Silicon Technologies Adjust to RF Applications

Silicon (Si), although not traditionally the material of choice for RF and microwave applications, has become a serious challenger to other semiconductor technologies for high-frequency applications. Fine-line electron- beam and photolithographic techniques are now capable of fabricating silicon gate sizes as small as 0.1 micron while commonly-available high-resistivity silicon wafers support low-loss microwave transmission lines. These advances, coupled with the recent development of silicon-germanium (SiGe), arm silicon integrated circuits (ICs) with the speed required for increasingly higher-frequency applications

Low-frequency noise in downscaled silicon transistors: Trends, theory and practice

By the continuing downscaling of sub-micron transistors in the range of few to one deca-nanometers, we focus on the increasing relative level of the low-frequency noise in these devices. Large amount of published data and models are reviewed and summarized, in order to capture the state-of-the-art, and to observe that the 1/area scaling of low-frequency noise holds even for carbon nanotube devices, but the noise becomes too large in order to have fully deterministic devices with area less than 10nm×10nm. The low-frequency noise models are discussed from the point of view that the noise can be both intrinsic and coupled to the charge transport in the devices, which provided a coherent picture, and more interestingly, showed that the models converge each to other, despite the many issues that one can find for the physical origin of each model. Several derivations are made to explain crossovers in noise spectra, variable random telegraph amplitudes, duality between energy and distance of charge traps, behaviors and trends for figures of merit by device downscaling, practical constraints for micropower amplifiers and dependence of phase noise on the harmonics in the oscillation signal, uncertainty and techniques of averaging by noise characterization. We have also shown how the unavoidable statistical variations by fabrication is embedded in the devices as a spatial “frozen noise”, which also follows 1/area scaling law and limits the production yield, from one side, and from other side, the “frozen noise” contributes generically to temporal 1/f noise by randomly probing the embedded variations during device operation, owing to the purely statistical accumulation of variance that follows from cause-consequence principle, and irrespectively of the actual physical process. The accumulation of variance is known as statistics of “innovation variance”, which explains the nearly log-normal distributions in the values for low-frequency noise parameters gathered from different devices, bias and other conditions, thus, the origin of geometric averaging in low-frequency noise characterizations. At present, the many models generally coincide each with other, and what makes the difference, are the values, which, however, scatter prominently in nanodevices. Perhaps, one should make some changes in the approach to the low-frequency noise in electronic devices, to emphasize the “statistics behind the numbers”, because the general physical assumptions in each model always fail at some point by the device downscaling, but irrespectively of that, the statistics works, since the low-frequency noise scales consistently with the 1/area law

Fabrication and characterization of germanium-on-silicon photodiodes

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 155-165).Germanium is becoming an increasingly popular material to use in photonic systems. Due to its strong absorption in the near infrared and its relative ease of integration on silicon, it is a promising candidate for the fabrication of CMOS-compatible photodetectors. The goal of this thesis is to understand the physics of Ge-on-Si photodiodes, especially the dark current. Low-pressure chemical vapor deposition was used to deposit thick (1 - 2 [mu]m) films on silicon substrates either selectively in oxide windows or in blanket films. Photodetectors were fabricated in both types of films and their optical and electronic properties are discussed. It was found that the main source of leakage current in these detectors is the generation of carriers at the Ge/passivation interface. This especially affects small devices, as the perimeter/area ratio is much larger than for large devices. A post-metallization anneal in nitrogen at 400°C was found to reduce the dark current of small devices (10 x 10 pm) by ~1000X at -1 V. The same anneal reduces the dark current of larger devices (100 x 100 [mu]m) by ~140X. Through metal-oxide-semiconductor capacitor and doping studies, it was found that the anneal draws holes to the surface of the germanium, leading to better isolation of the devices and reduced leakage current. It was also found that threading defects play a role in leakage current. Threading defects arise because of the 4% lattice mismatch between germanium and the underlying silicon. For 1 jim-thick germanium films, as-grown samples are expected to have -5 x 108 cm- 2 threading defects. At this level, these defects are the dominant leakage current mechanism. Annealing the films at high temperatures can reduce the defect density. Large-area (300 x 300 pm) devices fabricated with a post-metallization anneal and with a threading defect density of -2 x 107 cm-2 were found to have a dark current density of ~1 mA/cm2 and a responsivity of 0.32 A/W at -1 V and 1550 nm.by Nicole Ann DiLello.Ph.D

