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Several experimental groups have reported recently an intriguing high level of gain (Photomultiplication) in vertical organic photoresistance (as well as in other technologies, such as perovskite for instance). This mechanism is sometimes named as "Trap-Assisted Photomultiplication." This paper investigates the origin of this mechanism by means of drift diffusion simulations, analytical theory, and experiments, considering the particular case of PCDTBT:PC60BM photoresistors, although some conclusions are likely to apply in other technologies. It turns out that an excess of charges (induced by electron–hole carrier generation) may trigger additional carrier injection, leading to photomultiplication, under specific circumstances. We call this mechanism "gain by injection enhancement." Electron (respectively, hole) trapping for P only (respectively, N only) devices can play this role efficiently. As these additional carriers came from contacts, significant dark current injection is thus needed to achieve a large value of gain, explaining why this mechanism can occur only in P (or N) only photoresistors (and not photodiodes or intrinsic photoresistors, i.e., with midgap contacts). In such devices, however, the detectivity remains intrinsically limited by the high level of dark injection currents required to get gain, and consequently, this type of device may be interesting, in particular, in technologies where it is not possible to achieve low dark currents using photodiodes. However, penalized by the slow trap dynamics, the cut-off frequency of these devices remains extremely low (<100 Hz). Also, this gain takes a high value only at low irradiance, making photoresistor responsivity light dependent. All these results bring new light in analyzing and optimizing photoresistors, opening a large field of investigation to take advantage of gain by injection enhancement. [ABSTRACT FROM AUTHOR]

Single photon avalanche diodes (SPADs) are widely used to monitor fast and weak optical signals. The modeling of two main figures of merit of SPAD, namely the dark count rate (DCR) and the photon detection probability, requires one to calculate the avalanche triggering probability (ATP), usually obtained by numerically solving two transcendental equations (the McIntyre model) as post processing of technology computer-aided design simulations. This paper proposes an analytical alternative to this approach, exploiting an approximation of the impact ionization rates, in principle valid only under high field conditions, but extended to all fields by a simple fitting procedure. The proposed approximated/analytical ATP calculation can be efficient and relevant for SPAD compact modeling that is compatible with a spice-like simulator. As an illustration, a full analytical calculation for DCR based on both ATP and generation terms for a P+N abrupt diode junction is presented. [ABSTRACT FROM AUTHOR]

The frequency response of unipolar organic Schottky diodes used in a rectifying circuit, such as an RFID tag, has been investigated in detail. The time dependent response of rectifying circuits has been simulated solving both the Drift Diffusion and Poisson equations to model the hole transport within the diode, coupled with time dependent circuit equations. Several approximations have also been discussed. It turns out that the cut off frequency of the rectifying circuit is indeed limited by the carrier time-of-flight and not by the diode equivalent capacitance. Simulations have also been confirmed by comparison with experiments, involving diodes with different mobilities and thicknesses. This work confirms that the 13.56MHz frequency can be reached using polymer semiconductors, as already experimentally demonstrated in the literature, by an adequate control of the active layer thickness. [ABSTRACT FROM AUTHOR]

A simple analytical expression of the 3-D potential distribution along the channel of lightly doped silicon trigate MOSFETs in weak inversion is derived, based on a perimeter-weighted approach of symmetric and asymmetric double-gate MOSFETs. The analytical solution is compared with the numerical solution of the 3-D Poisson's equation in the cases where the ratios of channel length/silicon thickness and channel length/channel width are ≥ 2. Good agreement is achieved at different positions within the channel. The perimeter-weighted approach fails at the corner regions of the silicon body; however, by using corner rounding and undoped channel to avoid corner effects in simulations, the agreement between model and simulation results is improved. By using the extra potential induced in the silicon film due to short-channel effects, the subthreshold drain current is determined in a semianalytical way, from which the subthreshold slope, the drain-induced barrier lowering, and the threshold voltage are extracted. [ABSTRACT FROM AUTHOR]

A simple threshold voltage model of an undoped symmetrical double-gate MOSFET has been developed, based on an analytical solution of Poisson's equation for the potential distribution. The model has been verified by comparing the threshold voltage roll-off with the channel length with simulation results for different silicon thicknesses, gate oxide thicknesses, and drain voltage values. Good agreement between model and simulation results is obtained by calibrating the minimum carrier charge sheet density adequate to achieve the turn-on condition. [ABSTRACT FROM AUTHOR]

