolve the 2-mm line width. The proposed detector exhibits a signal-to-noise ratio of 49 dB with regard to the THz imaging performance, which is 9 dB higher than that of the previous CMOS detector core circuits with gate-drain capacitors.

**Citation:** Lee, M.-J.; Lee, H.-N.; Lee, G.-E.; Han, S.-T.; Yang, J.-R. Concurrent-Mode CMOS Detector IC for Sub-Terahertz Imaging System. *Sensors* **2022**, *22*, 1753. https:// doi.org/10.3390/s22051753

Academic Editor: Roman Sotner

Received: 7 January 2022 Accepted: 22 February 2022 Published: 23 February 2022

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**Keywords:** CMOS detector; concurrent-mode; differential detector IC; imaging SNR; integrated folded-dipole antenna; sub-terahertz imaging; voltage responsivity
