Seminar - Yair Paska...

dehisceforkElectronics - Devices

Nov 2, 2013 (3 years and 9 months ago)

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תדובע
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:אשונב

"
Understanding the Sensing Mechanism of
Nonpolar Analytes with Field Effect Transistor
s
"


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"

Understanding the Sensing Mechanism of
Nonpolar Analytes with Field Effect Transistor
s
"

Yair Paska

Advisor
:
Prof.
Hossam Haick


Silicon nanowire field effect transistors (Si NW FETs) have been used as powerful sensors for direct
detection of biological and chemical species. The detection of polar species has been attributed to
variations in the electric

field at the conduction channel due to molecular gating with the field induced
by the dipoles of the polar molecules. However, the detection of nonpolar species with Si NW FETs
has been lacking and not well understood to date. In this study, we experiment
ally study the detection
of nonpolar species using molecularly modified Si NW FETs and model the detection process based on
changes in the carrier mobility, voltage threshold, off
-
current, off
-
voltage, and subthreshold swing of
the Si NW FET.
We show that
elimination of the free (un
-
reacted) hydroxyl groups (Si
-
OH) located on
the oxide surface of the Si NW, by controlled tricholorosilane (TS) adsorption process, improves both
the electrical and sensing properties of the gated Si NW FET devices. The improvem
ents are expressed
through a decrease in the hysteresis magnitude and hysteresis drift of the gated Si NW sensors, through
a dramatic decrease in the sensitivity to polar analytes and humidity, and through
significant

increase in
the sensitivity to nonpola
r analytes.

The sensing mechanism of nonpolar
analytes

was explained in
terms of molecular gating, due to two
indirect

effects: (i) a change in the charged surface states induced
from conformational changes at the functionality of the Si NW surface because

of analyte
-
functionality
interactions; and (ii) a change in the dielectric medium close to the Si NW surface induced from the
formation of condensed nonpolar analyte layer. In contrast, polar analytes are sensed
directly

via
analyte
-
induced changes in the

Si NW charge carriers, most probably due to electrostatic interaction
between the Si NW and the polar analytes. The contribution of these effects to the overall measured
sensing signal was determined and discussed. A semi
-
empirical physical models for the

analyte
-
induced conductivity change in the Si NW FETs and for the fundamental feature of Si NW FET
characteristic are presented and discussed. The results provide a launching pad for real
-
world sensing
applications, such as environmental monitoring, homel
and security, food quality control, and medicine.