Through Technology Computer-aided Design Research Articles

Reliability of PERC Solar Cells under Alpha Irradiation: A Simulation-Based Study for Space Applications

The effects of alpha particles’ irradiation on the performance of simplified PERC silicon solar cells are studied through technology computer aided design (TCAD) simulations. The methodology adopted in this work incorporates the non-ionizing energy loss (NIEL) phenomenon to understand the radiation effects, which does not depend on extensive irradiation experimentations. Simulation results show that alpha particles in the range of ∼10 MeV and with fluence >1014 cm−2 lead to maximum performance degradation of the solar cell. Moreover, it is found that upon irradiating the solar cell with 1 MeV alpha particles, the output power reduces proportionally with increasing fluence due to the reduction of minority carrier lifetime in the base. On the other hand, the fill factor (FF) degrades by only <10% up to a fluence of ∼1013 cm−2, after which it decreases sharply. This trend reveals that the FF deteriorates significantly only when the irradiation process creates a Frenkel pair density that approaches the majority carrier density in the base. The methodology presented in this work opens new research avenues for potentially optimizing the solar cell design not only for better efficiency but also for improved reliability in radiation rich environments.

Comparative Analysis of Hot Carrier Degradation (HCD) in 10-nm Node nMOS/pMOS FinFET Devices

In 10-nm node core FinFETs, we analyzed the cause of higher hot carrier degradation (HCD) in pFinFETs than in nFinFETs. Self-heating effect is severe in pFinFETs because SiGe is used as the source/drain materials, which makes the device temperature higher than nFinFETs. Theoretically, because the lifetime of multiple particle (MP) mechanism decreases as temperature increases, degradation due to MP decreases. Therefore, it is difficult for the pure HCD mechanisms to occur more in pFinFETs, which has higher temperature than nFinFETs. However, in pFinFETs, unlike nFinFETs, interface traps can be generated due to negative-bias temperature instability (NBTI) that occurs by the reaction between inversion holes and electrons of Si-H bonds. Also, since NBTI deteriorates more as the temperature increases, the phenomenon of higher degradation in pFinFET than nFinFET can be explained with the NBTI mechanism. Therefore, we propose an additional NBTI mechanism that is caused by high device temperature in pFinFETs even in the HCD condition. In addition, the main components were investigated through measurements of current degradation rate in various voltage conditions, and it was found that NBTI is dominant in pFinFETs. Finally, NBTI that can occur in the HCD condition was predicted through technology computer-aided design (TCAD) simulation. As a result, degradation due to pure hot carriers without NBTI occurs more in nFinFETs than in pFinFETs.

Normally-Off AlGaN/GaN MOSHEMT as Lebel Free Biosensor

In this paper, a systematic analytical model of biosensing parameters such as transfer characteristic, output characteristic, channel potential, transconductance-to-drain current ratio (gm/Ids),change in threshold voltage (ΔVTh), and sensitivity for a dielectric modulated normally off AlGaN/GaN metal oxide semiconductor high electron mobility transistor (MOSHEMT) is reported. A detailed analysis of the concept of gate cavity dielectric modulation in MOSHEMTs is performed with focus on the enhancement of sensing parameters to signify the presence of biomolecules such as Streptavidin, Biotin, and APTES. The accuracy of the model result is validated through Technology Computer Aided Design (TCAD) device simulator by incorporating nano gaps in MOSHEMT structure as well as suitable physics-based mathematical models are considered in simulation. Results highlighted a new view points and deep insight the design of Normally off AlGaN/GaN MOSHEMT-based dielectric modulated biosensors. Furthermore, this heterostructure provide a useful approach towards biomolecular detection and meeting the demand of interdisciplinary novel semiconductor device design.

A Quantum-Well Charge-Trap Synaptic Transistor With Highly Linear Weight Tunability

In this work, a novel synaptic transistor has been proposed and analyzed through technology computer-aided design (TCAD) simulation. The proposed device has merits of full-Si processing compatibility, short- and long-term plasticity, high energy efficiency, and linear and symmetric conductance adjustability. The proposed device consists of a quantum-well structure and a charge-trap unit for realizing both short- and long-term memories, respectively. The quantum-well charge-trap synaptic transistor (QW CTS) employs two independent gates to separate inference and weight adjustment operation. An optimally designed and validated QW CTS has demonstrated a highly linear and symmetric weight tunability, with an ultra-low energy consumption of ~1.5 fJ per synaptic event. The QW CTS can be a core element in the hardware-driven Si neuromorphic system.

Variability-Aware Simulation Strategy for Gate-All-Around Vertical Field Effect Transistor.

