Ambipolar 2D Semiconductors and Emerging Device Applications

Abstract

the emergence of nanoscience and technology. Reviewing the history over the past 60 years, new devices based on new materials and new physics have been driving the development of integration circuits (ICs) along the Moore's law. Therefore, it has always been a frontier to explore new materials and novel physical properties that can help to push forward the development of ICs. In addition, currently, the multiple requirements for ICs are becoming increasingly important, such as flexible, transparent, and wearable. Just in this context, graphene, an ultrathin graphitic layer with a honeycomb structure, has aroused a wide attention since its discovery, which was demonstrated as a metallic transistor and proposed for high-frequency electronic circuits. [1] Due to its outstanding electronic, optical and mechanical properties, graphene was highly expected as a material to meet the increasing requirements for high-performance flexible, transparent and wearable electronic and optoelectronic devices. [2-8] The emergence of graphene inspired researchers to look for other 2D materials from van der Waals solids (VDWSs) since the one-atomic-layer graphene was revealed to be thermodynamically stable under ambient conditions. Up to now, 2D atomic crystals have been found in various VDWSs like hexagonal boron nitride (hBN), black phosphorus (BP), metal dichalcogenides (MDCs, like MoS 2 , WSe 2 , MoTe 2 , SnS 2 , and so on), and layered oxide, the basic of which can be seen in hundreds of reviews. [9-16] These graphene-like 2D materials are the thinnest crystals with the thickness usually less than 1 nm. They often exhibit distinctive properties different from conventional bulk materials. For example, bulk MoS 2 is of indirect bandgap but the monolayer is of direct one; [17,18] the bandgap of few layer semiconducting 2D layers often increases with the thickness decreasing, and can be modulated by electric field; [19-23] theoretical calculations indicate that the sunlight absorptivity of MDC monolayers like MoS 2 , MoSe 2 , and WS 2 is up to 5-10% that is 1 order higher than Si and GaAs, enabling them promising for ultrathin optoelectronic devices. [24] Besides, the 2D family covers a wide conductive range from metals, semimetals, semiconductors to insulators, offering a possibility to construct ultrathin devices totally based on 2D crystals. [25-27] And, as the study continues With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, lightemitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrate...

WSe2 N‐Type Negative Capacitance Field‐Effect Transistor with Indium Low Schottky Barrier Contact

“…Layered WSe 2 often exhibits ambipolar behavior, which can be tuned by defining the source/drain metal contact. [32] Indium has a pretty low work function of 4.12 eV, which is very close to the minimum conduction band of few-layer WSe 2 (≈4.0 eV). [30] Previous studies indicate that indium prefers to obtain a small ohmic contact resistance to WSe 2 to form N-type FETs due to the d-orbitals.…”

“…[1][2][3] However, when the conventional silicon-based field-effect transistors (FETs) enter the sub-5 nm process node, the channel length and the gate dielectric thickness are only of nanometers, making the applications. [29][30][31][32] Up to now, WSe 2 based P-type NC-FETs have been successfully obtained through engineering the electrical contact. [33][34][35] These P-type NC-FETs have exhibited pretty low SS down to 14.4 mV dec −1 , [33] and shown prominent power consumption in heterogeneously integrated CMOS circuits (as low as 68 pW).…”

Ambipolar transport compact models for two-dimensional materials based field-effect transistors

“…Tunneling through the forbidden gap (i.e., E falls in the energy gap) is banned, as shown in Eqs. (17) and (18). The coefficients at the interface barrier are evaluated using the semiclassical WKB approximations, shown by Eqs.…”

“…1 Tunable bandgap of the ambipolar 2D material family (adapted from Refs. [16] and [17]). The inset shows the typical transfer curve of ambipolar 2D-FETs.…”

Recent Advances in Two‐Dimensional Heterostructures: From Band Alignment Engineering to Advanced Optoelectronic Applications

“…[ 38 ] Moreover, dangling bond free surfaces of 2D materials also enable the flexible construction of vdW heterostructures through vdW integration, [ 39,40 ] providing promising material platforms for developing new devices with advanced functions. [ 41–65 ] From the point of device application, band alignment between semiconductors is the key that dominates the device performances. [ 66–68 ] For example, both holes and electrons in type‐I heterostructures tend to transfer to the components with narrow bandgap, and the high efficient recombining rate originating from the fine carrier confinement further enables the light‐emitting applications.…”