1. ECE699 – 004 Sensor Device Technology
Chapter 2 Electrochemical Sensors
Fall 2018
George Mason University

2. 2.1 Introduction to EC sensors
Conductimetric, potentiometric and amperometric
An example

3. 2.2 Conductimetric sensors
3 configurations

4. The circuit model

5. Metal-insulating oxide-conducting sensing film

6. The contact resistance of a junction
Richardson constant
Current through the metal/semiconductor junction

7. The resistance of the active layer
If it is semiconductor: 𝜎

8. Types of conductimetric sensors

9. 3. Semicoducting metal oxide sensors

10. 3. Semicoducting metal oxide sensors
Variety of semiconducting metal oxide materials
Tunable conductivity
Can be functionalized to detect target chemical
Adaptive to different environment
Low cost
Most frequently used: ZnO, SnO2, WO3, TiO2, CuO

11. N-type

12. P-type

13. Electrical properties
Scattering of carriers is dominated by phonon scattering - mobility
However, the carrier concentration increases exponentially with increasing temperature – greater than the decrease by mobility

16. Surface of metal oxide films

19. The surface potential barrier:
The electrons need to overcome this barrier to contribute to the current

20. Surface/space charge capacitance

22. Measurement setup

24. Accumulation and inversion layers

26. Semiconductor metal oxide film in the following forms:
Single crystal
Amorphous
Polycrystalline
Compressed powders

27. Compressed powders

28. Gas-semiconductor film interactions
Three gas adsorptions:
Physisorption: bonding is physical, is weak
Chemisorption: 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

29. Adsorption of O2 :
Ambient O2 picks
Up an electron
O2 is broken into
two oxygens on
metal oxide surface,
then attract e from
metal oxide bulks

30. The reaction with a reducing agent R
 more often

31. In the case of carbon:
The amount of ionsorbed oxygen decreased, barrier potential decreased, excess electrons is produced  metal oxide film resistance decreased

32. Reaction with an oxidizing gas
The resistance of metal oxide film increased because electrons were used.

34. Interaction with O2  decrease free electrons
Interaction with H2  H2 donate electrons
Interaction with water  H+ ion attracted to the film  decrease O- in the metal oxide

35. 2.5 other Electrochemical gas sensors
Solid-state capacitive gas sensors
MOS capacitive gas sensors
Micromachine capacitive polymer gas sensors
Schottky diode type gas sensors
Based on MIS capacitive structure

36. MOS capacitive gas sensors
Usually,
Oxidizing gases increase the barrier potential
Reducing gases decrease the barrier potential
The 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

37. Pt detect H2 containing gases

39. Capacitive polymer gas sensors
The polymer is the chemical selective layers

40. Schottky diode gas sensors

42. Pt/TiO2 MS diode gas sensor

43. Mid term exam
Prepare a 20-slides review and 6-page report on one of the following subject
Gas sensors on H2, CO, CO2 and O2
Gas sensors on toxic gases: Nox, Sox, H2S and S- containing gases
Gas sensors on organic solvent gases