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Electronic metadevices for terahertz applications

Nature volume 614, pages 451–455 (2023)Cite this article

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Abstract

The evolution of electronics has largely relied on downscaling to meet the continuous needs for faster and highly integrated devices1. As the channel length is reduced, however, classic electronic devices face fundamental issues that hinder exploiting materials to their full potential and, ultimately, further miniaturization2. For example, the carrier injection through tunnelling junctions dominates the channel resistance3, whereas the high parasitic capacitances drastically limit the maximum operating frequency4. In addition, these ultra-scaled devices can only hold a few volts due to the extremely high electric fields, which limits their maximum delivered power5,6. Here we challenge such traditional limitations and propose the concept of electronic metadevices, in which the microscopic manipulation of radiofrequency fields results in extraordinary electronic properties. The devices operate on the basis of electrostatic control of collective electromagnetic interactions at deep subwavelength scales, as an alternative to controlling the flow of electrons in traditional devices, such as diodes and transistors. This enables a new class of electronic devices with cutoff frequency figure-of-merit well beyond ten terahertz, record high conductance values, extremely high breakdown voltages and picosecond switching speeds. This work sets the stage for the next generation of ultrafast semiconductor devices and presents a new paradigm that potentially bridges the gap between electronics and optics.

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Fig. 1: Concept of electronic metadevices based on the microscopic manipulation of radiofrequency fields leading to extraordinary electrical properties.
Fig. 2: High-frequency characterization of electronic metadevices.
Fig. 3: Electronic metadevices with ultrahigh conductances and breakdown voltages.
Fig. 4: High-performance THz data transmitters using electronic metadevices.

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Data availability

All the data supporting the claims of this paper are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank H. Massler and Fraunhofer IAF Karlsruher Institut für Technologie (KIT) for helping with terahertz measurements. We also thank B. Rejaei and M. Rezaei for discussions.

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  1. Mohammad Samizadeh Nikoo

    Present address: Integrated Systems Laboratory, ETH Zurich, Zurich, Switzerland

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  1. Power and Wide-Band-Gap Electronics Research Laboratory (POWERlab), Institute of Electrical and Micro Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

    Mohammad Samizadeh Nikoo & Elison Matioli

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Contributions

M.S.N. and E.M. conceived the project. M.S.N. fabricated the devices, designed and performed the experiments. M.S.N. and E.M. interpreted the results and wrote the manuscript.

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Correspondence to Mohammad Samizadeh Nikoo or Elison Matioli.

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Extended data figures and tables

Extended Data Fig. 1 Port definition and electric field patterns and current densities in metadevices.

a, Three-dimensional schematic of a straight-gap device showing the integration of the two terminals with a coplanar waveguide. The ports are defined in between the terminals and the ground pads. Simulated vertical electric field (real part) at the barrier in the b, ON state and c, OFF state. The lateral depletion length due to fringing fields72 in the OFF state was considered to be 20 nm. The field pattern in the OFF state does not exhibit the oscillatory shape. It can be seen that an electrostatic voltage can control electromagnetic interactions in the devices, which is the basis for the switching mechanism in electronic metadevices. Simulated current density (absolute value) at the semiconductor channel in the d, ON state, and e, OFF state. The current density in the OFF state does not show any confinement. f, Real and g, imaginary parts of the current density at the barrier (Jz) in the ON state. The real part of the input current is totally dominant in the ON state, which makes the device impedance to be resistive. h, Real and i, imaginary parts of the current density at the barrier (Jz) in the OFF state. The imaginary part of the input current is dominant in the OFF state, which makes the device impedance mostly reactive.

Extended Data Fig. 2 Three terminal metadevices.

a, Micrograph of a fabricated three-terminal metadevice. One terminal is pumped with a radiofrequency signal and the state of the device is controlled by the gate terminal. In an integrated-circuit form factor, the gate terminal of the metadevice can be controlled similarly to a field-effect transistor, by either applying an over-threshold bias (OFF state) or by floating the terminal (ON state). b, Simulated electric field pattern (real part) at the barrier for a gated metadevice in the ON state showing that the gate does not perturb the electric field pattern. c, Simulated electric field pattern (real part) at the barrier for a gated metadevice in the OFF state, in which the gate electrode depletes the 2DEG underneath it. d, Measured output signal of the gated metadevice showing a sub-10 ps switching. The measurement were done using a three-port setup with one GSGSG 67-GHz probe (for the gate and the input RF signals) and one GSG 67-GHz probe (for the output RF signal).

