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We use a master equation model for numerical simulations of triplet loss mechanisms in phosphorescent organic light-emitting diodes (OLEDs). In addition to the IV characteristics, which were successfully simulated in our previous work, the measurements of luminous efficacy (LE) for the OLEDs with various doping profiles of the emitter fac-tris(2-phenylpyridine)iridium (III) (Ir(ppy) 3) can be reproduced based on a single set of excitonic parameters, which is a strong indication of the validity of the model. In order to minimize the strong Dexter diffusion of triplets out of the emission layer (EML) and the nonradiative decays, an additional exciton blocking layer and stronger exciton confinement on guest molecules are investigated in the simulations to improve the device efficiency. With the modifications, the LE of the lowest-doped OLED with 4% Ir(ppy) 3 can be more than twofold increased at a luminance up to 20 000 cd / m 2 . However, due to the enhanced triplet-triplet annihilation, the roll-off in electrophosphorescence efficiencies under high current injections of the upgraded devices becomes stronger, which can nevertheless be suppressed by a higher emitter concentration in the EML. [ABSTRACT FROM AUTHOR]

A very detailed investigation of high-speed silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs) showed an underestimation of the measured peak cutoff frequency by simulations with a hydrodynamic (HD) model. It was speculated that this might be due to a breakdown of the HD approximation or unknown additional mechanical stress. We repeated those simulations with the more fundamental Boltzmann transport equation (BTE) based on the same device model (doping profiles, 2-D geometry, parasitics, and so on) and obtained almost similar results showing that this failure was not due to a breakdown of the HD approximation. Since additional uniaxial stress along the direction of the lateral base has been shown to increase the cutoff frequency, we investigated this effect. We found by 2-D device simulations that the increase in the peak cutoff frequency is rather small, and at high stress levels, it even decreases, if the uniaxial stress is applied homogenously. This is due to the reduction of the conductivity of the highly doped collector layer by the stress. If the stress is limited to the intrinsic transistor, the increase in the cutoff frequency is monotonic with growing stress. On the other hand, the collector current for a given base–emitter voltage also increases with stress leading to an overestimation of the collector current compared with the measurements. If this increase is corrected by a slight decrease in the germanium profile, the gain in the peak cutoff frequency is lost. Thus, the underestimation of the peak cutoff frequency cannot be explained by an additional homogeneous uniaxial stress in the intrinsic transistor. [ABSTRACT FROM AUTHOR]

The noise behavior resulting from impact ionization (II) is assessed at room temperature for silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) with box Ge profile, featuring a maximum transit frequency of [f.sub.T] = 80 GHz. The noise parameters (NPs) measured over a wide range of collector-emitter voltages revealed that the weak avalanche model is accurate to capture II noise in examined SiGe HBTs.

A microscopic approach to model the hole inversion-layer mobility of strained-SiGe homo- and heterostructure-on-insulator in ultrathin-body MOSFET is presented.

A new deterministic approach to electron noise based on a spherical harmonics expansion (SHE) of Langevin-Boltzmann equation in the frequency domain for silicon devices is presented. This approach has many advantages including that it can handle the full frequency range from zero to terahertz, low current levels, and slow processes.

A novel heterostructure double-gate FET (DGFET) that reduces large band-to-band tunneling (BTBT) leakage currents while retaining its high mobility, making it suitable for scaling into the sub-20-nm regime, is presented. The BTBT simulations have shown a dramatic reduction in BTBT and an excellent electrostatic control of the channel while maintaining very high drive currents and switching frequencies in these nanoscale transistors.

The experimental results obtained with various prototypes of impact ionization (I-MOS) transistors are described for verifying the basic concept and further understanding the device physics. These first experimental prototypes of the I-MOS showed significant hot carrier effects resulting in threshold voltage shifts and degradation of subthreshold slope with repeated measurements.

A comprehensive investigation of the SPICE and unified compact noise models is performed by comparison with the more fundamental hierarchical hydrodynamic device model. It is shown that these models yield good results for frequencies up to about 10 GHz for state-of-the-art SiGe HBTs (heterojunction bipolar transistor).

The Langevin-type drift-diffusion (DD) and hydrodynamic (HD) noise models suitable for TCAD (Technology computer aided design) are verified by Monte Carlo (MC) simulation. The new models are more accurate without a loss in computational efficiency.

The Langevin-type hydrodynamic model for SiGe is presented with the CPU efficient Green's function technique. The results of noise models for homogeneous bulk conditions are discussed.

