First, microwave diodes including varactor, positive-intrinsic-negative (PIN), and Schottky diodes are discussed, whose equivalent circuit models are clear to experienced microwave engineers. In this paper, we give a comprehensive review of the physical mechanisms of semiconductor materials and devices, design strategies combing both passive and active components, practical implementations, and emerging applications. However, their material uniformity and bias control methods remain to be explored. On the other hand, new semiconductor materials bring in many interesting advances, such as high modulation speed and nonvolatility, and enable promising applications in new spectra, such as terahertz. The diodes and transistors have been widely applied in active microwave circuits for decades, and were demonstrated as a beneficial method for digital coding and programmable metamaterials in the last few years. While several review papers have discussed the developments and advances of active metasurfaces and metamaterials, this work approaches this hot topic from a distinct aspect of semiconductors, including the conventional devices, such as diodes and transistors, and newly rising materials with low dimensions. In addition, new semiconductors with high electron mobility and large-modulation range, have high-frequency applications. With the development of semiconductor materials and fabrication technologies, semiconductor devices provide pixel-control capability due to their small footprint and friendly circuit integration and achieve small switching times down to several picoseconds due to the high electron mobility and high-speed structures. In this regard, semiconductor-based control approaches are the most important candidate for active metasurfaces, particularly programmable and coding ones. The active stimuli manipulate the amplitude, frequency, phase, and polarization states of electromagnetic waves dynamically, which typically requires electrical bias control. In recent years, active metasurfaces have gained concentrated interest from microwave to optical frequencies, enabling many promising applications, such as reconfigurable intelligent surfaces (RIS) in communication, tunable flat lenses in optics, and non-reciprocal devices in physics. Alternatively, tunable metasurfaces can be achieved by integrating active components with pixelated metasurface architectures, which are well-known as active metasurfaces. Mechanical means, such as deformable mirrors and elastic materials, can reconfigure the metasurface properties, however, which is typically limited by the system complexity and size as well as mechanical actuation accuracy. Recently, various approaches have been frequently investigated to tune these passive metasurfaces. Typically, the optical properties of metasurfaces are set in stone post design due to their fixed spatial structures and material properties, which is referred to as “passive”. Metasurfaces are two-dimensional metamaterials with strongly scattering subwavelength atoms, which can manipulate the properties of electromagnetic waves unnaturally and have unlocked many important applications, including clocking, flat lens, perfect absorption, optical computing, biosensing, etc. Retrieved from Īuthor(s): Can Cui Junqing Ma Kai Chen Xinjie Wang Tao Sun Qingpu Wang Xijian Zhang Yifei Zhang (corresponding author)
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