Frontiers in Electronics

The following sections are included:

• PREFACE

• CONTENTS

• PICTURES

An overview of III-V MOSFET technological challenges in comparison to well-established heterostructure-based FET technologies is presented with an emphasis on required properties and possible solutions. Possible approaches to achieve thermodynamically stable high-k gate stack with low interface trap density are reviewed, followed with our results on amorphous Si interface passivation layer (IPL) in-situ deposited on top of GaAs or strained InGaAs MOSFET channels grown by molecular beam epitaxy. Main issues of Si IPL, namely increased equivalent oxide thickness due to IPL oxidation and Si diffusion into the semiconductor channel, are addressed using an in-situ deposited HfO₂ with ultrathin (down to 0.25 nm) Si IPL and controlling its bonding state at the interface. Enhancement mode inversion-type MOSFET with HfO₂ high-k oxide is demonstrated. The device employs amorphous Si interface passivation layer, sputter-deposited high-k oxide and metal TaN gate and modulation p-doped GaAs/AlGaAs heterostructure with inversion n-channel formed at the interface with the oxide. The MOSFET with equivalent oxide thickness of 3.7 nm and long 100 μm channel have maximum DC transonductance of 0.9 mS/mm, I_on/I_off = 2×10⁴ (at low I_off of 30 nA) and effective channel mobility exceeding 1000 cm²/V−s at sheet electron density <2×10¹² cm⁻².

We have investigated the short-channel effect (SCE), floating-body effect, and three-dimensional coupling effect in triple-gate MOSFET with various fin widths, gate lengths and number of fins. It is found that the SCE of these devices is alleviated as the fin width shrinks and does not depend on the number of fins. The gate-induced floating-body effect (GIFBE) is visible even in fully depleted (FD) triple-gate transistors when the film-buried oxide (BOX) interface is swept from depletion to accumulation by the back-gate bias. The 3-D coupling effect in vertical, lateral, and longitudinal directions was investigated for different channel geometries. The biasing condition which enables the simultaneous activation of all channels and gives rise to volume inversion is discussed.

The Screen-Grid Field Effect Transistor (SGrFET) is a planar MOSFET-type device with a gating configuration consisting of metal cylindrical fingers inside the channel perpendicular to the current flow. The SGrFET operates in a MESFET mode using oxide insulated gates. The multi-gate configuration offers advantages for both analog and digital applications, whilst the gate cylinder holes can be exploited for bio-applications. In this manuscript TCAD results are presented on the analog and digital performance of the Screen-Grid Field Effect Transistor. The results are compared to the operation of an SOI-MOSFET and a finFET.

A new modeling and parameter extraction methodology to represent the parasitic effects associated with shielded test structures is presented in this paper. This methodology allows to accurately account for the undesired effects introduced by the test fixture when measuring on-wafer devices at high frequencies. Since the proposed models are based on the physical effects associated with the structure, the obtained parameters allow the identification of the most important parasitic components, which lead to potential measurement uncertainty when characterizing high-frequency devices. The proposed methodology is applied to structures fabricated on different metal levels in order to point out the advantages and disadvantages in each case. The validity of the modeling and characterization methodology is verified by achieving excellent agreement between simulations and experimental data up to 50 GHz.

This paper explores the potential of germanium on sapphire (GeOS) wafers as a universal substrate for System on a Chip (SOC), mm wave integrated circuits (MMICs) and optical imagers. Ge has a lattice constant close to that of GaAs enabling epitaxial growth. Ge, GaAs and sapphire have relatively close temperature coefficients of expansion (TCE), enabling them to be combined without stress problems. Sapphire is transparent over the range 0.17 to 5.5 μm and has a very low loss tangent (α) for frequencies up to 72 GHz. Ge bonding to sapphire substrates has been investigated with regard to micro-voids and electrical quality of the Ge back interface. The advantages of a sapphire substrate for integrated inductors, coplanar waveguides and crosstalk suppression are also highlighted. MOS transistors have been fabricated on GeOS substrates, produced by the Smart-cut process, to illustrate the compatibility of the substrate with device processing.

