Above-threshold Region Research Articles

Characteristics of Fully-Depleted Poly-Si Thin Film Transistors Operated in Above-Threshold Region with Low Drain Bias

Transfer and output characteristics of fully-depleted polycrystalline silicon thin film transistors, including both tail and deep acceptor-like trap states in bulk, in the above-threshold region with low drain bias are presented under low or high state density in the situation without or with interface charge, respectively. The characteristics are calculated by a simple surface-potential-based drain current model in the strong inversion region with valid bias condition explained, and 2D-device simulation. The above-threshold region is found to be divided into Regions I and II, with Vsi , indicating the channel beginning to be completely strongly-inverted and large currents, and explication of deviations between the model and simulation in Region I. The large-traps effect on the range of Region I and Vth in the high state density situation is discovered.

Two-dimensional nonlinear spectroscopy in the extreme ultraviolet to study dynamics of atomic and molecular gases

We present a two-dimensional (2D) nonlinear four-wave mixing scheme in the extreme ultraviolet (XUV) to investigate ultrafast electronic wave-packets dynamics of multi-electron states in the above-threshold ionization region, using atomic argon and molecular nitrogen as examples. Coherent light sources in the range between 24 and 45 eV is generated by phase-matched cascaded four-wave mixing processes with a collinear configuration of laser fields 800 nm and 1400 nm. Motion of the electrons involving the excited states 3s3p6np (1P10) of argon gas, theF2Σg+ state and the predissociation state C2Σu+ of molecular nitrogen ion N2+ are unravelled through the ‘on-axis’ and ‘off-axis’ features of the two-dimensional spectra. Interpretation of the experimental data is supported by a theoretical dipole control model. Hence, results of this study may be promising for studies of dynamics of electrons in more complex systems.

Device Physics of Vertical Static Induction Transistors

Vertical static induction transistors (SITs), with vertically arranged source, porous gate, and drain electrodes, were proposed in 1950s and have recently gained new attention because of their suitability as flexible substrates and for hybrid integration with light-emitting diodes. However, the understanding of them is hindered by Schottky gate leakage and relies on case-by-case simulations. Here, we derive concise expressions for the channel potential and current-voltage characteristics for ideal SITs, including the subthreshold swing, threshold voltage, and above-threshold region. The theory is verified by two-dimensional device simulation and agrees well with the reported experimental results. An ideal SIT can be approximated as a partially gated transistor in parallel with a resistance and needs a sub-500-nm pore diameter to exhibit sharp switching in transfer scanning and good saturation in output scanning. The proposed theories connect the device structure and electrical characteristics of SITs concisely, and conclusions are also applicable to permeable base transistors.

Below- and near-threshold harmonic generation from multiple orbitals

We report an unexplored observation of multi-orbital contribution in the below- and near-threshold harmonic generation of aligned molecules. A typical pump–probe configuration is used in our experiments. By scanning the time delay and crossing angle of polarization directions between the pump and probe pulses, we find that the harmonic yield in this region exhibits abnormal intensity-dependent modulation patterns. Further comparison and analysis show that this observation can be interpreted as due to the contribution of deeper-lying molecular orbitals, which are much more prominent than in the above-threshold region and can overall surpass the contribution of the highest occupied molecular orbital, leading to reversed modulation patterns. The particular importance of the deeper-lying orbitals in this regime is closely related to the multi-photon excitation pathways involved in the generation process. Our work will advance further investigations on the mechanism of below- and near-threshold harmonic generation.

