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

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

2.2 Conductimetric sensors 3 configurations

The circuit model

Metal-insulating oxide-conducting sensing film

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

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

Types of conductimetric sensors

3. Semicoducting metal oxide sensors

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

N-type

P-type

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

Surface of metal oxide films

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

Surface/space charge capacitance

Measurement setup

Accumulation and inversion layers

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

Compressed powders

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

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

The reaction with a reducing agent R  more often

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

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

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

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

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

Pt detect H2 containing gases

Capacitive polymer gas sensors The polymer is the chemical selective layers

Schottky diode gas sensors

Pt/TiO2 MS diode gas sensor

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