A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

All-2D-Materials Subthreshold-Free Field-Effect Transistor with Near-Ideal Switching Slope

The Boltzmann Tyranny, set by thermionic statistics, dictates the lower limit of switching slope (SS) of a MOSFET to be 60 mV/dec, the fundamental barrier for low-dissipative electronics. The large SS leads to nonscalable voltage, significant leakage, and power consumption, particularly at short channels, making transistor scaling an intimidating challenge. In recent decades, an array of steep-slope transistors has been proposed; none is close to an ideal switch with ultimately abrupt switching (SS ∼ 0 mV/dec) between the binary logic states. We demonstrated an all-2D-materials van-der-Waals-heterostructure (vdW)-based FET that exhibits ultrasteep switching (0.33 mV/dec), a large on/off current ratio (∼107), and an ultralow off current (∼0.1 pA). The "Subthreshold-Free" operation achieved by the collective behavior of functional materials enables FET switching directly from the OFF-state to the ON-state with entirely eliminated subthreshold region, behaving as the ideal logic switch. Two-inch wafer-scale device fabrication is demonstrated. Boosted by device innovation and emerging materials, the research presents an advancement in achieving the "beyond-Boltzmann" transistors, overcoming one of the CMOS electronics' most infamous technology barriers that have plagued the research community for decades.

Highly Tunable Negative Differential Resistance Device Based on Insulator-to-Metal Phase Transition of Vanadium Dioxide

Negative differential resistance (NDR) based on the band-to-band tunneling (BTBT) mechanism has recently shown great potential in improving the performance of various electronic devices. However, the applicability of conventional BTBT-based NDR devices is restricted by their insufficient performance due to the limitations of the NDR mechanism. In this study, we develop an insulator-to-metal phase transition (IMT)-based NDR device that exploits the abrupt resistive switching of vanadium dioxide (VO₂) to achieve a high peak-to-valley current ratio (PVCR) and peak current density (J_peak) as well as controllable peak and valley voltages (V_peak/valley). When a phase transition is induced in VO₂, the effective voltage bias on the two-dimensional channel is decreased by the reduction in the VO₂ resistance. Accordingly, the effective voltage adjustment induced by the IMT results in an abrupt NDR. This NDR mechanism based on the abrupt IMT results in a maximum PVCR of 71.1 through its gate voltage and VO₂ threshold voltage tunability characteristics. Moreover, V_peak/valley is easily modulated by controlling the length of VO₂. In addition, a maximum J_peak of 1.6 × 10⁶ A/m² is achieved through light-tunable characteristics. The proposed IMT-based NDR device is expected to contribute to the development of various NDR devices for next-generation electronics.

An Ultra-steep Slope Two-dimensional Strain Effect Transistor

We introduce a high-performance and ultra-steep slope switch, referred to as strain effect transistor (SET), with a subthreshold swing < 0.68 mV/decade at room temperature for 7 orders of magnitude change in the source-to-drain current based on atomically thin 1T'-MoTe₂ as the channel material, piezoelectric lead zirconate titanate (PZT) as the gate dielectric, and nickel (Ni) as the source/drain contact metal. We exploit gate-voltage induced strain transduction in PZT leading to abrupt and reversible cracking of the metal contacts to achieve the abrupt switching. The SET also exhibits a low OFF-state current < 1 pA/μm, a high ON-state current > 1.8 mA/μm at a supply voltage of 1 V, a large current ON/OFF ratio > 1 × 10⁹, and a high transconductance of > 100 μS/μm. The switching delay for the SET was found to be < 5 μs, and no device failure was observed even after 1 million (1 × 10⁶) switching cycles.

Steep-slope transistors enabled with 2D quantum coupling stacks

As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS₂transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS₂devices had steep SS (27.2 mV/decade), validating our experimental approach (49.2 mV/decade). Furthermore, the simulations demonstrated very steep SS (8.6 mV/decade) in WTe₂/MoS₂devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance.

Ultra-Steep-Slope High-Gain MoS2 Transistors with Atomic Threshold-Switching Gate

The fundamental Boltzmann limitation dictates the ultimate limit of subthreshold swing (SS) to be 60 mV dec-1 , which prevents the continued scaling of supply voltage. With atomically thin body, 2D semiconductors provide new possibilities for advanced low-power electronics. Herein, ultra-steep-slope MoS2 resistive-gate field-effect transistors (RG-FETs) by integrating atomic-scale-resistive filamentary with conventional MoS2 transistors, demonstrating an ultra-low SS below 1 mV dec-1 at room temperature are reported. The abrupt resistance transition of the nanoscale-resistive filamentary ensures dramatic change in gate potential, and switches the device on and off, leading to ultra-steep SS. Simultaneously, RG-FETs demonstrate a high on/off ratio of 2.76 × 107 with superior reproducibility and reliability. With the ultra-steep SS, the RG-FETs can be readily employed to construct logic inverter with an ultra-high gain ≈2000, indicating exciting potential for future low-power electronics and monolithic integration.