Low-Frequency Noise Modeling of Amorphous Indium–Zinc-Oxide Thin-Film Transistors

An improved model for the low-frequency noise (LFN) of amorphous indium–zinc-oxide thin-film transistors (a-IZO TFTs) is developed in this article. For a-IZO TFTs, the LFN is not only attributed to the oxide traps in the gate insulator but also affected by the discrete trap centers in the active-layer film. This makes the device has excess noise in the subthreshold region, which is obviously different from monocrystalline silicon (c-Si) MOSFETs. Therefore, we put forward an accurate and physically meaningful model to characterize this excess noise. For the subthreshold region, the charge of tail states in the channel is considered to describe the excess subthreshold noise, and the channel density of states (DOS) is extracted, confirming the effectiveness of the model. For the above-threshold region, the classic carrier number with correlated mobility fluctuation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta N$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \mu $ </tex-math></inline-formula> ) mechanism has been investigated to explain the noise behavior. Furthermore, the proposed unified model is validated, and it has been successfully applied to a-IZO TFTs with different tail-state densities.

A Computationally Efficient Model for FDSOI MOSFETs and Its Application for Delay Variability Analysis

This paper proposes a compact, physics-based current model for fully depleted silicon-on-insulator (FDSOI) MOSFETs and applies it to delay variability analysis. An analytical method is applied to avoid the numerical iterations required in the evaluation of surface potential, which directly improves the computational efficiency. The accuracy of the explicit surface potential approximation is 190.3 nV, which allows for fast convergence. Surface potential and current calculations achieve 1.8× and 1.4× acceleration compared with BSIM-IMG, respectively. To establish the relationship between delay and underlying process parameters, we introduce the effective current and propose a process variation-aware delay prediction model. Higher-order derivatives are calculated to compensate the nonlinearity of delay variations with respect to process parameters. Experiments show a significant improvement in the prediction accuracy with higher-order derivatives, which are proved to be able to handle nonlinearity under process variations. The front gate work function contributes the most to the nonlinearity of the delay variation and the accuracy of the third-order prediction is 4.07%. Under the variation in the channel length and width, front and back gate oxide thickness and body thickness, delay variations have similar characteristics and the second-order prediction is found to be sufficient to model the nonlinearity with a maximum relative error of 1.22%. The delay prediction model only requires a single-point HSPICE DC or transient simulation and is universal for different voltages and different cells. Compared with the Monte Carlo (MC) simulation, the accuracy of the first-order prediction in the above-threshold region (0.8 V) is 0.94%. In the sub-threshold region (0.3 V), a prediction accuracy of 2.01% can be obtained while achieving a 21× reduction in computational time.

Quantified density of performance-degrading near-interface traps in SiC MOSFETs

Characterization of near-interface traps (NITs) in commercial SiC metal–oxide–semiconductor field-effect transistors (MOSFETs) is essential because they adversely impact both performance and reliability by reducing the channel carrier mobility and causing threshold-voltage drift. In this work, we have applied a newly developed integrated-charge technique to measure the density of NITs that are active in the above-threshold region of commercial SiC MOSFETs. The results demonstrate that NITs trap about 10% of the channel electrons for longer than 500 ns.

A STUDY ON THE RELATION BETWEEN MORPHOLOGICAL AND CHEMICAL CHANGES AND WETTABILITY MODIFICATION OF POLYETHERSULFONE SURFACE AFTER IRRADIATION WITH A PULSED KrF LASER

Laser irradiation is used for surface modification of polymers aiming to improve their properties for different applications. The wettability of a polymeric surface can significantly affect its performance for biological applications. In this paper, the interaction of high-energy KrF laser photons in the two above- and below-threshold regimes with polyethersulfone polymer is studied. The role of morphological and chemical changes of the irradiated polymer and their correlation in the modification of the wetting property of this polymer is investigated. The obtained results show that the morphological parameter of surface roughness is the dominant mechanism in the below-threshold regime, while in the above-threshold region, the competition between this parameter and the carbonization amount of the surface determines the final hydrophilic response.

