Graphene and Carbon Nanotube Heterojunction Transistors with Individual Gate Control

Abstract

Heterogeneously integrated nanomaterial devices show interesting characteristics for transistors and sensors due to their band diagram or steep material junctions. If these junctions and band alignments can be tuned by an electrical input bias, the device platform not only could be expanded but also could be used to explore fundamental characteristics. However, most reports on hetero-nanomaterial junctions use a global back-gate voltage, which makes it difficult to control band alignment at an interface. To explore device junctions, this study reports a laterally integrated heterojunction of graphene and a carbon nanotube (CNT) network film with individual gate electrodes to tune the band alignment corresponding to the Fermi level shift of graphene in contact with the semiconducting CNT network film. By developing the fabrication process, multiple gate structures are designed to apply a gate bias to CNTs and graphene separately. The threshold voltage shift of the CNT transistor depends on the gate voltage of graphene. Based on the thermionic emission theory, the barrier height between graphene and CNTs for both the conduction and valence band sides varies from 70 to 85 meV, with a linear change as a function of the applied gate voltage to graphene. Although the current Fermi level shift is small, this device platform may realize the exploration of fundamental properties and device concepts.

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“…Device characteristics under dry nitrogen show threshold voltage of −1.32 ± 0.77 V and device on-off ratios on the order of 10 3 (1247 ± 1556 V). Charge mobilities were calculated to be 1.34 ± 0.92 cm 2 V −1 s −1 at a gate voltage of −5 V, which is on the same order as other reported organic and carbon-based back gated transistors that range from 0.37 to 30 cm 2 V −1 s −1 ( Pei et al, 2020 ; Shiomi et al, 2019 ; Snow et al, 2005 ).

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“…[59,60] Graphene is one of the most heavily researched ones, whose lattice is composed of six-membered rings and sp 2 -hybridized carbons with a honeycomb structure (Figure 5j) and the basic structure of 2D bio-FET is shown in Figure 5k. [61][62][63][64] The working principle of 2D bio-FET is the amplification effect of effective signal transduction close to the 2D material surface.…”

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“…(b) Electrostatic modulation of the conduction band and valence band barrier heights using contact gating of graphene in CNT FETs. Reproduced with permission from ref . Copyright 2019 American Chemical Society.…”

“…Multiterminal gating schemes have also been developed to separate electrostatic control of the Fermi level of the graphene contacts from that of the semiconductor channel. − As shown in Figure b, Shiomi et al developed a device architecture for carbon nanotube (CNT) FETs where the bias applied to the contact gates, which are distinct from the channel gate, enabled barrier height modulation of approximately 15 meV for both the conduction and valence bands . In a similar approach, Chuang et al used an ionic-liquid top gate to control the band alignment of graphene contacts with either the conduction or valence band of WSe 2 while employing a back gate to modulate the response of the resulting n-type or p-type FET, thereby allowing the study of electron and hole transport in the same device .…”

“…45−47 As shown in Figure 1b, Shiomi et al developed a device architecture for carbon nanotube (CNT) FETs where the bias applied to the contact gates, which are distinct from the channel gate, enabled barrier height modulation of approximately 15 meV for both the conduction and valence bands. 45 In a similar approach, Chuang et al used an ionicliquid top gate to control the band alignment of graphene contacts with either the conduction or valence band of WSe 2 while employing a back gate to modulate the response of the resulting n-type or p-type FET, thereby allowing the study of electron and hole transport in the same device. 46 Decoupling electrostatic control of the semiconducting channel from electrostatic control of the contact band alignment enables a deeper understanding of the inherent material properties while also allowing control over specific device characteristics.…”