hv photonics Article Novel Broadband Slot-Spiral Antenna for Terahertz Applications Zhen Huang 1,2 , Zhaofeng Li 1,3,4 , Hui Dong 1,5, Fuhua Yang 3,4,6, Wei Yan 1 and Xiaodong Wang 1,3,4,7,8,* 1 Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China; [email protected] (Z.H.); [email protected] (Z.L.); [email protected] (H.D.); [email protected] (W.Y.) 2 College of Materials Science and Opto-Electronics Technology, University of Chinese Academy of Sciences, Beijing 100049, China 3 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China; [email protected] 4 School of Microelectronics, University of Chinese Academy of Sciences, Beijing 100049, China 5 The Department of Electronic Science and Technology, University of Science and Technology of China, Hefei 230026, China 6 State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 7 Beijing Academy of Quantum Information Science, Beijing 100193, China 8 Beijing Engineering Research Center of Semiconductor Micro-Nano Integrated Technology, Beijing 100083, China * Correspondence: [email protected] Abstract: We report a novel broadband slot-spiral antenna that can be integrated with high-electron- mobility transistor (HEMT) terahertz (THz) detectors. The effect of various antenna parameters on the transmission efficiency of the slot-spiral structure at 150–450 GHz is investigated systematically. The performances of the slot-spiral antenna and the spiral antenna both integrated with HEMTs are compared. The results show that the slot-spiral structure has a better transmission and miniatur- ization capability than the spiral structure. A formula for the responsivity is derived based on the transmission line principle and antenna theory, and results show that the detector responsivity is Citation: Huang, Z.; Li, Z.; Dong, H.; correlated with the antenna absorptivity. Additionally, guidelines for HEMT THz detector design Yang, F.; Yan, W.; Wang, X. Novel are proposed. The results of this study indicate the excellent application prospects of the slot-spiral Broadband Slot-Spiral Antenna for antenna in THz detection and imaging. Terahertz Applications. Photonics 2021, 8, 123. https://doi.org/ Keywords: terahertz detector; broadband antenna; slot-spiral; high-electron-mobility transistor; 10.3390/photonics8040123 antenna design; responsivity Received: 23 February 2021 Accepted: 12 April 2021 Published: 14 April 2021 1. Introduction Terahertz (THz) radiation (which is broadly defined within the 0.1–10 THz frequency Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in range) has been increasingly studied for radioscopic testing [1,2], nondestructive biopsies [3,4], published maps and institutional affil- high-frequency communication [5,6], ultrahigh temporal resolution observation [7,8], etc. iations. Investigating emission and detection over the THz range is more challenging than studying optics and electronics. It is difficult to obtain a high sensitivity and rapid response at room temperature for THz detectors, for which the fundamental operation is based on the conversion of optical signals into electrical signals. These problems have been efficiently solved by using field-effect transistor THz detectors, which have become good candidates Copyright: © 2021 by the authors. for applications. The so-called self-mixing theory was first presented by Dyakonov and Licensee MDPI, Basel, Switzerland. This article is an open access article Shur in early 1990s [9,10] and has been used to describe the mechanism of THz detection. distributed under the terms and The asymmetrical coupling of the THz field to a transistor results in AC voltage (Va) at conditions of the Creative Commons the terminals of the gate and source [11]. A two-dimensional electron gas (2DEG) exhibits Attribution (CC BY) license (https:// hydrodynamic behavior as a whole and forms a plasma wave. The gate voltage (Vgs) creativecommons.org/licenses/by/ approaches the pinch off value. The nonlinear property of the plasma wave in the channel 4.0/). Photonics 2021, 8, 123. https://doi.org/10.3390/photonics8040123 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 123 2 of 13 rectifies the AC signal such that a DC signal (DU) can be detected at the output port. The above detection process is illustrated schematically in the right panel of Figure1. Although many studies have been performed based on this principle, high respon- sivity remains difficult to achieve because of the low coupling efficiency between the detector and the incident THz wave. There are three ways to improve the responsivity of field-effect transistor THz detectors. The first one is to replace the traditional gate structure with the grating-gate structure [12], and the second method is to adopt silicon lens or medium-size dielectric cube [13], and the last one is to integrated antenna structure or array structure [14]. Especially for antenna structures, an appropriately designed planar antenna can remarkably enhance