Semiconductor Device Simulation Research Articles

Performance assessment of SiGe extended four corner source TFET for biosensing applications

In this paper, the performance of a novel SiGe extended four corner source tunneling field–effect transistor (SiGe EFCS TFET) based dielectrically modulated label-free biosensor has been investigated for biosensing applications. Using a combination of heterostructure (SiGe/Si) and extended four corner source (EFCS) lead to increased band–to–band tunneling (BTBT) probability, improved gate control and superior drain current sensitivity for biomolecule conjugation in comparison with a conventional TFET and Si EFCS TFET structures of similar dimensions. The influence of both the charge–neutral and charged biomolecules on the sensitivity performance is investigated using the Silvaco TCAD ATLAS semiconductor device simulator with a calibrated nonlocal BTBT model. Four different kinds of biomolecules such as Streptavidin, Ferro–cytochrome c, Keratin and Gelatin with various charge density values were used for this purpose. In order to model steric hindrance effects in partially filled cavities, in addition to various fill factors, four different step profile patterns have considered such as convex, concave, decreasing and increasing step profiles. Also, the dependence of the nanogap cavity properties on the sensing performance in terms of the length and thickness is investigated. A maximum drain current sensitivity of 1.69 × 105 is achieved in SiGe EFCS TFET for Gelatin biomolecules in a fully filled nanogap cavity at overdrive and drain voltages of 0.5 V.

A robust hybridizable discontinuous Galerkin scheme with harmonic averaging technique for steady state of real-world semiconductor devices

Solving real-world nonlinear semiconductor device problems modeled by the drift-diffusion equations coupled with the Poisson equation (also known as the Poisson-Nernst-Planck equations) necessitates an accurate and efficient numerical scheme which can avoid non-physical oscillations even for problems with extremely sharp doping profiles. In this paper, we propose a flexible and high-order hybridizable discontinuous Galerkin (HDG) scheme with harmonic averaging (HA) technique to tackle these challenges. The proposed HDG-HA scheme combines the robustness of finite volume Scharfetter-Gummel (FVSG) method with the high-order accuracy and hp-flexibility offered by the locally conservative HDG scheme. The coupled Poisson equation and two drift-diffusion equations are simultaneously solved by the Newton method. Indicators based on the gradient of net doping and solution variables are proposed to switch between cells with HA technique and high-order conventional HDG cells, utilizing the flexibility of HDG scheme. Numerical results suggest that the proposed scheme does not exhibit oscillations or convergence issues, even when applied to heavily doped and sharp PN-junctions. Devices with circular junctions and realistic doping profiles are simulated in two dimensions, qualifying this scheme for practical simulation of real-world semiconductor devices.

Combined effect of total ionizing dose and electromagnetic pulse on MOSFET devices

Theoretical and experimental research on the combined effect of total ionizing dose (TID) and electromagnetic pulses (EMP) is helpful to develop hardening techniques for semiconductor devices. In this work, a numeric model for the comprehension of the combined effect of TID and EMP on MOSFET devices is developed with the semiconductor device simulation software TCAD. Simulation results show that the NMOS and PMOS devices will exhibit more severe leakage current increase and sub-threshold characteristics degradation in the combined environment. In addition, the potential influence of doping concentration and gate length on the performance of the MOSFET in the combined environment is discussed.

Resistor-to-Schottky barrier analytical model for ohmic contact test structures

Analytical models for investigating Metal–Semiconductor (M–S) ohmic contacts in test structures have conventionally included resistive-only contact interfaces. Given that M–S contacts are fundamentally governed by electron tunnelling across the potential energy barrier at the M–S interface, this simplified approach may result in misinterpretation. This paper describes, in detail, a novel Resistor-to-Schottky (RSB) barrier analytical model that enables a more in-depth exploration of the physics underlying ohmic contacts. The proposed model is analysed and compared with models constructed using the semiconductor device simulator tool TCAD. The study reveals significant differences in outcomes when employing the RSB model rather than the conventional Transmission Line model and contributes to a more comprehensive understanding of M–S ohmic contacts in test structures.

