Ultra-short Channel Research Articles

Thermal transport in graphene field-effect transistors with ultrashort channel length

Thermal management has been widely studied to enhance the reliability of future organic nanoelectronics. Organic Field-Effect Transistors (OFETs) represent a novel technology for future nanomanufacturing of electronic devices. However, enhancing the thermal stability of transistors become a major challenge for next-generation of electronics devices. In this work, we report the thermal transport of graphene FETs (GFETs) with ultrashort channel length. In the first step, we investigate the ability of our model to characterize the heat transport in nanotransistors. To clarify the nature of the phonon-wall collisions along the channel, we have considered the effect of the temperature jump boundary condition in the oxide-graphene interface. In addition, our proposed effective thermal conductivity (ETC) model agrees with experimental results. Furthermore, we have found that graphene FETs are more thermally stable than the classical transistors based on Silicon MOSFETs.

Sub 10 nm Bilayer Bi2O2Se Transistors

AbstractDue to high carrier mobility and excellent air stability, emerging 2D semiconducting Bi2O2Se is attracting much attention as a potential channel candidate for the next‐generation field effect transistor (FETs). Although the fabricated bilayer (BL) and few layers Bi2O2Se FETs exhibit a large current on/off ratio (>106) and a near‐ideal subthreshold swing value (≈65 mV dec−1), the performance limit of ultrashort channel Bi2O2Se FET is obscure. Here the ballistic performance upper limit of the sub 10 nm BL Bi2O2Se metal‐oxide‐semiconductor FETs (MOSFETs) is simulated for the first time by using ab initio quantum transport simulations. The optimized BL Bi2O2Se n‐type MOSFETs can fulfill the high performance device requirements on the on‐state current, delay time, and power dissipation of the International Technology Roadmap for Semiconductors in 2028 until the gate length is scaled down to 5 nm. Therefore, Moore's law can be extended to 5 nm by taking BL Bi2O2Se as the channel.

A PbSe nanocrystal vertical phototransistor with graphene electrode

Recently, low-voltage phototransistors have become promising candidates for optoelectronic applications. In this study, a general strategy for the fabrication of high-performance lead selenide (PbSe) nanocrystal (NCs)-based vertical phototransistor (VPT) using graphene as electrode was presented. Within the vertical geometry, channel length was determined by measuring the thickness of NCs thin film, thus enabling the device with ultrashort channel length (213 nm) without expensive lithography. Moreover, a high current density of 863 A cm−2 was obtained at low voltage. Utilizing the unique tunable Fermi energy (FE) of graphene, the vertical carrier transport could be effectively modulated by the Schottky barrier height between the graphene and PbSe NCs active material. As a result, the device exhibited excellent photoelectric characteristics, including a responsivity of 1.1 × 104 A W−1, an external quantum efficiency of 1.7 × 106%, a detectivity of 1.3 × 1010 Jones, and a temporal response of 7 ms at an illumination irradiance of 36 mW cm−2. Through analysis of energy levels, the carrier transport was modulated by the barrier height at the graphene-PbSe NCs interface, which is attributed to the tunable FE of graphene. Fabrication of VPT with high performance and low energy consumption represents a significant step forward for high-speed opticalswitch applications and future nanoscale complementary circuits.

High performance flexible multilevel optical memory based on a vertical organic field effect transistor with ultrashort channel length

Optical memory based on a vertical organic field effect transistor with ultrashort channel length exhibits excellent device performance with distinct storage levels.

High-Performance All-Solution-Processed Flexible Photodetector Arrays Based on Ultrashort Channel Amorphous Oxide Semiconductor Transistors.

Amorphous oxide semiconductor (AOS) field-effect phototransistors (FEPTs) are promising candidates for emerging photodetectors. Unfortunately, traditional lateral AOS FEPTs suffer from low photosensitivity, slow response time and inadequate mechanical flexibility, which restrict their widespread commercial application. In this work, novel AOS-based vertical field-effect phototransistor (VFEPT) arrays are presented, where the semiconducting layer and source and drain electrodes are deposited by inkjet printing. Benefitted from the unique vertical structure and ultrashort channel length, the exciton dissociation, carrier transfer, and collection efficiency were dramatically enhanced, resulting in excellent photoelectric performance in VFEPT devices, which was better than that of the traditional lateral AOS phototransistors. Moreover, flexible AOS VFEPT arrays were investigated for the first time on polyimide substrates. Due to the unique vertical architecture, the carrier transport was negligibly affected by the strain-induced in-plane cracks of the semiconductor channel layer during the mechanical bending process, which overcame the maximum bending limit of traditional lateral AOS thin-film transistors to ensure a transistor technique that gives notable mechanical robustness against repeated mechanical bending. Hence, this work provided a new pathway in emerging applications for AOS photodetectors with sensitivity, transparency, and flexibility.