On the effects of total ionizing dose in silicon-germanium BiCMOS platforms

The objective of the proposed research is to analyze the effects of total ionizing dose (TID) on highly scaled CMOS and Silicon-Germanium Heterojunction Bipolar Transistors (SiGE HBTs). TID damage is caused by a build-up of charge at sensitive Si-SiO₂ interfaces and may cause device or circuit failure. TID damage is due to an accumulation of radiation particle strikes seen in extreme environments, such as space.M.S

DESIGN OF A BURST MODE ULTRA HIGH-SPEED LOW-NOISE CMOS IMAGE SENSOR

Ultra-high-speed (UHS) image sensors are of interest for studying fast scientific phenomena and may also be useful in medicine. Several published studies have recently achieved frame rates of up to millions of frames per second (Mfps) using advanced processes and/or customized processes. This thesis presents a burst-mode (108 frames) UHS low-noise CMOS image sensor (CIS) based on charge-sweep transfer gates in an unmodified, standard 180 nm front-side-illuminated CIS process. By optimizing the photodiode geometry, the 52.8 μm pitch pixels with 20x20 μm^2 of active area, achieve a charge-transfer time of less than 10 ns. A proof-of-concept CIS was designed and fabricated. Through characterization, it is shown that the designed CIS has the potential to achieve 20 Mfps with an input-referred noise of 5.1 e− rms

Silicon Nanodevices

This book is a collection of scientific articles which brings research in Si nanodevices, device processing, and materials. The content is oriented to optoelectronics with a core in electronics and photonics. The issue of current technology developments in the nanodevices towards 3D integration and an emerging of the electronics and photonics as an ultimate goal in nanotechnology in the future is presented. The book contains a few review articles to update the knowledge in Si-based devices and followed by processing of advanced nano-scale transistors. Furthermore, material growth and manufacturing of several types of devices are presented. The subjects are carefully chosen to critically cover the scientific issues for scientists and doctoral students

Strain engineered Si-Ge nanowire heterostructures and Josephson junction field-effect transistors for logic device applications

There has been relentless effort on the physical scaling of silicon (Si) metal-oxide-semiconductor field-effect transistors (MOSFETs) in pursuit of higher computing power in the past decades. Silicon and germanium (Ge) based nanowires are compatible with the standard Si process and promising for the ultimately scaled devices, by allowing the gate-all-around geometry and integration of strain engineering through radial heterostructures to address device-scaling limitations. In the first part of the thesis, advances in probing the strain of radial nanowire heterostructures and carrier mobility enhancement through strain engineering are presented. We present a sequence of structural characterization techniques for Ge-Si [subscript x] Ge [subscript 1-x] and Si-Si [subscript x] Ge [subscript 1-x] core-shell nanowires that extends to all types of Si-Ge radial nanowire heterostructures examined in the thesis. We combine planar and cross-sectional transmission electron microscopy to identify the crystal structure, orientation and morphology of the nanowire heterostructures. We then apply continuum elasticity model to calculate the strain distribution, which coupled with the lattice dynamic theory yields the Ge-Ge or Si-Si Raman modes under strain, showing good agreement with the experimental values acquired via Raman spectroscopy. We also study the electrical properties of Si [subscript x] Ge [subscript 1-x]-Si core-shell nanowires by fabricating and characterizing n-type MOSFETs, and show that the tensile strain in the Si shell leads to a 40% electron mobility enhancement compared to bare Si nanowire MOSFETs. Additionally, we demonstrate both n-type and p-type MOSFETs using Si [subscript x] Ge [subscript 1-x]-Ge-Si core-double-shell nanowires as channel, designed so that holes populate the Ge shell and electrons populate the Si shell, with mobility enhancement of both carriers thanks to the compressive and tensile strain in the respective region. We also extract the valence band offset from the decoupled hole transport in the two shells at low temperature, overcoming the issue that most techniques available to probe the band structure in planar heterostructures are not promptly applicable. Reducing the operation temperature provides an additional path for system optimization in addition to the shrinking of device geometry. In the second part of the thesis, we explore a Boolean logic device suitable for cryogenic computing. We execute a combined effort of modeling and experimental characterization to examine the feasibility of Josephson junction field-effect transistors (JJ-FETs) for logic device applications at low temperatures. JJ-FETs are similar to MOSFETs, with their source and drain electrodes being superconducting at the operation temperature. We develop a compact model for JJ-FETs operating in the short ballistic regime, and perform circuit level simulations to investigate the criteria of signal restoration and fan-out for JJ-FET logic gates. We also experimentally demonstrate the operation of JJ-FETs based on an InAs quantum well heterostructure platform. We perform self-consistent Poisson-Schrödinger simulations, finding different gate voltage regimes where carriers populate one or more subbands in different vertical positions of the heterostructure. Furthermore, we extend the short ballistic model to interpret the experimental data, and discuss the impact of a low oxide/channel interface quality on the implementation of practical JJ-FET logic devices.Electrical and Computer Engineerin

