# Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review
## 5 Fine-Tuning of Transport Properties
### Top-Gate Graphene Field-Effect Transistors  
The ability to control the graphene transport properties via an external voltage is the heart of future electronics [29]. Graphene field-effect transistors show ambipolar field-effect due to its unique electronic band structure with a zero band gap [58]. Under a gate voltage control, charge  
Figure 19: ARPES measured \(\pi\) bands for around K point for the Brillouin zone for (**a**) as-grown graphene monolayer on SiC (0001) (**b–e**) increasing amount of F4-TCNQ. Reprinted with permission from Reference [79]. carriers in graphene can be tuned continuously between electrons and holes with the Fermi level crossing the Dirac point [143]. Charge carriers can be tuned up to 10\({}^{13}\) cm\({}^{-2}\) with mobilities over 15,000 cm\({}^{2}\).V\({}^{-1}\)-s\({}^{-1}\) at 300 K [29]. The mobilities depend on temperature weakly, which indicates charge impurity scattering and, therefore, the gate control may be able to improve those numbers further, even up to \(\approx\)100,000 cm\({}^{2}\).V\({}^{-1}\)-s\({}^{-1}\)[58]. The first report of epitaxial graphene field-effect transistors (EG FETs) patterned on a single epitaxial graphene chip both on the SiC (0001) and SiC (000\(\overline{1}\)) was made in 2008 [60]. Figure 20 demonstrates the Id-Vg characteristics of the C-face and Si-face transistors. The results from this work show that the gate-controlled tuning can lead to mobility as high as 5000 cm\({}^{2}\).V\({}^{-1}\)-s\({}^{-1}\) on the C-face, and an on/off ratio up to 7 on the Si-face SiC. [PERSON] et al. [144] measured the first small-signal radio-frequency (RF) performance of EG FETs on 6H-SiC(0001). [PERSON] et al. [145] fabricated high-speed graphene RF FETs on 4H-SiC (0001) and obtained a cutoff frequency of 100 GHz for a gate length of 240 nm. [PERSON] et al. [146] revealed the RF performance of bilayer GFETs up to 200 \({}^{\circ}\)C. [PERSON] et al. [147] formed a QFSBLG FET on 4H-SiC (0001), (100-nm gate length) with an intrinsic current gain cutoff frequency 407 GHz. [PERSON] et al. [148] demonstrated the first top-gated graphene nanoribbon FETs of 10 nm width on 6H-SiC (0001). The results exhibited exceptionally high drive currents (for fast switching) and opening of a substantial band gap of 0.14 eV. [PERSON] et al. [149] determined the detection of terahertz radiation at room temperature using antenna-coupled EG FETs on SiC, which introduced the potential of plasmonic detectors using epitaxial graphene on silicon carbide. Additionally, the gate control in FET fabricated on bilayer EG enables us to selectively adjust the carrier concentration in each layer and induce a band gap in bilayer graphene. [PERSON] et al. demonstrated that, by doping the bilayer of graphene, the band gap of 200 meV could be achieved [34]. Alternatively, [PERSON] et al. showed the band gap of up to 250 meV is achievable by varying the electric field and by using an external gate control. [PERSON] et al. [45] reported a substrate-induced energy gap of 260 meV [37]. [PERSON] et al. demonstrated the first ambipolar behavior of wafer-scale EG FET on 3C-SiC(111)/Si substrates formed via thermal decomposition (see Figure 21) [80]. A comparison of the graphene FETs fabricated on Si(110) and Si(111) substrates were reported by [PERSON] et al. [20]. The graphene FETs on Si(111) exhibited higher channel currents, which is an order of magnitude larger than those  
Figure 20: Id–Vg characteristics for FET fabricated on (**a**) MEG on SiC (000\(\overline{1}\)) transistor for 0.5 V and 1.5 V drain voltages, (**b**) EG on SiC (0001) transistor for two drain voltages. Reprinted with permission from Reference [60]. Copyright (2008) Institute of Electrical and Electronics Engineers. on Si(110) substrates. However, the measurements indicated the presence of a large amount of gate-leakage current. Fabrication of graphene FETs involves the challenges of selecting an appropriate gate insulator and controlling its thickness [150].