Fully Integrated Indium Gallium Zinc Oxide NO2 Gas Detector.

We report an amorphous indium gallium zinc oxide (IGZO)-based toxic gas detection system. The microsystem contains an IGZO thin-film transistor (TFT) as a sensing element and exhibits remarkable selectivity and sensitivity to low concentrations of nitrogen dioxide (NO2). In contrast to existing metal oxide-based gas sensors, which are active either at high temperature or with light activation, the developed IGZO TFT sensor is operable at room temperature and requires only visible light activation to revive the sensor after exposure to NO2. Furthermore, we demonstrate air-stable sensors with an experimental limit of detection of 100 ppb. This is the first report on metal oxide TFT gas sensors without heating or continuous light activation. Unlike most existing gas sensing systems that take care of identifying the analytes alone, the developed IGZO microsystem not only quantifies NO2 gas concentration but also yields a 5-bit digital output. The compact microsystem, incorporating readout and analog-to-digital conversion modules developed using only two TFTs, paves the way for inexpensive toxic gas monitoring systems.

Continuous Monitoring of pH and Blood Gases Using Ion-Sensitive and Gas-Sensitive Field Effect Transistors Operating in the Amperometric Mode in Presence of Drift.

Accurate and cost-effective integrated sensor systems for continuous monitoring of pH and blood gases continue to be in high demand. The capacity of ion-selective and Gas-sensitive field effect transistors (FETs) to serve as low-power sensors for accurate continuous monitoring of pH and blood gases is evaluated in the amperometric or current mode of operation. A stand-alone current-mode topology is employed in which a constant bias is applied to the gate with the drain current serving as the measuring signal. Compared with voltage-mode operation (e.g., in the feedback mode in ion-selective FETs), current-mode topologies offer the advantages of small size and low power consumption. However, the ion-selective FET (ISFET) and the Gas-sensitive FET (GasFET) exhibit a similar drift behavior, imposing a serious limitation on the accuracy of these sensors for continuous monitoring applications irrespective of the mode of operation. Given the slow temporal variation associated with the drift characteristics in both devices, a common post-processing technique that involves monitoring the variation of the drain current over short intervals of time can potentially allow extraction of the measuring signal in presence of drift in both sensor types. Furthermore, in the amperometric mode the static sensitivity of a FET-based sensor, given by the product of the FET transconductance and the sensitivity of the device threshold voltage to the measurand concentration, can be increased by adjusting the device design parameters. Increasing the sensitivity, while of interest in its own right, also enhances the accuracy of the proposed method. Rigorous analytical validation of the method is presented for GasFET operation in the amperometric mode. Moreover, the correction algorithm is verified experimentally using a Si3N4-gate ISFET operating in the amperometric mode to monitor pH variations ranging from 3.5 to 10.

Water-stable organic transistors and their application in chemical and biological sensors.

The development of low-cost, reliable sensors will rely on devices capable of converting an analyte binding event to an easily read electrical signal. Organic thin-film transistors (OTFTs) are ideal for inexpensive, single-use chemical or biological sensors because of their compatibility with flexible, large-area substrates, simple processing, and highly tunable active layer materials. We have fabricated low-operating voltage OTFTs with a cross-linked polymer gate dielectric, which display stable operation under aqueous conditions over >10(4) electrical cycles using the p-channel semiconductor 5,5'-bis-(7-dodecyl-9H-fluoren-2-yl)-2,2'-bithiophene (DDFTTF). OTFT sensors were demonstrated in aqueous solutions with concentrations as low as parts per billion for trinitrobenzene, methylphosphonic acid, cysteine, and glucose. This work demonstrates of reliable OTFT operation in aqueous media, hence opening new possibilities of chemical and biological sensing with OTFTs.

Comparison of angular free-in-air and tissue-equivalent phantom response measurements in p-MOSFET dosimeters.

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) radiation dosimeters have found recent application in providing real-time measurement in diagnostic radiology as well as in radiotherapy. Due to the design of the MOSFET dosimeter, the response is dependent on both energy and angulation with respect to the direction of primary radiation. The axial angular dependence has been characterized for both free-in-air and for tissue-equivalent phantoms. However, neither the angular dependence normal (90-degree) to the axial rotation, nor the effects of various tissue compositions on angular dependence, have been investigated for radiation energies in the diagnostic range. To characterize the angular dependence normal to the axial rotation, we exposed three "high sensitivity" MOSFET dosimeters simultaneously to x-rays from a medical diagnostic x-ray unit over a 360-degree rotation, at 22.5-degree increments, for both free-in-air and in lung, skeletal, and soft tissue-equivalent phantoms. The MOSFET dosimeters clearly showed an angular dependence in the orientation normal-to-axial as well as in the axial rotation, both for free-in-air and in tissue-equivalent phantoms. Significant variations in response occurred when the MOSFETs were exposed at incident angles between 90 degrees and 180 degrees normal-to-axial, as compared to the normal position (i.e., the zero-degree position with the bubble-side of the MOSFETs facing the radiation source). A maximum decrease in response to 32% of normal was observed when the distal ends (end opposite the wire lead) of the dosimeters were pointing directly away from the x-ray source (270-degree position). To avoid significant errors in MOSFET dosimeter readings, placement of the dosimeters should be consistent, and care should be taken to avoid orienting the dosimeter with its sensitive region (bubble side) facing away from the source of primary radiation at particular angles.