Ultrashort intrinsic-like channel FETs with nanodot-type floating gate utilizing biomaterial

We investigated the electrical characteristics of an n–i–n-type FET of a sub-10 nm channel with nanoparticles (NPs). To fabricate the FET, a V-groove was formed in a silicon-on-insulator substrate by anisotropic wet etching. Cage-shaped proteins (ferritin) was used for the formation and placement of NPs. A solution of ferritin containing a metal oxide core was dropped onto the substrate, and spin drying was performed to arrange the NPs at the bottom of the V-groove. NPs served as a floating gate for this device. Threshold voltage (Vth) shift was confirmed as a memory operation in devices with NPs. The Vth shift was clearly observed in different types of NPs. The Vth shift was controlled along the channel length direction by a single nanodot floating gate.

A Colloidal-Quantum-Dot Infrared Photodiode with High Photoconductive Gain.

The photodiode is a prevailing architecture for photodetection with the merits of fast response and low dark current. However, an ideal photodiode is also desired for both high responsivity and high external quantum efficiency (EQE), which may facilitate more applications. Here the photoconducting effect in a photodiode is discussed and an Au-PbS colloidal quantum dot (CQD)-indium tin oxide Schottky junction photodiode is fabricated. The long carrier lifetime and improved carrier mobility in tetrabutylammonium iodide-modified PbS CQDs cooperating with the proper band structure and an ultrashort channel in the diode enable the photodiode with high photoconductive gain, realizing an EQE of ≈400% and a responsivity (R) of 5.15 A W-1 while simultaneously achieving a response time of 110 µs and a specific detectivity of 1.96 × 1010 Jones under 1550 nm illumination. In addition, this CQD-based photodiode is stable, low cost, and compatible with complementary metal oxide semiconductor technology. All of these promise this device great potential in applications.

Impact Ionization and Interface Trap Generation in 28-nm MOSFETs at Cryogenic Temperatures

The characterization of impact-ionization (II) and stress-induced damage in 28-nm bulk n-channel MOSFETs is used to identify the mechanisms for carrier-induced interface trap generation (ITG) from 300 K to 77 K. The energy-driven paradigm of hot carrier effects is used to study carrier-induced ITG and its temperature dependence, and to analyze the influence of electron-electron scattering (EES) on carrier-induced degradation of ultra-short channel devices. The analysis clearly illustrates the significant role of EES in the hot (i.e., high-energy) carriers single particle mechanism of ITG, as well as its contribution to the cold (i.e., low-energy) carriers multiple particle mechanism of ITG. This is evidenced through extractions of ITG lifetime as a function of temperature and stress current from measurements of 28 nm n-channel devices. We apply a simple model to discuss the impact of EES on the experimentally observed trends for II and ITG rates.

High Performance Flexible Organic Phototransistors with Ultrashort Channel Length

Organic phototransistors with high responsivity and sensitivity to light irradiance have great potential applications in environmental monitoring, space exploration, security, image sensors and healthcare systems. In this manuscript, a novel polymer bulk heterojunction field effect phototransistor with ultrashort channel length (tens of nanometers) and ultrahigh sensitivity to visible light was proposed. Due to the nanoscale channel and bulk heterojunction structure, a high-performance phototransistor with high responsivity of 750 A/W, photosensitivity of 1.0 × 106, and detectivity as high as 4.54 × 1015 Jones was demonstrated under 720 nm light illumination with 0.1 mW/cm2 intensity, which was even better that those lateral organic phototransistors. Moreover, organic phototransistors with ultrashort channel length were investigated for the first time on a flexible substrate, which exhibited outstanding mechanical flexibility due to their unique designs. Further investigation of the correlation between th...