the coupling efficiency to improve detector responsivity. A variety of antennas have been employed to date, the most commonly used of which are the dipole [15], bow-tie [16–21], patch [22–25], loop [26], and etc. [27–30]. The above antennas are used in linear polarized THz detection. However, for circularly polarized or non-polarized broadband detection, spiral antennas are usually adopted [31–33]. Here, we propose the slot-spiral antenna integrated HEMT THz detector. This structure has better performance than the spiral structure in circularly polarized or non-polarized broad- band detection. The simulation results show that the slot spiral structure not only has the broadband characteristic of a spiral structure but also can improve the responsivity of THz detectors by nearly two orders of magnitude greater than the spiral structure. In this paper, the numerical simulation results of slot-spiral antenna integrated with HEMT are presented. This paper is organized as follows. Section2 shows the detection model and specific parameters. Section3 presents the theoretical analysis of THz respon- sivity. Section4 shows the simulation results and discussion about the slot-spiral structure. Section5 makes detailed discussions about the comparison between slot-spiral antenna with spiral antenna. Section6 concludes this paper. 2. Detection Model Considering detection and imaging applications, the simulations were mainly carried out at an optimized transmission efficiency with a 150–450 GHz wave radiated by a Backward-Wave Oscillator (BWO), which has an extremely high radiation power in this region. Figure1 shows the simulation model, which consists of a cylindrical substrate for which the radius is 400 µm and height is 15.24 µm; a slot-spiral antenna; and a coupling field-effect transistor (shown as a partially enlarged image on the right side). The metal part of the slot-spiral antenna is composed of Ni/Au (20/200 nm), and Ni is the adhesion layer. A HEMT is in the center of the structure shown in Figure2. Sapphire is the substrate material; it possesses the advantage of excellent electron transport property and could reduce THz absorption loss. Above the sapphire is GaN/AlGaN (1.8 µm/22 nm). The source and drain ohmic contacts are formed by Ti/Al/Ni/Au (20 /120 /55 /45 nm), and the gate is made of Ni/Au (20/200 nm). It is worth mentioning that the transistor and slot-spiral antenna are not wire-connected; this kind of antenna is called floating antennas, which have also been used in THz detection [16]. We adopted the floating antenna structure here to avoid short circuits between the source and drain (or gate). The near-field electric field distribution and far-field parameters of the structures were accurately obtained by using the commercial program COMSOL. The antenna in the simulation was created using the formulas given below: 8x = wscos(s) 8x = g + wscos(s) <> <> y = wssin(s) , y = g + wssin(s) (1) > > :s 2 (0, np) :s 2 (0, np) The two curves presented above form a spiral arm, which revolves around the center to create a second spiral arm, where w controls the increase in the distance of one rotation, n is the slot-spiral lap, and g is the slot-spiral width. These three structure parameters Photonics 2021, 8, 123 3 of 13 are the main factors that control the properties of the slot-spiral antenna integrated with HEMTs and will be discussed in detail in the following sections. Figure 1. Slot-spiral antenna-coupled field-effect transistor. Figure 2. Simulation model of slot-spiral antenna integrated with FET (left) and specific parameters of transistor (right). 3. Terahertz Responsivity In this section, a formula for the responsivity is derived based on the transmission line principle and antenna theory. This formula considers in detail the influence of various antenna parameters and gives a reasonable explanation of how the antenna affects the responsivity, which is instructive and meaningful to the design of the antenna in THz detectors. Our derivation is based in part on the pioneering work of other researchers. A leading study by Knap [34,35] performed further crucial analyses on resonant and nonresonant detection, showing the following: eV 2 DU = a (2) 4hKBT This formula is only effective when VGS <Vth. All parameters related to transistors are taken into account in h. Va is the AC voltage induced by THz radiation, and it is correlated with radiation frequency, where e is the electron charge, h is the ideality factor (assumed to be 1 in the simulation), KB is the Boltzmann constant, and T is temperature (300 K at room temperature). According to antenna theory, the total power that the transistor absorbs from the electromagnetic wave (PTransistor) can be calculated as follows [21]: Photonics 2021, 8, 123 4 of 13 l2 P = P G (3) Transistor in 4p 0abs In the equations above, Pin is the incident power density, l is the wavelength of electromagnetic wave, and G0abs is the maximum absolute gain.