In this episode, Bob and Mike explore the world of metal oxide semiconductor (MOS) sensors, which are used to detect toxic gasses.
Our hazardous materials training manual, National Emergency Response Hazmat Drills: 50 Drills for Use with Hazardous Materials Personnel, is finally available on Amazon!
Complete Show Notes
4:45 What Are Metal Oxide Semiconductor (MOS) Sensors?
- Used for detecting toxic gasses (most commonly carbon monoxide and combustible gasses) and work via a gas-sensitive film composed of tin or tungsten oxides
- Film reacts with the gasses, which triggers the device whenever toxic levels are present
- Metal oxide sensors are considered due to their ability to operate in low humidity ranges
- They can detect a range of gasses, including combustibles
- To completely understand how MOS sensors work, it’s important to understand reduction reactions, oxidation reactions, and the basic functions of semiconductors
8:10 What is Reduction?
- Chemical reaction that involves the gaining of electrons by one of the atoms involved in the reaction
- Refers to the element that accepts electrons – the oxidation state of the element that gains electrons is lowered
- Example: When iron reacts with oxygen, it forms a chemical called rust – the iron is oxidized, and the oxygen is reduced
9:55 What is Oxidation?
- Chemical reaction that involves the moving of electrons – specifically, it means the substance that gives away electrons is oxidized
- When iron reacts with oxygen, it forms a chemical called rust because it’s been oxidized – the iron lost some electrons, and the oxygen has been reduced because it gained some electrons
- Oxidation is the opposite of reduction:
- A reduction reaction always comes together with an oxidation reaction
- Oxidation and reduction together are called redox
- Oxygen doesn’t have to be present in a reaction for it to be a redox reaction
- Oxidation is the loss of electrons
- We can remember the difference between reduction and oxidation using the acronym LEO-GER: Loss of Electron Oxidation and Gain of Electron Reduction
13:25 What is An Oxidizing Agent?
- Two meanings:
- Definition 1: A chemical that releases oxygen atoms
- Definition 2: A chemical that accepts electrons from a reducing agent
- Example: Potassium permanganate has an oxidation state of +7
- In acid solution, it gains 5 electrons (e-) to become a manganese compound with an oxidation state of +2
- Most oxidizing agents in this definition have oxygen, but not all of them (like fluorine (F2))
- When fluorine (the most powerful oxidizing agent) acts as an oxygen agent, it gains an electron to transfer from an oxidation state of 0 to an oxidation state of -1
17:25 What is a Reducing Agent?
- A chemical that gives electrons away to another chemical compound – the oxidizing agent
- For example, zinc is a reducing agent – when it reacts with an oxidizing agent, it gives up 2 electrons, thereby changing its oxidation state from 0 to +2
- All chemical elements have an oxidation state of 0
19:25 What is a Semiconductor?
- Material with an electrical conductivity value falling between that of a conductor (i.e. copper or gold) and an insulator (i.e. glass)
- Resistance decreases as temperature increases, which is opposite of what happens to a metal
- Conducting properties may be altered in useful ways by the deliberate and controlled introduction of impurities (doping) into the crystal structure
- When two differently doped regions exist in the same crystal, a semiconductor junction is created
- The behavior of charge carriers (i.e. electrons, ions, and electron holes) at these junctions is the basis of diodes, transistors, and all modern electronics
24:20 How Do Semiconductor Sensors Work?
- Detect gasses by a chemical reaction that takes place when a gas comes in direct contact with the sensor
- Tin dioxide is the most common material used in semiconductor sensors
- The electrical resistance in the sensor is decreased when it comes in contact with the monitored gas
- The resistance of the tin dioxide is typically around 50 kΩ in air, but can drop to around 3.5 kΩ in the presence of 1% methane – this change in resistance is used to calculate the gas concentration
- Semiconductor sensors are commonly used to detect hydrogen, oxygen, alcohol vapor, and harmful gasses (i.e. carbon monoxide)
- Two of the most common uses for semiconductor sensors are carbon monoxide sensors and breathalyzers
- Because the sensor must come in contact with the gas to detect it, semiconductor sensors work over a smaller distance than infrared point or ultrasonic detectors
26:45 Operating Principles of MOS Sensors
- When semiconductor particles (typically tin dioxide) are heated in air at high temperatures, oxygen is adsorbed on the particle surface to capture free electrons
- In the most extreme cases (where oxygen concentration is 0%), free electrons flow through the conjoined parts (grain boundary) of the tin dioxide crystals when MOS materials are heated at high temperatures (like 400 degrees Celsius)
- In clean air (approximately 21% O2), oxygen is adsorbed on the metal oxide surface
- Due to its high electron affinity, adsorbed oxygen attracts free electrons inside the metal oxide to form a potential barrier (eVs in air) at the grain boundaries
- This potential barrier prevents electron flow to cause high sensor resistance in clean air
- When the sensor is exposed to combustible or reducing gasses (i.e. carbon monoxide), the oxidation reaction with adsorbed oxygen occurs at the surface of the tin dioxide
- As a result, the density of adsorbed oxygen on the tin dioxide surface decreases as the height of the potential barrier is reduced
- Electrons flow easily through the potential barrier of reduced height as the sensor resistance decreases
- Gas concentration in air can be detected by measuring the resistance change of MOS-type gas sensors
- The chemical reaction of gasses and adsorbed oxygen on the tin dioxide surface varies depending on the reactivity of sensing materials and the working temperature of the sensor
- The adsorbed oxygen formed in clean air will be consumed on contact with carbon monoxide
- The resulting decrease of resistance is used to estimate the concentration of carbon monoxide
- The sensor recovers the original level of resistance when carbon monoxide is off
Have a question? Send an email to firstname.lastname@example.org or leave a message on our Haz Mat Guys comment hotline: 843-628-1484
- THMG014 – Metering Overview and Basic Sensors
- THMG071 – Meter Reading, Sensor Response, and Measurement Metering Ranges
- THMG063 – Top 5 Chemicals That Will Hurt You