In this work, the work function variation (WFV) and global variability (GV) sources on 5 nm node gate-all-around (GAA) silicon vertical field-effect transistor (VFET) devices are studied through technology computer-aided design (TCAD) simulations and spice simulation based on BSIM-CMG model. Compared to conventional lateral FET devices, VFETs can increase the gate area effectively while minimizing the loss of layout area due to their structural characteristics. Considering VFET devices below 5 nm node, an expansion of the gate area of the device reduces the influence of WFV. However, the effect of GV is exacerbated by weakening gate controllability. In order to analyze the exact variability issues, it is necessary to consider not only the influence by the WFV but also the influence by the GV. Therefore, we propose accurate guideline by analyzing the integrated variability issues in a various VFET device structures in a single device and a 6-T SRAM bit cells.

Transient Analysis of Tunnel Field-Effect Transistor with Raised Drain.

This paper investigates the electrical performance of a proposed tunnel field-effect transistor (TFET) via transient analysis. The proposed TFET has three features: a SiGe channel, a fin structure, and an elevated drain. As the SiGe channel and fin structure make a small tunnel resistance, the ONstate current can be increased. In addition, an elevated drain can suppress an ambipolar current (IAMB). The transient characteristics should be confirmed in terms of inverter switching for the high applicability of the proposed device to the logical circuit. The analysis is verified through technology computer-aided design (TCAD) simulations calibrated with rigorously fabricated devices. Based on the simulation results, we conclude that the proposed TFET shows better ON/OFF transient characteristics when compared to conventional TFETs and the small gate-to-drain capacitance (CGD) can improve the transient characteristics in TFET.

A Silicon-Compatible Synaptic Transistor Capable of Multiple Synaptic Weights toward Energy-Efficient Neuromorphic Systems

In order to resolve the issue of tremendous energy consumption in conventional artificial intelligence, hardware-based neuromorphic system is being actively studied. Although various synaptic devices for the system have been proposed, they have shown limits in terms of endurance, reliability, energy efficiency, and Si processing compatibility. In this work, we design a synaptic transistor with short-term and long-term plasticity, high density, high reliability and energy efficiency, and Si processing compatibility. The synaptic characteristics of the device are closely examined and validated through technology computer-aided design (TCAD) device simulation. Consequently, full synaptic functions with high energy efficiency have been realized.

TCAD calibration and performance investigation of an ISFET-based TNT (explosive) sensor

This article proposes a calibrated methodology as well as computational analyses for designing an explosive sensor for trinitrotoluene (TNT) detection using ion-sensitive field-effect transistor (ISFET) through technology computer-aided design (TCAD) tools. Although industrial device simulators are quite efficient in computing the various electrical parameters of a semiconductor device such as MOSFET, the absence of electrochemical models has posed a challenge to the investigation of ISFETs through TCAD simulations. A nanowire MOSFET has been calibrated using thirteen different concentrations of TNT from a fabricated sensor, and different sensitivities based on on current, threshold voltage and conductance have been proposed. The effect of gate oxide on the different sensitivities is also reported.

Ground Plane Impact on Performance of Relaxed Ge FinFETs

The implementation of a barrier potential layer underneath the channel region, well known as Ground Plane (GP) implantation, and its influence on the performance of relaxed germanium pFinFET devices is investigated in this manuscript. This study aims to explain the fin width dependence of the threshold voltage from experimental data and evaluates the ground plane doping concentration and its depth influence on relaxed p-type channel germanium FinFET parameters, as threshold voltage, transconductance and subthreshold swing, through Technology Computer-Aided Design (TCAD) numerical simulations. The threshold voltage variation reaches up to 80 mV from the narrowest device to the widest one, considering the studied range of ground plane doping concentration. Concerning the subthreshold swing parameter, neither the GP doping concentration, nor its depth play a significant role since the electrostatic coupling is predominant.

Efficient Sensitivity and Variability Analysis of Nonlinear Microwave Stages Through Concurrent TCAD and EM Modeling

An accurate, yet computationally efficient, computer-aided design (CAD) framework is proposed for the concurrent variability analysis of the active and a passive part of a radio frequency (RF)/microwave nonlinear stage. Both the active and passive parts are modeled, fully retaining a link to their physical and technological parameters. This allows for a global assessment of the nonlinear stage sensitivity and variability due to process variations. The active device is first modeled through technology CAD (TCAD); then, the model is implemented within an RF/microwave electronic design automation (EDA) circuit simulator through X-parameters. The passive part is modeled by means of accurate electromagnetic (EM) simulations and then taken into account in the circuit simulator through parametrized S-parameters. The method is demonstrated by analyzing a deep class AB power amplifier (PA) at 12 GHz in GaAs field-effect transistor (FET) technology. In particular, a Monte Carlo analysis of the output power variability around the nominal value of 26.4 dBm due to technological variations of both active device and output matching network is presented. The active device variability is shown to dominate over the passive structure one, even if up to 30% of the overall variance is due to the passive elements at intermediate input power levels.

