MMOS Sensor Technology

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MMOS Sensor Technology

• Kristin Lee
• Allison Douglas
• Scott Chao

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Mixed Metal Oxide Semiconductor (MMOS)

• When surface absorbs gas, there is a large change
  in electrical conductivity
• Commonly used oxides
• Cr2TiO3
• WO3
• SnO2

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Oxidizing and Reducing Gases

• Change in resistance caused by loss or gain of
  surface electrons
• Absorbed oxygen reacts with target gas
• Reducing gases
• Donate electrons and decrease hole concentration
• CO, CH4
• Oxidizing gases
• Accept (subtract) electrons and increase hole
  concentration
• NO2, O3

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Semiconductor Types

• n-type semiconductor
• Electrons are majority carrier
• Electron concentration (n) of conduction band
  affected by gas
• p-type semiconductor
• Holes are majority carrier
• Hole concentration (p) of valence band affected
  by gas

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Energy Band Diagram

• Ec corresponds to the potential energy of the
  electron at rest
• Kinetic energy of an electron is measured upward
  from the conduction band
• Ev corresponds to the potential energy of the
  hole at rest
• Kinetic energy of a hole is measured downward
  from the valence band

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Semiconductor Resistivity

• Resistivity, ? 1/(q?nn q?pp)
• q 1.6x10-19 C
• ?n,p electron, hole mobility (cm2/V-s)
• n electron concentration
• p hole concentration
• For n-type, ngtgtp, ? ?1/(q?nn)
• For p-type, pgtgtn, ? ?1/(q?pp)

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MMOS Ozone Sensors
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MMOS Ozone Sensors

• Heated
• Reduces interference from humidity
• Increases response time
• Gas selectivity optimization
• Oxide microstructure and thickness
• Catalytic additives to oxide
• Protective coatings
• Activated-carbon filters

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MMOS Ozone Sensors

• O3 is an oxidizing gas (i.e. reduces electron
  concentration)
• Recall, for n-type semiconductors
• ? ?1/(q?nn)
• As n decreases, ? will increase, and therefore
  resistance increases
• R ?(L/A)
• For p-type semiconductors
• ? ?1/(q?pp)
• As p increases, ? and R decrease

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MicroChemical Systems Ozone Sensor

• RS determined by voltage division
• VS VCC(RS/(RL RS)
• VSRL VSRS VCCRS
• RS (RL/(VCC - VS))VS
• As O3 concentration increases,
• VS increases and RS increases. Therefore, the
  MiCS sensors are n-type semiconductors.

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Resources

• Casey, H. Craig Jr. Devices for integrated
  Circuits Silicon and III-IV Compound
  Semiconductors. John Wiley Sons, Inc. 1999.
• City Technology. http//www.citytech.com/technolog
  y/semiconductors.asp
• Massoud, Hisham Z. Class notes for ECE 216,
  Department of Electrical and Computer
  Engineering, Duke University.
• MicroChemical Systems. Datasheet for MiCS-2610
  Ozone Sensor.