Scaling of complementary metal oxide semiconductor (CMOS) technologies to the sub-100 nm dimension regime increase the sensitivity to pervasive terrestrial radiation. Diminishing levels of charge associated with information in electronic circuits, interactions of multiple transistors due to tight packing densities, and high circuit clock speeds make single event effects (SEE) a reliability consideration for advanced electronics. The trend to adapt and apply commercial IC processes for space and defense applications has provided a catalyst to the development of infrastructure for analysis and mitigation that can be leveraged for advanced commercial electronic devices. In particular, modeling and simulation, leveraging the dramatic reduction in computing cost and increase in computing power, can be used to analyze the response of electronics to radiation, to develop and evaluate mitigation approaches, and to calculate the frequency of problematic events for target applications and environments.

In this paper, an efficient numerical model applicable for wide varieties of long channel field-effect transistors (MOSFET, MESFET, HEMT, etc.) is developed. A set of available data is used to calculate the model parameters and another set of data is used to verify the accuracy of the model. This model provides a single expression that is applicable for the entire range of device biasing and can predict the output parameters with less than 1% error compared to the experimental results. Lagrange polynomial, the highest degree of polynomial for any given set of data, is used to derive the model from available data. This method is efficient in the sense that it can be derived from a limited number of experimental data and since it uses only one equation for entire range of the device operation hence its computational cost is also small.

This paper gives a system designer's point of view regarding the emergence of the many new devices that are being considered as possible replacement of the Metal Oxyde Semiconductor Field effect transistor. The first part is a tentative to define criteria for the systemability of nanodevices. The second part is a short overview of the evolution of information processing systems. The third part is a more specific study of the demultiplexing and fault tolerance problems.

Recent advances in successful operation of silicon-based devices where transport is dependent on electron magnetic moment, or “spin”, could provide a future alternative to CMOS for logic processing. The basics of this spin electronics (Spintronics) technology are discussed and the specific methods necessary for application to silicon are described. Fundamental measurements of spin polarization and spin precession are demonstrated.

Material characterization is challenged by continuously decreasing device dimensions placing significant demands on characterization instruments and measurement interpretation. Numerous techniques exist and a few are highlighted here. Some of these have existed for a long time, while others have only emerged from the laboratory recently. Generally they are more user-friendly and have reasonable turn-around times. The trend in many techniques is clearly toward characterization of smaller dimensions. Among the myriad of characterization techniques in use today, I will discuss recent advances in transmission electron microscopy (TEM), electron holography, magnetic exchange force microscopy (MExFM), atom probe ion field ion microscopy, and X-ray tomography. They have made significant advances in the last few years and in some cases have produced very impressive results. For example, TEM is now able to generate images with 0.05 nm resolution, allowing display of individual atoms. MExFM in conjunction with magnetic fields has demonstrated vertical resolution of 0.0015 nm. Helium ion microscopy is also highlighted because it contributes a new application of ion beams, which had been largely the domain of Rutherford backscattering. Progress in developing further advances in nm dimensional characterization will, no doubt, continue to satisfy the demand for such measurements in the future.

A simulation study using molecular dynamics and the density-functional-theory/non-equilibrium-Green's-function approach has been carried out to investigate the potential of carbon nanotubes (CNT) as molecular-scale biosensors. Single molecules of each of two amino acids (isoleucine and asparagine) were used as the target molecules in two separate simulations. The results show a significant suppression of the local density of states (LDOS) in both cases, with a distinct response for each molecule. This is promising for the prospect of CNT-based single-molecule sensors that might depend on the LDOS, e.g., devices that respond to changes in either conductance or electroluminescence.

Interferometric lithography offers a facile, inexpensive, large-area fabrication capability for the formation of large areas of nanoscale periodic features. A self-aligned frequency doubling process to a 22-nm half-pitch is demonstrated. Many investigations of nanoscale phenomena require large-area samples, both for scientific investigations and certainly for ultimate large-scale applications. The utility of interferometric lithography is demonstrated to applications in nanophotonics and nanofluidics. For nanophotonics, metamaterial fabrication, negative index metamaterials and plasmonic applications are discussed. Two approaches to the fabrication of nanofluidic structures: etching and oxidation of silicon substrates, and colloidal deposition of silica nanoparticles to form porous walls and roofs followed by calcination to remove the photoresist and sinter the particles. These later structures have evident biomimetic functionality.

