Alireza Kargar

Modeling of a Silicon Nanowire pH Sensor with Nanoscale Side Gate Voltage

A silicon nanowire (Si-NW) sensor for pH detection is presented. The conductance of the device is analytically obtained, demonstrating that the conductance increases with decreasing oxide thickness. To calculate the electrical conductance of the sensor, the diffusion-drift model and nonlinear Poisson–Boltzmann equation are applied. To improve the conductance and sensitivity, a Si-NW sensor with nanoscale side gate voltage is offered and its characteristics are theoretically achieved. It is revealed that the conductance and sensor sensitivity can be enhanced by adding appropriate side gate voltages. This effect is compared to a similar fabricated structure in the literature, which has a wire with a rectangular cross section. Finally, the effect of NW length on sensor performance is investigated and an inverse relation between sensor sensitivity and NW length is achieved.

Theoretical investigation of silicon nanowire pH sensor

In this paper, Si nanowire (NW) sensor in pH detection is presented. The conductance of device is analytically obtained and demonstrated the conductance increases with reducing the oxide thickness. To calculate the electrical conductance of sensor, the diffusion-drift model and nonlinear Poisson-Boltzmann equation are used. To improve the conductance a Si NW sensor with nanoscale side gate voltage is presented and its conductance is theoretically achieved. It is shown the conductance and consequently sensor sensitivity can be enhanced by adding suitable side gate voltage. Finally this effect is compared to the almost similar fabricated structure in literature which has a wire with rectangular cross section.

Sensitivity Analysis of Silicon Nanowire Chemical Sensor

The Si nanowire sensors with different geometries in the chemical detection are described. To analyze the sensor performance, the sensitivity of sensors with different cross sections including circular, rectangular, and trapezoidal are derived by two definitions. It is demonstrated that the sensitivity of Si nanowire sensors with different structures in the chemical detection is a function of geometrical parameters and doping density. It is illustrated that the sensitivity varies inversely with cross-section area, doping density, and also the length of nanowire which has not analytically been shown before as a significant parameter for sensor performance. Finally, it offers a general formula for the Si nanowire chemical sensors sensitivity with different structures.

Optical surface sensing by bent slot waveguides

This paper presents the surface sensing characteristics of bent slot waveguides through a combined effective-index method/modified transfer-matrix method. The effect of waveguide parameters, including waveguide width, slot width, bending radius, and asymmetry coefficient on the waveguide surface sensitivity is investigated. We reveal that bent single-slot waveguides provide higher surface sensitivity than conventional bent waveguides.

Comparison of Current-Voltage Characteristics of Carbon Nanotube and Nanowire FETs

In this paper two types of nano transistors: coaxial nanowire and carbon nanotube field effect transistors are presented. The current-voltage transfer characteristics near the source of these nano transistors are achieved by using an approach based on WKB approximation and ballistic transport. The simulated current-voltage characteristics for these nano transistors are compared. It is shown the nanowire FET has a higher Ion/ Ioff ratio rather than carbon nanotube FET. Moreover, it is demonstrated the carbon nanotube FET can provide the higher on current depending on its structure and parameters. The presented comparisons can be used to understand the necessary characteristics of a nano transistor in order to be used in a specific VLSI circuit design or other specific applications.

Bending loss analysis of multiple-slot waveguide microrings

Bending efficiency of three-dimensional bent multiple-slot waveguides are studied applying a combined method of effective-index and modified transfer-matrix methods. The effects of asymmetric structure (in which the widths of inner and outer strips are unequal), asymmetric slots (slots with different widths), and asymmetric middle strips (middle strips with dissimilar widths) on the bending efficiency are investigated. We show that the bending efficiency can be improved through the use of asymmetric structures and asymmetric middle strips. The bending efficiency of different slot waveguides (up to quintuple-slot structure) is compared. We reveal that although in general, single-slot waveguide provides the lowest bending loss for the same waveguide parameters; it is possible that multiple-slot waveguide can present a lower bending loss than the single-slot one.

Carbon Nanotube FET with Asymmetrical Contacts

We compute the current-voltage transfer characteristics of a new structure of coaxial carbon nanotube field effect transistors and simulate its behavior. In the proposed structure one end of a carbon nanotube is contacted to the metal intrinsically. The other end is doped and then contacted to the metal. Thus, the source nanotube-metal interface forms a Schottky barrier and the drain contact exhibits Ohmic properties. The simulation results of the new device presented show unipolar characteristics but are lower on current than when both contacts are Schottky or Ohmic. The coaxial single Schottky barrier CNTFET (SSB-CNTFET) is proposed for the first time and is modeled based on a semi-classical approach for the calculation of tunneling probability of electrons through the barrier. Both analytical and numerical computations have been used in order to solve Laplace and Schrodinger equations and also to compute tunneling probability for the device current.

Quantum Mechanical Analysis of Nanowire FETs

In this paper, we present an analysis of a coaxial Nanowire FET by combining the analytical and numerical approaches. We achieve electrical characteristics of cylinder cal Nanowire FET and investigate the effects of changing metal/semiconductor Schottky barrier height and gate oxide thickness on electrical characteristics. First, we solve 1-D Laplace equation analytically to achieve the potential distribution within the wire. Then, we show the effect of changing the gate oxide thickness on the potential distribution. Second, by applying WKB approximation to tunneling problem and assuming ballistic current transport, we numerically achieve transfer characteristics of FET. Finally, we investigate the effect of varying Schottky barrier height (SB) on electrical characteristics of Nanowire FET.

Comparison of Sensitivity in Single and Dual Waveguide Coupled Microring Resonator Sensors

The single and dual waveguide coupled microring resonator sensors are presented. The sensitivity of each structure based on intensity variation and homogeneous mechanism is derived analytically which is a function of geometric and waveguide parameters. It is demonstrated that each of two structures has a optimal half single-pass phase shift so that in this phase shift, the maximum sensitivity of two devices is achieved. Then the maximum sensitivities are compared. Finally, it is shown the dual waveguide can present the higher sensitivity in a (a) determinable range of its parameters.

Modeling of Carbon Nanotube Schottky Diode Based on Coaxial Geometry

The coaxial carbon nanotube Schottky diode is presented. It is demonstrated when in the coaxial geometry with two different Schottky contacts, only the hole carriers (p-type carriers) are considered, the device shows typical diode characteristic. To achieve the current-voltage characteristic of the diode, the WKB approximation and Ballistic transport are applied. The effects of change of three parameters: the gate voltage, band gap, and oxide thickness on the diode characteristic are investigated. It is shown the threshold voltage depends on the gate voltage and band gap. These factors affect the threshold voltage inversely. Moreover, the diode resistance decreases when these parameters increase. Finally, it is demonstrated, the change of the oxide thickness almost has not any effect on the threshold voltage while it affects the diode resistance.