Acceleration of solving drift-diffusion equations enabled by estimation of initial value at nonequilibrium

<abstract><p>In this study, a novel method enabled by estimation of initial value guess at nonequilibrium was proposed to accelerate drift-diffusion equations in semiconductor device simulation. The initial value guess was obtained by solving analytical model about electrical potential with the decoupling algorithm. By obtaining the initial value directly at the target bias voltage, the proposed method eliminated time-consuming bias ramping process in the classical method starting from the equilibrium state, thereby accelerating the whole process. The method has been applied to a junction barrier Schottky (JBS) diode for validation. Numerical results showed that the proposed method achieves convergence within 10 iterations at several reverse bias voltages, achieving significant reduction of iteration number compared to the classical method using the bias ramping process. It demonstrated that the proposed method holds high feasibility to facilitate the semiconductor device property prediction in relatively regular device structure in the case of low current. With further improvements, this method can also be applied to more complex devices.</p></abstract>

Adaptive Local Mesh Refinement in 2D Finite Volume Method for Steady State and Transient Simulations of Semiconductor Devices

Adaptive Local Mesh Refinement in 2D Finite Volume Method for Steady State and Transient Simulations of Semiconductor Devices

Electrical Characteristics of 3D Trench Electrode Germanium Detector with Nested Complementary Cathodes.

High-purity germanium detectors, widely employed in fields such as aerospace applications based on radiation detection principles, have garnered attention due to their broad detection range and fast response time. However, these detectors often require larger sensitive area volumes to achieve larger signals and higher detection efficiency. Additionally, the large distance between the electrodes contributes to an issue of incomplete charge collection, which significantly restricts their application in space applications. To enhance the electrical performance of high-purity germanium detectors, this study introduces a strategy: designing the detector's cathode electrode into a 3D trench shape with nested complementary cathodes. This design greatly reduces the electrode spacing, endowing the detector with superior electrical characteristics, such as a smaller dead zone and improved charge collection efficiency. Performance simulations of the novel detector structure were conducted using the semiconductor device simulation software Sentaurus TCAD (2019.03). The simulation results confirmed that the nested complementary 3D trench electrode high-purity germanium detector exhibits excellent electrical features, including a larger sensitive area volume, rapid charge collection, and good cell isolations. This approach has the potential to effectively expand the application scenarios of high-purity germanium detectors. Depending on different operational environments and requirements, nested complementary 3D trench electrode high-purity germanium detectors of appropriate structural dimensions can be chosen. The experimental findings of this study hold a significant reference value for enhancing the overall structure of high purity germanium detectors and facilitating their practical application in the future.

A weak Galerkin finite element method for 1D semiconductor device simulation models

In this paper, we study a weak Galerkin (WG) finite element method for semiconductor device simulations. We consider the one-dimensional drift–diffusion (DD) and high-field (HF) models, which involves not only first derivative convection terms but also second derivative diffusion terms, as well as a coupled Poisson potential equation. The main difficulties in the analysis include the treatment of the nonlinearity and coupling of the models. The weak Galerkin finite element method adopts piecewise polynomials of degree k for the approximations of electron concentration and electric potential in the interior of elements, and piecewise polynomials of degree k+1 for the discrete weak derivative space. The optimal order error estimates in a discrete H1 norm and the standard L2 norm are derived. Numerical experiments are presented to illustrate our theoretical analysis. Moreover, numerical schemes also work out for the discontinuous diffusion coefficient problems.

Switching mechanisms of CMOS-compatible ECRAM transistors—Electrolyte charging and ion plating

To elucidate the internal chemical physics of measured CMOS-compatible electrochemical random-access memory (ECRAM) devices, we constructed a 2D semiconductor device simulation, including ions and electrochemical reactions, and used it to fit measured devices. We present the results of a device simulation model that includes Cu+ ions’ diffusion and the charge transfer reaction between the WOx conduction band electron and Cu+ (i.e., “Cu plating”). Reproducing the linear response of ECRAM devices, the effect of charging HfOx by the Cu+ ions is sufficient, and WOx is not being doped by the Cu+ ions. While potentiation is supported by the formation of an electron channel, an efficient depression requires the formation of high positive charge density at the channel material. At higher Cu+ flux, Cu+ reaches and penetrates the WOx layer. While this effect enhances the potentiation response, it also initiates the “plating” reactions. Including this reaction is essential to reproducing the data of devices exhibiting sub-linear responses. We suggest that electron trapping by ions (i.e., plating) would constitute a long-term degradation process even for H+ based devices.

