Carrier Injection Mechanism Research Articles

Ultimate-scaled one-dimensional transistors: Surpassing the subthreshold swing limit

The continuous miniaturization of field-effect transistors (FETs) has propelled microprocessors to unprecedented levels of integration. However, further scaling encounters a critical trade-off between integration density and power efficiency. To transcend this limitation, a new class of steep-slope transistors has emerged, capable of surpassing the subthreshold swing (SS) limit but imposing stringent requirements on gate electrostatics. In this Perspective, we propose atomically-thin, one-dimensional (1D) materials as promising candidates for steep-slope switches. These materials offer distinct advantages in electrostatic integrity, crucial for overcoming traditional scaling bottlenecks. We delve into the underlying operational mechanisms to elucidate design methodologies and optimization strategies for 1D steep-slope transistors. We anticipate that by engineering the density of states distribution and carrier injection mechanisms, these transistors will achieve unparalleled performance and energy efficiency, representing a new frontier in device scaling. Our insights seek to facilitate future research and development in this transformative area of semiconductor technology.

Comparative analysis of heavy ions and alpha particles impact on gate-all-around TFETs and gate-all-around MOSFETs

The single-event-transient (SET) effect due to heavy ions and alpha particles irradiation on n-type gate-all-around tunnel field effect transistor (GAA TFET) and n-type gate-all-around MOSFET (GAA MOSFET) has been carried out. Due to differences in carrier injection mechanisms, the generated electron-hole pairs (EHPs) due to high-energy particles (HEPs) act differently on the device. In addition, the impact of HEPs (i.e., heavy ions and alpha particles) on several locations along the channel are analyzed followed by the analysis of different energies of heavy ions and alpha particles irradiation on the device. Further, the impact of varying striking angles of HEPs on the device is also analyzed to get a close match as practically exposed device characteristic. Finally, the bipolar gain of the device has been analyzed which shows GAA TFET device has more immunity toward heavy ions strike and weak immunity toward alpha particle strikes when compared to the GAA MOSFET counterpart.

Effect of temperature modulation on the performance of top contact bottom gate organic thin film transistor

This research article investigates the influence of temperature variations on the performance parameters of Top Contact Bottom Gate (TCBG) Organic Thin Film Transistors (OTFTs). Employing the Arrhenius equation and Schottky emission model, the study explores temperature-dependent charge carrier mobility and injection mechanisms. Analyzing a wide range of parameters, the research unveils a complex interplay between temperature and OTFT behavior. The linear and saturation mobilities exhibit a decline due to enhanced carrier-phonon interactions and lattice vibrations. Transconductance and ION/IOFF ratio display a concomitant decrease at higher temperatures, indicative of the heightened susceptibility of OTFT performance to temperature fluctuations. The depreciation in subthreshold slope and threshold voltage with rising temperatures underscores the influence of temperature on trap states. The research emphasizes the importance of considering temperature’s impact during OTFT design for diverse applications, enhancing the reliability and functionality of organic electronic devices.

Improved Electrical Characteristics of Field Effect Transistors with GeSeTe-Based Ovonic Threshold Switching Devices.

Hyper-field effect transistors (hyper-FETs) are crucial in the development of low-power logic devices. With the increasing significance of power consumption and energy efficiency, conventional logic devices can no longer achieve the required performance and low-power operation. Next-generation logic devices are designed based on complementary metal-oxide-semiconductor circuits, and the subthreshold swing of existing metal-oxide semiconductor field effect transistors (MOSFETs) cannot be reduced below 60 mV/dec at room temperature owing to the thermionic carrier injection mechanism in the source region. Therefore, new devices must be developed to overcome these limitations. In this study, we present a novel threshold switch (TS) material, which can be applied to logic devices by employing ovonic threshold switch (OTS) materials, failure control of insulator-metal transition materials, and structural optimization. The proposed TS material is connected to a FET device to evaluate its performance. The results demonstrate that commercial transistors connected in series with GeSeTe-based OTS devices exhibit significantly lower subthreshold swing values, high on/off current ratios, and high durability of up to 108.

