Graphene is a relatively new material with unique properties that holds promise for electronic applications. Since 2004, when the first graphene samples were intentionally fabricated, the worldwide research activities on graphene have literally exploded. Apart from physicists, also device engineers became interested in the new material and soon the prospects of graphene in electronics have been considered. For the most part, the early discussions on the potential of graphene had a prevailing positive mood, mainly based on the high carrier mobilities observed in this material. This has repeatedly led to very optimistic assessments of the potential of graphene transistors and to an underestimation of their problems. In this paper, we discuss the properties of graphene relevant for electronic applications, examine its advantages and problems, and summarize the state of the art of graphene transistors.

Graphene Transistors: Status, Prospects, and Problems

With CMOS scaling reaching the limits in the next decade, new approaches are required to enhance the functionality of electronic systems. Reconfigurability on the device level promises to realize more complex systems with a lower device count. In the last 5 years a number of interesting concepts have been proposed to realize such a device level reconfiguration. Among these the reconfigurable field effect transistor (RFET), a device that can be configured between an n-channel and p-channel behavior by applying an electrical signal, can be considered as an end of roadmap extension of current technology with only small modifications to the process flow [1]. This paper gives a review on the RFET basics and current status. In the first sections the state of the art of reconfigurable devices will be summarized [2] and the RFET will be introduced together with related devices based on silicon nanowire technology [3]. The device optimization with respect to device symmetry and performance will be discussed next [4,5]. The potential of the RFET device technology will then be shown by discussiing circuit implementations making use of the unique advantages of this device concept [6,7,8]. The basic device concept was also extended towards applications in flexible devices and sensors [9,10] extending the capabilities also towards so called More than Moore applications were new functionalities are implemented in CMOS base processes. Finally the prospects of the RFET device technology will be discussed.

https://iopscience.iop.org/article/10.1088/1361-6641/aa5581/pdf

The RFET—a reconfigurable nanowire transistor and its application to novel electronic circuits and systems

Germanium is a promising material for future very large scale integration transistors, due to its superior hole mobility. However, germanium-based devices typically suffer from high reverse junction leakage due to the low band-gap energy of 0.66 eV and therefore are characterized by high static power dissipation. In this paper, we experimentally demonstrate a solution to suppress the off-state leakage in germanium nanowire Schottky barrier transistors. Thereto, a device layout with two independent gates is used to induce an additional energy barrier to the channel that blocks the undesired carrier type. In addition, the polarity of the same doping-free device can be dynamically switched between p- and n-type. The shown germanium nanowire approach is able to outperform previous polarity-controllable device concepts on other material systems in terms of threshold voltages and normalized on-currents. The dielectric and Schottky barrier interface properties of the device are analyzed in detail. Finite-element...

Enabling Energy Efficiency and Polarity Control in Germanium Nanowire Transistors by Individually Gated Nanojunctions

The polarity of the charge carriers injected through Schottky junctions of α-phase molybdenum ditelluride (α-MoTe2) and various metals was characterized. We found that the Fermi-level pinning in the metal/α-MoTe2 Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS2. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/α-MoTe2 junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/α-MoTe2 and Ni/α-MoTe2 junctions was also observed with Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt-Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes.

Carrier Polarity Control in α-MoTe2 Schottky Junctions Based on Weak Fermi-Level Pinning

Reconfigurable nanowire transistors are multifunctional switches that fuse the electrical characteristics of unipolar n- and p-type field effect transistors (FETs) into a single universal type of four-terminal device. In addition to the three known FET electrodes the fourth acts as an electric select signal that dynamically programs the desired polarity. The transistor consists of two independent charge carrier injection valves as realized by two gated Schottky junctions integrated within an intrinsic silicon nanowire. The transport properties that provide unipolar n- and p-type behavior will be elucidated. Further, solutions to the major device challenges toward the implementation of these novel transistors at the circuit level are proposed, by exploiting specific nanowire geometries and dimensions. These include methods that deliver equal on-currents and symmetric transfer characteristics for n- and p-type, and that eliminate supra-linear output characteristics at low source–drain biases. We will further show that circuits built of these symmetric transistors successfully exhibit complementary operation. Finally, the prospects in building reconfigurable circuits and systems will be briefly summarized.

Reconfigurable Nanowire Electronics-Enabling a Single CMOS Circuit Technology

We found that a transistor with a graphene channel irradiated with He ion beams can have a transport gap of up to 380 meV. We made novel dual-gated transistors using such a channel and obtained an on-off ratio up to 103 at 200 K. This novel device has a channel region between dual gates, and the polarity of the transistor (n- or p-type) can be electrostatically reversed by simply flipping the bias polarity of one of the dual gates.

S. Ogawa

Papers

Electrostatically-reversible polarity of dual-gated graphene transistors with He ion irradiated channel: Toward reconfigurable CMOS applications

High-precision alignment of electron tomography tilt series using markers formed in helium-ion microscope