Ambipolar carbon nanotube transistors with hybrid nanodielectric for low-voltage CMOS-like electronics

The proliferation of place-and-forget devices driven by the exponentially-growing Internet of Things industry has created a demand for low-voltage thin-film transistor (TFT) electronics based on solution-processible semiconductors. Amongst solution-processible technologies, TFTs based on semiconducting single-walled carbon nanotubes (sc-SWCNTs) are a promising candidate owing to their comparatively high current driving capability in their above-threshold region at low voltages, which is desirable for applications with constraints on supply voltage and switching speed. Low-voltage above-threshold operation in sc-SWCNTs is customarily achieved by using high-capacitance-density gate dielectrics such as metal-oxides fabricated via atomic layer deposition (ALD) and ion-gels. These are unattractive, as ALD requires complex-processing or exotic precursors, while ion-gels lead to slower devices with poor stability. This work demonstrates the fabrication of low-voltage above-threshold sc-SWCNTs TFTs based on a high-capacitance-density hybrid nanodielectric, which is composed of a readily-made AlO x nanolayer and a solution-processed self-assembled monolayer (SAM). The resultant TFTs can withstand a gate-channel voltage of 1–2 V, which ensures their above-threshold operation with balanced ambipolar behavior and electron/hole mobilities of 7 cm2 V−1 s−1. Key to achieving balanced ambipolarity is the mitigation of environmental factors via the encapsulation of the devices with an optimized spin-on polymer coating, which preserves the inherent properties of the sc-SWCNTs. Such balanced ambipolarity enables the direct implementation of CMOS-like circuit configurations without the use of additional dopants, semiconductors or source/drain electrode metals. The resultant CMOS-like inverters operate in the above-threshold region with supply voltages in the 1–2 V range, and have positive noise margins, gain values surpassing 80 V/V, and a bandwidth exceeding 100 kHz. This reinforces SAM-based nanodielectrics as an attractive route to easy-to-fabricate sc-SWCNT TFTs that can operate in the above-threshold region and that can meet the demand for low-voltage TFT electronics requiring moderate speeds and higher driving currents for wearables and sensing applications.

Supply and Threshold Voltage Scaling for Minimum Energy Operation over a Wide Operating Performance Region

A method for runtime energy optimization based on the supply voltage (Vdd) and the threshold voltage (Vth) scaling is proposed. This paper refers to the optimal voltage pair, which minimizes the energy consumption of LSI circuits under a target delay constraint, as a Minimum Energy Point (MEP). The MEP dynamically fluctuates depending on the operating conditions determined by a target delay constraint, an activity factor and a chip temperature. In order to track the MEP, this paper proposes a closed-form continuous function that determines the MEP over a wide operating performance region ranging from the above-threshold region down to the sub-threshold region. Based on the MEP determination formula, an MEP tracking algorithm is also proposed. The MEP tracking algorithm estimates the MEP even though the operating conditions widely change. Measurement results based on a 32-bit RISC processor fabricated in a 65-nm Silicon On Thin Buried oxide (SOTB) process technology show that the proposed method estimates the MEP within a 5% energy loss in comparison with the actual MEP operation.

Modeling the Influence of the Acceptor-Type Trap on the 2DEG Density for GaN MIS-HEMTs

In this article, an analytical model on the influence of the acceptor-type trap on the 2-dimensional electron gas (2DEG) density is proposed for GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). Based on the charge-control method, a numerical analysis of the 2DEG in both the subthreshold and above-threshold regions is carried out with the deep-level and band-tail acceptor-type traps at the AlGaN/insulator interface and the AlGaN/GaN interface. In particular, the influence of the acceptor-type trap on the 2DEG density and the gate-control capability in the subthreshold region is modeled for the first time. The results have shown that the acceptor-type trap plays an important role in weakening the gate-control capability of the 2DEG density in the subthreshold region. The experimental results, together with the modeling and numerical calculations, have shown consistent 2DEG density values under various gate voltages, which verify the proposed model.