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

Force and light tuning vertical tunneling current in the atomic layered MoS2

In this work, the vertical electrical transport behavior of bilayer MoS2 under the coupling of force and light was explored by the use of conductive atomic force microscopy. We found that the current–voltage behavior across the tip–MoS2–Pt junction is a tunneling current that can be well fitted by a Simmons approximation. The transport behavior is direct tunneling at low bias and Fowler–Nordheim tunneling at high bias, and the transition voltage and tunnel barrier height are extracted. The effect of force and light on the effective band gap of the junction is investigated. Furthermore, the source–drain current drops surprisingly when we continually increase the force, and the dropping point is altered by the provided light. This mechanism is responsible for the tuning of tunneling barrier height and width by force and light. These results provide a new way to design devices that take advantage of ultrathin two-dimensional materials. Ultrashort channel length electronic components that possess tunneling current are important for establishing high-efficiency electronic and optoelectronic systems.

46‐2: Invited Paper: Ultimate Resolution Active Matrix Display with Oxide TFT Backplanes for Electronic Holographic Display

Oxide TFT backplane with 1μm channel length was developed for the application to electronic holographic display. Spatial light modulator (SLM) panel with 3μm pixel pitch was designed and successfully fabricated. The resolution of SLM panel was 16K by 3.2K and the diagonal length of active area was 2 inch. Fabricated SLM panel was ECB mode reflective type LCD panel, which could modulate the phase of incident light more than 2π. 3D image was successfully reconstructed with fabricated SLM panel. Vertical channel oxide TFTs are proposed for the switching device of 1μm pixel pitch. Oxide TFTs with ultra short channel length can be an enabler of the active matrix display with ultimate resolution.

The Possibility of mW/cm2-Class On-Chip Power Generation Using Ultrasmall Si Nanowire-Based Thermoelectric Generators

A simple structure of planar silicon nanowire (SiNW)-based thermoelectric (TE) generators (TEGs) is presented in this paper, which has the ability to sustain temperature difference along SiNW under ultrashort channel length for achieving mW/cm2-class power output from environmental heat energy. The TE performance of the proposed SiNW-based TEGs was evaluated by finite-element simulation and analytic modeling. The channel length, pad length, and the thickness of the SiO2 layer were varied in the model while keeping a series of constant proportions toward finding the optimal SiNW-based TEG structure for high power generation. Based on the analysis, we demonstrated that decreasing the dimension of SiNW-based TEG is beneficial to improving the total TE power density. A very high power density of 4.2 mW/cm2 is possible to be achieved at SiNW length of $0.1~\mu \text{m}$ and pad length of $0.1~\mu \text{m}$ under a temperature difference of 5 K across the hot and cold regions. The miniaturized SiNW-based TEG has a great potential to obtain the output power density which is 100–1000 times higher than conventional planar TEGs with air cavity but has a simple structure and can easily be fabricated.

1T Non-Volatile Memory Design Using Sub-10nm Ferroelectric FETs

In this letter, we propose one-transistor ferroelectric NOR type (Fe-NOR) non-volatile memory based on HfZrOx ferroelectric FETs (FeFETs). The enhanced drain-channel coupling in ultra-short channel FeFETs is utilized to dynamically modulate the memory window of storage cells, thereby resulting in simple erase-, program-, and read-operations. The simulation analysis predicts sub-1V program/erase voltages in the proposed Fe-NOR memory array and, therefore, presents a significantly lower power alternative to conventional FeRAM and NOR flash memories.

Ambipolar Graphene-Quantum Dot Hybrid Vertical Photodetector with a Graphene Electrode.

A strategy to fabricate an ambipolar near-infrared vertical photodetector (VPD) by sandwiching a photoactive material as a channel film between the bottom graphene and top metal electrodes was developed. The channel length in the vertical architecture was determined by the channel layer thickness, which can provide an ultrashort channel length without the need for a high-precision manufacturing process. The performance of VPDs with two types of semiconductor layers, a graphene-PbS quantum dot hybrid (GQDH) and PbS quantum dots (QDs), was measured. The GQDH VPD showed better photoelectric properties than the QD VPD because of the high mobility of graphene doped in the channel. The GQDH VPD exhibited excellent photoresponse properties with a responsivity of 1.6 × 104 A/W in the p-type regime and a fast response speed with a rise time of 8 ms. The simple manufacture and the promising photoresponse of the GQDH VPDs reveal that an easy and effective way to fabricate high-performance ambipolar photodetectors was developed.

