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Performance Evaluation of Dual-Channel Armchair

Graphene Nanoribbon Field-Effect Transistor

Adila Syaidatul Azman, Zaharah Johari and Razali Ismail

Department of Electronics and Computer Engineering

Faculty of Electrical Engineering

Unversiti Teknologi Malaysia

81310 UTM Johor Bahru, Johor, Malaysia

dila.syaida@gmail.com

Abstract—Graphene has become a potential successor to silicon

in electronic devices. In this paper, the performance of dual-

channel armchair graphene nanoribbon field-effect transistor

(AGNR FET) is investigated. Both physical and electrical

properties of dual-channel AGNR FET are simulated using

Atomistic Tool Kit from Quantum Wise. Their band structures

and transmission spectra are analyzed. Current-voltage

characteristic is then extracted and the performance of single and

dual-channel AGNR FETs is compared. From the simulation, it

is found that dual-channel AGNR FET exhibits significant

improvement in ON current over two fold. Results obtained will

give insight in the implementation of dual-channel AGNR FET

for performance enhancement in future electronic devices.

Keywords—armchair graphene nanoribbon, dual-channel, field-

effect transistor.

I. INTRODUCTION

Graphene has attracted numerous research attentions since it was discovered in 2004 [1]. Its unique structure makes it another successor to Silicon for future electronic devices. Graphene in its two-dimensional (2D) form do not have band gap that is essential in switching devices. As a solution, the 2D graphene is carved down into narrow width becoming a one-dimensional graphene nanoribbon (GNR). GNR is useful as switching devices as it has bandgap that allow devices to switch on and off. Therefore, GNR is of great interest to be used as the conducting channel in FET for channel length less than 10 nm to replace the conventional silicon material [2].

GNR may present in two types depending on its edges namely zigzag and armchair. Zigzag GNR (ZGNR) has intrinsic zero band gap due to its metallic behavior. However, it is necessary to induce bandgap in ZGNR by applying voltage to the electrodes and metal gate [3]. On the contrary, armchair GNR (AGNR) possesses semiconducting properties and its bandgap is tunable by its width [3]. Therefore, AGNR will be the focus of the study in this paper.

Optimized performance and ability to deliver sufficient drive current are desirable in electronic devices like transistor. Researchers have started to apply multiple conducting channels in FET. For example,a local back-gated aligned carbon nanotube (CNT) array transistor has been proven to exhibit current density of more than 40 A/µm [4]. The practicality of implementing array channel in FET is also evident in array vertical nanowire transistor with gated

surround structure where it exhibits improvement in current flowing [5]. The enhancement in array FET performance is promising and yet to be demonstrated using graphene. This is of great interest since graphene and CNT originated from the same source and the fabrication process of graphene is simpler compared to CNT. A number of researchers have demonstrated various ways of fabricating array graphene-based FET [6-8]. It is shown that the array graphene field-effect transistor (GFET) exhibits enhancement in on/off current ratio Obviously, the back-gated graphene FETs array based on the nano-wall graphene channel demonstrated typical field-effect behavior with linear and saturation regions [9] This observation is fascinating as saturation is a hot debate topic in graphene-based FET. This indicates that multiple conducting channels of graphene may play significant role in some applications.

Although the array-channel graphene-based device has been fabricated, additional works particularly at computational level need to be done. This is to clarify the relation between the numbers of conducting channel with the FET performance. This has not been reported in Ref. [6-8]. Therefore, in this paper the performance of dual-channel AGNR FET is investigated. Hence, performance comparisons using the single and dual-channel AGNR FETs are assessed. In the next section, details of the computational approach used in this study are described. Fig. 1 below shows the structure of single and dual-channel AGNR FETs.

Fig. 1. Structures of single and dual-channel AGNR FET without the top-

gate

.

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II. COMPUTATIONAL METHOD

The simulation is carried out using Atomistix Tool Kit (ATK)

software from Quantum Wise version 13.8.1.

