Rakesh Prasher

Measurement of soil moisture using microwave radiometer

The science of microwaves owes its origin to the development of radar. Microwaves are part of electromagnetic spectrum. This field became vitally important as man reached out to space. Frequency range of these waves are from 3 GHz to 30 GHz. Microwaves have unique capabilities in remote sensing. The field of microwaves remote sensing has come to a stage of rapid growth. Microwaves can penetrate clouds and so the sensors can operate in all weather conditions. They are sensitive to the presence of moisture in the soil as well as in vegetation or any another material which absorbs moisture. Microwave sensors are of two types: active sensors and passive sensors. Passive sensors have been used for soil studies for the determination of moisture content, oceanographic application to determine winds over the ocean surface and water vapor content in atmosphere as well as liquid water content in clouds. Soils are composed of solids, liquids and gases mix in variable proportions. Soil texture depends upon the size of the particle and structure of soil depends on the way particles are arranged. Soil has physical as well as electrical properties. Colour, texture, grain soil etc. comprised the physical properties where the electrical properties include dielectric constant, conductivity and permeability. Dielectric constant is the primary electrical property which is used to estimate emissivity and brightness temperature of soil. Emissivity is an important parameter for microwave remote sensing, which provides information about soils. All substances at a finite absolute temperature radiate electromagnetic energy. Emissivity is the ratio of energy emitted by object to black body maintained at same physical temperature. Emissivity is a function of physical and electrical parameters of the object and electrical parameter of sensors. These are the moisture content in the object surface type (smooth or rough), dielectric constant (e), angle of incidence, polarization. Emissivity can be obtained from the measured dielectric constant (e) using the available models. The emissivity can be measured by instrument radiometer, which is highly sensitive receiver. Radiometers are passive microwave sensor, which collects the incoming radiations, amplify as well as process the signal, and gives the output, which is linearly related to incoming radiation collected by antenna. The electromagnetic radiations are measured by passive remote sensor in the form of brightness temperature. The radiometer system used here consists of LNBC, receiver and power meter. The LNBC (low noise block down converter) converts the signal to a lower frequency and sends them out to the cable connector, which is connected to satellite receiver via co-axial cable. LNBC have input frequency range 10.75 to 12.75 GHz and noise temperature of 35 degK. The receiver output is connected to microwave power meter. The power monitored is related to incoming EM radiations. The radiometer is calibrated using liquid nitrogen and sky as the targets. Then the EM radiations emitted by dry and wet soil are measured at different look angles (10deg to 60deg) with step of 5deg. The measured power is converted into brightness temperature and the brightness temperature is co-related to soil moisture. In the paper the relation between soil moisture and brightness, temperature is presented. This provides input for determination of soil moisture using passive sensors.

Characterization of Carbon Nanotube Field Effect Transistor Using Simulation Approach

As the size of the Si MOSFET approaches towards its limiting value, various short channel effects appear to affect its performance. Carbon nanotube field effect transistor (CNTFET) is one of the novel nanoelectronic devices that overcome those MOSFETs limitations. In this paper we have studied the effect of scaling carbon nanotube (CNT) diameter, insulator thickness and high-k dielectric materials on current-voltage characteristics of co-axial gated ballistic n-type CNTFET. The device metrics such as drive current (Ion), leakage current (Ioff), Ion/Ioff ratio, transconductance, subthreshold slope (S) and drain induced barrier lowering (DIBL) are also studied in this paper. The simulation results obtained are then compared with conventional nanoscale n-type MOSFET. It has been concluded that CNTFET seem to provide better performance than conventional nanoscale n-type MOSFET in term of high speed capability and lower switching power consumption.

