A Complete Guide to Semiconductor Gas Sensors: From Principles to Concentration Conversion

In modern industrial production, urban environmental monitoring, and household safety protection, gas leaks or excessive concentrations of harmful gases are often hidden killers behind major accidents. Semiconductor gas sensors (also known as metal oxide semiconductor gas sensors, MOS sensors), with their high sensitivity, low cost, and long service life, have become one of the most widely used technologies in the gas detection field.

Starting from fundamental physical and chemical principles, this article provides an in-depth analysis of their working mechanisms, influencing factors, technical limitations and optimization paths, typical application scenarios, and focuses in particular on the precise conversion formulas between ppm, %VOL, and %LEL, as well as practical engineering considerations.

As Nexisense, a company dedicated to gas sensing technology for more than 40 years, we hope this systematic content will help engineers, system integrators, and end users better understand and scientifically select this type of sensor.

Basic Definition and Core Materials of Semiconductor Gas Sensors

A semiconductor gas sensor is essentially a resistive-type sensing element. It primarily utilizes the property that the surface conductivity of n-type or p-type metal oxide semiconductor materials (such as SnO₂, ZnO, WO₃, In₂O₃, etc.) changes with the concentration of the target gas.

The most classic and widely used material is tin dioxide (SnO₂), which has a bandgap of approximately 3.6 eV and exhibits excellent gas-sensing characteristics at elevated temperatures. A typical sensor consists of the following components: a gas-sensitive layer (metal oxide thin film or porous ceramic), a heater (providing an operating temperature of 200–450°C), electrodes, and a substrate.

Nexisense’s product line covers everything from classic MQ series sensors to customized composite film structure sensors. By optimizing dopants (such as noble metal catalysts Pd, Pt, and Au) and microstructure design for different gases, we improve selectivity and anti-interference performance while maintaining high sensitivity.

In-Depth Working Principle: The Full Process of Adsorption, Redox Reaction, and Electron Transfer

The detection mechanism of semiconductor gas sensors relies on chemical adsorption and redox reactions on the material surface. The entire process can be divided into several key stages:

1. Oxygen adsorption in clean air
After heating, oxygen molecules (O₂) in the air capture free electrons from the conduction band of SnO₂, forming negatively charged oxygen ions (mainly O⁻ or O₂⁻). This creates an electron depletion layer on the grain surface, increasing the surface potential barrier and significantly increasing the resistance (baseline resistance R₀).

2. Adsorption and reaction of target gas
When reducing gases (such as CH₄, CO, H₂, C₃H₈) approach the sensor surface, they react with the adsorbed oxygen ions, releasing electrons back to the conduction band.

Example (CO): CO + O⁻ → CO₂ + e⁻

As a result, the depletion layer becomes thinner, the potential barrier is reduced, the carrier concentration increases, and the sensor resistance drops sharply (working resistance Rs).

3. Resistance change and signal output
The resistance change usually follows an exponential relationship:

Rs / R₀ = k · C^−β

Where C is the gas concentration, and k and β are empirical constants that vary with gas type and material. The resistance change is converted into a voltage signal through a voltage divider or constant current source, then collected by an ADC and processed by algorithms to obtain the concentration value.

For oxidizing gases (such as NO₂ and O₃), the reaction is reversed, further capturing electrons and increasing the resistance.

In actual products, Nexisense reduces activation energy through noble metal doping, controls particle sizes within the 10–50 nm range, and adopts porous nanostructures to increase specific surface area. This significantly improves response speed (typically <30 s) and recovery characteristics.

Technical Advantages, Common Limitations, and Engineering Optimization Strategies

Main Advantages

• High sensitivity: stable detection down to ppm or even ppb levels, suitable for early leak warnings.

• Extremely low cost: single sensors typically cost only a few to tens of RMB, enabling large-scale deployment.

• Fast response: stable output achieved within 10–60 seconds in most cases.

• Long service life: typically 3–8 years, with some industrial-grade models reaching up to 10 years.

• Robust structure: resistant to mechanical vibration and suitable for harsh environments.

Typical Limitations and Influencing Factors

• Limited selectivity: responds to multiple reducing gases and is easily affected by ethanol and VOCs.

  
  

• Temperature and humidity cross-sensitivity: a 10°C temperature change may cause over 20% reading drift; humidity affects oxygen adsorption.

