Taiuk Rim

Enabling communication and cooperation in bio-nanosensor networks: toward innovative healthcare solutions

Bio-nanosensors and communication at the nanoscale are a promising paradigm and technology for the development of a new class of ehealth solutions. While recent communication technologies such as mobile and wireless combined with medical sensors have allowed new successful eHealth applications, another level of innovation is required to deliver scalable and cost-effective solutions via developing devices that operate and communicate directly inside the body. This work presents the application of nano technology for the development of miniaturized bio-nanosensors that are able to communicate and exchange information about sensed molecules or chemical compound concentration and therefore draw a global response in the case of health anomalies. Two communication techniques are reviewed: electromagnetic wireless communication in the terahertz band and molecular communication. The characteristics of these two modes of communication are highlighted, and a general architecture for bio-nanosensors is proposed along with examples of cooperation schemes. An implementation of the bio-nanosensor part of the nanomachine is presented along with some experimental results of sensing biomolecules. Finally, a general example of coordination among bio-nanomachines using both communication technologies is presented, and challenges in terms of communication protocols, data transmission, and coordination among nanomachines are discussed.

High efficiency silicon solar cell based on asymmetric nanowire

Improving the efficiency of solar cells through novel materials and devices is critical to realize the full potential of solar energy to meet the growing worldwide energy demands. We present here a highly efficient radial p-n junction silicon solar cell using an asymmetric nanowire structure with a shorter bottom core diameter than at the top. A maximum short circuit current density of 27.5 mA/cm2 and an efficiency of 7.53% were realized without anti-reflection coating. Changing the silicon nanowire (SiNW) structure from conventional symmetric to asymmetric nature improves the efficiency due to increased short circuit current density. From numerical simulation and measurement of the optical characteristics, the total reflection on the sidewalls is seen to increase the light trapping path and charge carrier generation in the radial junction of the asymmetric SiNW, yielding high external quantum efficiency and short circuit current density. The proposed asymmetric structure has great potential to effectively improve the efficiency of the SiNW solar cells.

Thermally Phase-Transformed In2Se3 Nanowires for Highly Sensitive Photodetectors

The photoresponse characteristics of In2Se3 nanowire photodetectors with the κ-phase and α-phase structures are investigated. The as-grown κ-phase In2Se3 nanowires by the vapor-liquid-solid technique are phase-transformed to the α-phase nanowires by thermal annealing. The photoresponse performances of the κ-phase and α-phase In2Se3 nanowire photodetectors are characterized over a wide range of wavelengths (300-900 nm). The phase of the nanowires is analyzed using a high-resolution transmission microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The electrical conductivity and photoresponse characteristics are significantly enhanced in the α-phase due to smaller bandgap structure compared to the κ-phase nanowires. The spectral responsivities of the α-phase devices are 200 times larger than those of the κ-phase devices. The superior performance of the thermally phase-transformed In2Se3 nanowire devices offers an avenue to develop highly sensitive photodetector applications.

Suspended honeycomb nanowire ISFETs for improved stiction-free performance

This paper reports high performance ion-sensitive field-effect transistors (ISFETs) with a suspended honeycomb nanowire (SHNW) structure. The SHNW can provide a longer, stiction-free channel than that which is possible with a suspended straight nanowire (SSNW) for the realization of gate-all-around biosensors. Devices with SHNWs, SSNWs and conventional nanowires on the substrate have been fabricated using a top-down approach in order to compare their electrical performances. The SHNW devices exhibit excellent electrical characteristics such as lower subthreshold swing, higher transconductance and higher linear drain current. In addition, the SHNW ISFETs show better pH sensitivity than other ISFETs. Based on the results, the SHNW device appears promising for enhancing the intrinsic performance and ensuring the reliable operation of biosensor applications.

Analysis of Contact Effects in Inverted-Staggered Organic Thin-Film Transistors Based on Anisotropic Conduction

In this paper, we propose an analytic model for inverted-staggered organic thin-film transistors, and we use the proposed model to investigate the dependence of contact effect on the voltage bias, the film thickness of the organic semiconductor, and the channel length. In our model, the variable-range-hopping transport is adopted for the conduction in the horizontal direction to the semiconductor-insulator interface, and the space-charge-limited conduction is adopted for the conduction in the vertical direction by considering the molecular orientations. Qualitative agreement is obtained between simulation and measurement in the steady-state characteristics. From simulation study, we notice that the contact resistances vary with the source-gate voltage and with the source-drain voltage, the film thickness requires to be optimized to improve the on-current and the linearity in the linear operating regime, and the overlap length between the gate electrode and the source/drain contact needs to be guaranteed for the short-channel devices because it would not be scaled as much as the channel length.

