In this paper the potentiality of impact avalanche transit time (IMPATT) devices based on different semiconductor materials such as GaAs, Si, InP, 4H-SiC and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz frequencies. Drift-diffusion model is used to design double-drift region (DDR) IMPATTs based on different materials at millimeter-wave (mm-wave) and terahertz (THz) frequencies. The performance limitations of these devices are studied from the avalanche response times at different mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of GaAs and Si DDR IMPATTs are 220 GHz and 0.5 THz, respectively, whereas the same for InP and 4H-SiC DDR IMPATTs is 1.0 THz. Wz-GaN DDR IMPATTs are found to be excellent candidate for generation of RF power at THz frequencies of the order of 5.0 THz with appreciable DC to RF conversion efficiency. Further, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs as regards their RF power outputs. Thus, the wide bandgap semiconductors such as Wz-GaN and 4H-SiC are highly suitable materials for DDR IMPATTs at both mm-wave and THz frequency ranges.

With the ever-increasing depletion of nonrenewable fossil fuel reserve, greater attention has been directed towards renewable energy storage devices. One of the most important of such devices is the supercapacitor, which exhibits high specific capacitance. Polyaniline (PAni) is a versatile conducting polymer, which has demonstrated excellent electrochemical properties along with good stability and ease of synthesis.Therefore, PAni has been extensively used in the fabrication of supercapacitors. In the last few decades, researchers have studied the effect of morphology, developed during the synthesis of PAni, on its electrochemical properties. It is known that the electrical conductivity and the electrochemical properties of PAni get influenced by the level and type of dopant used, the method of synthesis adopted, and the surface area and porosity possessed. However, it has been realized that supercapacitors based on PAni suffer from short cycle life. This led to development of PAni composites with carbon-based materials and transition metal oxides. In this review, focus has been laid on the achieved performance levels of the recently developed PAni-based supercapacitors. In addition, an attempt has been made to study the fundamental aspects of the conductivity and the electrochemical properties of PAni and their effect on the supercapacitor performance. Moreover, several new interesting applications of PAni-based supercapacitors have also been included in this review.1 | INTRODUCTION Supercapacitors, for the last few decades, have accrued immense attention from the scientific society for their unique properties like long cycle life, wide operating temperature window, high power density, and fast charge-discharge cycle. 1,2 It can store charge in two different ways-either by electrostatic way (electrical double layer capacitors, EDLCs) or by thorough fast and reversible Faradaic reaction (pseudocapacitors). In case of EDLCs, the opposite charges are stored on the surface of the two parallel conductive plates with electrolytes between them that help in shuttling of ions between the two.EDLCs type of supercapacitors have shown higher power densities and hence higher rate capabilities. It has been studied that carbon-based materials have shown EDLC-type behavior with high power density, low cost, and tunable porosity but lacks in energy density. [3][4][5] Pseudocapacitors store charges through fast and reversible reaction, which can store charges near the surface of the electrode, and thus possess high capacitance but low power density. Transition metal oxides (TMOs), mixed TMOs, 6 and conducting polymers (CPs) exhibit pseudocapacitance. TMOs exhibit multiple oxidation states with low activation energy but lack high capacitance and flexibility and suffer from instability. In recent years, extensive studies have been done on TMOs and carbon-based materials. Another group of fascinating materials is CPs, which show high specific capacitance, flexibility, and ease of fabrication. The performance of the supercapacitor is...

The authors have developed a large-signal simulation technique extending an in-house small-signal simulation code for analyzing a 94 GHz double-drift region impact avalanche transit time device based on silicon with a non-sinusoidal voltage excitation and studied the effect of junction temperature between 300 and 550 K on the large-signal characteristics of the device for both continuous wave (CW) and pulsed modes of operation. Results show that the large-signal RF power output of the device in both CW and pulsed modes increases with the increase of voltage modulation factor up to 60%, but decreases sharply with further increase of voltage modulation factor for a particular junction temperature; while the same parameter increases with the increase of junction temperature for a particular voltage modulation factor. Heat sinks made of copper and type-IIA diamond are designed to carry out the steady-state and transient thermal analysis of the device operating in CW and pulsed modes respectively. Authors have adopted Olson's method to carry out the transient analysis of the device, which clearly establishes the superiority of type-IIA diamond over copper as the heat sink material of the device from the standpoint of the undesirable effect of frequency chirping due to thermal transients in the pulsed mode.