Short Series Superconductor Density of States Integral Approximations

Accurate and fast Fermi-Dirac integral approximations are used in semiconductor device simulators to compute carrier concentrations where Boltzmann statistics cannot be applied. A similar integral can be used to compute carrier densities in the subgap region of a superconductor. The primary difference between the two integrals is the density of states (DOS) functional used. Electrons and holes in semiconductors use the spherical band approximation yielding a DOS functional proportional to the square root of state energy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$E$</tex-math></inline-formula> ). Superconductors in the subgap region can be modeled with a DOS of the form <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(1-\frac{E^{2}}{\Delta ^{2}})^{-0.5}$</tex-math></inline-formula> . A short series approximation using Gaussian quadrature is computed for the superconductor DOS integral. Short series approximations are also applied to the Fréchet derivatives of the integral with respect to its parameters. All short series approximations will be compared against numeric integration solutions and results in a six hundred fold reduction in integration time. A table containing the short series approximation roots and weights is given. Error plots are shown for the short series approximations at different temperatures for the niobium pair-breaking potential energy.

Combined effects of total ionizing dose and electromagnetic pulse on a bipolar junction transistor

Bipolar Junction Transistors (BJTs) are widely used in the presence of ionizing radiation, such as in space or in the surroundings of nuclear reactors. For the application in these fields, the influence of Total Ionization Dose (TID) and Electromagnetic Pulse (EMP) on BJTs should be carefully considered. In this work, for the comprehension of the combined effects of TID and EMP on BJT, a numerical model of a NPN BJT (2N2222) is developed with the semiconductor device simulation software TCAD. The simulated results indicate that the TID of gamma-rays can increase the depletion layer area of the BJT and lead to a reduction of its current gain and that a negative base voltage may in these conditions cause the breakdown of the emitter junction. Therefore, the results obtained indicate that the combined TID and EMP can lower the breakdown voltage of the BJT.

Mathematical model of potential distribution in a pinned photodiode of an indirect time offlightCMOS image sensor.

This paper focuses on the rapid charge transfer of lock-in pixels in time of flight 3D image sensors. Through the principal analysis, a mathematical model of potential distribution in a pinned photodiode (PPD) in different comb shapes is established. Based on this model, the influence of different comb shapes on the accelerating electric field in PPD is analyzed. The semiconductor device simulation tool SPECTRA is applied to verify the effectiveness of the model, and the simulation results are analyzed and discussed. When the width of comb tooth is in narrow and medium range, the potential changes more obviously with the increase of comb tooth angle α, whereas the potential becomes stable even if the comb tooth angle α increases sharply with the wide comb tooth width. The proposed mathematical model contributes to instructing the design of pixel transferring electrons rapidly and resolving image lag.

A proposal of quantum computing algorithm to solve Poisson equation for nanoscale devices under Neumann boundary condition

We present an implementation study of gate-type quantum computing algorithms for the purpose of semiconductor device simulations. As one of the representative quantum algorithms we consider the use of HHL (Harrow–Hassidim–Lloyd) algorithm to solve the Poisson equation in semiconductor nanowire p–n junction under the Neumann boundary condition that the electric field is zero at the electrode boundaries. Our proposed model of the quantum gate to implement the Neumann boundary condition along with the appropriately designed non-uniform mesh grid has been found to successfully reproduce the solution obtained by conventional method.

Acceleration of semiconductor device simulation using compact charge model

In this work, we propose a method to get an initial guess for the semiconductor device simulation with a compact charge model. By using the obtained initial guess, we can perform the device simulation directly at the target bias condition without a time-consuming bias ramping process. In order to verify our method, rectangular Gate-All-Around (GAA) metal–oxide–semiconductor field-effect transistors (MOSFETs) having a long channel length are considered. Results clearly show that the device simulation can be accelerated through our method.

