Abstract
Graphene devices for radio frequency (RF) applications are of great interest due to their excellent carrier mobility and saturation velocity. However, the insufficient current saturation in graphene field effect transistors (FETs) is a barrier preventing enhancements of the maximum oscillation frequency and voltage gain, both of which should be improved for RF transistors. Achieving a high output resistance is therefore a crucial step for graphene to be utilized in RF applications. In the present study, we report high output resistances and voltage gains in graphene-on-silicon (GoS) FETs. This is achieved by utilizing bare silicon as a supporting substrate without an insulating layer under the graphene. The GoSFETs exhibit a maximum output resistance of 2.5 MΩ∙μm, maximum intrinsic voltage gain of 28 dB, and maximum voltage gain of 9 dB. This method opens a new route to overcome the limitations of conventional graphene-on-insulator (GoI) FETs and subsequently brings graphene electronics closer to practical usage.
Highlights
The enormous interest in graphene for electronic device applications originates from its outstanding charge transport properties and atomic-scale thickness[1,2,3]
FT values as high as 400 GHz were reported from state-of-the-art graphene radio frequency (RF) FETs7–16
The poor fmax and AV0 values of conventional GoIFETs mainly originate from the absence of current saturation in the output characteristics due to the absence of a band gap and the Klein tunneling effect[18]
Summary
For the conventional GoIFETs, the graphene was transferred onto thermally grown SiO2 (100 nm) on a heavily doped p-type silicon wafer (resistivity of ~0.01–0.05 Ω∙cm). For the GoSFETs, the thermal oxide (50 nm) in the active region on lightly doped n- or p-type silicon (resistivity of ~1–5 Ω∙cm) was patterned. The subsequent processes were identical for both the GoSFETs and GoIFETs. After patterning the channel graphene, the source and drain were defined using a Pd (5 nm)/Au (50 nm) lift-off process. 20 nm of Al2O3 was deposited by atomic layer deposition (ALD) process on the graphene surface. Prior to the ALD process, 0.5 nm aluminum layer was evaporated on the graphene surface as a nucleation layer. The ultrathin aluminum nucleation layer is soon oxidized during the ALD process. The input signal was sinusoidal with an amplitude of 20 mV and a frequency of 1 kHz
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