## 1 Introduction  
Over the past few decades, various terahertz (THz) technologies have been developed rapidly. Among them, THz imaging and sensing technology have broad application prospects in science and other fields, such as safety and security screening, process monitoring, non-contact material testing, radio astronomy, and earth observation [1; 2; 3; 4; 5; 6; 7; 8; 9]. In recent years, many active or passive imagingsystems based on all-solid-state electronics technology have been reported at the submillimeter and THz band [9; 10; 11; 12; 13; 14; 15]. Transmitters with sufficient output power are inevitably required for active imagers, and the core device is usually frequency multipliers. Low-noise and broadband receivers are necessarily needed in THz imaging systems. These receivers are mainly based on Schottky diodes or III/V compound semiconductor transistors. THz sources based on gallium nitride (GaN) or complementary metal oxide semiconductor (CMOS) technology have been widely studied. GaN Schottky diode-based sources show better performance in high power dissipation, up to 1.1 W, but the efficiency is much lower than that of GaAs-based circuits [16; 17]. THz sources based on silicon/SiGe CMOS around 300 GHz have also been reported, which generally show great advantages of high integration. However, the maximum output power of only \(-4.6\) dBm and \(-6.5\) dBm and optimum efficiency of only 1.4% and 0.052% are presented, respectively [18; 19]. THz sources based on Schottky diodes possess many advantages, including compactness, broadband, frequency stability, temperature stability, and capability of room temperature operation [20; 21; 22]. In recent years, remarkable progress has been made in the field of solid-state THz sources based on planar Schottky diodes. In pursuit of high working frequency, a tripler operating from 2.5 to 2.7 THz was reported by Jet Propulsion Laboratory [23]. In terms of high efficiency, a 220 GHz tripler with a peak efficiency of 16% was reported by Virginia Diodes, Inc. [24]. However, discreet GaAs Schottky varactor diode chips are used in [24]. Since the highly consistent performance of the circuits is a big concern for subsequent research on active radar imaging arrays, GaAs monolithic integrated diode chips are preferred to design THz triplers. THz subharmonic mixers (SHMs) based on GaAs Schottky diodes have also been rapidly developed due to the advantage of low noise and broadband performance at room temperature [25; 26; 27; 28]. They directly help improve the signal-to-noise rate (SNR) of the entire receiver, especially when there is no proper wideband THz low noise amplifier (LNA) in the system. In [29], a 190-240 GHz Schottky-diode SHM with lower than 1500 K noise temperature is presented. However, the use of flip-chip anti-parallel Schottky diodes may no longer be suitable for higher frequency THz mixers, which face the problems of poor alignment and position uncertainty of the diode and the passive circuits. Therefore, GaAs monolithic integrated technology is an effective and necessary alternative when the operating frequency exceeds 300 GHz. In this paper, 0.34 THz triplers and SHMs based on GaAs Schottky diodes are proposed for THz active imaging applications. The tripler and mixer are used as the solid-state transmitter and receiver front end, respectively. First, the GaAs process, diodes modeling, and the development of 0.34 THz triplers and SHMs are described in detail in Section 2. Compared with the traditional method of mounting discrete diodes on circuits, the 12-\(\mu\)m-thick GaAs monolithic integrated process in this paper reduces manual assembly errors, lowers the defective rate, and improves performance consistency. Moreover, when the frequency goes beyond 0.3 THz, the parasitic parameters introduced by the physical geometry of the Schottky diodes have a non-negligible effect on the frequency conversion efficiency of either multipliers or mixers.