Dark current modeling of thick perovskite X-ray detectors

Metal halide perovskites (MHPs) have demonstrated excellent performances in detection of X-rays and gamma-rays. Most studies focus on improving the sensitivity of single-pixel MHP detectors. However, little work pays attention to the dark current, which is crucial for the back-end circuit integration. Herein, the requirement of dark current is quantitatively evaluated as low as 10−9 A/cm2 for X-ray imagers integrated on pixel circuits. Moreover, through the semiconductor device analysis and simulation, we reveal that the main current compositions of thick perovskite X-ray detectors are the thermionic-emission current (JT) and the generation-recombination current (Jg-r). The typical observed failures of p–n junctions in thick detectors are caused by the high generation-recombination current due to the band mismatch and interface defects. This work provides a deep insight into the design of high sensitivity and low dark current perovskite X-ray detectors.Graphical

Simulation of transient radiation upset in a 0.18-μm CMOS SRAM by accurately modeling a 6T memory cell

Static random-access memory (SRAM) circuits exposed to pulsed X- or γ-rays will suffer transient radiation upsets, which poses a significant challenge to their reliability. The transient radiation upsets are caused by global photocurrents, which are governed by the interconnect resistance of the power supply lines and the local photocurrents generated in each memory cell. Thus, accurate modeling of local photocurrents in each memory cell is essential for predicting the global photocurrents and transient radiation upset thresholds of SRAM circuits. In this study, the local photocurrents generated in a 0.18-μm complementary metal-oxide-semiconductor (CMOS) SRAM cell are extracted using a physics-based model of the actual device structure, rather than an analytical or experimental model. The memory cell containing six transistors and parasitic structures is accurately modeled in three-dimensions. Its responses to pulsed γ-rays are simulated with an in-house parallel semiconductor device simulation program, which enables the extraction of accurate local photocurrents. These local photocurrents, together with the interconnect resistance of the power supply lines in SRAM circuits, are modeled and simulated with an in-house parallel SPICE circuit solver. Using this approach, the global photocurrents in the 0.18-μm CMOS SRAM circuit irradiated with γ-rays are obtained and the known rail-span-collapse effect is observed. Finally, the transient radiation upset threshold of the SRAM circuit is predicted numerically and verified experimentally.

A class of finite element methods with averaging techniques for solving the three-dimensional drift-diffusion model in semiconductor device simulations

Obtaining a satisfactory numerical solution of the classical three-dimensional drift-diffusion (DD) model, widely used in semiconductor device simulations, is still challenging nowadays, especially when the convection dominates the diffusion. In this work, we propose a series of finite element schemes with different types of averaging techniques to discretize the three-dimensional continuity equations. Our methods are based on the classical finite element framework, quite different from those mixed finite element/volume methods that also employ inverse averaging techniques. At first, the Slotboom variables are employed to transform the continuity equations into self-adjoint second-order elliptic equations with exponentially behaved coefficients. Then four averaging techniques, denoted with A1-A4, are introduced to approximate the exponential coefficient with its average on every tetrahedral element of the grid. The first scheme calculates the harmonic average of the exponential coefficient on a whole tetrahedron, and the other three schemes calculate the average of the exponential coefficient on each edge of a tetrahedral element. Our methods can avoid the spurious non-physical numerical oscillations and guarantee the conservation of the computed terminal currents with a terminal current evaluation approach. In fact, these methods not only maintain numerical stability but also overcome the disadvantages of some stabilization methods that cannot guarantee the conservation of the terminal currents, such as the streamline-upwind Petrov-Galerkin (SUPG) method. Moreover, the derivation of these discretization methods does not need the dual Voronoi grid as that of the finite volume Scharfetter-Gummel (FVSG) method or other mixed finite element/volume methods with inverse averaging techniques, which greatly reduces the complexity of parallel implementations of our methods. Simulations on two realistic three-dimensional semiconductor devices are carried out to test the accuracy and stability of our methods. According to numerical results, we conclude that scheme A4 can produce more accurate numerical solutions than the other three schemes, especially when the bias applied on the electrode is high. Numerical results also show that the scheme A4 is more robust than the Zlámal finite element method [1] in high-bias cases, and it also performs better than the FVSG method and a tetrahedral mixed finite element method [2] on poor-quality grids. Scheme A4 is also employed to study rich physical properties of the n-channel MOSFET.