The formation of nano sized Si structures during the annealing of silicon rich oxide (SRO) films was investigated. These films were synthesized by low pressure chemical vapor deposition (LPCVD) and used as precursors, a post-deposition thermal annealing leads to the formation of Si nano crystals in the SiO₂ matrix and Si nano islands (Si nI) at c-Si/SRO interface. The influences of the excess Si concentration, the incorporation of N in the SRO precursors, and the presence of a Si concentration gradient on the Si nI formation were studied. Additionally the influence of pre-deposition substrate surface treatments on the island formation was investigated. Therefore, the substrate surface was mechanical scratched, producing high density of net-like scratches on the surface. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the synthesized nano islands. Results show that above mentioned parameters have significant influences on the Si nIs. High density nanosized Si islands can epitaxially grow from the c-Si substrate. The reported method is very simple and completely compatible with Si integrated circuit technology.

This paper provides an overview of recent work and future directions in Gallium Nitride transistor research. We discuss the present status of Ga-polar AlGaN/GaN HEMTs and the innovations that have led to record RF power performance. We describe the development of N-polar AlGaN/GaN HEMTs with microwave power performance comparable with state-of-art Ga-polar AlGaN/GaN HEMTs. Finally we will discuss how GaN-based field effect transistors could be promising for a less obvious application: low-power high-speed digital circuits.

Hot-electron fluctuation techniques were developed for experimental investigation of picosecond and subpicosecond electronic and phononic processes in voltage-biased 2DEG channels of interest for microwave low-noise and high-power transistors. Examples illustrate real-space transfer, hot-electron energy relaxation, and occupancy relaxation of hot-phonon modes. The pioneering results on hot-electron energy relaxation and hot-phonon lifetime are confirmed by time-resolved response experiments. The fluctuation technique for measuring the hot-phonon lifetime as a function of the hot-phonon temperature is unique, no datum has been reported for comparison as yet.

We describe a new analytical model of a Heterostructure Field Effect Transistors (HFETs) that accounts for electron trapping in the gate-drain spacing of the device and for related non-ideal device behavior. Under conditions of a very strong trapping, the electron velocity saturates outside the gate, in the trapping region, and the negative trapped charge leads to relatively large differential output conductance at the drain voltages exceeding the knee voltage. Also under the conditions of severe trapping, the negative trapped charge leads to the positive offset of the output current-voltage (I-V) characteristic. The model describes quite well numerous experimental data for passivated and unpassivated AlGaN/GaN HFETs with and without field plates (FP) under different bias conditions.

Detection of THz radiation by a high electron mobility (HEMT) GaAs/GaAlAs transistor was investigated at 4 K as a function of the magnetic field B. The detection signal (a source - drain photovoltage appearing as a response to THz radiation) was found to be periodic in B⁻¹, i.e., it showed Shubnikov - de Haas oscillations. A Fourier transform of the signal showed a large amplitude component independent of the gate voltage, and a small amplitude component dependent on it. This shows that a HEMT response to the radiation cannot be described either by a plasma instability in gated (“shallow water”) or in ungated (“deep water”) parts of the channel, but rather by a response of the channel as a whole. This is in a good correspondence with recent experimental evidence of antenna effects in detection of radiation by HEMTs and advanced theoretical models of instability of coupled gated - ungated plasma in HEMTs.

Experimental measurements of photoresistivity under terahertz (THz) radiation in low magnetic fields at conditions of cyclotron resonance (CR) in two-dimensional electron system (2DES) of GaAs/AlGaAs nanostructures are presented and discussed. We report the experimental discovery of “CR-vanishing effect” (CRV) in GaAs/AlGaAs heterostructures with high mobility as a well-defined gap on CR-line that is independent on incident THz power. Our analysis shows that the CRV may appear in systems with well correlated state of 2D electrons such as plasma waves and others. Fundamental nature of these correlated states of electrons in 2DES is discussed. Future THz detectors utilizing the new correlated states in 2DES may expand horizons for supersensitive detection in sub-THz and THz frequencies ranges.

A semiconductor scintillation-type gamma radiation detector is discussed in which the gamma-ray absorbing semiconductor body is impregnated with multiple small direct-gap semiconductor inclusions of bandgap slightly narrower than that of the body. If the typical distance between them is smaller than the diffusion length of carriers in the body material, the photo-generated electrons and holes will recombine inside the impregnations and produce scintillating radiation to which the wide-gap body is essentially transparent. In this way it is possible to implement a semiconductor scintillator of linear dimensions exceeding 10 cm.

We report our development of terahertz (THz) quantum-cascade lasers with record performance. Using those high-power lasers as the illumination sources and a focal-plane array camera, we are able to perform real-time THz imaging at video rate.

