ECE699 004 Sensor Device Technology Chapter 2 Electrochemical Sensors Fall 2018 George Mason University 1 21 Introduction to EC sensors Conductimetric potentiometric and amperometric An example ID: 773289
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ECE699 – 004Sensor Device Technology Chapter 2 Electrochemical SensorsFall 2018George Mason University 1
2.1 Introduction to EC sensors Conductimetric, potentiometric and amperometric An example 2
2.2 Conductimetric sensors 3 configurations 3
The circuit model 4
Metal-insulating oxide-conducting sensing film 5
The contact resistance of a junction Richardson constant Current through the metal/semiconductor junction 6
The resistance of the active layer If it is semiconductor: 7
Types of conductimetric sensors 8
3. Semicoducting metal oxide sensors 9
3. Semicoducting metal oxide sensors Variety of semiconducting metal oxide materialsTunable conductivityCan be functionalized to detect target chemical Adaptive to different environment Low cost Most frequently used: ZnO, SnO2, WO3, TiO2, CuO 10
N-type 11
P-type 12
Electrical properties Scattering of carriers is dominated by phonon scattering - mobilityHowever, the carrier concentration increases exponentially with increasing temperature – greater than the decrease by mobility 13
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Surface of metal oxide films 16
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19 The surface potential barrier: The electrons need to overcome this barrier to contribute to the current
Surface/space charge capacitance 20
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22 Measurement setup
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Accumulation and inversion layers 24
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Semiconductor metal oxide film in the following forms: Single crystalAmorphousPolycrystallineCompressed powders 26
Compressed powders 27
Gas-semiconductor film interactions Three gas adsorptions:Physisorption: bonding is physical, is weakChemisorption: chemically bonded, the bonding is >2.5 times stronger Ionosorption: bo bonding occurs, by an electrostatic interaction, a surface state capturing an electron. Focus on the transfer of charge from film to gas 28
Adsorption of O2 :Ambient O2 picksUp an electronO2 is broken into two oxygens on metal oxide surface, then attract e from metal oxide bulks 29
The reaction with a reducing agent R more often 30
In the case of carbon: The amount of ionsorbed oxygen decreased, barrier potential decreased, excess electrons is produced metal oxide film resistance decreased 31
Reaction with an oxidizing gas The resistance of metal oxide film increased because electrons were used. 32
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Interaction with O2 decrease free electronsInteraction with H2 H 2 donate electrons Interaction with water H + ion attracted to the film decrease O- in the metal oxide 34
2.5 other Electrochemical gas sensors Solid-state capacitive gas sensorsMOS capacitive gas sensorsMicromachine capacitive polymer gas sensors Schottky diode type gas sensors Based on MIS capacitive structure 35
MOS capacitive gas sensors Usually,Oxidizing gases increase the barrier potentialReducing gases decrease the barrier potentialThe selectivity depends on the materials in the device Gases can be absorbed in metal, oxide or semiconductor The insulating layer should be thicker than 10nm to prohibit electron tunneling 36
37 Pt detect H 2 containing gases
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Capacitive polymer gas sensors 39 The polymer is the chemical selective layers
Schottky diode gas sensors 40
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Pt/TiO2 MS diode gas sensor 42
Mid term exam Prepare a 20-slides review and 6-page report on one of the following subjectGas sensors on H2, CO, CO2 and O 2 Gas sensors on toxic gases: Nox, Sox, H2S and S-containing gasesGas sensors on organic solvent gases 43