# Monolithically Integrated THz Detectors Based on High-Electron-Mobility Transistors
## 1 Introduction  
The frequency range between 100 GHz and 1.5 THz, bridging the microwave and infrared frequency regions, is usually referred as _ THz spectrum_ and presents unique advantages for various application contexts, such as detection defects in tablet coating, industrial product inspection, spectroscopy for chemistry and astronomy, material characterization, security (weapons concealed under clothing), wireless communications and power transfer, and medicine (detection of cancer and caries) [1, 2]. THz waves in this range can indeed penetrate non-conductive materials like plastics, fabrics, and ceramics while remaining non-ionizing and safe for biological tissues. The potential of THz technology in medical diagnostics [3], security screening [4], and material inspection [5] and characterization [6, 7, 8] is not just exciting, but also intriguing, as it opens up a wide range of applications. In this frequency range, powerful electronic sources, such as solid-state oscillators and frequency multipliers, are already available, enabling the development of high-performance systems. THz direct detectors, which convert THz waves directly into electrical signals, are essential for real-time, high-resolution imaging without the need for complex intermediary steps [9]. They do not require local oscillators and their integration with antenna structures. Still, they have poorer sensitivity than the heterodyne methods and deliver no phase information. Schottky diodes are mainlyused as sensitive room-temperature power detectors with limited array capabilities [10]. Many previous approaches to THz direct detection have been based on silicon field-effect transistors (FETs) [11; 12]. III-V FET THz detectors have been demonstrated using GaAs FET devices [13] and GaN high electron mobility transistors (HEMTs) [14]. Results on THz detection using InP heterojunction bipolar transistor (HBT) were preliminary investigated in [15] with poor DC characteristics and published in [16] without specialized antenna structures and with limited sensitivity. A monolithically integrated one- or two-dimensional array of Schottky diodes has yet to be shown. The focal plane arrays demonstrated so far are based on silicon [17] and GaN technology [18]. Due to their limited sensitivities, the output currents from the individual THz detectors must be time-integrated, which dramatically reduces the readout speed. Passive imaging without illumination is possible with THz direct detectors, as demonstrated in [19]. However, to acquire a THz image with a size of 2.7 cm x 4 cm of a hand at room temperature with an uncooled broadband THz FET detector, it takes several hours with an integration time of 200 ms, a scanning spatial resolution of 1 mm, and a scanning speed of 1 cm\({}^{2}\)/100 s. Hence, increasing sensitivity is paramount for the applicability of large THz detector arrays. Therefore, to fully capitalize on the potential of this frequency range, there is an urgent and growing need to develop larger and more sensitive monolithic integrated detector chips. These larger detector arrays are not just a development, but a necessity that would significantly improve image resolution and enable faster scanning over broader areas, making THz imaging systems more practical for real-world applications. In this paper, we show various implementations of FET-based direct THz detectors. All designs are based on resistive mixing. Narrowband and broadband single THz detectors, together with a THz focal plane array (FPA), were designed and measured. A GaN HEMT based on a monolithic microwave integrated circuit (MMIC) process was employed using transistors with only a transition frequency \(f_{T}\) of 50 GHz. All designs feature best-in-class detection properties at room temperature and the monolithic integrated detectors enable scaling to two-dimensional arrays, allowing for the fabrication of THz camera chips.