3rd MFEM Community Workshop, October 26, 2023

MFEM is a free, open source, lightweight, scalable C++ library for finite element methods.

Features

• Arbitrary high-order finite element meshes and spaces.
• Wide variety of finite element discretization approaches.
• Conforming and nonconforming adaptive mesh refinement.
• Scalable from laptops to GPU-accelerated supercomputers.
• ... and many more.
• See MFEM Gallery, Publications, Videos and News pages.

MFEM is used in many projects, including BLAST, Cardioid, Palace, VisIt, RF-SciDAC, FASTMath, xSDK, and CEED in the Exascale Computing Project.

Annual workshop 

MFEM host an annual workshop and FEM@LLNL seminar series. The MFEM team has  announced the 3rd MFEM Community Workshop, which will take place on October 26, 2023, virtually, using Zoom for videoconferencing. The goal of the workshop is to foster collaboration among all MFEM users and developers, share the latest MFEM features with the broader community, deepen application engagements, and solicit feedback to guide future development directions for the project.

Registration
If you plan to attend, please register no later than October 19th. There is no registration fee. Zoom details will be distributed to participants prior to the event date. For questions, please contact the meeting organizers at mfem@llnl.gov.

[paper] Model for Cryo-CMOS Subthreshold Swing

Arnout Beckers, Jakob Michl, Alexander Grill, Member IEEE; Ben Kaczer, Marie Garcia Bardon, Bertrand Parvais, Bogdan Govoreanu, Kristiaan De Greve, Gaspard Hiblot, 

and Geert Hellings, Senior Member IEEE

Physics-Based and Closed-Form Model for Cryo-CMOS Subthreshold Swing

in IEEE Transactions on Nanotechnology, vol. 22, pp. 590-596, 2023,

DOI 10.1109/TNANO.2023.3314811.

IMEC, Leuven (B)

Institute for Microelectronics, TU Vienna (A)

Vrije Universiteit Brussel (B)

KU Leuven (B)

Abstract: Cryogenic semiconductor device models are essential in designing control systems for quantum devices and in benchmarking the benefits of cryogenic cooling for high-performance computing. In particular, the saturation of subthreshold swing due to band tails is an important phenomenon to include in low-temperature analytical MOSFET models, as it predicts theoretical lower bounds on the leakage power and supply voltage in tailored cryogenic CMOS technologies with tuned threshold voltages. Previous physics-based modeling required to evaluate functions with no closed-form solutions, defeating the purpose of fast and efficient model evaluation. Thus far, only the empirically proposed expressions are in closed form. This article bridges this gap by deriving a physics-based and closed-form model for the full saturating trend of the subthreshold swing from room down to low temperature. The proposed model is compared against experimental data taken on some long and short devices from a commercial 28-nm bulk CMOS technology down to 4.2 K.

FIG: (a) TEM picture of a mature imec technology node. (b) Electrostatic potential fluctuations near the channel/oxide interface. (c) Gaussian distributed depths of the potential wells. (d) Including the binding energy in the wells in the quantum picture gives a Laplace distribution of P(Eb). (e-f) Convolution (*) of P(Eb) with the sharp-edged 2-D DOS leads to a logistic/Fermi-like DOS function with an exponential tail.

[paper] Characterization and Modeling of SOI LBJTs at 4K

Yuanke Zhang, Yuefeng Chen, Yifang Zhang, Jun Xu, Chao Luo, and Guoping Guo

Characterization and Modeling of Silicon-on-Insulator 

Lateral Bipolar Junction Transistors at Liquid Helium Temperature

IEEE TED Vol. XX, No. XX, preprint arXiv:2309.09257 (2023).

University of Science and Technology of China (USTC), Hefei 230026, Anhui, China

CAS Key Lab ofQuantum Information, Hefei 230026, Anhui, China.

