## 1 Introduction  
Over the last two decades, the progress in new semiconductor materials and devices has fostered the research of room-temperature Terahertz (THz) detectors [1]. The portion of the electromagnetic (EM) spectrum located between the infrared region and the RF/microwaves one is commonly known as the Terahertz or submillimeter region and THz radiation is also referred as T-rays. The broad range of potential applications of THz radiation has fueled the research on THz sources and detectors to the point that these new devices start to be exploited in real-world applications which will be brought to market [2, 3]. Accordingly, different emerging applications based on THz science developments have been investigated so far [4]. An extensive list of different fields that may potentially benefit from new THz sensing devices and techniques includes astronomy [5], spectroscopy (many chemicals can have distinctive spectroscopic fingerprints in the THz range) [6; 7], telecommunications (systems operating in the submillimeter range have a bandwidth inherently higher than those working in microwaves: for instance, THz systems will be able to meet the needs of upcoming 5-10 Gb/s data rates in point-to-point wireless short-range communications [8; 9]), security screening (THz beams can be used to detect hidden objects in clothing, envelopes and materials used in packaging, additionally, some hidden substances such as plastic explosives can be detected using their Terahertz fingerprint) [10], metrology [11], etc. At the beginning of the nineties, [PERSON] and [PERSON] [12; 13] presented a ground-breaking work that proposed and demonstrated the ability of sub-micron FETs (Field-Effect Transistors) to detect terahertz radiation. The detection was based on the nonlinear properties of the two-dimensional (2D) electron plasma in the channel of FETs. While the proposal of [PERSON] and [PERSON] was a theoretical work, in the course of the following years the detection of sub-terahertz radiation based on plasma waves was also experimentally demonstrated using different types of FETs such as silicon MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor) [14], strained-Si MODFETs (Modulation-Doped Field-Effect Transistor) [15], GaN FETs [16], and graphene FETs [17]. Terahertz detectors based on the properties of the 2D electron plasmas in FET channels perform direct detection of the incoming THz radiation using a frequency down-conversion technique. In direct detection, the power of THz signals impinging on the FET top-surface is converted into dc current or voltage levels via a non-linear mechanism [12; 13]: for instance, in an open-drain field-effect transistor under THz light a DC voltage will set up between its source and drain contacts as a non-linear response to the THz excitation (phovoltaic mode). Dimensions and doping-profiles of short gate FETs can be designed to tailor the frequency response of the plasma channel plasma within the terahertz range. A non-linear voltage tuning of the value of this frequency can be performed through the gate-to-source bias voltage. By combining plasma-wave detectors and new continuous-wave (CW) solid-state THz sources [18], innovative THz systems will eventually be developed and brought to market, which will benefit from the well-known properties associated with solid-state sources: scalability, reliability, and compactness. The present paper reports on the use of field-effect transistors (FETs) based on the SiGe/Si/Ge double-heterojunction as sub-THz plasma-waves detectors. A \(\Pi\)-gate Si/Si\({}_{0.7}\)Ge\({}_{0.3}\) MODFET was used as a photovoltaic mode direct detector of CW sub-THz radiation. A key advantage of plasma-waves THz detectors based on the Si/SiGe material system when compared to the ones fabricated using III-V materials, is that the first ones can be integrated on the same chip with CMOS (Complementary Metal-Oxide-Semiconductor) circuits, since both CMOS and Si/SiGe MODFETs can share the same Si wafer.