Simulation of Heterojunction Bipolar Transistors in Two Dimensions

This work describes the formulation and, development of a two-- dimensional drift-diffusion simulation program for accurate modeling of heterojunction bipolar transistors (HBT\u27s). The model described is a versatile tool for studying HBT\u27s, allowing the user to determine the terminal characteristics and physical operation of devices. Nonplanar structures can be treated, response to transient conditions can be computed, and the high frequency characteristics of transistors may be projected. The formulation of an electron energy balance equation is presented and included in the model in an attempt to more accurately compute high-field transport characteristics. The model is applied to some common design questions and experimental results are reproduced

Unified simulation of transport and luminescence inoptoelectronic nanostructures

Computer simulation of microscopic transport and light emission in semiconductor nanostructures is often restricted to an isolated system of a single quantum well, wire or dot. In this work we report on the development of a simulator for devices with various kinds of nanostructures which exhibit quantization in different dimensionalities. Our approach is based upon the partition of the carrier densities within each quantization region into bound and unbound populations. A bound carrier is treated fully coherent in the directions of confinement, whereas it is assumed to be totally incoherent with a motion driven by classical drift and diffusion in the remaining directions. Coupling of the populations takes place through electrostatics and carrier capture. We illustrate the applicability of our approach with a well-wire structur

A crossbar network for silicon quantum dot qubits

Copyright © 2018 The Authors. The spin states of single electrons in gate-defined quantum dots satisfy crucial requirements for a practical quantum computer. These include extremely long coherence times, high-fidelity quantum operation, and the ability to shuttle electrons as a mechanism for on-chip flying qubits. To increase the number of qubits to the thousands or millions of qubits needed for practical quantum information, we present an architecture based on shared control and a scalable number of lines. Crucially, the control lines define the qubit grid, such that no local components are required. Our design enables qubit coupling beyond nearest neighbors, providing prospects for nonplanar quantum error correction protocols. Fabrication is based on a three-layer design to define qubit and tunnel barrier gates. We show that a double stripline on top of the structure can drive high-fidelity single-qubit rotations. Self-aligned inhomogeneous magnetic fields induced by direct currents through superconducting gates enable qubit addressability and readout. Qubit coupling is based on the exchange interaction, and we show that parallel two-qubit gates can be performed at the detuning-noise insensitive point. While the architecture requires a high level of uniformity in the materials and critical dimensions to enable shared control, it stands out for its simplicity and provides prospects for large-scale quantum computation in the near future

On the numerical modeling of terahertz photoconductive antennas

This paper shows the relevance of mobility models to describe the car- rier dynamics for the analysis of radiative semiconductor photoconductive devices in the terahertz regime. We have built a simulator that self-consistently solves the device physics and Maxwell’s equations to study the radiated fields. In particu- lar, we show a significant influence of an accurate description of the steady-state regime of the semiconductor device for calculating radiated electromagnetic fields in the broadside direction. Comparison with measurements shows the accuracy of our simulator and demonstrates the superior performance of numerical schemes based not only on the description of the carrier, electric potential, and field dis- tributions, but also on reliable local mobility models.This work was supported in part by the Spanish Ministry of Educa- tion under Project CSD2008-00068, the Junta de Andalucia Project P09-TIC-5327, the EU FP7/2007-2013, under grant 205294 (HIRF-SE project), and the Spanish National Project TEC2010-20841-C04-04

Generalized Scharfetter--Gummel schemes for electro-thermal transport in degenerate semiconductors using the Kelvin formula for the Seebeck coefficient

Many challenges faced in today's semiconductor devices are related to self-heating phenomena. The optimization of device designs can be assisted by numerical simulations using the non-isothermal drift-diffusion system, where the magnitude of the thermoelectric cross effects is controlled by the Seebeck coefficient. We show that the model equations take a remarkably simple form when assuming the so-called Kelvin formula for the Seebeck coefficient. The corresponding heat generation rate involves exactly the three classically known self-heating effects, namely Joule, recombination and Thomson--Peltier heating, without any further (transient) contributions. Moreover, the thermal driving force in the electrical current density expressions can be entirely absorbed in the (nonlinear) diffusion coefficient via a generalized Einstein relation. The efficient numerical simulation relies on an accurate and robust discretization technique for the fluxes (finite volume Scharfetter--Gummel method), which allows to cope with the typically stiff solutions of the semiconductor device equations. We derive two non-isothermal generalizations of the Scharfetter--Gummel scheme for degenerate semiconductors (Fermi--Dirac statistics) obeying the Kelvin formula. The approaches differ in the treatment of degeneration effects: The first is based on an approximation of the discrete generalized Einstein relation implying a specifically modified thermal voltage, whereas the second scheme follows the conventionally used approach employing a modified electric field. We present a detailed analysis and comparison of both schemes, indicating a superior performance of the modified thermal voltage scheme