Technology computer-aided design (TCAD) is an integral part of the development process of semiconductor technologies and devices, a field which has become increasingly complex and heterogeneous. Processing of integrated circuits requires nowadays over 400 process steps, and the resulting devices often have an intricate 3-D structure and contain various specifically designed materials. The full device behavior can only be understood by considering effects on all length scales from atomistic (material properties, interfaces, defects, and so on), to nanometric (quantum confinement, non-bulk properties, tunneling, ballistic transport, and so on), to full-chip dimensions (strain, heat transport, and so on), and time scales from femtoseconds (scattering, ferroelectric switching time, and so on) to seconds (trapping times, degradation, and so on). Voltages, currents, and charges have been scaled to such low levels that statistical effects and process variations have a strong impact. Devices based on new materials (e.g., 2-D crystals) and physical principles (ferroelectrics, magnetic materials, qubits, and so on) challenge standard TCAD approaches. While the simulation methods developed by the physics community can describe the basic device behavior, they often lack important simulation capabilities like, for example, transient simulations or integration with other TCAD tools, and are often too slow for daily use. Due to the complexity of semiconductor technology, it becomes more and more difficult to assess the impact of a change in processing or device structure on circuit performance by looking at a single aspect of an isolated device under idealized conditions. Instead, a TCAD tool chain is required which can handle realistic device structures embedded in a chip environment. New methodologies are required for all aspects of TCAD to ensure an efficient tool chain covering from atomistic effects to circuit behavior based on flexible simulation models that can handle new materials, device principles, and the ensuing large-scale simulations and that make use of artificial intelligence for well-chosen (sub)routines to decrease the overall simulation time. This Special Issue features six invited and 18 regular papers that address these problems. [ABSTRACT FROM AUTHOR]

A stabilization method is presented for the Boltzmann equation (BE) that is based on Godunov’s approach applied directly in the phase space. This makes it possible to perform transient simulations under quasiballistic conditions and to include the Pauli principle in the scattering integral without further approximations. The method ensures positive distribution functions by construction. The BE is solved self-consistently together with the Poisson and Schrödinger equations by the Newton–Raphson method for a nanowire silicon nMOSFET, for which the stationary ${I}$ – ${V}$ characteristics, the small-signal admittance parameters, and the switching behavior are calculated. The large-signal behavior of the MOSFET as an amplifier is evaluated by cyclostationary simulations including power gain and the output power at higher harmonics. The current responsivity of the MOSFET operated as a passive mixer is simulated up to terahertz (THz) frequencies including the quasiballistic case with negligible scattering. [ABSTRACT FROM AUTHOR]

A numerical framework for DC and RF small-signal simulations of nanowire transistors is presented, which is based on the self-consistent solution of the Poisson, Schrödinger, and Boltzmann transport equations and is stable for the entire range from weak to strong particle scattering. The proposed approach does not suffer from the deficiencies due to the transformation of the Boltzmann transport equation into the energy space and can handle the quasi-ballistic case. This is a key requirement for the investigation of plasma resonances and other high-mobility phenomena. The in-house solver is validated with results of a previously developed simulator based on the H-transformation for a conventional N + NN + silicon transistor with strong scattering. Then, its results are compared with those of moments-based models and it is shown that these do not provide a satisfactory description of the electron dynamics in the quasi-ballistic transport regime. Furthermore, the internal boundary conditions of the transport models at the contacts are found to have a significant impact on plasma resonances and the physics-based thermal-bath boundary condition strongly suppresses them. [ABSTRACT FROM AUTHOR]

Characterizing mixed hot-carrier/bias temperature instability (BTI) degradation in full VG, VDbias space is a challenging task. Therefore, studies usually focus on individual degradation mechanisms, such as BTI and hot-carrier degradation (HCD). However, a simple superposition of these mechanisms at an arbitrary VG, VD combination often fails to predict the cumulative damage. We experimentally acquired a large data set covering the full bias space of a pMOSFET which allows us to obtain detailed degradation and recovery maps. Our models for describing oxide and interface defects provide physical insights into the underlying mechanisms and a possible interplay between the degradation modes. Additionally, we perform a dedicated experiment to reveal the implications of different stress regimes onto the various types of defects by switching BTI and HCD stress conditions. The results clearly reveal the conceptual limits of the assumption of independent degradation regimes. [ABSTRACT FROM AUTHOR]

The noise characteristics of today’s short-channel devices are shown to have a better resemblance to ballistic devices than to long-channel metal oxide semiconductor field effect transistors (MOSFETs). Therefore the noise characteristics of these devices are best modeled using a ballistic-MOSFET-based noise model. Extensive hydrodynamic device simulations are presented in support of this hypothesis and a simple compact model is introduced. This model is used for predicting the noise behavior of future nanoscale devices. Most of the findings of this work can also be applied to carbon nanotubes and nanowires because of their similarities to MOSFETs. [ABSTRACT FROM AUTHOR]