Novel structure light emitting diodes (LEDs) made of InN/GaN multiple quantum wells (MQWs) are proposed and demonstrated. The MQWs consisted of very fine and narrow 1 monolayer (ML)-thick InN wells embedded in GaN matrix, which were successfully fabricated by radio-frequency molecular beam epitaxy. The thickness of InN wells can be fractional ML and/or two MLs depending on the growth conditions, resulting in different wavelength light emissions from deep violet to blue. Epitaxy processes for the MQWs fabrication are very unique on the basis of the selfordering and coherent growth mode for atomically flat ˜ 1 ML InN well deposition on GaN template. It is shown that the epitaxy temperature for 1 ML InN wells can be much higher than the highest epitaxy temperature of thick InN layer due to the effects of GaN matrix. Bright electroluminescence (EL) emission is observed at 418 nm at room temperature in LEDs fabricated by the MQWs. Further it is confirmed that the quantum confined Stark effect (QCSE) in InN wells is remarkably reduced due to the effects with using ultimately thin InN wells as active layers, resulting an extremely small blue shift in the EL peak wavelengths for two orders different injection current levels.

Terahertz time-domain measurements using air as the emission and detection medium are shown to enable broadband spectroscopy with continuous coverage of the entire terahertz band, from 0.1 to over 10 THz. It is shown that the unique properties of using a gaseous medium allow for excellent phase matching without interruptions from crystalline phonon modes or antenna resonant structures from conventional solid state emitters and sensors, allowing the full bandwidth of the laser pulse to be used. It is also shown that the limiting factor on the system bandwidth is the temporal profile of the probe pulse used for detection, and how modification of this profile can further enhance the detection bandwidth.

Employing Monte-Carlo simulations we investigate effects of an electric field on electron kinetics and transport in quantum-dot structures with potential barriers created around dots via intentional or unintentional doping. Results of our simulations demonstrate that the photoelectron capture is substantially enhanced in strong electric fields and this process has an exponential character. Detailed analysis shows that effects of the electric field on electron capture in the structures with barriers are not sensitive to the redistribution of electrons between valleys and these effects are not related to an increase of drift velocity. Most data find adequate explanation in the model of hot-electron transport in the potential relief of quantum dots. Electron kinetics controllable by potential barriers and an electric field may provide significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors.

We have developed a detector that can detect single photons at terahertz frequencies. The detector is based on quantum dot and single electron transistor technology, and the structure is scalable. One can scale the detector of nearly the same structure to be used for different frequencies. Our numerical calculation, which was performed along with our experimental efforts, indicates that the same detector structure can be used for millimeter-wave and far-infrared frequencies.

Optical MEMS technology combined with broadband infrared sensor technology is used to realize wavelength-tunable infrared sensors. This paper describes the ongoing research into one such sensor design based on an electrically tunable Fabry-Pérot cavity. Theory, measured results and future research directions are presented and discussed for the single-sensor design currently being developed, in the context of the intended application of this technology; the development of lightweight, portable and robust multi-spectral imaging systems.

In this work we present the process flow for the fabrication of un-cooled IR detectors employing surface micro-machining techniques over silicon substrates. These detectors are based on thin films deposited by plasma at low temperatures. The thermo sensing film used is an intrinsic a-Si_xGe_1−x:H film, which has demonstrated a very high temperature coefficient of resistance (TCR), and a moderated resistivity, these properties are better than those of the a-Si:H intrinsic film, which is commonly used in commercial IR devices. Two device configurations have been designed and fabricated, labeled planar and sandwich. The former is the configuration commonly used in commercial micro-bolometers, while the latter is proposed in order to reduce the high cell resistance observed in this kind of devices, without the necessity of doping the intrinsic film, which results in a decrement of the TCR and therefore in responsivity. Finally some performance characteristics of the devices studied are discussed in comparison with data reported in literature.

Nanocrystalline silicon (nc-Si) thin film transistors (TFTs) have potential for high-performance applications in large area electronics, such as next generation of flat panel displays and medical x-ray imagers, for pixel drivers, readout circuits, as well as complementary channel logic circuits for system-on-panel integration. This potential stems from reduced threshold voltage shift and higher transconductance, compared to amorphous silicon counterpart. In this paper, we discuss various TFT structures, their associated design and performance considerations, including leakage current and threshold voltage stability mechanisms.

The following sections are included:

• AUTHOR INDEX