Planar Electrostatically Doped Reconfigurable Schottky Barrier FDSOI Field-Effect Transistor Structures

In the last 50 years, our economy and society have obviously been influenced and shaped to a great extent by electronic devices. This substantial impact of electronics is the result of a continuous performance improvement based on the scaling, i.e. shrinking, of MOSFET devices in complementary integrated circuits, following Moore's law. As the MOSFET feature sizes are approaching atomistic dimensions, the scaling trend slowed down considerably and is even threatened for sub-10 nm technology nodes. Further, additional advancements are increasingly difficult to realize both from the technological and especially the economical perspective. Therefore, technologies that have the potential to supersede the CMOS technology in the future are the topic of intensive investigation by both researchers and the industry. An attractive solution is the leveraging of existing semiconductor technologies based on emerging research devices (ERD) offering novel characteristics, which enable new circuit architectures in future nanoscale logic circuits. A possible ERD contender are polarity controllable or reconfigurable MOSFET (RFET) concepts. Generally, RFET devices are able to switch between n- and p-type conduction by the application of an electrical signal. Therefore, RFET promise increased complex systems with a lower device count decreasing the costs per basic logic function based on their higher logic expressiveness. The focus of this work lies in the successful transfer of a predecessor silicon nanowire (NW) RFET technology into a planar RFET device, while simultaneously optimizing the resulting RFET for reconfigurable as well as conventional CMOS circuits. As for the predecessor NW RFET, the planar approach features a doping-less CMOS compatible fabrication process on a conventional SOI substrate and obtains its reconfigurability by electrostatic doping. The device can be regarded as a entanglement of two MOSFET in one structure, i.e. a depletion mode FET centered on top of a backside enhancement mode Schottky barrier FET (SBFET). The backside SBFET establishes the conductive channel consisting of the desired charge carrier type via an appropriate potential on its gate electrode. The topside FET controls the charge carrier flow between source and drain by locally depleting this channel given an opposite potential on its gate electrode with respect to the backside gate electrode. Two generations of devices have been successfully processed, while different gate electrode materials, i.e. nickel, aluminum and reactively sputtered tungsten-titanium-nitride, have been introduced to the device structure. As n- and p-type symmetry of the very same device is essential for RFET circuit design, tungsten-titanium-nitride is experimentally identified as a possible mid-gap metal gate electrode for RFET devices. Also, a Schottky barrier adjustment process for ideal n- and p-type symmetry based on silicide induced dopant segregation is experimentally demonstrated. Extensive electrical characterizations supported by calibrated TCAD simulations are presented, demonstrating experimental sub-threshold slopes of 65 mV/dec and on-to-off current ratios of over 9 decades. Based on TCAD simulations and supported by experimental results, the design space of the device concept is explored in order to gather predictive results for future scaled device optimization. Further, the high temperature (HT) performance is evaluated and compared to the predecessor NW RFET devices as well as to a state-of-the-art industrial high reliability HT MOSFET clearly illustrating the on par performance of the planar RFET concept with respect to off-state leakage current

Miniaturized Transistors, Volume II

In this book, we aim to address the ever-advancing progress in microelectronic device scaling. Complementary Metal-Oxide-Semiconductor (CMOS) devices continue to endure miniaturization, irrespective of the seeming physical limitations, helped by advancing fabrication techniques. We observe that miniaturization does not always refer to the latest technology node for digital transistors. Rather, by applying novel materials and device geometries, a significant reduction in the size of microelectronic devices for a broad set of applications can be achieved. The achievements made in the scaling of devices for applications beyond digital logic (e.g., high power, optoelectronics, and sensors) are taking the forefront in microelectronic miniaturization. Furthermore, all these achievements are assisted by improvements in the simulation and modeling of the involved materials and device structures. In particular, process and device technology computer-aided design (TCAD) has become indispensable in the design cycle of novel devices and technologies. It is our sincere hope that the results provided in this Special Issue prove useful to scientists and engineers who find themselves at the forefront of this rapidly evolving and broadening field. Now, more than ever, it is essential to look for solutions to find the next disrupting technologies which will allow for transistor miniaturization well beyond silicon’s physical limits and the current state-of-the-art. This requires a broad attack, including studies of novel and innovative designs as well as emerging materials which are becoming more application-specific than ever before