In this paper, we examine the interplay of two serious reliability issues in MOSFET devices, namely, bias temperature instability (BTI) and hot-carrier degradation (HCD). Most publications are devoted to the characterization of either BTI or HCD, and complex models have been developed to independently describe each degradation mode. However, very limited data are available on the interplay of both degradation regimes, particularly the effect of a drain bias onto the charging and discharging dynamics of oxide traps. Part I of this paper provides an extensive experimental study toward the impact of mixed-mode stress conditions on the dynamics of oxide defects. Here, we present the first microscopic modeling approach beyond a simple electrostatic approximation. We extend the existing nonradiative multiphonon theory by taking nonequilibrium processes such as full carrier distribution functions which include the effect impact ionization along the channel into account. To ultimately validate our framework, we compare simulation results and experimental data for a single-oxide defect as well as a large ensemble of traps in a MOSFET device. We show that our modeling approach accurately captures the rather puzzling measurement trends for a broad stress regime and allows developing the knowledge on how oxide defects can be affected by an increased drain stress. [ABSTRACT FROM AUTHOR]

This paper presents a physical model to investigate the electroforming, set and reset processes in Redox-based resistive switching RAM based on the valence change mechanism. The model uses a kinetic Monte Carlo code in a three-dimensional structure. It is based on the formation and dissolution of an oxygen-deficient/oxygen-vacancy-rich filament in the resistive switching oxide material. In contrast to other proposed models, oxygen vacancies only form at the anode/oxide boundary due to an oxygen exchange reaction. The generated oxygen vacancies are mobile and move away from the interface allowing for further oxygen vacancy generation. The model includes electric field, temperature and temperature gradient as driving forces for the electroforming, set and reset transition of these devices. It is demonstrated that this alternative model could successfully reproduce $I-V$ characteristics observed in experimental results. [ABSTRACT FROM AUTHOR]

We present and validate a physics-based model to describe the underlying mechanisms of hot-carrier degradation in bipolar transistors. Our analysis is based on a deterministic solution of the coupled system of Boltzmann transport equations for electrons and holes. The full-band transport model provides the energy distribution functions of the charge carriers interacting with the passivated Si–H bonds along the oxide interface. The simulation results assert the dominant role of hot holes along the emitter–base spacer oxide interface in the long-term degradation of an n-p-n SiGe heterojunction bipolar transistor under low and high-current conditions at the border of the safe-operating area. The interface trap density is calculated by incorporating an energy driven paradigm for the microscopicmechanisms of defect creation into a reaction-limitedmodelwith dispersive reaction rates. These interface traps increase the forward-mode base current via Shockley–Read–Hall recombination and degrade the overall device performance. The Gummel characteristics of a degraded device and time evolution of the excess base current for different stress conditions are verified versus the experimental data obtained for a state-of-the-art toward-terahertz SiGe HBT. [ABSTRACT FROM AUTHOR]

A stationary deterministic solver based on a spherical harmonics expansion of the Boltzmann transport equations for electrons, holes, and phonons is presented to study self-heating in ultrascaled bipolar transistors. With the electrothermal device simulator, a state-of-the-art toward-terahertz SiGe heterojunction bipolar transistor is analyzed and the simulation results are verified against experimental data. To investigate nonequilibrium effects for the carrier-phonon system, the impact of hot longitudinal optical phonons on steady-state carrier transport is discussed. Furthermore, the self-consistent and deterministic solution of the coupled set of equations allows to extract the junction temperature by making use of a method based on the simulated DC characteristics. The resultant junction temperature is compared with the value obtained from the temperature profile within the nanoscale device. Good agreement is obtained for the average temperature in the base–emitter junction verifying the analytical approach used to extract the thermal resistance of the device by experiments. [ABSTRACT FROM PUBLISHER]

We present the simulations of the small-signal capacitance characteristics of organic light-emitting diodes based on the master equation for a 3-D lattice solved under the sinusoidal steady-state condition. Small-signal measurements and current–voltage characteristics by Martens et al. can be simultaneously reproduced with appropriate parameters. According to our simulations, the frequently observed effect of a negative differential capacitance at low frequencies is affected by several parameters. It becomes more pronounced with longer recombination lifetime, lower injection barrier, and higher energetic disorder. [ABSTRACT FROM PUBLISHER]

The performance of electronic devices for the detection and generation of THz waves might be potentially improved by resonances or instabilities due to plasma waves. The analytical modeling of these devices is based on very simple transport models, of which the accuracy is assessed by a systematic comparison with numerical solutions of the Boltzmann transport equation (BTE). To this end, the BTE is linearized and solved for plasma waves of arbitrary frequency and wave number. The comparison reveals that the simple transport models fail to capture even the most salient aspects of transport in the important frequency/wave number range and better models are required for reliable prediction of the performance of THz devices. [ABSTRACT FROM AUTHOR]