Advanced III-V / Si nano-scale transistors and contacts: Modeling and analysis

The exponential miniaturization of Si CMOS technology has been a key to the electronics revolution. However, the continuous downscaling of the gate length becomes the biggest challenge to maintain higher speed, lower power, and better electrostatic integrity for each following generation. Hence, novel devices and better channel materials than Si are considered to improve the metal-oxide-semiconductor field-effect transistors (MOSFETs) device performance. III-V compound semiconductors and multi-gate structures are being considered as promising candidates in the next CMOS technology. III-V and Si nano-scale transistors in different architectures are investigated (1) to compare the performance between InGaAs of III-V compound semiconductors and strained-Si in planar FETs and triple-gate non-planar FinFETs. (2) to demonstrate whether or not these technologies are viable alternatives to Si and conventional planar FETs. The simulation results indicate that III-V FETs do not outperform Si FETs in the ballistic transport regime, and triple-gate FinFETs surely represent the best architecture for sub-15nm gate contacts, independently from the choice of channel material. ^ This work also proves that the contact resistance becomes a limiting factor of device performance as it takes larger fraction of the total on-state resistance. Hence, contact resistance must be reduced to meet the next ITRS requirements. However, from a modeling point of view, the understanding of the contacts still remains limited due to its size and multiple associated scattering effects, while the intrinsic device performance can be projected. Therefore, a precise theoretical modeling is required to advance optimized contact design to improve overall device performance. In this work, various factors of the contact resistances are investigated within realistic contact-to-channel structure of III-V quantum well field-effect transistors (QWFET). The key finding is that the contact-to-channel resistance is mainly caused by structural reasons: 1) barriers between multiple layers in the contact region 2) Schottky barrier between metal and contact pad. These two barriers work as bottleneck of the system conductance. The extracted contact resistance matches with the experimental value. The approximation of contact resistance from quantum transport simulation can be very useful to guide better contact designs of the future technology nodes. ^ The theoretical modeling of these nano-scale devices demands a proper treatment of quantum effects such as the energy-level quantization caused by strong quantum confinement of electrons and band structure non-parabolicity. 2-D and 3-D quantum transport simulator that solves non-equilibrium Green\u27s functions (NEGF) transport and Poisson equations self-consistently within a real-space effective mass approximation. The sp3d5s* empirical tight-binding method is employed to include non-parabolicity to obtain more accurate effective masses in confined nano-structures. The accomplishment of this work would aid in designing, engineering and manufacturing nano-scale devices, as well as next-generation microchips and other electronics with nano-scale features

Bandwidth enhancement of dielectric resonator antennas

An experimental investigation of bandwidth enhancement of dielectric resonator antennas (DRA) using parasitic elements is reported. Substantial bandwidth enhancement for the HE(sub 11delta) mode of the stacked geometry and for the HE(sub 13delta) mode of the coplanar collinear geometry was demonstrated. Excellent radiation patterns for the HE(sub 11delta) mode were also recorded

Generalized Scharfetter-Gummel schemes for electro-thermal transport in degenerate semiconductors using the Kelvin formula for the Seebeck coefficient

Many challenges faced in today's semiconductor devices are related to self-heating phenomena. The optimization of device designs can be assisted by numerical simulations using the non-isothermal drift-diffusion system, where the magnitude of the thermoelectric cross effects is controlled by the Seebeck coefficient. We show that the model equations take a remarkably simple form when assuming the so-called Kelvin formula for the Seebeck coefficient. The corresponding heat generation rate involves exactly the three classically known self-heating effects, namely Joule, recombination and Thomson-Peltier heating, without any further (transient) contributions. Moreover, the thermal driving force in the electrical current density expressions can be entirely absorbed in the diffusion coefficient via a generalized Einstein relation. The efficient numerical simulation relies on an accurate and robust discretization technique for the fluxes (finite volume Scharfetter-Gummel method), which allows to cope with the typically stiff solutions of the semiconductor device equations. We derive two non-isothermal generalizations of the Scharfetter-Gummel scheme for degenerate semiconductors (Fermi-Dirac statistics) obeying the Kelvin formula. The approaches differ in the treatment of degeneration effects: The first is based on an approximation of the discrete generalized Einstein relation implying a specifically modified thermal voltage, whereas the second scheme follows the conventionally used approach employing a modified electric field. We present a detailed analysis and comparison of both schemes, indicating a superior performance of the modified thermal voltage scheme.Comment: 26 pages, 7 figure

Doctor of Philosophy

dissertationOptics is an old topic in physical science and engineering. Historically, bulky materials and components were dominantly used to manipulate light. A new hope arrived when Maxwell unveiled the essence of electromagnetic waves in a micro perspective. On the other side, our world recently embraced a revolutionary technology, metasurface, which modifies the properties of matter-interfaces in subwavelength scale. To complete this story, diffractive optic fills right in the gap. It enables ultrathin flat devices without invoking the concept of nanostructured metasurfaces when only scalar diffraction comes into play. This dissertation contributes to developing a new type of digital diffractive optic, called a polychromat. It consists of uniform pixels and multilevel profile in micrometer scale. Essentially, it modulates the phase of a wavefront to generate certain spatial and spectral responses. Firstly, a complete numerical model based on scalar diffraction theory was developed. In order to functionalize the optic, a nonlinear algorithm was then successfully implemented to optimize its topography. The optic can be patterned in transparent dielectric thin film by single-step grayscale lithography and it is replicable for mass production. The microstructures are 3?m wide and no more than 3?m thick, thus do not require slow and expensive nanopatterning techniques, as opposed to metasurfaces. Polychromat is also less demanding in terms of fabrication and scalability. The next theme is focused on demonstrating unprecedented performances of the diffractive optic when applied to address critical issues in modern society. Photovoltaic efficiency can be significantly enhanced using this optic to split and concentrate the solar spectrum. Focusing through a lens is no news, but we transformed our optic into a flat lens that corrects broadband chromatic aberrations. It can also serve as a phase mask for microlithography on oblique and multiplane surfaces. By introducing the powerful tool of computation, we devised two imaging prototypes, replacing the conventional Bayer filter with the diffractive optic. One system increases light sensitivity by 3 times compared to commercial color sensors. The other one renders the monochrome sensor a new function of high-resolution multispectral video-imaging