Al-doped ZnO films were deposited onto SiO 2 /Si substrates by rf magnetron sputtering system as a CO gas sensor. The dependence of the thin film thickness on CO gas sensing properties was investigated, where the film thickness was varied by controlling the deposition time. The structure of the deposited ZnO:Al films was determined by X-ray diffraction, scanning electron microscopy and atomic force microscopy. The CO gas sensing properties were determined by in situ measurement for surface resistance of the thin film as a function of film thickness, different atmosphere, and operation temperature. It was shown that the films were flat and smooth with (0 0 0 1) preferred orientation. The grain size was increased as the film thickness was increased during deposition. Here, the CO gas sensing properties were relative to the structural characteristics where the maximum sensitivity of 61.6% was obtained at 65 nm film thickness for the operation temperature of 400 °C.

The effects of thickness and operation temperature on ZnO:Al thin film CO gas sensor

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet- and dry-deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono- or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non-equilibrium or sub-stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor-deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.

Semiconductor Gas Sensors: Dry Synthesis and Application

Highly crystalline zinc oxide (ZnO) nanoparticles have been synthesized by hydrothermal route at 120 °C over a range of different time periods. The synthesized nanoparticles are characterized for their phase and morphology by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermogravimetric-differential thermal analysis (TG-DTA) follows the thermal changes accompanying the heat treatment and ultra-violet diffuse reflectance spectroscopic studies (UV-DRS) give a measure of the optical properties. Electrical properties of the synthesized nanoparticles are studied by AC impedance and DC conductivity measurement. The gas sensing properties were studied towards reducing gases like liquefied petroleum gas (LPG), ammonia, hydrogen, ethanol (EtOH), etc., and it is observed that the nanoparticles show high sensitivity to LPG and ethanol at relatively low operating temperatures. Pd incorporation results in a decrease in operating temperature by more than 100 °C, and improves the sensing characteristics in terms of response and recovery times.

Hydrothermal synthesis of highly crystalline ZnO nanoparticles: A competitive sensor for LPG and EtOH

Chemiresistive gas sensors with low power consumption, fast response, and reliable fabrication process for a specific target gas have been now created for many applications. They require both sensitive nanomaterials and an efficient substrate chip for heating and electrical addressing. Herein, a near room working temperature and fast response triethylamine (TEA) gas sensor has been fabricated successfully by designing gold (Au)-loaded ZnO/SnO2 core-shell nanorods. ZnO nanorods grew directly on Al2O3 flat electrodes with a cost-effective hydrothermal process. By employing pulsed laser deposition (PLD) and DC-sputtering methods, the construction of Au nanoparticle-loaded ZnO/SnO2 core/shell nanorod heterostructure is highly controllable and reproducible. In comparison with pristine ZnO, SnO2, and Au-loaded ZnO, SnO2 sensors, Au-ZnO/SnO2 nanorod sensors exhibit a remarkably high and fast response to TEA gas at working temperatures as low as 40 °C. The enhanced sensing property of the Au-ZnO/SnO2 sensor is also discussed with the semiconductor depletion layer model introduced by Au-SnO2 Schottky contact and ZnO/SnO2 N-N heterojunction.

Near Room Temperature, Fast-Response, and Highly Sensitive Triethylamine Sensor Assembled with Au-Loaded ZnO/SnO2 Core–Shell Nanorods on Flat Alumina Substrates

High transmittance CdO thin films obtained by the sol-gel method

To obtain a sensitive sensor for TMA gas, ZnO-based films were fabricated by a rf magnetron sputtering method using targets added with Al2O3, TiO2, or V2O5. The films were heat-treated at a temperature between 400°C and 800°C for 60 min in oxygen. It was found that the sensitivity and selectivity of the films for TMA gas at 300°C increased by doping with 4.0 wt% Al2O3, 1.0 wt% TiO2 and 0.2 wt% V2O5. The sensor with the doped film 120 nm thick and annealed at 700°C for 60 min in oxygen exhibited fairly excellent sensitivity and selectivity to TMA, and electrical stability. Upon exposure to odors from a mackerel, the sensor using this film could trace well its deterioration during storage.

Zinc oxide thin film doped with Al2O3, TiO2 and V2O5 as sensitive sensor for trimethylamine gas

This paper presents the analog adaptive phase-locked loop (PLL) architecture with a new adaptive bandwidth controller to reduce locking time and minimize jitter in PLL output for wireless communication. It adaptively controls the loop bandwidth according to the locking status. The adaptive bandwidth control is implemented by controlling charge pump current depending on the locking status. It is simulated by HSPICE and achieves the primary reference sidebands at the output of the VCO at approximately -80dBc

A low jitter phase-lock loop based on a new adaptive bandwidth controller

A Fast Lock and Low Jitter Phase Locked Loop with Locking Status Indicator

A low spur phase-locked loop (PLL) with FVCO-sampled feedforward loop-filter has been proposed. Conventional PLL has loop filter made of a resistor and capacitors. The proposed PLL is working stably with the filter consisted of capacitors and a switch. It has been designed with a 1.8V 0.18μm CMOS process and proved by simulation with HSPICE.

A Low Spur Phase-Locked Loop with FVCO-sampled Feedforward Loop-Filter

In this paper, a phase locked loop with suppressed power supply noise has been proposed. The added negative feedback loop of voltage controlled oscillator(VCO) and power noise detector suppresses the power noise induced jitter variation of VCO down to 1/3. The power noise detector is the modified circuit of frequency voltage converter. The proposed PLL has been designed based on a 1.8V 0.18um CMOS process and proved by HSPICE simulation.

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Hyek-Hwan Choi

Papers

Zinc oxide thin film doped with Al2O3, TiO2 and V2O5 as sensitive sensor for trimethylamine gas

A low jitter phase-lock loop based on a new adaptive bandwidth controller

A Fast Lock and Low Jitter Phase Locked Loop with Locking Status Indicator

A Low Spur Phase-Locked Loop with FVCO-sampled Feedforward Loop-Filter

Design of Phase Locked Loop with Supply Noise Detector for Improving Noise Reduction