Electroluminescence in organic light-emitting diodes is simulated by the master equations for free carriers and excitons. The IV characteristics of both unipolar and bipolar devices can be well reproduced. The luminous efficacies of the phosphorescent OLEDs, which are doped with Ir(ppy)3 in the emission layer, depend on both the triplet generation zone and the triplet transfer capability. Triplet diffusion into the hole-transport layer is primarily attributed to the decline in efficiencies of OLEDs with low emitter concentrations. Higher luminous efficacies can be obtained by graded doping profiles with the merits of broad triplet distribution within and confined to the emission layer. Moreover, triplet-polaron quenching plays a more significant role in the triplet loss than triplet-triplet annihilation does according to our simulations.
Terminal current noise calculations are performed for a SiGe heterojunction bipolar transistor in a wide range of
collector-emitter bias conditions. The generalized hydrodynamic (HD) model with a local temperature approach
for avalanche generation is used. The parameters of the local temperature model are calibrated by matching the
avalanche multiplication factor to results obtained by full-band Monte Carlo simulations. The noise figure calculation
results are compared with experimental values and overall good agreement is obtained. The hydrodynamic
and a drift-diffusion (DD) model are used to investigate terminal current noise due to impact-ionization. The
behavior of the current noise spectral intensity is found to be different for the two models. The Fano factor of
the collector current fluctuations is well described by the avalanche multiplication factor in the case of the DD
model, whereas the HD model evidences no correlation between the Fano factor and the avalanche multiplication
factor. The collector terminal electron transfer functions are used to discuss the difference.
A deterministic solver for the Langevin Boltzmann equation is presented, which is based on a spherical harmonics
expansion, box integration, and a maximum entropy dissipation principle. The numerical properties of this
method are very similar to the classical approaches (drift-diffusion or hydrodynamic models), and the same
numerical methods can be used (ac analysis, adjoint method, harmonic balance, etc). Since the equations can
be solved directly in the frequency domain, the full frequency range down to zero frequency is accessible. In
addition, rare events can be simulated without excessive CPU times. This is demonstrated for a silicon NPN
BJT. Not only the terminal current noise is calculated, but also the spatial origin of noise and the corresponding
A deterministic solver for the Langevin Boltzmann equation including the Pauli principle is presented based on
a spherical harmonics expansion. The solver can handle rare events, slow processes and low frequencies without
problems and without an increase in CPU time in contrast to the Monte Carlo method. This is demonstrated for strongly degenerate systems and deep traps. Although the two electron sub-ensembles for the different spin directions are correlated due to the deep traps, the spin variable can be eliminated without any approximations
resulting in a reduction of the number of unknowns by two. Approximations for the inclusion of the Pauli
principle are investigated and found to be so bad that it is better to neglect the Pauli principle than to use those approximations.
The noise performance of PNP and NPN SiGe structures is examined
by an experimentally verified hydrodynamic (HD) noise model.
This model is a hierarchical numerical noise model because all noise parameters required by this model are generated by full band Monte Carlo bulk simulations leading to the methodology of the hierarchical numerical noise simulation. The hierarchical HD noise model is applied to compare the performance of NPN and PNP SiGe HBTs. The simulations include AC, DC and noise characteristics like the minimum noise figure. A similar noise performance for both types of devices is found.
The accuracy of the SPICE and unified compact noise models
is assessed in the RF range by comparison with the hydrodynamic device model for a state-of-the-art SiGe HBT with a low base resistance. Despite the low base resistance, as a general result, it turns out that the noise is dominated by the thermal fluctuations of the holes within the base and the exact determination of the base
noise resistance is a prerequisite for accurate compact noise modeling. It is shown that the base noise resistance equals the base resistance and can be evaluated with standard parameter extraction schemes. Based on an accurate base resistance the SPICE model
yields good results as long as the frequency is considerably
below the peak cutoff frequency. The unified model, on the other hand, is found to yield good results even at frequencies comparable to the peak cutoff frequency. But this is achieved at the expense of an additional parameter which is difficult to determine without physics-based numerical noise simulation. Moreover, it is shown that the drift-diffusion model should not be used to assess the accuracy of compact noise models, because it yields erroneous noise results
for state-of-the-art SiGe HBTs.
Understanding the properties of close-in phase noise is crucial for analyzing the effects of low-frequency, colored noise on the frequency stability of electrical oscillators. This paper shows these properties are distinctly different from those of far-out phase noise, which are commonly studied in the literature. Unlike far-out phase noise, the spectrum of close-in phase noise caused by several uncorrelated noise sources is not the same as the sum of the phase noise spectra caused by individual sources. Furthermore, in the absence of colored noise, this spectrum is not necessarily Lorentzian as generally believed. We show that the phase noise spectrum of a periodic signal with zero cycle-to-cycle jitter is always Lorentzian and demonstrate the appearance of 1/f4 phase noise due to a Lorentzian noise source. We also study two methods for suppressing the effects of low-frequency, colored noise on phase noise: signal symmetrization and noise-source switching. We show that the suppression of 1/f3 phase noise in single-ended ring oscillators is due to switching and not because of symmetrization. Symmetrization is effective only for the noise sources which are constantly “on”, such as the tail current source in differential ring oscillators. These findings provide effective guidelines for designing low-phase-noise oscillators.