How Do Contact and Channel Contribute to the Dirac Points in Graphene Field‐Effect Transistors?

Abstract

electronics. [1][2][3][4][5] The cause of the amazing electronic properties of graphene is its unique line-up band structure, in which the valence and conduction bands meet in a single point at the Fermi level. This peculiar point is the so-called Dirac points and cones. [1] In graphene field-effect transistor (GFET), the channel conductance is modulated by the electric field from a back-or top-gate. The transfer characteristics, I DS -V GS curves typically display a V shape, with a hole-dominated conductance (p-branch) at lower V GS and electron type transport at more positive gate voltages (n-branch). The valley of the curves is the charge neutrality where the concentration of electron is equal to that of hole. This valley corresponds to the Dirac point in graphene band structure and quite important for electronic applications of graphene. For example, both the graphene frequency doubler and ambipolar mixer are biased at the Dirac point for their best performance. [6][7][8][9] In most case, only one unique Dirac point was found in the transfer characteristic of GFET. [10][11][12][13] Recently, several research groups found that the transfer characteristics in GFET show an additional minimum other than the major Dirac point, in other words, a doubledips curves. Nouchi and Tanigaki have found an anomalous distortion in GFET with ferromagnetic metal electrodes. [14] The transfer characteristics in these GFETs show two local minimal current points at hole branch. They attribute this phenomenon to the oxide layer induced weaker pinning of charge density at the metal contacts. Brenner and Murali have utilized hydrogen silsesquoxane film to partially cover the graphene channel, which leads to the different doping levels in channel region and a formation of p-n junction. An additional minimum conductance other than the Dirac point can also be observed in this GFET. [15] Chiu et al. found that an additional dip will appear on the left-hand side of the original Dirac point in I DS -V BG curve in the high field regime. They claim that this double dip structure can be attributed to the formation of a p-n junction in drain region, which is induced by the trapped charge doping under high bias conditions. [16] Bartolomeo et al. show that a double Dirac point can also been achieved while keeping the drain bias very low (20 mV). They have clarified that the double Dirac point is related to charge transfer between graphene and metal contact and that it is enhanced by the charge storage at The Dirac point(s) in graphene field-effect transistors (GFETs) are of great importance for electronic application. However, the lack of the effective means to distinguish the electrical properties of graphene at the contact and channel regions limits a clear understanding of their contributions to the Dirac point(s). A method, which can characterize the electrical properties of graphene under metal contact and in the channel, is developed, respectively. It is found that the Fermi levels of graphene at the contact and channel regions are quite ...

Metal‐Contact‐Induced Transition of Electrical Transport in Monolayer MoS2: From Thermally Activated to Variable‐Range Hopping

“…Unfortunately, there have been few reports concentrating on this issue because of the difficulty to measure the electrical propriety of MoS 2 beneath the metal. Recently, we developed a modified transfer length method to realize the electrical property and carrier transport of 2D graphene film under the metal contact . Through this method, the sheet resistances of MoS 2 both under metal contact ( R SK ) and in channel ( R SH ) are obtained through our method.…”

“…The current flowing into the metal from the MoS 2 sheet is highest near the contact edge x = 0 and drops nearly exponentially with the distance (seen in Figure a). The distance that the current drops to 1/ e of the total current is defined as transfer length L T . Based on the modified transmission line model that we developed, the potential distribution under the metal contact V ( x ) can be expressed as belowwhere L is the contact length, W is the contact width, ρ C is the specific contact resistivity, and I is the total current flowing into the contact region.…”

Reversible doping of graphene field effect transistors by molecular hydrogen: the role of the metal/graphene interface

“…Additionally, the splitting between both CNPs is also dependent on the H2 concentration. In figure 3(e) we present the 𝑅 𝑥 𝑛 curves for all concentrations analyzed (from Now, it is important to stress that apart from a double-gate structure that enables setting of the density of charges at different regions of the graphene channel independently, all the other processes that create a double-CNP are irreversible [21,28,31,43,47,49]. In our case, the formation of the second CNP is totally controlled and reversible in devices fabricated with a 1nm thick layer of Cr2O3, and under hydrogen exposure.…”

“…In the GFET configuration, an electric current between the source and drain terminals is modulated by the application of a gate potential [1,5,12,13], In this context, it is well known in the microelectronics industry that electric transport between the conducting channel and metallic electrodes plays an important role in the device characteristics and performance [14,15]. Thus, an intensive effort has been devoted to understand and improve the electrical junction between the metallic electrodes and graphene [16][17][18][19][20][21][22]. Typically, in a metalsemiconductor junction, there is a Schottky-barrier that actively affects the electronic device properties [3,5].…”

Emerging Internet of Things driven carbon nanotubes-based devices

“…The Pd/CNT device features an Ohmic contact, leading to ballistic transport behavior [139]. Indeed, the metal contact with low-dimensional carbon nanomaterials contributes to conductance regulation [140,141], electrical doping [142,143], and carrier type reversal [144,145]. In addition, the electric field could also lead to the increased tunneling probability [146] and ultralow noise level [147].…”