(Invited) Emerging Graphene Device Technologies

Abstract

Graphene has extraordinary electronic properties including a high carrier mobility [1], a Young’s modulus over 1 TPa [2], [3] and the ability to detect the presence of single molecules [4]. Here we explore a number of potential applications based on graphene including sensors, transistors, and supercapacitors. Its remarkable mechanical properties have made it an ideal candidate for nanoelectromechanical system (NEMS) applications. Specifically, its use as a pressure sensor has been thoroughly explored [5], [6]. Graphene has shown an extraordinary sensitivity to changes in pressure, drastically outperforming conventional silicon sensors as well as carbon nanotubes [5]. These devices function through the piezoresistive effect and the underlying physics has been examined in detail [5]. Fig. 1a shows the structure of the graphene pressure sensor along with a typical resistance response of the device to changes in pressure. Graphene based humidity sensors have been demonstrated, providing a comprehensive overview of their sensing properties [7]. These sensors have rapid response and recovery times and have a linear signal response over a wide humidity range. Fig. 1b shows a humidity sensor device and corresponding resistance change when exposed to a variety of gasses. Results are adapted from [7]. Two types of graphene based transistors have additionally been explored. The first example is a graphene field effect transistor (GFET) that has been fabricated and examined showing soft current saturation [8]. The other example is the graphene base transistor (GBT) which was first experimentally demonstrated in 2013 [9]. The difference between the GBT and the GFET are expressed in Fig. 1c and Fig. 1d. In the case of the GBT, charge transport occurs perpendicular to the graphene by tunneling through an emission barrier interface (EBI), then transported through the graphene layer (acting as the base), and lastly through the interface between the base and the collector (BCI). The GBT device structure and function is illustrated in a schematic and simplified band diagram (Fig. 1c). These GBTs have since had their performance improved by the use of an intermediate trap assisted tunneling layer between the EBI and BCI [10]. For GFETs, charge transport occurs along the graphene layer between the source and the drain and the current is electrostatically controlled by the gate (Fig. 1d). The combination of graphene’s electronic properties with this high surface to volume ratio makes graphene an ideal material for use in energy storage devices. Graphene based supercapacitors have been demonstrated which employ ink-jet printed graphene films to provide high surface area [13]. Fig. 1e and Fig. 1f shows ink-jet printed graphene on two substrates. Fig. 1e is an optical image of printed graphene on glass while Fig. 1f is a scanning electron microscope (SEM) image of ink-jet printed graphene on SiO2. These devices can be integrated with other graphene devices in a back end of the line (BEOL) integration scheme. Graphene’s integratability has promising implications for the heterogeneous integration of multiple devices on the same chip.