Tunnel field-effect transistors for sensitive terahertz detection

Abstract

The rectification of electromagnetic waves to direct currents is a crucial process for energy harvesting, beyond-5G wireless communications, ultra-fast science, and observational astronomy. As the radiation frequency is raised to the sub-terahertz (THz) domain, ac-to-dc conversion by conventional electronics becomes challenging and requires alternative rectification protocols. Here, we address this challenge by tunnel field-effect transistors made of bilayer graphene (BLG). Taking advantage of BLG's electrically tunable band structure, we create a lateral tunnel junction and couple it to an antenna exposed to THz radiation. The incoming radiation is then down-converted by the tunnel junction nonlinearity, resulting in high responsivity (>4 kV/W) and low-noise (0.2 pW/[Formula: see text]) detection. We demonstrate how switching from intraband Ohmic to interband tunneling regime can raise detectors' responsivity by few orders of magnitude, in agreement with the developed theory. Our work demonstrates a potential application of tunnel transistors for THz detection and reveals BLG as a promising platform therefor.

Fig. 1. Dual-gated bilayer graphene THz detector.

a Schematic of an hBN encapsulated dual-gated BLG transistor. THz radiation is incident on a broadband antenna connected to the source (S) and gate terminals yielding modulation of the top gate-to-source voltage (V_tg) while the back gate voltage (V_bg) is fixed. The build-up photovoltage V_ph is read out between the source and drain (D) terminals. b Band structure of the BLG at the interface between the n-doped bottom gate-sensitive region and dual-gated p-doped channel (Δ is the induced band gap). Blue and red colours illustrate conduction and valence bands fillings, respectively. Note, even for a single-gated region, a finite band gap appears in the energy dispersion due to the difference in on-site energies between the top and bottom graphene layers. c, d Optical photographs of the fabricated dual-gated detector. The source and top-gate terminals are connected to a broadband bow-tie antenna. e The two-terminal resistance of our BLG device, r_2pt, as a function of V_tg for two representative V_bg = 0 and V_bg = 2 V. Inset: Zoomed-in r_2pt(V_tg) for V_bg = 0 V. T = 10 K.

Fig. 2. Tunnelling-assisted THz detection.

a Detector responsivity, R_v, as a function of V_tg for V_bg = 0 V (black), V_bg = −1.4 V (blue) and V_bg = 1.5 V (red) measured in response to f = 0.13 THz radiation. T = 10 K. Inset illustrates band profiles in the vicinity of the single and dual-gated interface when V_bg and V_tg are of opposite polarities. Green arrows illustrate interband tunnelling. b, c Normalized transconductance F versus V_tg obtained by numerical differentiation of the device resistance for V_bg = 0 V (b) and V_bg = 1.5 V (c). Note, F(V_tg) dependencies are fairly symmetric whereas the R_v(V_tg) is highly asymmetric for the same V_bg (a). d, e R_v from a normalized to the channel resistance r_2pt as a function of V_tg for given V_bg.

Fig. 3. Performance of the BLG TFET detector.

a Responsivity of our detector as a function of V_bg and V_tg recorded in response to f = 0.13 THz radiation. The black lines demark the (V_tg, V_tg) regions where the tunnel junction configuration is realized. b r_2pt(V_tg, V_tg) map measured at T = 10 K. The appearance of highly resistive regions (red) points out to the band gap opening in BLG. c NEP of our detector at given V_bg determined using the Johnson–Nyquist relation for the noise spectral density. Horizontal line marks NEP level for SHEBs operating at the same f and T = 4.2 K (see Supplementary Note 3 for a detailed comparison of the BLG-TFET with other THz detectors). Green shaded region indicates the spread in NEP for SHEBs at higher f. d Temperature dependence of R_v(V_tg) and r_2pt(V_tg) (inset) at V_bg = 1.5 V.

Fig. 4. Modelling tunnelling-assisted THz detection.

a Calculated R_v(V_tg, V_bg) map of our dual-gated BLG device in response to f = 0.13 THz radiation. b Calculated band profiles for different (V_tg, V_bg) configurations indicated by the coloured symbols in a. White, grey, yellow, and pink symbols point to the band diagrams of the FET mode whereas the green and blue symbols correspond to the regime of interband tunnelling. Red, blue, and black lines illustrate conduction band minimum (E_C), valence band maximum (E_V), and the chemical potential (μ), respectively. c Line cuts of the map in (a) for V_bg = 0 V and V_bg = 2.5 V. The radiation resistance of the antenna Z_rad ≈ 75 Ω was used for these calculations (“Methods”). Inset: The ratio between the tunnel junction, R_TJ, contribution to the responsivity and that of the channel nonlinearity, R_ch, for V_bg = 2.5 V.

Fig. 5. Equivalent circuit of the BLG-TFET detector.

Antenna is modelled as an equivalent voltage source V_ant that generates ac current I_THz (red arrows) flowing into the source and escaping the FET channel through the gate capacitance. Rectification occurs mainly at the tunnel barrier between source and channel (see band alignment profile in the inset) with voltage-dependent conductance G_S. The doping level and the band gap size is controlled via a simultaneous action of the top and bottom gate voltages, V_tg and V_tg, respectively. The photovoltage, V_ph, is read out between the source and drain terminals.