Analytical Current–Voltage Model for Double-Gate a-IGZO TFTs With Symmetric Structure for Above Threshold

This article presents an analytical model to reproduce the ${I}-{V}$ characteristics of a double-gate (DG) amorphous indium–gallium–zinc oxide (a-IGZO) TFT with symmetric structure. This model for the above-threshold region is based on the gate voltage dependence of surface potential and potential at the center of the semiconductor layer, which are described by analytical expressions. Considering the densities of the free and localized carrier in the semiconductor, the total carrier density, which is equal to the effective carrier density, is calculated. The carrier mobility, calculated as the ratio between the free carrier and the effective carrier density, depends on the gate voltage as a power law with the power parameter $\gamma $ different from zero, as it is typically represented for the amorphous TFT. The drain current model has only 13 model parameters and was validated with six different simulated transistor structures, as well as with an experimental device. A good coincidence of the modeled characteristics with both the simulated and experimental devices was obtained.

Density-of-States-Based Physical Model for Ink-Jet Printed Thiophene Polymeric TFTs

We proposed a physical model for ink-jet printed polymeric thin-film transistors (PTFTs) all over the sub- and above-threshold regions by using an effective carrier density. The nonlinearity under the low lateral electric field in the printed thiophene PTFTs was reproduced by applying the back-to-back Schottky diode model based on simple Poole–Frenkel (PF) mobility formalism. The analytical ${I}{-}\!{V}$ model supplemented with ${C}{-}\!{V}$ model in a single framework was also verified by successfully reproducing the measured characteristics of TFTs with three different thiophene polymeric channel materials. Additionally, we applied the physics-based analytical model on the inkjet-printed PTFT-based inverter and confirmed that the proposed models could predict the inverter circuit characteristics of the gain and static noise margin (SNM) based on the physical parameters.

A threshold voltage definition for modeling asymmetric dual-gate amorphous InGaZnO thin-film transistors with parameter extraction technique

A physical definition of threshold voltage has been proposed for asymmetric dual-gate amorphous InGaZnO (a-IGZO) thin-film transistors. Based on the definition, a compact model for drain current has been derived with complete parameter extraction methods. According to the distribution characteristics of density of states (DOS) in the a-IGZO bandgap, the threshold voltage of bottom-gate (BG)-driven devices is defined by the ratio of the trapped-carrier density to the free-carrier density at the bottom surface of the channel layer. This definition has proven to give appropriate values of threshold voltage for devices with DOS parameters varying in a wide range. The trapped-charge density in the above-threshold region can be expressed based on the defined threshold voltage, with the form related to the BG voltage, the top-gate voltage, and the potential along the channel. The free-charge density and analytical drain current can thus be obtained in the above-threshold region. Finally, the continuous drain current expression is developed covering all operation regions. In the modeling process, the determination and extraction strategies for parameters are also presented.

Origin of Low-Frequency Noise in Triple-Gate Junctionless n-MOSFETs

The low-frequency noise in triple-gate junctionless n-MOSFETs, with channel lengths varying from 95 to 25 nm and operating in the bulk and accumulation modes, is investigated by measurements in the frequency and time domains. The experimental drain current noise spectra present $1/{f}$ and Lorentzian-type behavior components. The noise spectra in the time domain reveal that the Lorentzian-type behavior components are due to the capture and emission processes of carriers at discrete gate insulator traps, resulting in random telegraph noise (RTN). The $1/{f}$ behavior can be described by the carrier number with the correlated mobility fluctuations model. In the below-threshold region, the conducting channel is isolated from the interface by depletion region. In the above-threshold region (bulk conduction mode), the histograms of the time-domain data show multilevel switching events, from which one or more individual traps can be distinguished. The extracted time constants of two-level RTN signals indicate the interaction of a single trap either with the channel or with both channel and gate. The relative RTN amplitude is described with the carrier number with the mobility correlated fluctuations physics-based model or with the “hole in the inversion layer” stochastic simulation-based model, enabling estimation of the flat-band voltage fluctuation caused by the RTN.

A semiphysical current–voltage model with a contact ideality factor for disordered thin-film transistors

A semiphysical current–voltage model for disordered thin-film transistors is presented. In the proposed model, a power-law function is employed to describe the drift current in the above-threshold region while a Fermi–Dirac function-like exponential formula is introduced to describe the diffusion current in the subthreshold region. In particular, this subthreshold current model is more physically meaningful compared with the typical exponential model. In addition, it also contains an ideality factor for the source and drain contacts, explaining the vertical separation of the drain current for different drain voltages in the subthreshold region. Using the proposed approach, analytical expressions are derived, providing good agreement with experimental results in both the linear and saturation regimes.