Impact of hole trap-detrap mechanism on X-ray irradiation induced threshold voltage shift of radiation-hardened GAA TFET device

We demonstrate the X-ray irradiation effect on a radiation-hardened gate-all-around tunnel FET (GAA TFET) device. The radiation-hardened device has a high and fixed oxide electric field, which is responsible to attain equilibrium state of the device. Once the device reaches equilibrium, the threshold voltage starts to saturate for any variation in the gate bias. Therefore, the fixed electric field in the oxide region is reduced with the application of negative bias to the gate, which varies the equilibrium trapped charge concentration in the oxide. The negative gate bias causes electrons of oxide and holes from the bias terminal to recombine, which ultimately reduces the electric field in the oxide region. This variation in charge concentration results in a new threshold voltage when the device has been re-irradiated. Further, the unique carrier injection mechanism of the TFET device plays a significant role in trapping and detrapping mechanisms along the channel. The report consists of positive charge trapping and detrapping of charge carriers along the channel of the GAA-TFET device. In addition, the report also deals with the impact of applying excessive gate bias to the device and its impact on electric fields. For the X-ray irradiation analysis, several figures of merit are taken into consideration before coming to the conclusion of the report, such as geminate yield, trapped charges in the oxide region, and time-varying positive charge build-up in the oxide. The device under consideration has also been irradiated with several radiation doses and time durations to verify its robustness.

Reducing Contact Resistance in Organic Field-Effect Transistors: A Comprehensive Comparison between 2D and Microrod Single Crystals.

A comprehensive comparison of organic single crystals based on a single material but with different dimensions provides a unique approach to probe their carrier injection mechanism. In this report, both two-dimensional (2D) and microrod single crystals with the same crystalline structure of an identical thiopyran derivative, 7,14-dioctylnaphtho[2,1-f:6,5-f']bis(cyclopentane[b]thiopyran) (C8-SS), are grown on a glycerol surface with the space-confined method. Organic field-effect transistors (OFETs) based on the 2D C8-SS single crystal exhibit superior performance compared with those based on the microrod single crystal, particularly in their contact resistance (RC). It is demonstrated that the resistance of the crystal bulk in the contact region plays a key role in RC of the OFETs. Thus, among the 30 devices tested, the microrod OFETs typically appear contact-limited, whereas the 2D OFETs possess significantly reduced RC arising from the tiny thickness of the 2D single crystal. The 2D OFETs show high operational stability and high channel mobility up to 5.7 cm2/V·s. The elucidation of the contact behavior highlights the merits and great potential of 2D molecular single crystals in organic electronics.

Band-to-band tunneling switches based on two-dimensional van der Waals heterojunctions

Quantum mechanical band-to-band tunneling is a type of carrier injection mechanism that is responsible for the electronic transport in devices like tunnel field effect transistors (TFETs), which hold great promise in reducing the subthreshold swing below the Boltzmann limit. This allows scaling down the operating voltage and the off-state leakage current at the same time, and thus reducing the power consumption of metal oxide semiconductor transistors. Conventional group IV or compound semiconductor materials suffer from interface and bulk traps, which hinder the device performance because of the increased trap-induced parasitics. Alternatives like two-dimensional materials (2DMs) are beneficial for realizing such devices due to their ultra-thin body and atomically sharp interfaces with van der Waals interactions, which significantly reduce the trap density, compared to their bulk counterparts, and hold the promise to finally achieve the desired low-voltage operation. In this review, we summarize the recent progress on such devices, with a major focus on heterojunctions made of different 2DMs. We review different types of emerging device concepts, architectures, and the tunneling mechanisms involved by analytically studying various simulations and experimental devices. We present our detailed perspective on the current developments, major roadblocks, and key strategies for further improvements of the TFET technology based on 2D heterojunctions to match industry requirements. The main goal of this paper is to introduce the reader to the concept of tunneling especially in van der Waals devices and provide an overview of the recent progress and challenges in the field.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films.

Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Hot Electron-Driven Photocatalysis Using Sub-5 nm Gap Plasmonic Nanofinger Arrays.

Semiconductor photocatalysis has received increasing attention because of its potential to address problems related to the energy crisis and environmental issues. However, conventional semiconductor photocatalysts, such as TiO2 and ZnO, can only be activated by ultraviolet light due to their wide band gap. To extend the light absorption into the visible range, the localized surface plasmon resonance (LSPR) effect of noble metal nanoparticles (NPs) has been widely used. Noble metal NPs can couple incident visible light energy to strong LSPR, and the nonradiative decay of LSPR generates nonthermal hot carriers that can be injected into adjacent semiconductor material to enhance its photocatalytic activity. Here we demonstrate that nanoimprint-defined gap plasmonic nanofinger arrays can function as visible light-driven plasmonic photocatalysts. The sub-5 nm gaps between pairs of collapsed nanofingers can support ultra-strong plasmon resonance and thus boost the population of hot carriers. The semiconductor material is exactly placed at the hot spots, providing an efficient pathway for hot carrier injection from plasmonic metal to catalytic materials. This nanostructure thus exhibits high plasmon-enhanced photocatalytic activity under visible light. The hot carrier injection mechanism of this platform was systematically investigated. The plasmonic enhancement factor was calculated using the finite-difference time-domain (FDTD) method and was consistent with the measured improvement of the photocatalytic activity. This platform, benefiting from the precise controllable geometry, provides a deeper understanding of the mechanism of plasmonic photocatalysis.