Process variation dependence of total ionizing dose effects in bulk nFinFETs

The electrical characteristics of bulk nFinFETs after total-ionizing-dose (TID) irradiation were experimentally evaluated with respect to the process variation (PV). The inconsistent transfer characteristics, particularly the different threshold voltage shifts under an identical bias condition and irradiation dose, were investigated through technology computer aided design (TCAD) simulation, considering the shallow trench isolation (STI) footing and the asymmetrical sidewall. The influence that the bias condition had, as an important factor including ON, OFF, and transmission gate (TG), is included in this paper to better illustrate the PV dependence. The HfO2 high-ĸ metal gate, the STI, and the PV dependent geometry were some of the influences that occurred within the degradation of electrical performances as the TID increased. A radiation-hardness strategy was proposed in order to improve the electrical properties of the FinFET devices in the radiation environments.

Nanowire size dependence on sensitivity of silicon nanowire field-effect transistor-based pH sensor

In this study, we investigated the effects of nanowire size on the current sensitivity of silicon nanowire (SiNW) ion-sensitive field-effect transistors (ISFETs). The changes in on-current (Ion) and resistance according to pH were measured in fabricated SiNW ISFETs of various lengths and widths. As a result, it was revealed that the sensitivity expressed as relative Ion change improves as the width decreases. Through technology computer-aided design (TCAD) simulation analysis, the width dependence on the relative Ion change can be explained by the observation that the target molecules located at the edge region along the channel width have a stronger effect on the sensitivity as the SiNW width is reduced. Additionally, the length dependence on the sensitivity can be understood in terms of the resistance ratio of the fixed parasitic resistance, including source/drain resistance, to the varying channel resistance as a function of channel length.

Analysis on temperature dependent current mechanism of tunnel field-effect transistors

In this paper, the total drain current (ID) of a tunnel FET (TFET) is decomposed into each current component with different origins to analyze the ID formation mechanisms of the TFET as a function of gate voltage (VGS). Transfer characteristics are firstly extracted with fabricated Silicon channel TFETs (Si TFETs) and silicon germanium channel TFETs (SiGe TFETs) at various temperatures. The subthreshold swings (SS) of both Si TFETs and SiGe TFETs get degraded and the SSs of SiGe TFETs get degraded more as temperature becomes higher. Then, all the IDs measured at various temperatures are decomposed into each current component through technology computer aided design (TCAD) simulations with a good agreement with experimental data. As a result, it is revealed that Shockley–Read–Hall (SRH) recombination mainly contribute to the ID of a TFET before band to band tunneling (BTBT) occurs. Furthermore, the SS degradation by high temperature is explained successfully by the SRH recombination with electric field dependence.

Numerical Simulation and Modeling of Resistive and Recombination Losses in MWT Solar Cells

This study analyzes the impact of resistive and recombination losses in metal wrap through (MWT) solar cells through technology computer aided design (TCAD) numerical simulations. Two types of MWT architectures are considered in this study: “point busbar,” featuring one circular tabbing contact for each via at the back side, and “continuous busbar,” in which the rear busbar connects all the vias along a line. A comparison with conventional, H-pattern, front contact (FC) solar cells is performed by adopting the surface recombination velocity at the rear-contact isolation region as a parameter representative of possible passivation options. The differences under dark and light conditions are highlighted. Moreover, the following resistive losses in MWT cells are investigated: via resistance, shunting effect, and lateral conduction of charge carriers above rear busbar. An analytical model to account for the lateral conduction of charge carriers is proposed and validated by means of numerical simulations. While the advantage of MWT over FC cells is confirmed by simulation, we quantitatively show how the resistive and recombination losses limit the efficiency of MWT cells.

Fin shape fluctuations in FinFET: Correlation to electrical variability and impact on 6-T SRAM noise margins

Threshold voltage ( V T ) and drive current ( I ON ) variability of low stand-by power (LSTP)-32 nm FinFETs subject to fin line-edge roughness (LER) is investigated through Technology Computer-Aided Design (TCAD) simulations featuring quantum-corrected hydrodynamic transport. Statistical results provided by an ensemble Monte Carlo (MC) approach highlight an increase in the average V T and a decrease in the average I ON with respect to sensitivity analysis based predictions. Correlations of fin shape fluctuations to electrical performance are investigated, thus assessing further limitations of sensitivity analysis and proposing better alternatives to the expensive MC approach. An equivalent fin width is calculated, which allows reducing the spread in I ON scatter plots and highlights relative importance of LER in different fin regions. Simplified device instances with linearly varying fin width are simulated to better assess the impact of local thinning/thickening in the channel, source and drain extensions. Asymmetries in the device behavior are observed upon swapping the taper direction and the critical role of extensions is identified. Moreover, the impact of LER on noise margins of FinFET-based Static Random Access Memories (SRAMs) is investigated, considering the hold, read and write operating modes. Results are compared to published data on fabricated cells with similar device features. “ μ - 6 σ ” statistics extracted from 1000 mixed-mode simulations helps with assessing variability concerns for mainstream integration of aggressively scaled of FinFET-SRAMs.