THz Device Design for SiGe HBT under Sub-room Temperature to Cryogenic Conditions

BiCMOS technology can be a possible replacement for FDSOI and FinFET technology due to their higher transconductance, which allows them to operate at in THz range i.e. radio frequencies (RF) in addition to their higher voltage handling ability. The most advanced SiGe heterojunction bipolar transistor (HBT) technology (55-nm BiCMOS) demonstrates room temperature cut-off frequency ($f_{\mathrm{t}}$) and maximum oscillation frequency ($f_{\max}$) of 320 GHz and 370 GHz respectively. In this paper, we performed TCAD analysis to investigate the performance metrics, $f_{\mathrm{t}}$ and $f_{\max}$ of the SiGe HBT at different cryogenic temperatures. The calibrated Gummel characteristics reveals that a record DC current gain of $1.2\times 10^{4}$ is obtained at 77 K for $\mathrm{V}_{\text{BE}}=\mathrm{V}_{\text{CE}}=1.2\ \mathrm{V}$. The HBT device employs bandgap engineering by linearly varying the Ge concentration in the base region, which enhances the device performance. Both the bandgap engineering with linearly graded Germanium (Ge) profile (induces intrinsic drift field in the base) and the cryogenic operation of the HBT device results in enhancement of $f_{\mathrm{t}}$ and $f_{\max}$. Our simulations predict that the value of peak $f_{\mathrm{t}}$ decreases below 100 K due to increase in the emitter junction capacitance and the peak $f_{\max}$ increase is due to decrease in collector junction capacitance and base resistance. The aggregate metric $f_{\mathrm{t}}+f_{\max} > 1.2\ \text{THz}$ is achieved under cryogenic condition without scaling the device, this advantage can be utilized in the THz device applications. © 2020 IEEE

A Kinetic Monte Carlo Study of Retention Time in a POM Molecule-Based Flash Memory

The modelling of conventional and novel memory devices has gained significant traction in recent years. This is primarily because the need to store an increasingly larger amount of data demands a better understanding of the working of the novel memory devices, to enable faster development of the future technology generations. Furthermore, in-memory computing is also of great interest from the computational perspectives, to overcome the data transfer bottleneck that is prevalent in the von-Neumann architecture. These important factors necessitate the development of comprehensive TCAD simulation tools that can be used for modeling carrier dynamics in the gate oxides of the flash memory cells. In this work, we introduce the kinetic Monte Carlo module that we have developed and integrated within the Nano Electronic Simulation Software (NESS)-to model electronic charge transport in Flash memory type structures. Using the developed module, we perform retention time analysis for a polyoxometalate (POM) molecule-based charge trap flash memory. Our simulation study highlights that retention characteristics for the POM molecules have a unique feature that depends on the properties of the tunneling oxide. © 2002-2012 IEEE

Comprehensive study of cross-section dependent effective masses for silicon based gate-all-around transistors

The use of bulk effective masses in simulations of the modern-day ultra-scaled transistor is erroneous due to the strong dependence of the band structure on the cross-section dimensions and shape. This has to be accounted for in transport simulations due to the significant impact of the effective masses on quantum confinement effects and mobility. In this article, we present a methodology for the extraction of the electron effective masses, in both confinement and the transport directions, from the simulated electronic band structure of the nanowire channel. This methodology has been implemented in our in-house three-dimensional (3D) simulation engine, NESS (Nano-Electronic Simulation Software). We provide comprehensive data for the effective masses of the silicon-based nanowire transistors (NWTs) with technologically relevant cross-sectional area and transport orientations. We demonstrate the importance of the correct effective masses by showing its impact on mobility and transfer characteristics

The Use of TCAD Simulations in Semiconductor Devices Teaching

Semiconductor devices have come a long way since the invention of the point contact transistor in 1947. These tiny devices transform and shape our lives and will continue to do so in ways we have yet to discover. Skills and knowledge in semiconductor devices are therefore essential for the development of a more sustainable world. Nevertheless, the ways we teach semiconductor devices are still rooted in 20th century textbooks and methods. Learning methods and materials therefore need to be updated so that students learn in ways that are appropriate for the global, dynamic and transnational environments in which they will work. Our methodology relied on using modern Technology CAD (TCAD) device simulations to revolutionize the teaching of semiconductor devices. Based on student responses, we believe that our methodology will enhance student employability and encourage student understanding of advanced semiconductor concepts, which are required for managing the global sustainability challenges. We demonstrate how the application of state-of-the art simulation and visualization CAD tools can be used to teach undergraduate students how to understand the basic principles of key electronic devices. We provide examples of teaching materials and assessment exercises that were used to assist student learning of semiconductor devices within a transnational education (TNE) programmed. Moreover, we believe that exposing students to modern industry software tools will help embed skills development and employability