Unexpected role of hole and electron blocking interlayers controlling charge carrier injection and transport in TADF based blue organic light-emitting diodes

There has been increasing interest in blue organic light-emitting diodes based on thermally activated delayed fluorescence (TADF). The construction of a fully optimized device architecture is crucial in accordance with developing high-performance materials because highly efficient electroluminescence cannot be realized without balancing both carrier injection and transport with decreasing several exciton loss processes. Thus, the detailed mechanism of carrier injection, transport, and recombination in emitting layers has to be clarified. In this study, various device architectures for a recently emerged blue TADF molecular system based on multiple donors and acceptors were systematically investigated, especially by focusing on the interlayers. This work also aims to offer guidelines for improving device stabilities. Our findings clarify the role of each layer, providing in-depth insight into device design and the selection of proper materials for each constituted layer.

Nanosheet-Compatible Complementary-Field Effect Transistor Logic Non-Volatile Memory Device

This study proposes a logic non-volatile memory (NVM) device based on the state-of-the-art gate-all-around (GAA) nanosheet process. By adopting a complementary FET structure, we not only reduce the layout footprint area but also demonstrate the compatibility with sub-3nm nanosheet CMOS technology. We simulated the device in TCAD for program and erase operations, considering both Fowler Nordheim tunneling and hot carrier injection mechanisms. The memory window is optimized by adjusting the device structure, such as tunneling and blocking oxide thicknesses, top- and bottom-channel width, and the floating gate length, resulting in a significant enhancement in the memory window. Finally, NAND/NOR type CFET logic NVM are demonstrated to reveal the possible applications of gate-all-around stacked junctionless nanosheet transistor towards embedded flash.

Strong Fermi-Level Pinning in GeS-Metal Nanocontacts.

Germanium sulfide (GeS) is a layered monochalcogenide semiconductor with a band gap of about 1.6 eV. To verify the suitability of GeS for field-effect-based device applications, a detailed understanding of the electronic transport mechanisms of GeS–metal junctions is required. In this work, we have used conductive atomic force microscopy (c-AFM) to study charge carrier injection in metal–GeS nanocontacts. Using contact current–voltage spectroscopy, we identified three dominant charge carrier injection mechanisms: thermionic emission, direct tunneling, and Fowler–Nordheim tunneling. In the forward-bias regime, thermionic emission is the dominating current injection mechanism, whereas in the reverse-bias regime, the current injection mechanism is quantum mechanical tunneling. Using tips of different materials (platinum, n-type-doped silicon, and highly doped p-type diamond), we found that the Schottky barrier is almost independent of the work function of the metallic tip, which is indicative of a strong Fermi-level pinning. This strong Fermi-level pinning is caused by charged defects and impurities.

Cold Carrier Injection Mechanism for Gate Oxide Integrity of High Voltage NDMOS

This article studies a reliability mechanism for field-plate-assisted reduced surface field (RESURF) effect 225-V NDMOS devices based on anode hole oxide injection for carriers that are in thermal equilibrium with the surrounding lattice. The injection mechanism is facilitated by a unique combination of layout and application biases that result in electric field vectors pulling low-energy minority hole carriers into oxide traps that overlap the drain potential. The effect of this positive charge trapping along the field oxide is to inhibit the RESURF mechanism, while also weakening the gate oxide where it overlaps the drain that ultimately results in a rupture. To quantify the process, a new accelerated aging technique is described that uses the parasitic n-p-n bipolar parallel of the nMOS channel to significantly increase the number of holes while still maintaining the MOS application voltages needed to enable the mechanism. This provides a cost-effective way to accurately accelerate this reliability mechanism with significantly smaller lead times relative to using higher biases and temperatures. This technique is then used with technical computer-aided design (TCAD) analysis to determine the impact of different manufacturing variables, where process controls are introduced for targeted manufacturing limits that can eliminate this mechanism from occurring.

