Dual-gate graphene terahertz photodetectors cut internal resistance by an order of magnitude in experiment
In a laboratory study, tuning the carrier density of ungated channel regions lowered the internal resistance of graphene field-effect detectors, improving responsivity and noise-equivalent power.
Researchers have reported in Frontiers of Optoelectronics an experimental study of high-mobility dual-gate graphene field-effect transistors used as terahertz photodetectors. By tuning the carrier density in channel regions that the local top gate does not cover, the authors decreased the internal resistance of the device by an order of magnitude springer.com
Frontiers of Optoelectronicsspringer.comOpen source ↗. This improved the responsivity and the noise-equivalent power of the detector.
Why access resistance matters
Graphene field-effect transistors show fast response and high sensitivity in the terahertz range, which is needed for 6G wireless communications, quantum information, bioimaging and security screening. According to the authors, the overall performance of such detectors can be strongly compromised by internal access, or parasitic, resistances. The rectified signal under the top gate is modified by unmodulated channel segments between the gate and the contacts, and their contribution had been insufficiently quantified.
Detection in these devices relies on plasma waves excited in the electron gas of the graphene channel. Nonlinear transport in the channel rectifies the oscillating field into a direct-current voltage or photocurrent. This is the Dyakonov-Shur mechanism, also called resistive self-mixing.
Device design and measurements
The team used monolayer graphene encapsulated between hexagonal boron nitride flakes, assembled by polymer-assisted dry transfer on a silicon dioxide and doped silicon substrate. The silicon served as a global back gate. An on-chip bow-tie antenna concentrated the incident radiation between the source and the local top gate. On both sides, next to the source and drain contacts, short channel regions remained outside the control of the top gate.
Total resistance therefore consisted of the active part under the dual-gated segment and an access resistance: the metal-graphene contact resistances plus the resistance of the non-top-gated graphene. Changing the back-gate voltage varied the carrier density, and thus the resistance, of these regions without changing the top-gate field springer.com
Frontiers of Optoelectronicsspringer.comOpen source ↗.
In continuous-wave terahertz measurements the photocurrent changed sign with the top-gate voltage and crossed zero at the charge neutrality point. The authors link this antisymmetric profile to ambipolar transport in graphene and to the Dyakonov-Shur mechanism. They report that the device worked in an overdamped, non-resonant broadband regime.
Experimental data agreed well with a series resistance model, according to the authors. They state that the results help to understand and improve existing high-mobility graphene photodetectors.
