Hiranmay Saha

Design optimization of a high performance silicon MEMS piezoresistive pressure sensor for biomedical applications

In this paper the design optimization of high performance conventional silicon-based pressure sensors on flat diaphragms for low-pressure biomedical applications has been achieved by optimizing the doping concentration and the geometry of the piezoresistors. A new figure of merit called the performance factor (PF) is defined as the ratio of the product of sensor sensitivity (S) and sensor signal-to-noise ratio (SNR) to the temperature coefficient of piezoresistance (TCPR). PF has been introduced as a quantitative index of the overall performance of the pressure sensor for low-range biomedical applications. A comprehensive analysis considering the impact of doping concentration, length of the piezoresistor and thickness of the diaphragm indicates that for achieving the highest figure of merit an optimum concentration of 1018 atoms cc−1 and geometry of 100 µm long piezoresistor and 10 µm thick membrane for the specified biomedical application are required.

Role of parasitics in humidity sensing by porous silicon

Abstract Humidity sensing by porous silicon (PS) layer is commonly reported either by capacitive sensing or by conductive sensing. A critical analysis of both capacitive and conductive sensing by microporous PS layer is presented here. The influences of parasitic capacitances and resistances unavoidably associated with the active porous layer on the measured changes in capacitance and resistance of the humidity sensor with variation of humidity are analysed. The role of contact geometry, signal frequency and porosity of PS layer are also discussed. It is shown that capacitive sensing is more sensitive in low frequency range owing to the relative contributions of parasitic components.

Improved contacts on a porous silicon layer by electroless nickel plating and copper thickening

In this paper stable, low-resistance contacts on porous silicon have been realized by electroless nickel deposition from a very weakly alkaline solution followed by copper thickening. The porous silicon layer after nickel deposition has been analysed by x-ray diffraction, which shows that a porous silicon nickel structure is successfully achieved from the bath. FESEM studies have been performed which show that the surface morphology of the porous silicon layer remains intact after nickel–copper plating. The mean roughness of the porous silicon surface has been found to improve after nickel plating from the AFM studies. Electrical characterization shows that the J–V characteristics of the nickel–copper-plated porous silicon lateral structure contact is fairly linear up to a certain value of applied voltage. Specific contact resistance of the nickel on porous silicon has been measured for the first time and has been found to be of an order of magnitude lower than that of other metals. No significant ageing is visible in this electroless nickel contact contrary to the vacuum-evaporated nickel contact.

A hygrometer comprising a porous silicon humidity sensor with phase-detection electronics

A novel hygrometer is presented, comprising a capacitive humidity sensor with a porous silicon (PS) dielectric and electronics. The adsorption of water vapor by the PS layer leading to change of its effective dielectric constant is modeled with an effective medium approximation (EMA). A simple, but precise, phase-sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates to ambient humidity. It is outlined how the nonlinear response of the sensor is compensated through piecewise linearization. The sensor is tested in combination with the phase detection circuitry. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and an accuracy of 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 s with good stability.

Influence of surface texturization on the light trapping and spectral response of silicon solar cells

A quantitative model that explains the spectral response, internal quantum efficiency, total short-circuit current, open-circuit voltage, and efficiency of high-efficiency solar cells with textured front surface and Lambertian back-surface reflectors is presented. A comparison of the textured cell characteristics is made with those of planar cells, and the separate roles of the front surface reflection coefficient and internal quantum efficiency in enhancing the short-circuit current have been investigated. It is shown that, in the case of large diffusion lengths, almost all the contribution to the increase of spectral response on texturization is due to the reduced reflection coefficient whereas, for small diffusion lengths, there is a significant increase in internal quantum efficiency on texturization, especially in the region of higher wavelengths. However, there is a small decrease in open-circuit voltage for large diffusion lengths, whereas no significant change is observed for small diffusion lengths on texturization. Nevertheless, there is a net gain in power conversion efficiency which is larger for smaller diffusion lengths. >

Electrode design and planer uniformity of anodically etched large area porous silicon

The structure of electrodes in the electrochemical formation cell of porous silicon greatly influences the planar uniformity of the porous silicon layer. In this paper a systematic study of the planar uniformity of large area porous silicon layer formed by different electrode structure is reported based on photoluminescence and reflectance scanning of the surface. A new design of the electrode structure is developed for achieving satisfactory planar uniformity of large area porous silicon layer.