Ferroelectric Negative-Capacitance-Assisted Phase-Transition Field-Effect Transistor

An enormous study is being carried out in the field of emerging steep slope devices, specifically on negative-capacitance-based and phase transition-based devices. This article investigates the action of ferroelectric (FE) and phase transition material (PTM) on a hybrid device, negative-capacitance-assisted phase transition FinFET (NC-PT-FinFET). We encounter several unique phenomena resulting from this unified action and provide valid arguments based on these observations. A significant enhancement in the differential gain and transconductance, a unique variation in the effect of PTM on drain-channel coupling, tunability of hysteresis across PTM by FE thickness( [Formula: see text]), and ultralow subthreshold slope (SS) by lowering both of its factors are some of the major outcomes of the NC-PT-FinFET. Focus is built on comprehending the individual role of FE and PTM in the intriguing features observed in every device performance parameter with the help of mathematical expressions and physical interpretations. Various tunable parameters present in this hybrid device widen its applicability in digital and memory applications.

Investigation on Threshold Voltage Adjustment of Threshold Switching Devices with HfO2/Al2O3 Superlattice on Transparent ITO/Glass Substrate

Threshold voltage adjustment in threshold switching (TS) devices with HfO₂/Al₂O₃ superlattice (by means of changing the cycle ratio of HfO₂ to Al₂O₃ in atomic layer deposition) is investigated to implement a transparent cross-point array. TS devices with different cycle ratios (i.e., 3:1, 3:2, and 3:3) were fabricated and studied. The threshold voltage of the devices was increased from 0.9 V to 3.2 V, as the relative contents of Al₂O₃ layer in the superlattice were increased. At the same time, it is demonstrated that the off-resistance values of the devices were enhanced from 2.6 10⁹ to 6 10¹⁰ as the atomic layer deposition (ALD) cycle ratio of HfO₂ to Al₂O₃ layer was adjusted from 3:1 to 3:3. However, the hold voltage and the on-current values were almost identical for the three devices. These results can be understood using the larger barrier height of Al₂O₃ layer than that of HfO₂ layer.

A new approach to the fabrication of VO2 nanoswitches with ultra-low energy consumption

A new approach for the formation of free-standing vertical resistive nanoswitches based on VO2 nanocrystals (NCs) with embedded conductive nanosharp Si tips is demonstrated in the present article. This approach consists in the chemical vapor deposition synthesis of VO2 NCs on the apices of sharp conductive nanotips formed on a Si substrate by the standard methods of planar silicon technology. The amplification of the electric field and current density at the tip apex inside a high-quality VO2 NC leads to a record-breaking reduction of switching voltage (by a factor of 20-70) in comparison with conventional geometry devices with planar contacts. Our pulse measurements showed that the extremely low energy equal to 4.2 fJ was consumed for the switching in such NCs, and the total number of switching cycles in one NC without degradation exceeded 1011. The proposed approach can be extended to the formation of large arrays of such nanoswitches. We showed that periodic arrays of individual VO2 NCs were selectively synthesized on sharp Si tips. The nanosizes of the switches, ultra-low power consumption for switching and the possibility of forming dense arrays of such objects make the fabricated nanoswitches promising devices for future neuromorphic systems.

3D Smith charts scattering parameters frequency-dependent orientation analysis and complex-scalar multi-parameter characterization applied to Peano reconfigurable vanadium dioxide inductors

Recently, the field of Metal-Insulator-Transition (MIT) materials has emerged as an unconventional solution for novel energy efficient electronic functions, such as steep slope subthermionic switches, neuromorphic hardware, reconfigurable radiofrequency functions, new types of sensors, terahertz and optoelectronic devices. Employing radiofrequency (RF) electronic circuits with a MIT material like vanadium Dioxide, VO₂, requires appropriate characterization tools and fabrication processes. In this work, we develop and use 3D Smith charts for devices and circuits having complex frequency dependences, like the ones resulting using MIT materials. The novel foundation of a 3D Smith chart involves here the geometrical fundamental notions of oriented curvature and variable homothety in order to clarify first theoretical inconsistencies in Foster and Non Foster circuits, where the driving point impedances exhibit mixed clockwise and counter-clockwise frequency dependent (oriented) paths on the Smith chart as frequency increases. We show here the unique visualization capability of a 3D Smith chart, which allows to quantify orientation over variable frequency. The new 3D Smith chart is applied as a joint complex-scalar 3D multi-parameter modelling and characterization environment for reconfigurable RF design exploiting Metal-Insulator-Transition (MIT) materials. We report fabricated inductors with record quality factors using VO₂ phase transition to program multiple tuning states, operating in the range 4 GHz to 10 GHz.