A. Model

Different configurations of AGNR are designed with different widths and lengths for closed system of single-channel and dual-channel AGNR FETs as shown in Fig. 2. The red lines indicate the source and drain electrodes and the blue lines indicate the metal regions which act as the gate of the AGNR FETs. The channel width and length of these configurations is extracted and tabulated in Table I. The AGNR sheet is terminated with hydrogen atoms to avoid end-effect by calculation. The optimized bond lengths of carbon-carbon and carbon-hydrogen are 1.42Ǻ and 1.10Ǻ, respectively.

Fig. 2. Different configurations for closed systems of single and dual-channel

AGNRFETs. The name is according to the dimension of the configuration, which is “ac-W-L”, where ac stands for armchair, W is the width in terms of

number of carbon atoms, and L is the length of the scattering region in terms

of repetition of one pair of hydrogen atoms.

TABLE I: CHANNEL WIDTH AND LENGTH FOR EACH CONFIGURATION OF AGNR FET

Configuration Z-Matrix

Channel Width (Ǻ) Channel Length (Ǻ)

ac-4-8 5.59 32.68

ac-6-8 8.05 32.68

ac-8-8 10.51 32.68

ac-4-16 5.59 66.78

ac-8-16 10.51 66.78

Dual-channel ac-4-8

19.27 32.68

Dual-channel

ac-8-8 29.12 32.68

B. Extended-Huckel and Non-Equilibrium Green’s Function

Methods

Self-consistent Extended-Huckel and Non-Equilibrium

Green’s Functional (NEGF) methods [10] are used in order to study the physical and electrical properties of AGNR FETs. In the first stage, the physical properties of AGNR such as band structures, transmission spectra and density of states are calculated using the Recursion calculator. In the second stage, similar methods are used to analyze the current versus voltage characteristics. To solve Poisson equation in the “electrode” calculations, Fast Fourier transform (FFT) and multigrid techniques are used [10].

III. RESULTS AND DISCUSSION

First, energy band gap was calculated for different configurations of AGNR. According to Hou et. al. [10] and Son et al. [11], the energy band gap decreases as width of the AGNR increases. From Fig. 3, it is clearly shown that the energy band gap of the AGNR decreases as the width increases. For narrowest AGNR, which is ac-4-8 with channel width = 5.59 Ǻ, the energy band gap is approximately 2.25 eV. As we increase the channel width to 8.05 Ǻ for ac-6-8 configuration, the energy band gap is reduced to 1.2 eV. Meanwhile, for ac-8-8 configuration, with channel width of 10.51 Ǻ, the energy band gap is the smallest, which is 0.1 eV. In addition, for ac-4-16 configuration, the energy band gap is approximately 2.25 eV. On the other hand, the energy band gap for ac-8-16 configuration is approximately 0.08 eV. It is worth noting that the energy band gap is infinitesimal if the AGNR width is kept constant while varying the channel length.

The transmission spectra (TS) for single-channel and dual-channel AGNR FETs with ac-8-8 configuration are plotted in Fig. 4. It can be observed that the transmission coefficient is zero around the Fermi energy level for both single and dual-channel AGNR FETs. Additionally, the width of the zero transmission energy is approximately the same to the amount of band gap. This is in line with observation made by Hou et al. [10].

Comparing the TS for single and dual-channel AGNR FETs in Fig. 4, it is observed that beyond zero TS window the single and dual-channel AGNR FETs have transmission coefficient of 1 and 2 respectively. This indicates that there are only one π band appears at the top of the valence band and one π* band appears at the bottom of the conduction band in the single-channel AGNR FET [10]. Meanwhile, the transmission coefficient of 2 achieved in dual-channel AGNR FET indicates the present of two conducting channels. The increment by two fold in transmission coefficient as the number of conducting channel double may explain the enhancement in drain current in latter section.

To analyze the current-voltage characteristic, voltage is supplied to the electrodes. In this study, the voltage used at the drain (right electrode) is 2.0 V, while the source at the left electrode is grounded. The metal gate is biased with 1.0 V. Fig. 5(a) shows the difference in the drain current, ID for single-channel AGNR FET configurations with two different values

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of channel length at VD = 1.98 V, which are 32.68 Ǻ and 66.78 Ǻ. The ID for channel length 32.68 Ǻ is 0.1431 mA, while the ID for channel length 66.78 Ǻ is 0.1403 mA. This shows that the ID changes are insignificant although the length of the AGNR is reduced to almost halves. On the contrary, considerable effect can be observed when the width of the AGNR is reduced to halve. This is evident in Fig. 5(b) where ID is substantially decreased. This indicates that the width in AGNR plays an important role in FET performance.