Palladium-silver-activated ZnO surface: highly selective methane sensor at reasonably low operating temperature.

Metal oxide semiconductors (MOS) are well known as reducing gas sensors. However, their selectivity and operating temperature have major limitations. Most of them show cross sensitivity and the operating temperatures are also relatively higher than the value reported here. To resolve these problems, here, we report the use of palladium-silver (70-30%) activated ZnO thin films as a highly selective methane sensor at low operating temperature (∼100 °C). Porous ZnO thin films were deposited on fluorine-doped tin oxide (FTO)-coated glass substrates by galvanic technique. X-ray diffraction showed polycrystalline nature of the films, whereas the morphological analyses (field emission scanning electron microscopy) showed flake like growth of the grains mainly on xy plane with high surface roughness (107 nm). Pd-Ag (70-30%) alloy was deposited on such ZnO films by e-beam evaporation technique with three different patterns, namely, random dots, ultrathin (∼1 nm) layer and thin (∼5 nm) layer as the activation layer. ZnO films with Pd-Ag dotted pattern were found show high selectivity towards methane (with respect to H2S and CO) and sensitivity (∼80%) at a comparatively low operating temperature of about 100°C. This type of sensor was found to have higher methane selectivity in comparison to other commercially available reducing gas sensor.

Piezoresistive pressure sensing by porous silicon membrane

In this paper, the piezoresistive pressure-sensing property of porous silicon has been reported. The pressure sensitivity of a porous silicon membrane of 63% porosity and 20-/spl mu/m thickness has been observed to be about three times more than that of a conventional bulk silicon membrane of the same dimensions. The increased sensitivity is attributed to the improvement in piezoresistance due to quantum confinement in the porous silicon nanostructure. The piezoresistive coefficient of porous silicon is estimated for the first time and is observed to be about 50% larger than that of monocrystalline silicon for a 63% porosity porous silicon membrane. The response time has also been studied and observed to be significantly shorter. Power dissipation of the porous silicon pressure sensor is also much less compared to that of commercial bulk silicon piezoresistive pressure sensors.

Silica nanoparticles on front glass for efficiency enhancement in superstrate-type amorphous silicon solar cells

Antireflective coating on front glass of superstrate-type single junction amorphous silicon solar cells (SCs) has been applied using highly monodispersed and stable silica nanoparticles (NPs). The silica NPs having 300 nm diameter were synthesized by Stober technique where the size of the NPs was controlled by varying the alcohol medium. The synthesized silica NPs were analysed by dynamic light scattering technique and Fourier transform infrared spectroscopy. The NPs were spin coated on glass side of fluorinated tin oxide (SnO2: F) coated glass superstrate and optimization of the concentration of the colloidal solution, spin speed and number of coated layers was done to achieve minimum reflection characteristics. An estimation of the distribution of the NPs for different optimization parameters has been done using field-emission scanning electron microscopy. Subsequently, the transparent conducting oxide coated glass with the layer having the minimum reflectance is used for fabrication of amorphous silicon SC. Electrical analysis of the fabricated cell indicates an improvement of 6.5% in short-circuit current density from a reference of 12.40 mA cm−2 while the open circuit voltage and the fill factor remains unaltered. A realistic optical model has also been proposed to gain an insight into the system.

Green energy sources (GES) selection based on multi-criteria decision analysis (MCDA).

Purpose – Reducing greenhouse gas emissions from fossil fuel consumption is a big challenge on the view of global warming and climate changes caused by greenhouse gases as per recent scientific reports. This paper aims to identify the major challenges of green energy sources (GES) to the future power systems and suggests an appropriate GES based on the preference by the decision maker on the various issues to meet these challenges.Design/methodology/approach – The proposed work presents a multi‐criteria decision analysis (MCDA) – the analytic hierarchy process (AHP) to evaluate the GES – photovoltaic (PV), wind generator (WG), biomass (BM) and micro‐hydel (MH) and to find the appropriate selection in general, by evaluating its main operational characteristic. In this research, the choices of the green energy alternatives on the basis of various factors have been taken into consideration. MATLAB simulation of different criteria to ascertain their clear‐cut effects on GES selection under multiple uncertaint...