Home Podcasts THMG149 – Metal Oxide Semiconductor (MOS) Sensors

THMG149 – Metal Oxide Semiconductor (MOS) Sensors


In this episode, Bob and Mike explore the world of metal oxide semiconductor (MOS) sensors, which are used to detect toxic gasses.

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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
      • Example: Potassium chlorate has a chemical formula of KCIO3
      • When it oxidizes a reducing agent (i.e. powdered aluminum metal), it loses its oxygen to the aluminum and becomes potassium chloride (KCL)
    • 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

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