MOS type/World leader in gas sensing innovation

Operating principle

Figaro offers a wide range of gas sensor products for the detection of various gases, from explosive gases such as propane, toxic gases such as carbon monoxide, to air quality sensors for volatile organic compounds (VOCs) that are responsible for sick-house syndrome. Figaro offers a diverse portfolio of sensor technologies that can be matched to the unique requirements of each application.

  • MOS type
  • Catalytic type
  • Electrochemical type



In clean air, donor electrons in tin dioxide are attracted toward oxygen which is adsorbed on the surface of the sensing material, preventing electric current flow.


In the presence of reducing gases, the surface density of adsorbed oxygen decreases as it reacts with the reducing gases. Electrons are then released into the tin dioxide, allowing current to flow freely through the sensor.

Operating principle

When semiconductor particles (typically tin dioxide) are heated in air at high temperature, oxygen is adsorbed on the particle surface by capturing free electrons. The depletion layer thus formed is largely dependent on the radius of semiconductor particles used. If it is as small as conventionally used in gas sensors (tens nano-meters), the depletion can extend up to the whole area of each particle (volume depletion, high sensitive). If the size is far larger, on the other hand, depletion takes place conventionally on the periphery of each particle (regional depletion, low sensitive).

  Figure 1 shows how the energy band structure and the distribution of conduction electrons change with increasing the partial pressure of oxygen from zero (flat band state) to state I (regional depletion), II (border) and III (volume depletion). Until the border is reached, the adsorption equilibrium is attained by increasing the depletion layer thickness. Later (volume depletion), however, the Fermi level is lowered by p kT on going from II to III while the layer thickness is kept constant.

x : Distance in a radial direction
qV(x) : Potential energy
a : Particle radius
[O-] : Adsorbed oxygen concentration
EC : Conduction band energy
EF : Fermi level
pkT : Fermi level shift
[e] : Electron concentration
Nd : Donor density

Figure 1. Energy band structure (top) and distribution of conduction electrons (bottom)
for a semiconductor particle as correlated with an increase in adsorbed oxygen concentration

 In this stage, two important equations are derived theoretically for a sensor device consisting of spherical particles, as follows.

[e]S=Nd exp{-(1/6)(a/LD)2-p} ... (1)
R/R0=Nd/[e]S ... (2)

 Here [e]S is the surface electron concentration of particles and LD is the Debye length. R and R0 is the sensor resistances at the steady state and flat band state, respectively. For other symbols, see the caption of Fig.1. When sensor materials are selected, Nd, a, LD and R0 are fixed, while p is dependent on the actual gaseous conditions.

As described above, MOS type gas sensors change resistance (R) as a result of a change in adsorbed oxygen concentration. If this is used adequately, one can detect reducing gases like carbon monoxide. The adsorbed oxygen formed in clean air will be consumed on contact with carbon monoxide, the resulting decrease of R being used to estimate the concentration of carbon monoxide. The sensor recovers the original level of resistance when carbon monoxide is off. Such a detection mechanism is operative in tin dioxide based gas sensors.

Reference: Noboru Yamazoe, Kengo Shimanoe, Basic approach to the transducer function of oxide
semiconductor gas sensors, Sensors and Actuators B 160 (2011) 1352-1362

Warnings and Precautions for Use of MOS-type gas sensors

  • Carefully read product information and other technical information provided by Figaro before using our products, and confirm specifications and operating conditions.
  • When designing an application circuit, please make sure that an accidental short circuit or open circuit of other electronic components would not cause the sensor to be subjected to excessive voltage, current, or temperatures exceeding the rated values.
  • When designing application products, please make sure that a gas sensor malfunction would not 1) cause adverse effects on other components, 2) directly or indirectly impair the safety of application products that use gas sensors (e.g., emit smoke, cause fire, or other unstable states of application products).
  • Consider adding safety measures for fail-safe where necessary, such as a protection circuit.

Cautions for Safe Use of MOS-type gas sensors

Applied voltage
Do not use the gas sensor if higher than the rated voltage is applied. If higher than the rated voltage is applied to the sensor, the lead wires, the heater, and/or the sensor element may be damaged or sensor characteristics may be irreversibly impaired, even if no physical damage or breakage occurs.
Environmental conditions
  • Avoid exposing the sensor where adhesives or hair grooming materials containing silicone or silicone rubber/putty may be present. If silicone vapors adsorb onto the sensing element surface, the sensing material will be coated, irreversibly inhibiting sensitivity.
  • Avoid highly corrosive environments. High density exposure to corrosive gases such as hydrogen sulfide, sulfur oxide, chlorine, hydrogen chloride, etc. for extended periods may cause corrosion or breakage of the lead wires or of the heater material. For information on specific gases and conditions for corrosive gases, please consult with Figaro.
  • Avoid contamination by alkaline metals. Sensor characteristics may be significantly changed if the sensor is contaminated by alkaline metals, especially salt water spray.
  • Sensor performance may be affected if exposed to a high density of reactive gases for a long period of time, regardless of the powering condition. For information on specific gases and conditions, please consult with Figaro.
  • If water freezes on the sensing element surface, the sensing material may crack, which will irreversibly affect sensor characteristics.
  • If water condenses on the sensor element surface and remains for an extended period, sensor characteristics may temporarily drift. Light condensation under normal conditions of indoor usage would not pose a significant problem for sensor performance.
  • Regardless of its powering condition, if the sensor is exposed in extreme conditions such as very high humidity, high temperatures, or high contamination levels of organic vapors or other gases for a long period of time, sensor performance may be impaired.
  • MOS-type gas sensors cannot properly operate in a zero or low oxygen content atmosphere. They require the presence of normal ambient oxygen in their operating environment in order to function properly.
  • Sensor characteristics may be changed due to soaking or splashing the sensor with water.
  • Avoid mechanical shock. Breakage of lead wires may occur if the sensor is subjected to a strong shock.
  • Under no circumstances should the sensor be disassembled, nor should the sensor can and/or cap be deformed. Such action would void the sensor warranty and would cause irreversible change in characteristics.
Storage conditions
When stored without powering in normal air for a long period, or in an environment contaminated with organic vapors or volatile oils, the sensor may show a reversible drift in resistance according to the environment. The sensor should be stored in a sealed bag of which the material does not emit odor or gas. Do not store the sensor with silica gel.
Mounting process
  • Manual soldering is always recommended for mounting gas sensors.
  • Wave soldering may be used for MOS-type gas sensors if limited to the following conditions:
    (1) Suggested flux: Rosin flux with minimal chlorine
    (2) Transfer speed: 1-2 meters / min.
    (3) Pre-heating temperature: 100±20˚C
    (4) Solder bath temperature: 250±10˚C
    (5) Allowable soldering passes: 2 times maximum
    The results of wave soldering cannot be guaranteed if conducted outside the above guidelines since some flux vapors may cause poisoning or drift in sensor performance similar to the effects of silicone vapors.
  • When a resin coating is applied on a printed circuit board for improving its resistance to moisture and corrosive gases, the chemical solvent contained in the coating material may affect sensor characteristics. Sample testing should be conducted to see if this process would adversely affect sensor characteristics.
  • Excessive vibration may cause the sensor element and/or the lead wires to resonate and eventually break. Usage of compressed air drivers or ultrasonic welders on assembly lines may cause such vibration to the sensor. Before using such equipment, preliminary tests should be conducted to verify that there will be no influence on sensor characteristics.
  • MOS type
  • Catalytic type
  • Electrochemical type