2.1.2. Schottky Diode

Commercial off-the-shelf (COTS) diodes have been extensively used in 5G RIS for active tuning; the concept was adopted in the THz spectrum by constructing semiconductor metamaterials with a Schottky gate structure. This idea was first realized by an active metamaterial switch in 2006 [54]. The Schottky junction was formed by integrating a metallic SRR with a 1 μm thick n-doped gallium arsenide (GaAs) layer (Figure 3a) [55]. Applying a reverse gate-bias voltage alters the substrate charge-carrier density around the split gap, thus affecting the resonance response of the SRRs. Later, the same group used the unit cell to build a 4 × 4 pixelated spatial light modulator (Figure 3b). Each pixel, consisting of 50 × 50 split-ring resonators (SRRs) with a total size of 4 × <sup>4</sup> mm2, was independently controlled by the external bias voltage. An amplitude-modulation depth of ~3 dB was achieved at 16 V bias voltage in kilohertz (kHz) speed. The modulation speed was enhanced to megahertz (MHz) by placing the ohmic ground plane directly underneath the Schottky layer to minimize the device capacitance, as shown in Figure 3c [56]. This was utilized to build a four-color, 8 × 8 pixelated spatial light modulator by repeating a 2 × 2 four-color subarray, thereby realizing a more advanced spatial and spectral control (Figure 3d).

To realize beam steering, a switchable-diffraction grating with combined amplitude and phase modulation using Schottky gate structure for tuning was presented in [57] (Figure 3e). By applying a reverse bias voltage (−13 V) on alternate columns, the metasurface achieved 20 dB amplitude modulation at 36.1◦ with a speed of 1 kHz at 0.4 THz. An alternative method applied to achieve beam steering was shown using meta-atoms with specially designed 'C'-shaped structures covering a 2*π* phase range. Figure 3f shows eight 'C'-shaped SRRs with different gap sizes and orientations for a π/4 phase gradient [58]. The metasurface metallic pattern was made using gold film and embedded with a doped semiconductor substrate (Figure 3g) [58]. As a result, the metasurface realized broadband (0.55 to 0.83 THz) beam steering with a deflection angle from 59.09◦ to 34.88◦ at a modulation speed of 3 kHz (Figure 3h).

**Figure 3.** Schottky-diode-structure-enabled reconfiguration. (**a**,**b**)A4 × 4 pixel amplitude modulator. (**a**) Schematic of the cross section of the unit cell incorporating SRR with Schottky gate structure (top). The gray scale of the depletion region indicates the free charge-carrier density. A single pixel on the THz SLM for amplitude modulation (bottom). (**b**) THz SLM consisting of 4 × 4 pixels. Reprinted from Ref. [55]. (**c**,**d**) An 8 × 8 four-color spatial light modulator. (**c**) Schematic of the metamaterial absorber with a flip-chip-bonded, n-doped GaAs epitaxial layer. (**d**) An example of the spatial light modulator with different frequencies for each pixel. Reprinted from Ref. [56]. (**e**) A diffractive modulator with grating configuration realizing 22 dB amplitude modulation at 36.1◦. Reprinted from Ref. [57]. (**f**–**h**) A phase-modulated deflector. (**f**) An array consisting of eight unit cells realized 2π phase control with nearly the same transmission efficiency. (**g**) Microscopic image of the fabricated metasurface. (**h**) An illustration of the deflected wave transmission. Reprinted from Ref. [58].

### 2.1.3. High-Electron Mobility Transistor (HEMT)

The instability of two-dimensional electron gases (2DEGs) in short-channel highelectron mobility transistors (HEMTs) leads to a resonant response at the geometrical plasmon frequency, which depends on the size and shape of the channel [59–61]. A pseudomorphic HEMT-integrated metadevice was first introduced in 2011 by D. Shrekenhamer et al. (Figure 4a) [61]. HEMTs were integrated beneath the capacitive gaps of a square electric-LC (ELC) resonator. The resonance response was reconfigured by changing the channel-carrier density through external bias voltage (1 V). The metadevice was fabricated using a commercial GaAs process and realized a modulation depth of 33% at 0.46 THz with a rate of 10 MHz. In 2016, the authors adopted the same meta-atom to demonstrate a 2 × 2 spatial light modulator (Figure 4b) [62].

Various structures have been designed to explore 2DEGs in HEMTs [63–65]. One such embedded HEMT with metal–insulator–metal (MIM) capacitors for amplitude modulation (45%) was introduced in [63] through a biasing voltage of 3 V at 0.58 THz. Reconfigurability with both amplitude and phase modulation is desirable. Double-channel heterostructures with two split channels of decreased polarized-carrier concentration were designed to support a nanoscale 2DEG layer with high concentration and mobility [66] (Figure 4c). An equivalent collective dipolar array was combined with a double-channel heterostructure. An external electrical signal was applied to control the electron concentration in the 10 nm thick 2DEG layers, which led to a resonant mode conversion between two dipolar resonances, providing fast amplitude and phase modulations. Depletion of the 2DEG layer shifted the dipolar resonance from the long central wire to the short one, resulting in a blueshift of the resonance frequency. This design demonstrated 1 GHz modulation speed for the first time and achieved 85% modulation depth (Figure 4d) and 68◦ phase shift (Figure 4e) at ~0.351 THz. By combining inductance–capacitance resonance and dipole resonance, an enhanced-resonance active HEMT metasurface was designed (Figure 4f), realizing a phase modulation of 137◦ at 0.35 THz with a biasing voltage of 8 V (Figure 4g) [67].

**Figure 4.** High-electron-mobility-transistor (HEMT)-enabled reconfigurable metasurface. (**a**,**b**) An HEMT-embedded metamaterial modulator with a speed of 10 MHz. (**a**) A simulation unit-cell model with HEMT beneath each split gap. The cross-sectional view is shown on the right. Reprinted from Ref. [61]. (**b**)A2 × 2 spatial light modulator with a modulation depth of 33% at 0.46 THz. Reprinted from Ref. [62]. (**c**–**e**) Metasurface with 1 GHz modulation speed combining a dipolar array with a double-channel heterostructure. (**c**) Image of the fabricated metasurface. (**d**) Depth modulation of 85%. (**e**) Phase modulation of 68◦. Reprinted from Ref. [66]. (**f**,**g**) Large phase modulator with HEMT embedded and an enhanced-resonance metasurface. (**f**) Schematic of the unit-cell structure. (**g**) Phase modulation of more than 130◦. Reprinted from Ref. [67].