High Performance Flexible Nonvolatile Memory Based on Vertical Organic Thin Film Transistor

AbstractFlexible floating‐gate organic transistor memory (FGOTM) is a potential candidate for emerging memory technologies. Unfortunately, conventional planar FGOTM suffers from weak driving ability and insufficient mechanical flexibility, which limits its commercial application. In this work, a novel flexible vertical FGOTM (VFGOTM) is reported. Benefitting from new vertical architecture, VFGOTM provides ultrashort channel length to afford an extremely high current density. Meanwhile, VFGOTM devices exhibit excellent memory performance and outstanding retention property. The memory properties of VFGOTM devices are comparable or even better than traditional planar FGOTM and much better than the reported organic nonvolatile memory with vertical transistor structures. More importantly, organic nonvolatile memory with vertical transistor structures is investigated for the first time on a flexible substrate. The results show that VFGOTM architecture allows vertical current flow across the channel layer to effectively eliminate the effect of mechanical bending during current transport, which significantly improves the mechanical stability of the flexible VFGOTM. Hence, with a combination of excellent driving ability, memory performance, and mechanical stability, VFGOTM devices meet the practical requirements for high performance memory applications, which have great potential for the application in a wide range of flexible and wearable electronics.

Single Electron Transistor with Single Aromatic Ring Molecule Covalently Connected to Graphene Nanogaps.

We report a robust approach to fabricate single-molecule transistors with covalent electrode-molecule-electrode chemical bonds, ultrashort (∼1 nm) molecular channels, and high coupling yield. We obtain nanometer-scale gaps from feedback-controlled electroburning of graphene constrictions and bridge these gaps with molecules using reaction chemistry on the oxidized graphene edges. Using these nanogaps, we are able to optimize the coupling chemistry to achieve high reconnection yield with ultrashort covalent single-molecule bridges. The length of the molecule is found to influence the fraction of covalently reconnected nanogaps. Finally, we discuss the tunneling nature of the covalent contacts using gate-dependent transport measurements, where we observe single electron transport via large energy Coulomb blockade even at room temperature. This study charts a clear path toward the assembling of ultraminiaturized electronics, sensors, and switches.

Graphene-Contacted Ultrashort Channel Monolayer MoS2 Transistors.

2D semiconductors are promising channel materials for field-effect transistors (FETs) with potentially strong immunity to short-channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS2 . FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene-contacted ultrashort channel MoS2 FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material-based devices for ultrascaled electronics.

Physical Insights Into the Nature of Gate-Induced Drain Leakage in Ultrashort Channel Nanowire FETs

In this paper, we demonstrate that the lateral band-to-band tunneling component of gate-induced drain leakage (GIDL) leads to the formation of a parasitic bipolar junction transistor (BJT) in the OFF-state (gate voltage = 0.0 V) of nanowire FETs (NW FETs). We discuss in detail the difference in the nature of GIDL, i.e., drain current dependence on the negative gate voltage ( ${V}_{\text {GS}} \le 0$ V) for different NW FET configurations. Furthermore, we show that the parasitic BJT action is significant in NW junctionless accumulation mode FET (JAMFET) and NW MOSFET in the OFF-state and diminishes as the gate voltage becomes negative. Using calibrated 3-D simulations, we investigate the impact of scaling on the NW FETs and show that the enhanced band gap due to the quantum confinement effect facilitates the scaling of the NW FETs to the sub-5-nm regime. In addition, we propose the use of a lightly doped drain extension to increase the ON-state to OFF-state current ratio ( ${I}_{ {{\scriptscriptstyle {ON}}}}/{I}_{ {\scriptscriptstyle {OFF}}})$ of NW JAMFET and NW MOSFET.

In situ control of synchronous germanide/silicide reactions with Ge/Si core/shell nanowires to monitor formation and strain evolution in abrupt 2.7 nm channel length

The metal-semiconductor interface in self-aligned contact formation can determine the overall performance of nanoscale devices. This interfacial morphology is predicted and well researched in homogenous semiconductor nanowires (NWs) but was not pursued in heterostructured core/shell nanowires. We found here that the solid-state reactions between Ni and Ge/Si core/shell nanowires resulted in a protruded and a leading NiSiy segment into the channel. A single Ni2Ge/NiSiy to Ge/Si core/shell interface was achieved by the selective shell removal near the Ni source/drain contact areas. Using in situ transmission electron microscopy, we measured the growth rate and anisotropic strain evolution in ultra-short channels. We found elevated compressive strains near the interface between the compound contact and the NW and relatively lower strains near the center of the channel which increased exponentially below the 10 nm channel length to exceed 10% strain at ∼3 nm lengths. These compressive strains are expected to result in a non-homogeneous energy band structure in Ge/Si core/shell NWs below 10 nm and potentially benefit their transistor performance.