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(a) Schematics of the epitaxial layers forming the heterostructure together with fabricated ohmic and gate contacts. Two 2DEGs are present in the active InGaAs layer sandwiched between two AlGaAs barriers, and their charge density is modulated the Ti/Au gate electrode evaporated on top of the upper barrier layer. (b) SEM image of the HFET layout with the integrated bow-tie antenna connected to the ends of the transistor channel (source and drain contacts). The arrow indicates the polarization direction of the electric field of the radiation. (c) Transfer characteristics of the HFET. Channel conductance (blue thin line) and mutual transconductance (red thick line) were calculated from the drain current measured in dc with a drain-source voltage of 300 mV.
(a)–(f) Optical responsivity spectra of the lens-coupled HFET detector calculated from the lock-in signal at the drain port for different values of the dc gate voltage Vg . Data are normalized by the frequency-dependent input radiation power emitted in free-space by a tunable THz source in two available frequency ranges. Note the change of sign in panel (b) due to the crossover between two different detection mechanisms. Inset of panel (a): schematic section of the detector package with Si substrate lens and front-end optics (off-axis parabolic mirror (OPA)). The radiation was linearly polarized along the bow-tie axis so that a THz-oscillating electric field is established along the channel. Inset of panel (d): instantaneous electric field map in the vicinity of the channel calculated in a simplified scheme with an electromagnetic solver (CST microwave studio). A THz-oscillating potential is produced between source and gate and between gate and drain.
(a),(b) Comparison between experimental responsivity (red circles) at 183 GHz and 255 GHz and that one calculated from dc transport data (blue line) following Eqs. (2) and (3), respectively. Insets: examples of the instantaneous channel potential under the gate U(x) in the case of impedance matching (a) and severe mismatch (b) (full and dashed lines correspond to opposite phases of the oscillation). (c) Root-mean-square detector noise at 1 kHz within 1 Hz bandwidth.
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