We have investigated the agglomeration behaviour of two types of multi-walled carbon nanotubes (MWNTs; N-MWNTs and D-MWNTs), which have different chemical functionalities, average diameter, varying extent of agglomeration and agglomerations. The properties were altered by varying the agglomerated structure. The strength of the MWNT agglomerates was estimated via nanoindentation. The work done to indent D-MWNT agglomerates (3910.3 × 10(-8) erg) was higher than for N-MWNTs agglomerates (2316.4 × 10(-8) erg). An organic modifier, the Li salt of 6-aminohexanoic acid (Li-AHA), was used to deagglomerate the MWNTs in an aqueous medium. The stability of the aqueous dispersion of Li-AHA-modified MWNTs was analyzed by UV-vis spectroscopy and zeta potential measurements. An increase in Li-AHA concentration increased the dispersion of MWNTs in the aqueous medium. Furthermore, the mechanism of dispersion of the two types of MWNTs in the aqueous medium in the presence of Li-AHA was determined based on the electrostatic charge repulsion between the negatively charged species. A fluorescence-activated cell sorting technique was used to assess the debundling of MWNT agglomerates in the aqueous medium. We examined the morphology-property relationship in Li-AHA-modified MWNTs.

An improved activation process using a PdC1,-HF-NH4F system f or the activator solution is reported. The process is applicable for autocatalytic electroless deposition of metals on rough as well as smooth/polished, n-and p-tpe silicon substrates of all doping levels. Using the improved activation process, adhesion of more than 8.33 >< 10' N/m for palladium deposits and 7.25 X 106 N/m' for nickel deposits has been obtained. The etch rate of Si02 in the activator solution is low enough for it to be compatible with planar integrated-circui-t technology. The activator avoids the use of tin, which is undesirable in device fabrication.

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Large-signal (L-S) characterization of double-drift region (DDR) impact avalanche transit time (IMPATT) devices based on silicon designed to operate at different millimeter-wave (mm-wave) and terahertz (THz) frequencies up to 0.5 THz is carried out in this paper using an L-S simulation method developed by the authors based on non-sinusoidal voltage excitation (NSVE) model. L-S simulation results show that the device is capable of delivering peak RF power of 657.64 mW with 8.25% conversion efficiency at 94 GHz for 50% voltage modulation; whereas RF power output and efficiency reduce to 89.61 mW and 2.22% respectively at 0.5 THz for same voltage modulation. Effect of parasitic series resistance on the L-S properties of DDR Si IMPATTs is also investigated, which shows that the decrease in RF power output and conversion efficiency of the device due to series resistance is more pronounced at higher frequencies especially at the THz regime. The NSVE L-S simulation results are compared with well established double-iterative field maximum (DEFM) small-signal (S-S) simulation results and finally both are compared with the experimental results. The comparative study shows that the proposed NSVE L-S simulation results are in closer agreement with experimental results as compared to those of DEFM S-S simulation.

The solution in which Si is immersed for activation prior to autocatalytic electroless deposition (AED) of Ni, Cu, etc. is usually HF‐PdCl2‐normalHCl . However, we find that the AED of Pd on polished crystalline Si, which has important applications in modern planar integrated circuit technology, is nonadherent using this activator solution. Our study of the effects of modifying this solution and varying the substrate doping on the Pd deposition during activation and Pd AED adhesion reveals the following. The activation quality and hence AED adhesion depends not on the SiO2 etch rate of the solution but on the composition of the solution Pd complex and the substrate hole concentration. In spite of the highest Pd deposition rate during immersion, the activation quality of n‐Si with doping >1019/cm3 is poor unless a hole‐generating stimulus (e.g., illumination) is present. A change in substrate hole concentration influences AED adhesion by altering the substrate potential and/or density of nucleation sites during activation. Similarly, a change in the Pd complex affects AED adhesion by altering the solution Pd potential. Introduction of certain ammonium compounds into the HF‐PdCl2‐normalHCl bath creates a specific Pd‐ammine complex, most probably normalPdfalse(NH3)2 Cl2 , which gives Pd AED adhesions of ∼9 and 4 false(3false)×106N/m2 on heavily doped p‐Si and lightly doped p(n)‐Si, respectively, using a low SiO2 etch rate (∼90 Å/min) and a low temperature (200°C) for Pd nuclei silicidation during activation. Our results regarding the activation mechanism are believed to apply generally to all AED on Si. © 1999 The Electrochemical Society. All rights reserved.