Extended Data Fig. 3 Simulation of electronic metadevices operating at terahertz frequencies.

a, Simulated vertical electric field pattern (real part) at the barrier of the terahertz metadevice at different frequencies from 200 GHz to 1.6 THz. The input port of the devices were excited by a current source (15 mA with zero phase), and the real part of Ez is plotted. The absolute value of the input current was selected to result in a current density of ~1 A mm–1 in the 2DEG. b, Simulated current density at the semiconductor channel at different frequencies from 200 GHz to 1.6 THz. At 200 GHz, there is almost no interaction between the subwavelength mode and the stripe array, and therefore, current density is not confined. The current density has the most confinement at the highest frequency, 1.6 THz. c, Simulated electric field pattern (real part) at the barrier of a straight-gap device at different frequencies from 200 GHz to 1.6 THz. The input port of the devices were excited by a current source (3.2 mA with zero phase), and the real part of Ez is plotted. The absolute value of the input current was selected to have a current density of ~1 A mm–1 in the 2DEG. d, Simulated current density at the semiconductor channel at different frequencies from 200 GHz to 1.6 THz. The straight-gap device does not show the level of current confinement seen in the Metadevices at high frequencies. e, Simulated contact resistance of straight-gap and metadevices showing the superior performance of metadevices with RC < 30 Ω μm, which agrees well with the measurements. f, Simulated contact resistance versus total ON-state resistance of the terahertz metadevice.

Extended Data Fig. 4 Effect of number of stripes on the collective interaction, and the superlinear scaling.

a, Schematic (top view) of an electronic metadevice with an effective width of Weff. Microwave metadevices (Extended Data Table 1) were simulated at 70 GHz by COMSOL, where the first port was excited by a current source I = Weff× (1 A mm−1) to scale the total current of electronic metadevices with respect to their effective width. The second port was short circuited. b, Vertical electric field at the barrier Ez (real part; in-phase with input current) simulated for a device with two stripes. c, Current density (absolute value) at the 2DEG. d, Zoomed in current density (absolute value) at the 2DEG, showing confinement in about 2 μm. e, Vertical electric field at the barrier Ez (real part; in-phase with input current) simulated for an electronic metadevice with 8 stripes. f, Current density (absolute value) at the 2DEG. g, Zoomed in current density (absolute value) at the 2DEG, showing confinement in about 1 μm. The device shows two times more confinement compared to the device with two stripes. The results also indicates the capability of the devices in high-power operation, since at very high current densities of ~1 A mm–1, the devices exhibit Ez much below the critical electric field of wide-band-gap materials. h, Simulated conductance and stripe array capacitance (entire gap depleted) of devices with different number of stripes. By increasing the number of stripes from 2 to 8, the capacitance is increased by 4.9-fold, while the device gains 7.8-times in conductance. Such superlinear increase in the conductance, enhances the cutoff frequency FOM of the devices.

Extended Data Fig. 5 Electric field patterns and electrical characteristics of metadevices at partial depletion.

Simulated vertical electric field (Ez) at the barrier with different electron densities (sheet resistances) under port 1. a, \({N}_{S}={N}_{S}^{{\rm{ref}}}\) (identical to Fig. 1f), b, \({N}_{S}=0.8{N}_{S}^{{\rm{ref}}}\), c, \({N}_{S}=0.5{N}_{S}^{{\rm{ref}}}\), d, \({N}_{S}=0.1{N}_{S}^{{\rm{ref}}}\), e, \({N}_{S}=0.01{N}_{S}^{{\rm{ref}}}\), f, \({N}_{S}=0.001{N}_{S}^{{\rm{ref}}}\). g, Extracted impedance (real part, red, imaginary part, blue) from simulations at different electron densities. The field distributions and the impedances are quite flat up to ~50% depletion.

Extended Data Fig. 6 Circuit model of electronic metadevices.