Al/HfO2/Si Gate Stack with Improved Physical and Electrical Parameters

In this paper, we present the fabrication and characterization of Al/HfO2/Si gate stack with improved physical and electrical gate stack parameters such as dielectric constant (k), equivalent oxide thickness (EOT), interface trap density (Dit) and effective oxide charges (Qeff) extracted from the C-V and G-V characteristics. Atomic layer deposition (ALD) has been used for high-k dielectric formation. The effect of rapid thermal annealing (RTA) temperatures on the properties of the fabricated gate stack has also been investigated at 300 °C & 600 °C for 3 minutes in nitrogen (N2) ambient whereas forming gas annealing (FGA) was performed at 420°C for 20 minutes in 96% N2 and 4% H2 ambient. Fourier transform infrared spectroscopy (FTIR) was used to study of thermal annealing effects on the HfO2/Si interface. Filmetrics F20 thin film analyzer and XRD were used to find thickness and composition of the material respectively. The results reveal that the physical and electrical properties (K, Dit, Qeff, etc) have been improved by RTA in nitrogen ambient, which further improves the interface properties of HfO2/Si due to the densification of HfO2 thin films. It is also shown that the 600°C nitrogen annealed samples exhibit a reduced EOT 2.54 nm, interfacial trap density, effective oxide charges (Qeff), C - V hysteresis and leakage current.

Study of gate all around InAs/Si based nanowire FETs using simulation approach

In this paper, a gate-all-around Si Nanowire FET (NWFET) and InAs NWFET have been studied and compared with respect to various performance parameters. The device metrics considered at the nanometer scale are transfer characteristics, transconductance, output characteristics, drive and leakage current, switching speed (Ion/Ioff), conduction-band profile, subthreshold swing (SS) and drain induced barrier lowering (DIBL). It has been shown that InAs channeled NWFET has higher mobility and hence higher transconductance, whereas Si NWFET shows better immunity towards short channel effects with lower leakage current, lower sub-threshold slope, lower DIBL. Therefore, Si NWFET appears to be applicable for Low Operating Power applications whereas, InAs with its high ON currents and switching speeds prove to be a good candidate for high performance applications of the long term ITRS where reasonably high leakage currents are acceptable as a trade off for increased operating speeds.

Impact of Scaling Gate Oxide Thickness on the Performance of Silicon Based Triple Gate Rectangular Nwfet

Previously, simulations were carried out on the classical drift diffusion technique which no longer supports the present day criteria in a 3D domain. Now-a-days, we enhance our simulation capabilities by performing simulation at an atomistic level rather than bulk which gives us best result in a 3D domain. To fulfill this requirement, the first full-band quantum and atomistic transport simulator OMEN is designed for post CMOS devices. In this paper, we have investigated the effect of scaling gate oxide thickness of rectangular Si-NWFET on its device performance in terms of transfer characteristics, output characteristics, electron doping, drive current (Ion), leakage current (Ioff), switching speed (Ion/Ioff) and transconductance. We concluded that the conductivity of Si-NWFET and doping density of electrons in Si-NWFET enhances with the reduction in oxide thickness. Also, we have concluded that with the reduction in oxide thickness, drive current of the device increases and leakage current of the device decreases which is an improvement over CNTs and conventional MOSFETs. Further, the switching speed (Ion/Ioff) of the device and transconductance (gm) enhances by reducing the oxide thickness.

Novel Attributes in Scaling Issues of an InSb-Nanowire Field-Effect Transistor

Due to the inherently lower bandgap and larger permittivity of III–V materials, III–V MOSFETs are more susceptible to short-channel effects (SCE). They show promising improvement in drain-induced barrier lowering (DIBL), due to suppressed SCE. In this paper, we present a scaling study of nanowire field-effect transistors (NWFETs) using a two-dimensional model and explore the scaling issues in device performance focusing on transconductance characteristics, output characteristics, average velocity, Switching speed, subthreshold swing and with different gate oxide thicknesses (tox) and nanowire diameters. Also, our results show the output conductance, transconductance, voltage gain and average electron velocity at the top of the barrier get improved in NWFETs with thinner tox and larger nanowire diameter.

Impact of Silicon Body Thickness on the Performance of Gate-all-around Silicon Nanowire Field Effect Transistor

As the size of the MOSFET is reduced, various short channel effects (SCEs) appears that degrade its performance. Multigate nanowire FET is one of the novel nanoelectronic devices that overcome these MOSFET limitations. The silicon nanowire field effect transistors with multiple gates around the silicon channel can significantly improve the gate control and are considered to be promising candidates for the next generation transistors. In this paper, we have considered the performance limits of Si nanowire field effect transistors in a Gate All Around (GAA) structure. Furthermore, we have studied the effects of Silicon body thickness on the characteristics of GAA silicon nanowire FET. It has been observed that Si-NWFET afford high drive-current (Ion), high transconductance and hence high gain. Thus, GAA configuration has good control of gate, which reduces the short-channel effects to a great extent.