Electrical performance and use in logic of printed current switching transistors employing nanostructured silicon

Printed electronics seek to replace a full range of conventional electronic components and circuits with their printed counterparts, and offer an extraordinary range of advantages in producing exible, low-cost, large area coverage, stretchable, wearable devices and circuits. We already witness the incredible advantages and extraordinary contribution of printed electronics in our daily lives, as well as in the cutting-edge printed electronics innovations and research available today. At an in-depth level, and as an important contribution to printed devices, this work presents the design, production, and characterization of a novel fully printed current-driven switch, referred to here as a Current Switching Transistor (CST). The CST possesses the unique ability to operate as a two-way switch for both direct (DC) and alternating current (AC). In this thesis, CSTs were successfully used for the fabrication of exible fundamental "AND" and "OR" logic gates. At the fundamental level, this work sets out to illustrate that, a printed nanostructured silicon layer could be used as the active material for current switching transistors and other electronic devices. Also investigated was the temperature dependence of the transfer characteristics, in an extended range of temperature from 340 to 10 K, as well as their reliability when subjected to a constant current bias stress. Significantly in this work, the switching behavior observed and the electrical properties of the CSTs produced using nanostructured silicon remained excellent at temperatures as low as 10 K. Such transistor performance demonstrates the transistor's high potential as the candidate for cryogenic applications. The transistor's mechanism of operation was shown to be based on the activated percolation of charge carriers through the network of particles in the active silicon layer. Additionally, this work shows that the ON/OFF ratio of the transistors was temperature dependent, yielding the highest value of 10³ achieved at cryogenic temperatures below 150 K. A reliability test achieved through bias stressing the base terminal, with a constant voltage of 52 V or a current of 75 μA for up to 6 hours at room temperature proved the devices to be highly stable. Except for the reversible shift in the switching voltage, which could be attributed to self-heating, no alteration of the device characteristics was observed

Radio Astronomical Antennas in the Central African Region to Improve the Sampling Function of the VLBI Network in the SKA Era?

On the African continent, South Africa has world-class astronomical facilities for advanced radio astronomy research. With the advent of the Square Kilometre Array project in South Africa (SA SKA), six countries in Africa (SA SKA partner countries) have joined South Africa to contribute towards the African Very Long Baseline Interferometry (VLBI) Network (AVN). Each of the AVN countries aims to construct a single-dish radio telescope that will be part of the AVN, the European VLBI Network, and the global VLBI network. The SKA and the AVN will enable very high sensitivity VLBI in the southern hemisphere. In the current AVN, there is a gap in the coverage in the central African region. This work analyses the increased scientific impact of having additional antennas in each of the six countries in central Africa, i.e., Cameroon, Gabon, Congo, Equatorial Guinea, Chad, and the Central African Republic. A number of economic human capital impacts of having a radio interferometer in central Africa are also discussed. This work also discusses the recent progress on the AVN project and shares a few lessons from some past successes in ground stations retrofitting

A novel mode of current switching dependent on activated charge transport

We demonstrate a fully printed transistor with a planar triode geometry, using nanoparticulate silicon as the semiconductor material, which has a unique mode of operation as an electrically controlled two-way (double throw) switch. A signal applied to the base changes the direction of the current from between the collector and base to between the base and emitter. We further show that the switching characteristic results from the activated charge transport in the semiconductor material, and that it is independent of the dominant carrier type in the semiconductor and the nature of the junction between the semiconductor and the three contacts. The same equivalent circuit, and hence similar device characteristics, can be produced using any other material combination with non-linear current-voltage characteristics, such as a suitable combination of semiconducting and conducting materials, such that a Schottky junction is present at all three contacts

Selective atomic layer deposition on flexible polymeric substrates employing a polyimide adhesive as a physical mask

The rise of low-temperature atomic layer deposition (ALD) has made it very attractive to produce high- κ dielectric for flexible electronic devices. Similarly, selective deposition of ALD films is of great relevance for circuitry. We demonstrated a simple method of using a physical mask to block the film's growth in selected polymeric and flexible substrate areas during a low-pressure ALD process. A low-cost silicone adhesive polyimide tape was used to manually mask selected areas of bare substrates and aluminum strips deposited by evaporation. 190 cycles of aluminum oxide (Al 2O 3) and hafnium oxide (HfO 2) were deposited at temperatures ranging from 100 to 250 °C. Using x-ray photoelectron spectroscopy (XPS) analysis and energy dispersive x-ray spectroscopy (EDS), we showed that the mask was effective in protecting the areas under the tape. The mask did not show any modification of shape for an exposure of 10 h at 250 °C, hence keeping the form of the masked area intact. An analysis of the unmasked area by ellipsometry (632.8 nm) and x ray shows a regular film with a thickness variation under 2 nm for a given temperature and constant refractive index. EDS, selected-area XPS, and imaging XPS show an evident change of elemental content at the interface of two areas. By XPS, we established that the structure of the films was not affected by the mask, the films were stoichiometric, and there was no effect of outgassing from the adhesive film.publishedVersionPeer reviewe

Effect of traditional and oven roasting on the physicochemical properties of fermented cocoa beans

The objective of this study was to determine the effect of oven and traditional roasting on the polyphenol content, antioxidant activity, lipid quality, proximate composition and mineral content of fermented cocoa beans. Results showed that traditional roasting significantly decrease (p < 0.05) the polyphenol content and antioxidant activity of cocoa beans. The analysis of oil showed that oven and traditional roasting significantly increase (p < 0.05) the peroxide and thiobarbituric acid values of cocoa butter and that traditional roasting samples were the most deteriorated. Oven and traditional roasting for 10 min reduce the iodine value of the oil, but increase its acidity. The proximate and mineral composition of cocoa beans was also seriously affected during cooking. Drying and oven roasting for 5 and 10 min; and drying and traditional roasting for 5 min appear to be the best processing methods of cocoa beans for production of cocoa based foods like chocolate