Schematic of the cross section of metadevices in a, ON, b, OFF states. c, Proposed compact circuit model including four elements Rch (channel resistance), RC (contact resistances), \({X}_{S}=L\omega -{(C\omega )}^{-1}\) (reactive impedance due to the metal-semiconductor coupling and the extension of the metallic pads), and CP (capacitance between the interdigital metals). d, Extracted CS and LS at different voltages (discrete points). The inductance play a negligible role in the OFF-state reactance, and therefore, it can be considered constant (LS =50 pH) for the entire voltage range. The measured capacitance was empirically fitted by \({C}_{S}\left(V\right)=260{\left(1+\frac{1}{25}\left({|V|}{/V}_{{\rm{th}}}\right)+{\left(V{/V}_{{\rm{th}}}\right)}^{20}\right)}^{-1}+9.5{\rm{fF}}\), with Vth = 4.3 V (dashed line). e, Measured (discrete points) and modeled (solid lines) impedances at the resonance frequency (ω = ω0S). Red (real part), blue (imaginary part). The total resistance was modeled by \({R}_{{\rm{ch}}}+{R}_{S}=6.5+45{\left(1+{\left(V{/V}_{{\rm{th}}}\right)}^{-10}\right)}^{-1}\Omega \) . The device exhibits a very linear ON state followed by an abrupt switching. f, Broad-band measured impedance of the device in ON (|V| ≤ 3 V) and OFF states (V = 10 V) (discrete points), showing a very good agreement with the model (solid lines). In a wide range of frequencies around the resonance, the imaginary part of the impedance is very small and negligible compared to RON, which shows the wide band nature of the transmissive mode. g, Absolute values of the impedance of the 6-stripe microwave metadevice, as well as 4-stripe mm-wave and terahertz metadevices (parameters described in Extended Data Table 1) at intermediate frequencies. Considering data modulation with a 10% bandwidth, the devices show a high impedance for the control (data) signal, while exhibiting a low impedance for the carrier signal.

Extended Data Fig. 7 Dynamic performance of electronic metadevices under harsh switching conditions.

a, Schematic of the experimental setup to evaluate the dynamic performance of the metadevices under harsh switching conditions. b, Measured reflection (S11) from the device, under burst conditions. The device is relaxed for 10 ms followed by a sequence of 100 switching cycles. The measurement corresponds to the switching amplitude of 20 V. c, Measured reflection in a time period of 10 µs. At each period, the device holds a stress for about 4 µs, and then goes to the ON state for about 500 ns. The ON resistance of the device is extracted based on the reflection coefficient. d, Extracted ON resistance of the device (normalized by the reference resistance of the relaxed device, as shown in part b) for different amplitudes over the 100 cycles. Without any passivation, the device shows a very good performance with only a negligible RON degradation. e, Waveforms in a double-sweep experiment to evaluate possible hysteresis in the threshold voltage. The device is submitted to a large signal voltage and the reflection is measured in both switching cycles (ramp up and ramp down). f, Extracted double-sweep reflection showing a hysteresis-free operation.

Extended Data Fig. 8 Validation of the theoretical description for the subwavelength mode at the metal-semiconductor junction.

a, Schematic of a thin MIS structure showing the direction of electromagnetic fields. b, c, Simulated electric field Ez (cross section) with d = 100 nm (we considered a larger-than-usual thickness to have more data points in our numerical model which is used to support our analytical approach) at 50 GHz and 300 GHz, respectively. The structure is excited from the left end and terminated from the right end. d, e, Simulated electric field Ez (cross section) at 50 GHz and 300 GHz at the windows A and B of parts b and c, respectively. The results show a uniform Ez at the thin barrier, which is one of the assumptions in our theoretical modeling. The semiconductor layer (10 nm thin) is just below the barrier. f, g, Simulated electric field components Ez and Ex in the cutline shown in parts d and e at 50 GHz and 300 GHz, respectively, indicating a uniform Ez and a linear Ex at the barrier which is in agreement with the theoretical calculation of Eq. (13). h, i, Simulated displacement current at the barrier and derivation of current in the semiconductor layer (real and imaginary parts, respectively) validating the current continuity in Eq. (14). The results are presented at 50 GHz. j, k, Simulated displacement current at the barrier and derivation of current in the semiconductor layer (real and imaginary parts, respectively) at 300 GHz validating Eq. (14).

Extended Data Table 1 Geometries of metadevices for microwave, mm-wave, and THz operation
Full size table
Extended Data Table 2 Comparison of terahertz switches realized based on metadevice concept with classic devices
Full size table

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Samizadeh Nikoo, M., Matioli, E. Electronic metadevices for terahertz applications. Nature 614, 451–455 (2023). https://doi.org/10.1038/s41586-022-05595-z

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