This work presents simulated output characteristics of gas sensor transistors based on graphene nanoribbon (GNRFET). The device studied in this work is a new generation of gas sensing devices, which are easy to use, ultracompact, ultrasensitive, and highly selective. We will explain how the exposure to the gas changes the conductivity of graphene nanoribbon. The equations of the GNRFET gas sensor model include the Poisson equation in the weak nonlocality approximation with proposed sensing parameters. As we have developed this model as a platform for a gas detection sensor, we will analyze the current-voltage characteristics after exposure of the GNRFET nanosensor device to NH3 gas. A sensitivity of nearly 2.7% was indicated in our sensor device after exposure of 1 ppm of NH3. The given results make GNRFET the right candidate for use in gas sensing/measuring appliances. Thus, we will investigate the effect of the channel length on the ON- and OFF-current.
The gas detection is very important in both research and commercial applicability. There is an increasing need for a higher sensitivity and selectivity as well as for a faster response time. The most significant feature of a material in gas sensing applications is its elevated surface-to-volume ratio. For this motive, new materials are revealed in the construction of innovative sensors with an elevated surface-to-volume ratio. This grants a considerable active surface area for the gas molecule interactions. The different patterns of these nanostructures are graphene and carbon nanotubes (CNT) [
Graphene is defined as a single stratum of carbon atoms that are arranged in a two-dimensional honeycomb lattice. Since its inception in 2004, this material attracted enormous attention, which led to an intensive research activity [
Furthermore, CNT unique electronic characteristics like tunable conductance, ballistic transport, and highly charged mobility [
In the present research we will propose a basic model of how GNRs can be used in gas detection applications. The recommendations from the mathematical model will highlight the relevant properties of graphene in the context of gas sensing, which will elucidate the fundamental working principle of chemical sensors. We have employed the field effect transistor (FET) as a basic structure of our gas sensor model.
The model’s focus was supposed to merge the merits and advantages of graphene nanoribbon which has broad specific surface area (2630 m2 g−1) [
The structure of the proposed analytical device using graphene nanoribbon as a channel for gas sensor is the same as the metal-oxide semiconductor field effect transistors (MOSFETs) structure [
GNRFET-based gas sensor.
The nanoribbon surface could interact with gas molecules, changing the GNRFET conductivity and so its transfer characteristics due to the variability of the source-drain current which is a measurable parameter [
Recently, many theoretical studies on the molecular gas adsorption on the GNR have been carried out. Those studies indicate that the adsorption of CO2 and O2 molecules results in a
In our model we consider that the GNRFET operates with the condition that the channel’s electron gas is nondegenerate. Thus, the distribution functions of the electron and the hole in the subbands near the source and drain electrodes are shown in the following [
The proposed model is based on the correlation between current, gas concentration, and temperature as follows:
The nanoribbon sections adjacent to the source and drain contacts are serving as conducting pads and are highly conducting. Thus, these sections are equipotential and equal to
The Fermi energies, the electric potential, and the electron densities in the source and drain regions are related to each other as
If we consider that the current is determined by the electrons overcoming the potential barrier between GNR and electrodes, we can use the current formula as the follows (per unit length) [
We can associate
The sensing measurements of graphene-FETs were carried out using NH3 gases, which behave as electron donor [
Figure
The
In Figure
(a) Change in
The sensitivity of GNRFET is defined as a percentage conductance change after gas exposure. It can be calculated as
The behavior of the sensitivity of the sensor is practically linear. The GNR gas detectors reveal an increase of almost 2.7% in sensitivity to exposure of 1 ppm of NH3 concentration. This phenomenon could be explained considering that our device exploits an important transformation of the Schottky junctions built up between the graphene nanoribbon and the electrodes (Figure
Schottky junction built up between the GNR and the electrodes: (a) before gas exposure and (b) after exposure to NH3.
Consequently, we notice a change in the alignment of the Fermi level between the drain/source electrodes and the GNR bands. Moreover, the Schottky barrier characteristics at the junction change as well. This results in the electron injection at the barrier and the metal/GNR junction is decreased, giving rise to the current. This consideration is very important because it seems to confirm that the interaction between the gases and the metal/nanoribbon is the phenomenon at the base of sensing for this kind of sensors. Thus, by measuring the variation of the electrical conductivity, due to the interaction with the contacts, we can identify the detected molecule and also the concentration in the air of the found chemical species.
To confirm this point of view, we have simulated the variation of the
The (a) ON- and (b) OFF-current curve and (c)
This statement is demonstrated by the comparison between the measurements performed by changing the gas concentration exposed to the GNRFET. This consideration is in agreement with the analytical and experimental results in the instance of oxygen exposition attained by an IBM team in Yorktown Heights, led by Avouris et al. [
As is seen in Figure
This result can be credited to an essential reliance of the height of the prospective barrier built up between the GNR and the drain/source metal electrodes for the electrons propagating on the molecular gate length (the nanoribbon). In the long-channel limit, the OFF-current is mostly owed to the thermionic discharge of carriers over the impending barrier, whereas in the short-channel limit the OFF-current is dominated by the tunneling of carriers through the barrier [
As shown in Figure
GNR-based gas sensor
It can be seen that the drain current has increased when the temperature increases. However, at the first two temperatures the current values have no significant changes, comparing with that before exposure to the gas. The sensitivity of the device increases from 0.37% at
In conclusion, we replicated the GNRFET-based gas sensor using the analytic model. The dependencies of
The authors declare that they have no competing interests.