Chemical Gated Field Effect Transistor by Hybrid Integration of One-Dimensional Silicon Nanowire and Two-Dimensional Tin Oxide Thin Film for Low Power Gas Sensor

Gas sensors based on metal-oxide-semiconductor transistor with the polysilicon gate replaced by a gas sensitive thin film have been around for over 50 years. These are not suitable for the emerging mobile and wearable sensor platforms due to operating voltages and powers far exceeding the supply capability of batteries. Here we present a novel approach to decouple the chemically sensitive region from the conducting channel for reducing the drive voltage and increasing reliability. This chemically gated field effect transistor uses silicon nanowire for the current conduction channel with a tin oxide film on top of the nanowire serving as the gas sensitive medium. The potential change induced by the molecular adsorption and desorption allows the electrically floating tin oxide film to gate the silicon channel. As the device is designed to be normally off, the power is consumed only during the gas sensing event. This feature is attractive for the battery operated sensor and wearable electronics. In addition, the decoupling of the chemical reaction and the current conduction regions allows the gas sensitive material to be free from electrical stress, thus increasing reliability. The device shows excellent gas sensitivity to the tested analytes relative to conventional metal oxide transistors and resistive sensors.

Optimized operation of silicon nanowire field effect transistor sensors

Ion-sensitive field effect transistors have been advanced in recent years by utilizing silicon nanowires (Si-NWs), but establishing their optimized operation regime is an area of ongoing research. We propose a modified configuration of SiNWs in the form of a honeycomb structure to obtain high signal to noise ratio and high current stability. The low-frequency noise characteristics and the electrical stress are systematically considered for the optimization and compared against conventional SiNW devices. The operation voltage of the device severely affects the sensing stability; as the gate voltage is increased, the signal-to-noise ratio is enhanced, however, the stress effect becomes severe, and vice versa. The honeycomb nanowire structure shows enhanced noise characteristics in low voltage operation, proving to be an optimum solution for achieving highly stable sensor operation.

Statistical variability study of random dopant fluctuation on gate-all-around inversion-mode silicon nanowire field-effect transistors

Random dopant fluctuation effects of gate-all-around inversion-mode silicon nanowire field-effect transistors (FETs) with different diameters and extension lengths are investigated. The nanowire FETs with smaller diameter and longer extension length reduce average values and variations of subthreshold swing and drain-induced barrier lowering, thus improving short channel immunity. Relative variations of the drain currents increase as the diameter decreases because of decreased current drivability from narrower channel cross-sections. Absolute variations of the drain currents decrease critically as the extension length increases due to decreasing the number of arsenic dopants penetrating into the channel region. To understand variability origins of the drain currents, variations of source/drain series resistance and low-field mobility are investigated. All these two parameters affect the variations of the drain currents concurrently. The nanowire FETs having extension lengths sufficient to prevent dopant p...

Improved Electrical Characteristics of Honeycomb Nanowire ISFETs

Ion-sensitive field-effect transistors (ISFETs) with a honeycomb nanowire (HCNW) structure have been fabricated on a silicon-on-insulator wafer. The HCNW ISFET shows lower threshold voltage, lower subthreshold swing, higher drain current, and lower variability than the conventional nanowire device. Improved electrical characteristics are mainly due to the increased effective channel width and enhanced current drivability. The HCNW structure also exhibits improved current sensitivity in its pH response. These results suggest that the HCNW structure is promising for enhancing device performance and realizing sensors with high sensitivity.

Characteristics of gate-all-around silicon nanowire field effect transistors with asymmetric channel width and source/drain doping concentration

We performed 3D simulations to demonstrate structural effects in sub-20 nm gate-all-around silicon nanowire field effect transistors having asymmetric channel width along the channel direction. We analyzed the differences in the electrical and physical properties for various slopes of the channel width in asymmetric silicon nanowire field effect transistors (SNWFETs) and compared them to symmetrical SNWFETs with uniform channel width. In the same manner, the effects of the individual doping concentration at the source and drain also have been investigated. For various structural conditions, the current and switching characteristics are seriously affected. The differences attributed to the doping levels and geometric conditions are due to the electric field and electron density profile.