(Invited) The Electroluminescence of Graphene

Electroluminescence is the phenomenon by which a material emits light in response to the passage of an electric current. In solids, it is the prerogative of semiconductors and related organic materials, and it results from the radiative recombination of electrons and holes. Today, electroluminescent devices represent the majority of lighting and display devices and are a building block of the global information and telecommunication network.We report on two observations signaling the electroluminescence of high-mobility graphene field-effect transistors (FET). The semi-metallic nature of graphene a priori forbids electroluminescence: Indeed, in semiconductors, the bandgap energetically insulates the electrons from the holes. Such protection doesn’t exist in graphene which is gapless. Nonetheless, electroluminescence is possible, because (i) of the remarkable inefficiency of the non-radiative carrier relaxation in graphene, and (ii) thanks to an original carrier injection mechanism specific to 2D semimetals: the Zener-Klein (ZK) tunnel conductance. [1]Our first observation consists in the thermometry of graphene’s electrons with applied bias, see figure 1a. Using electronic noise thermometry, we observe that, after an initial warm up with the deposited Joule power, the electron gas cools down, starting at a doping-dependent threshold. The turning point is concomitant with the onset of electron-hole injection by Zener-Klein (ZK) tunneling. The cooling mechanism involves the radiative emission, due to the electron-hole recombination, in electromagnetic near-field modes of the hBN-graphene heterostructure. It is reminiscent of far-field electroluminescent cooling that can be observed in regular LEDs [2] except that it’s much more efficient (~10 mW vs. ~10 pW, i.e., 9 orders of magnitude larger). [3]Our second observation consists in the detection of the middle-wave infrared radiation (MIR) emitted by a high-mobility graphene FET operating in ambient conditions, see figure 1b. We observe a sizeable optical emission in the range 80–250 meV, starting at the ZK tunneling threshold bias. The spectral analysis of the emitted light indicates that it originates from the scattering of the hBN-graphene heterostructure near-fields, as observed in reference [4] by surface near-field optical microscopy. Bibliography [1] W. Yang et al, A graphene Zener-Klein transistor cooled by a hyperbolic substrate. Nat. Nanotechnol. 2018, Vol. 47, 13.[2] P. Santhanam et al, Room temperature thermo-electric pumping in mid-infrared light-emitting diodes. Appl. Phys. Lett. 2013, Vol. 103, 18, p. 183513.[3] E. Baudin et al, Hyperbolic Phonon Polariton Electroluminescence as an Electronic Cooling Pathway. Adv. Funct.. Mater. 2020, Vol. 30, 8, p. 1904783.[4] S. Dai et al, « Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial ». Nat. Nanotech. 2015, Vol. 10, 8, p. 682. Figure 1

Steep-Slope Gate-Connected Atomic Threshold Switching Field-Effect Transistor with MoS2 Channel and Its Application to Infrared Detectable Phototransistors.

For next‐generation electronics and optoelectronics, 2D‐layered nanomaterial‐based field effect transistors (FETs) have garnered attention as promising candidates owing to their remarkable properties. However, their subthreshold swings (SS) cannot be lower than 60 mV/decade owing to the limitation of the thermionic carrier injection mechanism, and it remains a major challenge in 2D‐layered nanomaterial‐based transistors. Here, a gate‐connected MoS2 atomic threshold switching FET using a nitrogen‐doped HfO2‐based threshold switching (TS) device is developed. The proposed device achieves an extremely low SS of 11 mV/decade and a high on‐off ratio of ≈106 by maintaining a high on‐state drive current due to the steep switching of the TS device at the gate region. In particular, the proposed device can function as an infrared detectable phototransistor with excellent optical properties. The proposed device is expected to pave the way for the development of future 2D channel‐based electrical and optical transistors.

Crossover between thermo- and field-assisted carrier injection in staggered pn heterojunction of MoTe2 and ReS2

In semiconductor devices, understanding and controlling the properties of metal–semiconductor and semiconductor–semiconductor junctions is vital. A wide variety of heterojunctions using transition metal dichalcogenides (TMDs) have been fabricated and characterized. Rectification of the drain-source current (Ids) has been observed from p/n-TMD junctions, which is responsible for thermionic emission. In this study, we investigated diode behaviors of MoTe2 and ReS2 heterostructures, whose built-in potential ranged between 0.18 and 0.37 eV; however, we observed that the device showed a crossover between thermionic and Fowler-Nordheim (F-N) tunneling for a carrier injection mechanism. Thermionic emission existed within a narrow temperature and bias windows. Our experimental and theoretical analyses revealed that the carrier injection can be controlled by modulating the potential barrier height, temperature, and applied strength. For instance, at 300 K under a low drain-source voltage Vds, the carriers were thermally injected. Subsequently, the injection mechanism quickly shifted to the thermal field and F-N tunneling as Vds increased. Furthermore, when the temperature was lower than 220 K, the field-enhanced tunneling of the carrier prevailed regardless of Vds magnitude. This result expands the insight into the charge transport mechanism in staggered TMD heterojunctions.