Abstract: Conventional silicon bipolars are not suitable for low-temperature operation due to the deterioration of current gain (β). In this paper, we characterize lateral bipolar junction transistors (LBJTs) fabricated on silicon-on insulator (SOI) wafers down to liquid helium temperature (4 K). The positive SOI substrate bias could greatly increase the collector current and have a negligible effect on the base current, which significantly alleviates β degradation at low temperatures. We present a physical-based compact LBJT model for 4 K simulation, in which the collector current (IC) consists of the tunneling current and the additional current component near the buried oxide (BOX)/silicon interface caused by the substrate modulation effect. This model is able to fit the Gummel characteristics of LBJTs very well and has promising applications in amplifier circuits simulation for silicon-based qubits signals.

Fig: IC (solid lines) and IB (dash lines) versus VBE of LBJT at different temperatures 

under (a) VBOX = 0 V; (b) VBOX = 12 V, VCE = 1 V.

Acknowledgement: The device fabrication was done by Prof. Zhen Zhang’s group in the Angstrom Microstructure Laboratory (MSL) at Uppsala University. Dr. Qitao Hu, Dr. Si Chen, Prof. Zhen Zhang are acknowledged for the device design and fabrication, and the technical staff of MSL are acknowledged for their process support.

Palace: 3D Finite Element Solver for Computational Electromagnetics

Palace, for PArallel LArge-scale Computational Electromagnetics, is an open-source, parallel finite element code for full-wave 3D electromagnetic simulations in the frequency or time domain, using the MFEM finite element discretization library.

Key features:

• Eigenmode calculations with optional material or radiative loss including lumped impedance boundaries. Automatic postprocessing of energy-participation ratios (EPRs) for circuit quantization and interface or bulk participation ratios for predicting dielectric loss.
• Frequency domain driven simulations with surface current excitation and lumped or numeric wave port boundaries. Wideband frequency response calculation using uniform frequency space sampling or an adaptive fast frequency sweep algorithm.
• Explicit or fully-implicit time domain solver for transient electromagnetic analysis.
• Lumped capacitance and inductance matrix extraction via electrostatic and magnetostatic problem formulations.
• Support for a wide range of mesh file formats for structured and unstructured meshes, with built-in uniform or region-based parallel mesh refinement.
• Arbitrary high-order finite element spaces and curvilinear mesh support thanks to the MFEM library.
• Scalable algorithms for the solution of linear systems of equations, including geometric multigrid (GMG), parallel sparse direct solvers, and algebraic multigrid (AMG) preconditioners, for fast performance on platforms ranging from laptops to HPC systems.

Crosstalk Between Coplanar Waveguides Example: Views of the mesh boundaries for these two configurations are shown below. In both cases the computational domain is discretized using an unstructured tetrahedral mesh. 

[workshop] gdsfactory

gdsfactory workshop, UCSB Photonics Society

August 24, 2023 in Henley Hall, UCSB

The first gdsfactory hands-on workshop organized as part of the  UCSB  Photonics Society. Thomas Dorch from Freedom Photonics and Andrei Isichenko presented gdsfactory. Last year gdsfactory seminar by Joaquin Matres, the maintainer of gdsfactory - you can access the video recording here. This workshop was a hands on; things to do before the workshop

1. Install anaconda python 3 on your computer. If you don't have it installed, the links below are for miniconda, a "lightweight" version of anaconda. Windows: link. Mac: link (select if Intel or M1).
2. Download a Python IDE. Either Visual Studio Code or Pycharm. Personally I prefer VS Code
3. Download klayout. Windows: link. Mac: link (works on M1 mac, Ventura 13.4).

gdsfactory team will be running the tutorial using python notebooks (.ipynb). These can be run through JupyterLab, VS code (install this extension), or through Google Colab. You have the option to skip all the steps above and run the notebooks entirely in Google Colab (but with some limitations in klayout integration). You can try it out using this notebooks in this repository, focused on workshop_part1.ipynb. In Google Drive you should have the option to select "open with Google Colaboratory"

Click the "Open in Colab" link above to get started, and save a copy of the notebook to your Google Drive.

Google Drive Links to the notebooks:

https://drive.google.com/file/d/1x6kHQ9nHb1HB4HOEiEr1BG_y5lQ8si3e/view?usp=sharing

https://drive.google.com/file/d/1Ppz-CDrFezfLTIAHeBLYopl6Q4oyt8a4/view?usp=sharing