Mobility of circular and elliptical si nanowire transistors using a multi-subband 1d formalism

We have studied the impact of the cross-sectional shape on the electron mobility of n-type silicon nanowire transistors (NWTs). We have considered circular and elliptical cross-section NWTs including the most relevant multisubband scattering processes involving phonon, surface roughness, and impurity scattering. For this purpose, we use a flexible simulation framework, coupling 3D Poisson and 2D Schrödinger solvers with the semi-classical Kubo-Greenwood formalism. Moreover, we consider cross-section dependent effective masses calculated from tight binding simulations. Our results show significant mobility improvement in the elliptic NWTs in comparison to the circular one for both 100 and 110 transport directions

Enhanced Capabilities of the Nano-Electronic Simulation Software (NESS)

The aim of this paper is to present a flexible TCAD platform called Nano-Electronic Simulation Software (NESS) which enables the modelling of contemporary future electronic devices combining different simulation paradigms (with different degrees of complexity) in a unified simulation domain. NESS considers confinement-aware band structures, generates the main sources of variability, and can study their impact using different transport models. In particular, this work focuses on the new modules implemented: Kubo-Greenwood solver, Kinetic Monte Carlo solver, Gate Leakage calculation, and a full-band quantum transport solver in the presence of hole-phonon interactions using a mode-space k⋅p approach in combination with the existing NEGF module

Schrödinger Equation Based Quantum Corrections in Drift-Diffusion: A Multiscale Approach

In this work, we report the development of a 3D drift-diffusion (DD) simulator for ultrascaled transistors with quantum corrections based on the solution of the Schrödinger equation. In a novel multi-scale simulation approach we use effective masses from tight-binding calculations, carrier mobility from the semi-classical Kubo-Greenwood formalism, and quantum corrections based on self-consistent Poisson-Schrödinger solution. This scheme has been implemented into the University of Glasgow TCAD tool called NESS (Nano Electronic Simulation Software). The approach is validated with respect to non-equilibrium Green's function (NEGF) simulations in the case of nanowire field effect transistors with different cross-sectional shapes

Nano-electronic Simulation Software (NESS): a flexible nano-device simulation platform

The aim of this paper is to present a flexible and open-source multi-scale simulation software which has been developed by the Device Modelling Group at the University of Glasgow to study the charge transport in contemporary ultra-scaled Nano-CMOS devices. The name of this new simulation environment is Nano-electronic Simulation Software (NESS). Overall NESS is designed to be flexible, easy to use and extendable. Its main two modules are the structure generator and the numerical solvers module. The structure generator creates the geometry of the devices, defines the materials in each region of the simulation domain and includes eventually sources of statistical variability. The charge transport models and corresponding equations are implemented within the numerical solvers module and solved self-consistently with Poisson equation. Currently, NESS contains a drift–diffusion, Kubo–Greenwood, and non-equilibrium Green’s function (NEGF) solvers. The NEGF solver is the most important transport solver in the current version of NESS. Therefore, this paper is primarily focused on the description of the NEGF methodology and theory. It also provides comparison with the rest of the transport solvers implemented in NESS. The NEGF module in NESS can solve transport problems in the ballistic limit or including electron–phonon scattering. It also contains the Flietner model to compute the band-to-band tunneling current in heterostructures with a direct band gap. Both the structure generator and solvers are linked in NESS to supporting modules such as effective mass extractor and materials database. Simulation results are outputted in text or vtk format in order to be easily visualized and analyzed using 2D and 3D plots. The ultimate goal is for NESS to become open-source, flexible and easy to use TCAD simulation environment which can be used by researchers in both academia and industry and will facilitate collaborative software development

Lateral P–N Junction Photodiodes Using Lateral Polarity Structure GaN Films: A Theoretical Perspective

In this work, we propose lateral p–n junction self-powered ultraviolet photodiodes using lateral polarity structure GaN (LPS-GaN) films. The design of the proposed photodiode is inspired by the recent demonstrations of obtaining laterally varying doping profiles by controlling the incorporation of donor and acceptor impurities in LPS-GaN. The proposed photodiodes provide a larger area for light absorption near the surface, and at the same time provide a large photocurrent even at zero external bias voltage, thanks to the electric field distribution of the lateral p–n junction. Through technology computer-aided design simulations and semi-analytical calculations, we show that the proposed photodiodes outperform the traditionally popular metal–semiconductor–metal (MSM) GaN photodiodes. The results shown in this study hold promise for realizing solar-blind photodetectors using polar GaN and AlGaN films