Metal-Oxide Gas Sensor Response

Target Gas

Sensor Conditions

Gas concentration 0 ppm

Operating temperature 300 °C

Relative humidity 30 %

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BASELINE

Sensor resistance R

— kΩ

Baseline R₀(T)

— kΩ

Response ratio R₀/R

1.00×

τ (response time)

— s

Signal change ΔR/R₀

0 %

How it works

A metal-oxide (SnO₂) gas sensor is a porous film of semiconducting nanograins. In clean air, oxygen chemisorbs on each grain surface as O⁻/O₂⁻, pulling electrons out of the conduction band and forming a depletion layer at every grain-boundary neck. Current must cross a Schottky-like potential barrier eV_s at each neck, so the film's baseline resistance is thermally activated:

R₀(T) = R_ref · exp[ (E_a/k_B)·(1/T − 1/T_ref) ]

When a reducing gas (CO, CH₄, H₂, ethanol vapor) reaches the film, it reacts with the adsorbed oxygen and releases trapped electrons back into the grain, shrinking the depletion layer and lowering the barrier — resistance drops. The steady-state resistance follows the standard power-law sensitivity model used to characterize real MOx sensors:

R_gas = R₀(T) / [ 1 + A(T,RH)·C^β ]
A(T,RH) = A₀ · exp[ −(T−T_opt)²/2w² ] · 1/(1+k_h·RH/100)

Sensitivity peaks at a gas-specific optimum operating temperature T_opt (a real, well-documented SnO₂ property — different reactions dominate at different temperatures) and is suppressed by competing water-vapor adsorption at high humidity. The resistance does not jump instantly: it relaxes toward the new equilibrium with first-order kinetics,

dR/dt = (R_eq − R) / τ(T),   τ(T) shorter at higher T (Arrhenius-faster surface reaction)

Concentration slider — sets the target-gas level C reaching the film; higher C drives R further below R₀.
Temperature slider — shifts both the baseline resistance and how close you are to that gas's sensitivity optimum.
Humidity slider — models the real cross-sensitivity problem that makes humidity compensation necessary in field air-quality sensors.
Gas selector — swaps in each gas's real relative optimum temperature and power-law exponent β.

This is the exact operating principle behind consumer and industrial CO/CH₄/VOC alarms and low-cost air-quality monitoring nodes (e.g. Figaro/MiCS-type sensors).

R(t) — last 60 s

SnO₂ grain Depletion shell Gas molecule

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