A computational study on electrical characteristics of a novel band-to-band tunneling graphene nanoribbon FET

doi:10.1016/j.spmi.2013.05.003

Superlattices and Microstructures

Volume 60, August 2013, Pages 169-178

https://doi.org/10.1016/j.spmi.2013.05.003 Get rights and content

Highlights

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We propose a new structure by modification of the BTBT–GNRFET.
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The gate metal replaced by a dual-material gate.
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A narrow lightly doped region was also used at the drain side of the channel.
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The proposed structure suppressed the ambipolar current.
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The proposed structure enjoys from better switching, hot electron effect and OFF-state behavior.

Abstract

In this study, a modified structure was proposed for the band-to-band tunneling field-effect transistor (BTBT–FET) mainly to suppress the ambipolar current with the assumption that the ON state characteristics, especially sub-threshold swing, must not be degraded. The proposed structure uses a dual-material gate as gate contact and a narrow lightly doped region at the drain side of the channel. Electrical characteristics of the proposed device were explored by a mode-space non-equilibrium Green’s function (NEGF) formalism in the ballistic limit. A significant reduction in the ambipolar current was seen in simulation results for different values of the drain–source voltages. The results also revealed that the ON current remained the same and the sub-threshold swing got slightly better than that of the main structure. The comparison with the main structure showed that the proposed structure benefited from improved switching characteristics such as delay, switching power-delay product and I_ON/I_OFF ratio. Further comparison indicated that the new structure had improved hot electron effect.

Introduction

In recent years, numerous studies have been done to find new materials, which can replace silicon in next-generation electronic devices. Graphene is one of the materials that has generated great interest due to its special electronic, mechanical, optical and thermal properties [1], [2], [3]. Because of its zero bandgap, graphene is used in the form of nanotubes (CNT) [4], [5] and nanoribbons (GNRs) [6], [7], the bandgaps of which can be tuned. A lot of work has been done on the use of CNTs as the channel of the field-effect transistors (FETs) [8], [9], [10]. Throughout the results, the proposed FETs can be categorized into three groups of Schottky-barrier FETs, tunneling FETs and MOS-Like FETs. All these structures can be constructed using GNRs with almost similar behaviors [11], [12], [13]. The last type acts as a conventional MOSFET and the current flow in the ON-state is dominated by the thermionic emission. Unfortunately, MOS-like FETs suffer from high leakage current resulted from the band-to-band tunneling (BTBT) of the carriers.

In the n-type (p-type) MOS-Like transistors, the current in the ON and OFF states is produced by the thermionic emission of the electrons (holes) and the BTBT of the holes (electrons), respectively. This phenomenon, in which there are two types of carriers in the transistor current but at different bias conditions, is called ambipolarity property, which is a serious problem for the use of transistor in the CMOS topology, in which both n-type and p-type transistors work together. In this situation, both transistors remain ON in any switching event, one by thermionic emission current and another one by the BTBT current (also called ambipolar current).

Some works have tried to suppress the BTBT phenomenon so that the MOS-Like topology could be used in this situation [14], [15], [16], [17]. Although the thermionic emission limits the sub-threshold swing (SS) of MOS-Like FETs to 60 mV/dec, it has been demonstrated that these transistors show lower SS in the BTBT region [18]. Then, it was proposed to use the MOS-Like FETs in the reverse mode [18]. That is, the ON-state of the MOS-Like FETs is used as the OFF-state of the new transistors and vice versa. Such a transistor, which has a structure exactly identical to the MOS-Like FET has been named BTBT–FET. This is because, in contrast to the MOS-Like FETs, the ON current is now generated by the BTBT phenomenon. To be further specified, let us consider an n-type MOSFET, which, as we know, that turns on and off at V_GS > 0 and V_GS < 0, respectively. This transistor can be also used as a BTBT–FET. In this case, it will be turned on and off at V_GS < 0 and V_GS > 0, respectively. Note that for all the above-mentioned situations, V_DS is positive. Low SS, which is an important parameter for ultra-low-power applications, is the main advantage of the BTBT–FETs in comparison with the other structures.

Unfortunately, the BTBT–FETs also suffer from the ambipolar current even more than the MOS-Like FETs. Thermionic emission leads to high leakage current, which should be suppressed in order for this structure to be actually used in the CMOS topology. Yet, up to the present time, no modified structures have been proposed. In this paper, a structure was introduced mainly to suppress the ambipolar current of the BTBT structures. The results showed that it not only strongly suppressed the thermionic emission of the carriers but also showed the same ON-state current and SS as the main BTBT–GNRFET structure. The simulation results also indicated that the proposed structure benefited from the improved switching characteristics and hot-electron effect compared to the original structure.

Section snippets

Device geometry

A cross section of the proposed structure and the conventional BTBT–GNRFET are shown in Fig. 1.

The devices were composed of an armchair carbon nanoribbon with n = 12 and a width of 1.35 nm, which was sandwiched by HfO₂ as the dielectric layer with 1.5 nm thickness and relative dielectric constant of ɛ_r = 16. The n-type source and drain contacts were uniformly doped with a high doping concentration of 0.01 dopant per carbon atom.

As can be seen from the figure, two modification were made in the

Simulation approach

The non-equilibrium Green’s function formalism was used to solve the Schrödinger equation in the ballistic limit. A brief description of the NEGF method has been explained in Refs. [19], [20]. The basis of this method was on obtaining the retarded Green’s function of the device as computed by the following equation, $G (E) = {[EI - H - Σ_{S} (E) - Σ_{D} (E)]}^{- 1}$ where Σ_S, Σ_D, H, E and I are self-energy of the source contact, self-energy of the drain contact, Hamiltonian matrix of the device, energy value and identity

Results and discussion

To evaluate electrical characteristics of the proposed structure, such as OFF current (I_OFF), ON current (I_ON), I_ON/I_OFF ratio, ambipolarity, hot-electron effect, intrinsic switching delay and power-delay product (PDP), the structures shown in Fig. 1 were simulated with different values of bias conditions. The work function values of Φ_GNR and Φ_GNR + 0.5 V were considered for the gate materials of the Metal1 and Metal2, respectively, where Φ_GNR is the GNR work function. The values of other

Conclusion

The NEGF formalism with the uncoupled mode space was used to investigate the electrical properties of a modified BTBT–GNRFET. Thermal emission of electrons from source to drain led to the high leakage currents at positive gate voltages. To decrease this, a structure was used in which a dual material gate was used as gate contact and a narrow lightly doped region was applied at the drain side of the channel. The simulation showed that the proposed structure had considerably lower leakage

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A computational study on electrical characteristics of a novel band-to-band tunneling graphene nanoribbon FET

Highlights

Abstract

Introduction

Section snippets

Device geometry

Simulation approach

Results and discussion

Conclusion

Phys. E. Low. Dimens. Syst. Nanostruct.

Superlattices Microstruct.

Science

Nature

Science

Nat. Nanotechnol.

Carbon Nanotubes

Phys. Rev. B. Condens. Matter.

Phys. Rev. Lett.

IEEE. Trans. Electron. Dev.

IEEE. Trans. Nanotechnol.

IEDM. Tech. Dig.