119
7
Semiconductors as Gas Sensors
7.1 Introduction
Solid-state chemical and gas sensors are becoming increasingly necessary
in todays global climate to detect malicious release of poisonous or toxic
substances present in the atmosphere. Personal safety issues in both indus-
try and residential environments demand highly sensitive detection of gases
such as CO
x
, NO
x
and NH
3
.
The advantages of an all-solid-state gas detection system are low power
consumption, integration with existing circuitry and miniaturization.
Microelectromechanical systems (MEMS) technology has allowed for the
integration of the gas sensor, heating element and temperature sensor on a
standard Si wafer with easy integration into standard complementary metal
oxide semiconductor (CMOS) circuitry. MEMS implementation is not an easy
task; it should be optimized rst for nanostructured sensing mechanisms.
However, one of the disadvantages of traditional semiconducting metal
oxide gas sensors has been low gas sensitivity due to the limited surface-
to-volume ratio. In addition, many of the ceramic and thin-lm gas sen-
sors must be operated at temperatures exceeding 500°C in order to improve
sensitivity.
Nanomaterials offer an exciting possibility for improving solid-state gas
detection. Nanostructured sensing materials such as nanowires, nanotubes
and quantum dots offer an inherently high surface area [110]. In fact, car-
bon nanotubes have one of the best available surface-to-volume ratios [4].
The increased surface area leads to high sensitivity and fast response and
often allows for lower operating temperatures. In addition, different types
of nanomaterials have different sensitivities. It is possible to apply differ-
ent nanomaterials on the same MEMS platform to detect more complex gas
mixtures.
120 MEMS and Nanotechnology for Gas Sensors
In order to determine what type and structure of nanomaterials would be
best suited for gas detectors, researchers have designed and fabricated gas
detectors using single-walled carbon nanotubes, multiwalled carbon nano-
tubes, ZnO nanowires and nanoparticles and GaN nanorods [11,12]. Gas sen-
sitivity, temperature response, initial response time and recovery times are
important parameters of a gas sensor. Different sensor structures have been
studied by many researchers [1–10], which are included in this chapter. Also
the sensing mechanism and performance of some solid‐state gas sensors are
reviewed in this chapter, which is still a grey area and has not been fully
understood.
The analysis of various parameters of metal oxides and the search of crite-
ria, which could be used during material selection for solid-state gas sensor
applications, are also the main objectives of this chapter.
7.2 Development of Semiconductor Sensors
Owing to its high surface/volume ratio, increased surface activity and strong
adsorption to the target gas molecules, nanomaterials are grasping all areas
of application. So the nanoparticle-built semiconducting lms are expected
to be of good gas sensing performances and hence have attracted much
attention in the past decades. For instance, the metal oxide semiconducting
(MOS) thin lms consisting of nanoparticles, such as nanotubes, nanowires,
nanorods and nanorings, have been extensively studied in their gas sensing
properties to different gases. Generally, such nanostructured thin lms dis-
play much better sensing performances than the corresponding bulk mate-
rials. These nanostructured porous thin lms are usually produced on the
electrode-equipped substrates with the use of a nanoprecursor. It is hard
to control the nanostructure of the lm, which results in non-uniformity in
lm thickness and poor reproducibility in lm production. These ultimately
result in inconsistency of sensing performances and hamper the long-term
reliability of thin-lm sensors. So there is a compromise while choosing a
specic sensing layer. A single semiconducting nanowire gas sensor is excel-
lent for its high sensing performance; still the use is restricted due to its com-
plicated device construction, high cost and non-repeatability. Many different
facile synthesis approaches with low cost have already been implemented
for the fabrication of nanostructure-based thin-lm gas sensors with high
performances [13–16].
While working with semiconductor pn junctions, researchers discovered
that the junction parameters were changing due to environmental gases. At
that time, this change was completely unwanted and was creating a problem.
Encapsulation solved the problem as it was no longer exposed to the outside
environment. At a later stage, this problem clicked the mind of researchers

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