2.1.1. Complementary Metal-Oxide-Semiconductor (CMOS) Transistor

Incorporating active electronic devices into a passive metasurface allows for fast dynamic control, but the application is limited to microwave frequencies. In 2020, Venkatesh et al. presented a programmable metasurface using complementary metal-oxide-semiconductor (CMOS) transistors operating at 0.3 THz beyond its cutoff frequencies [52]. They applied a 65 nm industry-standard CMOS process to fabricate the metasurface in a silicon chip tile, consisting of 12 × 12 arrays (Figure 2a). Each meta-atom contained eight n-type metaloxide-semiconductor (NMOS) transistors for an eight-bit reconfiguration at gigahertz (GHz) speed. Parallel subwavelength inductive microloops were added to the transistor for local resonance to suppress the non-negligible parasitic capacitance leakage. An amplitude modulation of 25 dB was achieved between all switches open (maximum transmission) and closed (minimum transmission) states. A demonstration of the 2 × 2 tiled chips (Figure 2b) for holographic projections of the letter 'P' is shown in Figure 2c. The unit-cell structure was derived from a general C-shaped split-ring resonator (SRR), which controls the transmitted amplitude and phase by varying the gap-opening orientation and size, respectively. This was realized by selectively switching the eight transistors of a meta-atom, leading to 256 states in total. Due to the symmetric configuration, each meta-atom had 84 unique codes and realized a phase coverage of 260◦. Beam steering from 0◦ to ±30◦ was achieved by configuring the unit cell in three different phase profiles and meta-atom digital settings (Figure 2d).

**Figure 2.** CMOS transistor-enabled reconfigurable metasurface. (**a**–**d**) GHz-speed programmable metasurfaces using CMOS-based chip tiles. (**a**) A single silicon chip tile consists of a 12 × 12 array (left).

The enlarged portion (right) shows the unit-cell structure with active NMOS transistors embedded in the gap of inductive microloops. Each unit cell has an eight-bit control, enabling 256 states for amplitude and phase control. (**b**) Photo of the fabricated 2 × 2 tiled chips, which were wire-bonded to a customized printed circuit board for external voltage control. (**c**) Amplitude modulation was experimentally demonstrated as a holographic projection of the letter 'P'. (**d**) Beam steering at ±30◦ with the corresponding three different phase profiles and meta-element digital settings. Reprinted from Ref. [52]. (**e**–**g**) Reconfigurable metasurface based on CMOS structures. (**e**) A bias voltage is applied for transmitted amplitudes and phase modulation. The reconfigurable metamaterial can be divided into subsections for greater functionality. (**f**) Cross section of the unit cell, consisting of six layers for the CMOS transistor configuration. (**g**) Layout with wire connection for the biasing control. Reprinted from Ref. [53].

By utilizing custom-designed CMOS-based semiconductor structures, it is possible to circumvent the cutoff frequency limitation of commercial transistors (Figure 2e) [53]. The meta-atom structure, consisting of a square SRR on top of a six-layer CMOS-based semiconductor structure, is shown in Figure 2f. The gap of the SRR was connected to the source and drain of a metal-oxide-semiconductor field-effect transistor (MOSFET) through vias (Figure 2g). The metasurface was fabricated with 180 nm CMOS technology. With a bias voltage from 0 V to 1.8 V, a redshifted frequency of 35 GHz and a phase difference of 3◦ were achieved at 0.3 THz. Although the phase modulation was limited, the authors presented an alternative solution for realizing a CMOS-based reconfigurable metasurface.