Localized Triggering of the Insulator-Metal Transition in VO2 Using a Single Carbon Nanotube

Vanadium dioxide (VO₂) has been widely studied for its rich physics and potential applications, undergoing a prominent insulator-metal transition (IMT) near room temperature. The transition mechanism remains highly debated, and little is known about the IMT at nanoscale dimensions. To shed light on this problem, here we use ∼1 nm-wide carbon nanotube (CNT) heaters to trigger the IMT in VO₂. Single metallic CNTs switch the adjacent VO₂ at less than half the voltage and power required by control devices without a CNT, with switching power as low as ∼85 μW at 300 nm device lengths. We also obtain potential and temperature maps of devices during operation using Kelvin probe microscopy and scanning thermal microscopy. Comparing these with three-dimensional electrothermal simulations, we find that the local heating of the VO₂ by the CNT plays a key role in the IMT. These results demonstrate the ability to trigger IMT in VO₂ using nanoscale heaters and highlight the significance of thermal engineering to improve device behavior.

Enhanced Phase Transition Properties of VO2 Thin Films on 6H-SiC (0001) Substrate Prepared by Pulsed Laser Deposition

For growing high quality epitaxial VO₂ thin films, the substrate with suitable lattice parameters is very important if considering the lattice matching. In addition, the thermal conductivity between the substrate and epitaxial film should be also considered. Interestingly, the c-plane of hexagonal 6H-SiC with high thermal conductivity has a similar lattice structure to the VO₂ (010), which enables epitaxial growth of high quality VO₂ films on 6H-SiC substrates. In the current study, we deposited VO₂ thin films directly on 6H-SiC (0001) single-crystal substrates by pulsed laser deposition (PLD) and systematically investigated the crystal structures and surface morphologies of the films as the function of growth temperature and film thickness. With optimized conditions, the obtained epitaxial VO₂ film showed pure monoclinic phase structure and excellent phase transition properties. Across the phase transition from monoclinic structure (M1) to tetragonal rutile structure (R), the VO₂/6H-SiC (0001) film demonstrated a sharp resistance change up to five orders of magnitude and a narrow hysteresis width of only 3.3 °C.

Nano-Particle VO2 Insulator-Metal Transition Field-Effect Switch with 42 mV/decade Sub-Threshold Slope

The possibility of controlling the insulator-to-metal transition (IMT) in nano-particle VO2 (NP-VO2) using the electric field effect in a metal-oxide-VO2 field-effect transistor (MOVFET) at room temperature was investigated for the first time. The IMT induced by current in NP-VO2 is a function of nano-particle size and was studied first using the conducting atomic force microscope (cAFM) current-voltage (I-V) measurements. NP-VO2 switching threshold voltage (VT), leakage current (Ileakage), and the sub-threshold slope of their conductivity (Sc) were all determined. The cAFM data had a large scatter. However, VT increased as a function of particle height (h) approximately as VT(V) = 0.034 h, while Ileakage decreased as a function of h approximately as Ileakage (A) = 3.4 × 10−8e−h/9.1. Thus, an asymptotic leakage current of 34 nA at zero particle size and a tunneling (carrier) decay constant of ~9.1 nm were determined. Sc increased as a function of h approximately as Sc (mV/decade) = 2.1 × 10−3eh/6 and was around 0.6 mV/decade at h~34 nm. MOVFETs composed of Pt drain, source and gate electrodes, HfO2 gate oxide, and NP-VO2 channels were then fabricated and showed gate voltage dependent drain-source switching voltage and current (IDS). The subthreshold slope (St) of drain-source current (IDS) varied from 42 mV/decade at VG = −5 V to 54 mV/decade at VG = +5 V.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor

Vanadium dioxide (VO₂) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO₂ is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO₂ microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe₂) semiconductor by van der Waals stacking. We fabricated the WSe₂ transistor using the VO₂ wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe₂ transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO₂. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO₂ is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO₂-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.

Optical switching in hybrid VO2/Si waveguides thermally triggered by lateral microheaters

The performance of optical devices relying in vanadium dioxide (VO₂) technology compatible with the silicon platform depends on the polarization of light and VO₂ properties. In this work, optical switching in hybrid VO₂/Si waveguides thermally triggered by lateral microheaters is achieved with insertion losses below 1 dB and extinction ratios above 20 dB in a broad bandwidth larger than 30 nm. The optical switching response has been optimized for TE and TM polarizations by using a homogeneous and a granular VO₂ layer, respectively, with a small impact on the electrical power consumption. The stability and reversibility between switching states showing the possibility of bistable performance is also demonstrated.

Steep switching devices for low power applications: negative differential capacitance/resistance field effect transistors

Simply including either single ferroelectric oxide layer or threshold selector, we can make conventional field effect transistor to have super steep switching characteristic, i.e., sub-60-mV/decade of subthreshold slope. One of the representative is negative capacitance FET (NCFET), in which a ferroelectric layer is added within its gate stack. The other is phase FET (i.e., negative resistance FET), in which a threshold selector is added to an electrode (e.g., source or drain) of conventional field effect transistor. Although the concept of the aforementioned two devices was presented more or less recently, numerous studies have been published. In this review paper, by reviewing the published studies over the last decade, we shall de-brief and discuss the history and the future perspectives of NCFET/phase FET, respectively. The background, experimental investigation, and future direction for developing the aforementioned two representative steep switching devices (i.e., NCFET and phase FET/negative resistance FET) are to be discussed in detail.