This paper discusses the optimization of series resistance of nonplanar metal-insulator-metal capacitor, i.e., an original 3-D capacitor with a capacitance density of 35 nF/mm², used in very large scale integration. A fully analytical and physically based model of its series resistance versus material and geometrical parameters has been developed, in excellent agreement with both 3-D numerical simulations and experiments. Based on the modeling results, possible design strategies of series-resistance reduction are suggested; showing a reduction of the series resistance by approximately a factor of four, without any degradation of the capacitance density. [ABSTRACT FROM AUTHOR]

The aim of this paper is to establish a well-defined theoretical background in which the kT-layer concept and the empirical expression of the backscattering coefficient introduced by Lundstrom and co-workers to describe the role of scattering in nanoscale devices under high-field conditions and quasi-ballistic transport regime are derived analytically. To this purpose, one-dimensional (1-D) Boltzmann transport equation is solved in the framework of a ‘relaxation length’ approximation, leading to a set of drift-diffusion like transport equations. This set of equations is then solved in a template 1-D structure (with a linear potential-energy profile), leading to an analytical expression for the back-scattering coefficient, which is equal, in the limit of low- and high-electric fields, to the formulas proposed by Lundstrom et al. This approach allows us to identify the exact assumptions and qualitative validity limits of these formulas. [ABSTRACT FROM AUTHOR]

Objective: To assess the prediction accuracy of recent optical and numerical models for the spectral reflectance and color of monolithic samples of dental materials with different thicknesses., Methods: Samples of dental resin composites of Aura Easy Flow (Ae1, Ae3 and Ae4 shades) and Estelite Universal Flow Super Low (A1, A2, A3, A3.5, A4 and A5 shades) with thicknesses between 0.3 and 1.8 mm, as well as Estelite Universal Flow Medium (A2, A3, OA2 and OA3 shades) with thicknesses between 0.4 and 2.0 mm, were used. Spectral reflectance and transmittance factors of all samples were measured using a X-Rite Color i7 spectrophotometer. Four analytical optical models (2 two-flux models and 2 four-flux models) and two numerical models (PCA-based and L*a*b*-based) were implemented to predict spectral reflectance of all samples and then convert them into CIE-L*a*b* color coordinates (D65 illuminant, 2°Observer). The CIEDE2000 total color difference formula (ΔE 00 ) between predicted and measured colors, and the corresponding 50:50% acceptability and perceptibility thresholds (AT 00 and PT 00 ) were used for performance assessment., Results: The best performing optical model was the four-flux model RTE-4F-RT, with an average ΔE 00 = 0.72 over all samples, 94.87% of the differences below AT 00 and 65.38% below PT 00 . The best performing numerical model was L*a*b*-PCHIP (interpolation mode), with an average ΔE 00 = 0.48, and 100% and 79.69% of the differences below AT 00 and PT 00 , respectively., Significance: Both optical and numerical models offer comparable color prediction accuracy, offering flexibility in model choice. These results help guide decision-making on prediction methods by clarifying their strengths and limitations., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Optical characterization and appearance prediction of translucent materials are required in many fields of engineering such as computer graphics, dental restorations or 3D printing technologies. In the case of strongly scattering materials, flux transfer models like the Kubelka-Munk model (2-flux) or the Maheu's 4-flux model have been successfully used to this aim for decades. However, they lead to inaccurate prediction of the color variations of translucent objects of different thicknesses. Indeed, as they rely on the assumption of lambertian fluxes at any depth within the material, they fail to model the internal reflectance at the interfaces, penalizing the accuracy of the optical parameter extraction. The aim of this paper is to investigate the impact of translucency on light angular distribution and corresponding internal reflectances by the mean of the radiative transfer equation, which describes more rigorously the impact of scattering on light propagation. It turns out that the light angular distribution at the bordering interfaces is often far from being lambertian, and that the internal reflectance may vary significantly according to the layer's thickness, refractive index, scattering and absorption coefficients and scattering anisotropy. This work enables to better understand the impact of scattering within a translucent layer and also invites to revisit the well-known Saunderson correction used in 2- or 4-flux models.