Role of intermediate continuum states in exterior complex scaling calculations of two-photon ionization cross sections

The calculation of partial two-photon ionization cross sections in the above-threshold energy region is discussed in the framework of exterior complex scaling. It is shown that with a minor modification of the usual procedure, which is based on the calculation of the outgoing partial waves of the second-order scattering wave function, reliable partial ionization amplitudes can be obtained. The modified procedure relies on a few-term least-squares fit of radial functions pertaining to different partial waves. To test the procedure, partial and total two-photon ionization cross sections of the helium atom have been calculated for a broad range of incident photon energies. The calculated cross sections may be seen to agree well with the results found in the literature. Furthermore, it is shown that, using a similar approach, partial photoionization cross sections of an atom in an autoionizing (resonance) state may be calculated in a relatively straightforward way. Such photoionization cross sections may find their use in enhanced few-parameter models describing the atom-light interaction in cases where a direct solution of the time-dependent Schroedinger equation becomes too resource-intensive.

Controlling electron quantum paths for generation of circularly polarized high-order harmonics by H2+ subject to tailored ( ω , 2ω ) counter-rotating laser fields

Recently, studies of high-order harmonics (HHG) from atoms driven by bichromatic counter-rotating circularly polarized laser fields as a source of coherent circularly polarized extreme ultraviolet (XUV) and soft-x-ray beams in a tabletop-scale setup have received considerable attention. Here, we demonstrate the ability to control the electron recollisions giving three returns per one cycle of the fundamental frequency $\ensuremath{\omega}$ by using tailored bichromatic ($\ensuremath{\omega}$, $2\ensuremath{\omega}$) counter-rotating circularly polarized laser fields with a molecular target. The full control of the electronic pathway is first analyzed by a classical trajectory analysis and then extended to a detailed quantum study of ${\mathrm{H}}_{2}{}^{+}$ molecules in bichromatic ($\ensuremath{\omega}$, $2\ensuremath{\omega}$) counter-rotating circularly polarized laser fields. The radiation spectrum contains doublets of left- and right-circularly polarized harmonics in the XUV ranges. We study in detail the below-, near-, and above-threshold harmonic regions and describe how excited-state resonances alter the ellipticity and phase of the generated harmonic peaks.

New Design Perspective for Ferroelectric NC-FETs

This letter presents a new angle for the design of negative-capacitance FETs, where the target is not a steep sub-threshold swing, but instead an enhancement of the device transconductance in near- and above-threshold region. The new design is robust against an undesired complete ferroelectric switching and the resulting onset of hysteresis, and it is validated using numerical simulations, calibrated against recent experimental data, and for an ultra-thin body, double-gate FET architecture.

A threshold voltage and drain current model for symmetric dual-gate amorphous InGaZnO thin film transistors

Based on the drift-diffusion theory, a simple threshold voltage and drain current model for symmetric dual-gate (DG) amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) is developed. In the subthreshold region, most of the free electrons are captured by trap states in the bandgap of a-IGZO, thus the ionized trap states are the main contributor to the diffusion component of device drain current. Whereas in the above-threshold region, most of the trap states are ionized, and free electrons increase dramatically with gate voltage, which in turn become the main source of the drift component of device drain current. Therefore, threshold voltage of DG a-IGZO TFTs is defined as the gate voltage where the diffusion component of drain current equals the drift one, which can be determined with physical parameters of a-IGZO. The developed threshold voltage model is proved to be consistent with trap-limited conduction mechanism prevailing in a-IGZO, with the effect of drain bias being also taken into account. The gate overdrive voltage-dependent mobility is well modeled by the derived threshold voltage, and comparisons of the obtained drain current with experiment data show good verification of our model.