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Ultra-Sensitive Terahertz Detection With 2D Tunnel Field-Effect Transistors
Tunnel field-effect transistors (TFETs) are considered as main candidates for future low-power electronic circuits. The origin of low-power switching is the steep dependence of source-drain interband tunneling current on the overlap between conduction and valence bands [1]. Despite numerous applications of TFETs in logic circuits, it was not realized that strong (nonlinear) sensitivity of tunnel current to gate and drain voltages also implies efficient rectification of high- frequency signals. Here, we study the operation of tunnel field-effect transistors based on graphene bilayer with gate- induced tunnel junctions as detectors of sub-THz (0.13 THz) radiation [2]. We show high responsivity (~4 kV/W) and low noise equivalent power (~0.2 pW/Hz1/2) at T = 10 K, which are competing to those of superconducting and semiconducting hot-electron bolometers. Our devices were made of bilayer graphene encapsulated in hexagonal boron nitride. Induction of finite band gap and excess carrier density was achieved by simultaneous action of back and top gates. Short sections of graphene bilayer (~300 nm) close to the source and drain contacts were not covered by the top gate, and were controlled by the bottom one only. Thus, by application of gate voltages of opposite polarity to the gates, it was possible to induce a p-n junction between single-gated and double-gated regions. Further enhancement of gate voltages could result in overlap between conduction and valence bands at the two sides of the junction, thus pushing it to tunneling regime. The radiation is fed to TFET from THz antenna coupled between source and gate, the photovoltage was read out between source and drain. We have found that dependence of photoresponse on top gate voltage differs drastically in the gapless and gapped regimes of transport. In the gapless regime, the voltage responsivity is quite low (max ~0.1 kV/W) and symmetric with respect to charge neutrality point. In the gapped regime (i.e. at finite back gate voltage), the responsivity reaches 4 kV/W, is highly non-linear in intensity, and is strongly asymmetric with respect to charge neutrality. The strongest responsivity is achieved at opposite doping of channel and contacts. We have verified that both current and voltage responsivities grow in the tunneling regime, compared to the regime of intraband ohmic transport.
The observed dependences are in a good agreement with the theory of rectification at gate-controlled junctions near the contacts. Being in a good agreement with current measurements, our theory shows that even higher responsivity can be achieved in TFETs with junction at the middle of the channel, as well as in TFETs with extra ‘doping gates’ [3].
Preliminary experiments with split-gate field effect transistors based on bilayer graphene confirm this suggestion [4]. We show that both THz photovoltage and photocurrent grow in such split-gate detectors approximately linearly with the induced band gap in graphene bilayer. Moreover, photoresponse is absent in case of uniform channel doping, and re-appears rapidly in the presence of induced tunnel junctions in the channel.
The work was supported by Russian Science Foundation, grant 21-79-20225.