Fig. 3. Band structures for different configurations of AGNR.

However, the ID for the AGNR FET does not saturate. This could possibly due to the relatively long linear region of graphene, supported by the experiment that had been done by Liu et al. using graphene FETs array. They had proven that the ID of graphene approximately saturates at VDS = 20 V [9]. This is not attainable in this study since VDS was swept up to only 2.0 V considering in actual case, graphene could not often withstand high voltage without breaking. Nevertheless, saturation region is desirable in devices like switches and amplifiers. Most importantly, saturated ID is necessary for a fast switching transistor in high frequency electronic devices. The unsaturated behavior of ID prohibits the device to switch on and off. Hence, further studies need to be done to investigate the optimized conditions for the AGNR FET to achieve saturation and to reduce the saturation voltage using more than one conducting channel.

Fig. 4. Transmission spectra for single and dual-channel AGNR FETs for ac-

8-8 configuration biased at 1.0V.

Fig. 5. (a) ID-VD characteristic biased at 1.0V for single-channel AGNR FET W = 10.5083 Ǻ for two different L. (b) ID-VD characteristic for single-

channel AGNR FET configurations with L = 66.78 Ǻ for two different W.

a) ac-4-8 b) ac-6-8

c) ac-8-8 d) ac-4-16

e) ac-8-16

0 0.5 1 1.5 20

5

10

15x 10

4

Vd (V)

Id (

nA

)

Id vs Vd Characteristics

ac-8-8

ac-8-16

1.94 1.96 1.98 2

1.4

1.41

1.42

1.43

x 105

Id = 143.1 uA

Id = 140.3 uA

0 0.5 1 1.5 20

5

10

15x 10

4

Vd (V)

Id (

nA

)

Id versus Vd Characteristics

ac-12-16

ac-8-16

Id = 7.626 uA

Id = 126.2 uA

(a)

(b)

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To evaluate the dual-channel AGNR FET performance, the current-voltage characteristics of single and dual-channel ac-8-8 configuration biased at 1.0V are compared in Fig. 6. From Fig. 6, it can be seen that the ID for dual-channel AGNR FET is higher than the ID of single-channel AGNR FET. The value of the drain current for single-channel AGNR FET is 0.1293 mA, while the ID for dual AGNR FET is 0.2594 mA at VD= 1.8 V which is approximately double the value of the ID for single-channel AGNR FET. Since it has two conducting channels, the number of electrons flowing from the drain to the source is double the number of electrons flowing in the single-channel AGNR FET. As the current flowing is directly proportional to the number of electrons flowing from the drain to the source, therefore, it is obvious that the current flowing in dual-channel AGNR FET is double the current flowing in single-channel AGNR FET. To the best of the knowledge, there is no experimental data to compare with the simulation result present. Despite, the trend of the conductivity is similar to that achieved in Ref. [4] where the ON current linearly increase with number of CNT channels.

Fig. 6. Drain current versus drain voltage characteristics for single-channel and dual-channel armchair GNR FET ac-8-8 configurations.

IV. CONCLUSION

In this paper, the performance of dual-channel AGNR FET is studied using Extended-Huckel and NEGF methods. From the simulation, it is found that the dual-channel AGNR FET exhibits ID improvement over two fold compared to single-channel AGNR FET. This shows feasibility of using multiple conducting channels in future electronic devices. The outcome of this study will stimulate experimental effort to confirm the finding.

ACKNOWLEDGMENT

Authors would like to acknowledge the financial support from Research University grant of the Ministry of Higher Education (MOHE), Malaysia under Project Q.J130000.2523.05H23. Also thanks to the Research Management Center (RMC) of Universiti Teknologi Malaysia

(UTM) for providing excellent research environment in which to complete this work.

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