## 1 Introduction  
Terahertz waves exhibit characteristics, such as high absorption for the water molecules, high reflectivity for conductive metals, and transparency for dielectric materials with non-polarity, which are useful in several industrial applications, including imaging systems [1; 2]. A terahertz imaging system mainly consists of a terahertz source, a quasi-optical system for irradiating the output terahertz wave to the detector and the object, and a detector to illustrate the output voltage depending on the incident power of the terahertz wave [2; 3]. The quality of the terahertz imaging system is determined by the responsivity and the noise equivalent power (NEP), which are characteristics of the terahertz detector. In industrial applications of the imaging system, it is important to realize a detector with high responsivity and low NEP [2; 3; 4]. Various terahertz detectors including a microbolometer and Schottky barrier diodes are examined to improve detection characteristics [5; 6; 7; 8]. Recently, a highly sensitive detector based on the complementary metal-oxide-semiconductor (CMOS) process was proposed by considering integration with a signal conditioning block and control circuits fabricated in the same CMOS process and the importance of implementing a large scale focal plane array [9; 10; 11]. A CMOS plasmon detector that detects terahertz signals higher than the operating frequency of the CMOS process is a promising structure for an imaging system because of the implementation of the large-scale pixel arrays and integration with the signal conditioning blocks [12; 13]. The operation of the plasmon detector is described with the potential difference in the channel of the transistor that varies proportionally to the magnitude of an incident terahertz wave. The physical behavior of the plasmon detector is explained in detail by using the resonance characteristics of the plasma wave generated in the channel by the terahertz input signals from the Dyakonov-Shur plasma wave theory [14]. The detector operation is represented by the non-quasi-static analysis that models the channel of the transistor as a two-dimensional electron gas [14; 15; 16; 17]. With respect to the electronics viewpoint, the plasmon detector is considered as a resistive mixer that is operated by the variation in the channel resistance in the transistor that changes due to the input signals [18]. A non-quasi-static model considering the effects of parasitic capacitors and distributed self-mixing operation, and a quasi-static model that describes the low-frequency operation as a square-law power detector, are used in the operating analysis of the terahertz detector [19; 20; 21]. Extant studies physically and accurately describe the operating characteristics of the terahertz detector. However, they are not useful in terms of providing an intuitive understanding of designing a plasmon detector. The analysis of detector characteristics in the weak inversion region is necessary to determine optimum design conditions because previous studies indicated that the photoresponse of the detector is maximized at the bias condition of the subthreshold region of the detector [16; 17; 19; 20]. Additionally, power combining efficiency and impedance matching between the antenna and detector should be considered important factors in the improvement of the detector performances [21; 22]. This study shows a quasi-static analysis based on an equivalent circuit model of the CMOS plasmon detector to improve the detector performances. The equivalent circuit model consists of the superposition method that describes the relationship of the characteristic impedances including the effect of the transmission line length and small signal analysis method that simplifies the operation of the plasmon detection in the subthreshold region. The analysis using the proposed equivalent model makes it easy to understand the design specifications and detector structure to improve the performances of the plasmon detector.