A Geometrical Optimization Rule of the Synaptic Pass-Transistor for a Low Power Analog Accelerator

We present a simulation framework on a geometrical optimization rule of the synaptic pass-transistor (SPT) for a low power analog accelerator (AA). Here, the SPT is a synaptic transistor (Syn-Tr) in series with a load resistor to be scaled with respect to a geometrical ratio between a channel length and width of the Syn-Tr. When only the load resistance is increased for the reduction of the power consumption of the SPT, a synaptic characteristics (e.g. a synaptic dynamic ratio, DR_w) is hard to be maintained. To overcome this, the channel geometrical ratio and scaling factor of the load resistance are required to be increased equally, thus a geometrical optimization rule. Here, the load resistance is equivalent to a geometric mean where two extreme cases of the synaptic full facilitation and full depression are considered. To verify the proposed rule, we perform a semiconductor device simulation for a static and pulsed characteristics of the SPT. When the SPT is scaled based on the geometrical optimization rule, from the simulation results, it is found that the static-power consumption is decreased while maintaining the DR_w. As a trade-off relation, however, the noise power-spectral-density is found to be increased due to a higher thermal noise associated with a higher total resistance of the scaled SPT. Here, the increased noise power-spectral-density of a single SPT may affect the performance of the AA based on the SPT-array, so we also show a crossbar simulation, checking the classification accuracy against a standard dataset (e.g. MNIST).

Modeling and Simulations of 4H-SiC/6H-SiC/4H-SiC Single Quantum-Well Light Emitting Diode Using Diffusion Bonding Technique.

In the last decade, silicon carbide (SiC) has emerged as a potential material for high-frequency electronics and optoelectronics applications that may require elevated temperature processing. SiC exists in more than 200 different crystallographic forms, referred to as polytypes. Based on their remarkable physical and electrical characteristics, such as better thermal and electrical conductivities, 3C-SiC, 4H-SiC, and 6H-SiC are considered as the most distinguished polytypes of SiC. In this article, physical device simulation of a light-emitting diode (LED) based on the unique structural configuration of 4H-SiC and 6H-SiC layers has been performed which corresponds to a novel material joining technique, called diffusion welding/bonding. The proposed single quantum well (SQW) edge-emitting SiC-based LED has been simulated using a commercially available semiconductor device simulator, SILVACO TCAD. Moreover, by varying different design parameters, the current-voltage characteristics, luminous power, and power spectral density have been calculated. Our proposed LED device exhibited promising results in terms of luminous power efficiency and external quantum efficiency (EQE). The device numerically achieved a luminous efficiency of 25% and EQE of 16.43%, which is at par performance for a SQW LED. The resultant LED structure can be customized by choosing appropriate materials of varying bandgaps to extract the light emission spectrum in the desired wavelength range. It is anticipated that the physical fabrication of our proposed LED by direct bonding of SiC-SiC wafers will pave the way for the future development of efficient and cost-effective SiC-based LEDs.

On the Optoelectronic Mechanisms Ruling Ti‐hyperdoped Si Photodiodes

AbstractThis work deepens the understanding of the optoelectronic mechanisms ruling hyperdoped‐based photodevices and shows the potential of Ti hyperdoped‐Si as a fully complementary metal‐oxide semiconductor compatible material for room‐temperature infrared photodetection technologies. By the combination of ion implantation and laser‐based methods, ≈20 nm thin hyperdoped single‐crystal Si layers with a Ti concentration as high as 1020 cm−3 are obtained. The Ti hyperdoped Si/p‐Si photodiode shows a room temperature rectification factor at ±1 V of 509. Analysis of the temperature‐dependent current–voltage characteristics shows that the transport is dominated by two mechanisms: a tunnel mechanism at low bias and a recombination process in the space charge region at high bias. A room‐temperature sub‐bandgap external quantum efficiency (EQE) extending to 2.5 µm wavelength is obtained. Temperature‐dependent spectral photoresponse behavior reveals an increase of the EQE as the temperature decreases, showing a low‐energy photoresponse edge at 0.45 eV and a high‐energy photoresponse edge at 0.67 eV. Temperature behavior of the open‐circuit voltage correlates with the high‐energy photoresponse edge. A model is proposed to relate the optoelectronic mechanisms to sub‐bandgap optical transitions involving an impurity band. This model is supported by numerical semiconductor device simulations using the SCAPS software.