Realization and optimization of optical logic gates using bias assisted carrier-injected triple parallel microring resonators

We propose a p-i-n diode embedded parallel triple microring resonator (MRR) configuration to simultaneously realize optical OR and AND, or NAND and NOR logic gates using a bias-assisted carrier injection mechanism. The applied bias on the rings induces refractive index change in the intrinsic region through bandfilling, bandgap shrinkage and free carrier absorption effects, leading to intensity variation at the output ports of the MRR due to respective resonant wavelength shift. The optical logic gate operational outputs are represented as the light intensities at the output ports of the MRR with the wavelength of the input optical signal launched into the input port being at the resonant wavelength of the microring resonator, while the operands are represented as the bias applied onto the rings. The proposed microring resonator configuration is theoretically optimized for achieving a high contrast ratio and an optical confinement factor by optimizing the intrinsic region width, applied bias, coupling coefficient between ring-bus waveguide and lifetime of carriers in the intrinsic region.

Bias-polarity dependent electroluminescence from a single platinum phthalocyanine molecule

By using scanning tunneling microscope induced luminescence (STML) technique, we investigate systematically the bias-polarity dependent electroluminescence behavior of a single platinum phthalocyanine (PtPc) molecule and the electron excitation mechanisms behind. The molecule is found to emit light at both bias polarities but with different emission energies. At negative excitation bias, only the fluorescence at 637 nm is observed, which originates from the LUMO→HOMO transition of the neutral PtPc molecule and exhibits stepwise-like increase in emission intensities over three different excitation-voltage regions. Strong fluorescence in region (I) is excited by the carrier injection mechanism with holes injected into the HOMO state first; moderate fluorescence in region (II) is excited by the inelastic electron scattering mechanism; and weak fluorescence in region (III) is associated with an up-conversion process and excited by a combined carrier injection and inelastic electron scattering mechanism involving a spin-triplet relay state. At positive excitation bias, more-than-one emission peaks are observed and the excitation and emission mechanisms become complicated. The sharp molecule-specific emission peak at ~911 nm is attributed to the anionic emission of PtPc− originated from the LUMO+1→LUMO transition, whose excitation is dominated by a carrier injection mechanism with electrons first injected into the LUMO+1 or higher-lying empty orbitals.

A statistical study into reliability of FPGA implemented circuits: Simulation and modelling

In this work, we report on the development of a methodology to study the reliability of digital circuits implemented in FPGA. For this, the design and simulation tools have been extended to introduce aging. The aging laws for Look-Up Tables, describing the drifts in the propagation time, caused by Hot Carrier Injection and Negative Bias Temperature Instability degradation mechanisms, have been integrated into the simulation environment by introducing additional variables and equations. Moreover, the study has been further extended by considering the statistical dispersion in the failure time due to different sources of dispersion. Furthermore, an analytical and a statistical model of the failure time of the digital circuit as a function of the clock frequency, aging law parameters and dispersion parameters have been proposed. Finally, the developed methodology has been applied to two different digital circuits implemented in FPGA.

Plasmon-enhanced S2 electroluminescence from the high-lying excited state of a single porphyrin molecule

We demonstrate the B-band electroluminescence from the high-lying S2 excited state of a single zinc porphyrin molecule with the scanning tunneling microscope-induced luminescence technique by using an aluminum tip. The nanocavity plasmon mode is found to be critical for the occurrence of S2 electroluminescence. When using a silver tip to excite the molecule electronically decoupled from the Ag(100) substrate by an ultrathin sodium chloride spacer, we only observe the Q-band electroluminescence originating from the radiative decay of the S1 first excited state, without any B-band emission due to the lack of effective plasmonic enhancement for the B-band. However, when the nanocavity plasmon resonance is tuned to a bluer range by using an aluminum tip, the S2 electroluminescence from a single zinc porphyrin shows up because the nanocavity plasmon mode can now spectrally overlap with the B-band emission to generate efficient plasmonic enhancement for the radiative decay directly from the S2 state. Interestingly, the excitation mechanisms for these two types of emission are found to be different. While the Q-band emission is attributed mainly to a carrier-injection mechanism, the B-band electroluminescence is found to be excited via an inelastic electron scattering process. Our results open a route to investigate the photophysical property and dynamic behavior of isolated molecules in their excited states.