shows an optical microscopy image of the graphene deposited on the Si substrate. It is evident that many graphene slices or islands of about 10 μm in diameter are uniformly scattered on the substrate. Figure
is an enlarged picture of the graphene film obtained via AFM. The large-area graphene film has a smooth and uniform surface free of carbon clusters. The film is about 2-nm thick, which is equal to a few layers of graphene. From Figure
, we can confirm that the growth of the graphene film is characteristic of the layer-island mode. C atoms from the decomposition of CH4
at 950°C (the decomposition temperature of CH4
is about 900°C) absorbed, condensed, and formed islands on the Si substrate. The graphene film was formed by the combination of many islands over time.
Graphene deposited on the Si substrate. (a) Optical microscopy image of the graphene deposited on the Si substrate, and (b) the enlarged AFM picture.
shows the Raman spectrum of the graphene film, which contains two major scattering peaks: a 2D-band peak at 2,690 cm−1
and a G-band peak at 1,590 cm−1
. The intensity ratio of the peaks (I2D
) is 14, which confirms that the graphene film comprises a few layers and is of high quality. The surface carrier concentration, carrier mobility, and current–voltage (I-V
) properties of the graphene film were determined via Hall effect measurements. The carrier concentration of the samples is about 1010
, while the electron mobility is 5.1 × 104
, which is very close to the known ideal value of 2 × 105
]. The surface I-V
behavior of the graphene film is shown in Figure
. Clearly, the graphene surface shows a linear current–voltage relationship, which suggests good transport of electrons on the graphene film. For the Hall effect measurements, four measuring pole points are arranged in a square on the graphene surface. The I-V
behaviors presented in Figure
indicate the relationships between the measured points. Since graphene film is a highly conductive material
], electrons in graphene have high mobility. The voltage between two points changes in a linear relationship with the applied current, which closely obeys Ohm’s law.
Properties of the graphene film. (a) Raman spectrum of the graphene film and (b) the surface voltage–current properties of the graphene film.
The gate current, Ig
, versus gate voltage, Vg
, behavior of the graphene/Si transistor is shown in Figure
. Remarkably, the device shows good rectification properties. The current increases exponentially with the applied positive voltage but is almost zero under the revised voltage. This shows that the GFET device does not have any significant gate leakage. Figure
shows the drain current, Ids
, versus the source-drain voltage, Vds
, under different gate voltages; the GFET exhibits a nearly linear plot when Vds
is less than 2.5 V for different gate voltages. As Vds
increases above 2.5 V, Ids
tends towards saturation. The results indicate that the IV
plot for a Vg
of 4 V is optimal as a ratio of current switch, Ion
, of 100 is obtained. Moreover, the GFET has a smaller threshold voltage, and the Ids-Vds
plots of the graphene transistor show significant current saturation characteristics, which is typical of Si FETs but rare for GFETs. It has been suggested that the velocity saturation of the carrier at higher biases may lead to the current saturation phenomenon in graphene transistors
]. The saturation velocity depends on the charge carrier concentration and scattering by interfacial phonons in the SiO2
layer that supports the graphene channels
]. The achieved current saturation of the GFET makes this device well suited for analog applications. These results demonstrate the feasibility of two-dimensional graphene devices for analog and radio-frequency circuit applications without the need for bandgap engineering.
Figure 4 The current–voltage behavior of the graphene/Si transistor. (a) Gate current, Ig, versus gate voltage, Vg, behavior of the graphene/Si transistor and (b) the source-drain current, Ids, versus the source-drain voltage, Vds, with different gate (more ...)
Moreover, we determined the transfer characteristics of Ids
at a Vds
value of 1 V (Figure
). The plot clearly shows that the graphene transistor can be modulated via the local graphene gate within the range of −2 to 3.5 V, which demonstrates that graphene can act as an effective gate for graphene transistors. Figure
shows the measured electron transconductance, gm
), of the GFET as a function of the gate voltage. A peak transconductance of 3 mS/μm was obtained at a Vds
value of 1 V in this device. Significantly, this scaled transconductance is nearly one order of magnitude better than that of the reported graphene transistors
]. The sign and magnitude of gm
is strongly dependent on the gate voltage. The branch where gm
is less than zero represents p-type transport that is dominated by holes; when gm
becomes positive with increasing gate voltage, the channel becomes n-type.
The drain-source current, I ds, versus top-gate voltage (a), V g, and the electron transconductance, g m(b).