• Preheating required: stabilization usually takes 1–5 minutes or longer after startup.

• Long-term drift: material aging and surface contamination cause baseline drift; annual drift can reach 10–30% without compensation.

• High power consumption: heater power typically 200–800 mW, unsuitable for ultra-low-power applications.

Nexisense Optimization Practices

We enhance selectivity using Pd/Pt noble metal catalysts, suppress interference through multilayer composite film structures (such as SnO₂/In₂O₃ heterojunctions), and integrate digital temperature and humidity sensors with neural network compensation algorithms. This allows field accuracy to remain within ±5% across −20°C to 50°C and 20–95% RH. We also provide factory multi-point calibration data, significantly reducing system integration effort.

Application Scenarios and Typical Product Examples

• Residential and commercial safety: gas alarms for CH₄/C₃H₈ leaks; CO alarms for carbon monoxide poisoning risk.

• Industrial process safety: multi-point monitoring of combustible and toxic gases in petrochemical plants, coal mines, and wastewater treatment facilities, supporting 4–20 mA and RS485 outputs for PLC or SCADA integration.

• Environmental and indoor air quality: TVOC and formaldehyde detection in vehicles and fresh air systems; linkage with air purifiers.

• Special fields: refrigerant leak detection (e.g., R134a), ammonia and hydrogen monitoring, intelligent recognition of pet odors or burning smells.

Representative Nexisense modules include the NX-D8 series for hydrogen, NX-D series for air quality, and dedicated ammonia sensors. All support both digital and analog outputs and integrate preliminary concentration linearization algorithms.

Gas Concentration Units and Engineering Conversion Formulas

Three gas concentration units are commonly used: ppm (parts per million by volume), %VOL (volume percentage), and %LEL (percentage of lower explosive limit). Correct conversion is fundamental to system design and alarm threshold configuration.

1. ppm ↔ %VOL

%VOL = ppm / 10,000
ppm = %VOL × 10,000

Example: 4000 ppm CO = 0.4 %VOL

2. %VOL ↔ %LEL (for combustible gases)

%LEL = (%VOL / LEL value of the gas) × 100%
%VOL = (%LEL / 100) × LEL value

Example: 1.0 %VOL methane → %LEL = (1.0 / 5.0) × 100 = 20% LEL

3. ppm ↔ %LEL (Complete Formula)

ppm = %LEL × LEL value (%VOL) × 100
%LEL = ppm / (LEL value × 100)

Example: 10,000 ppm methane → %LEL = 10,000 / (5 × 100) = 20% LEL

Engineering Notes

• LEL values vary slightly by standard (e.g., methane 4.4–5.3%); always reference applicable national or industry standards (such as GB 15322).

• Alarm thresholds are typically set at 10–50% LEL depending on risk level.

• Nexisense sensors provide standardized output curves from the factory, allowing direct mapping to required units and reducing on-site calibration workload.

How to Scientifically Select a High-Quality Semiconductor Gas Sensor

1. Clarify the target gas and concentration range: ppm-level or %LEL-level?

2. Evaluate interfering gases: presence of alcohols, silicones, or sulfur compounds? Review cross-sensitivity data.

3. Check environmental adaptability: temperature and humidity range, preheating time, and power consumption limits.

4. Focus on long-term stability: annual drift <5–10% with compensation mechanisms.

5. Verify certifications and reliability: explosion-proof (Ex), SIL level, EMC testing, manufacturer patents, and production experience.

With 40 years of accumulated expertise, Nexisense offers one-stop solutions from sensing elements to complete modules, supporting customized algorithms and interface protocols.

Frequently Asked Questions (FAQ)

1. Why do semiconductor sensors require heating?
High temperature accelerates adsorption/desorption reactions and maintains optimal sensing states, typically at 200–400°C.

2. How can sensor failure be identified?
Abnormal baseline resistance, significantly extended response time, sensitivity reduction >30%, or excessive annual drift.

3. How significant is temperature and humidity influence, and how is it compensated?
Without compensation, deviations can reach tens of percent. Modern products use algorithms with auxiliary sensors for correction.

4. Which is better for alarm thresholds: ppm or %LEL?
%LEL is recommended for combustible gases to uniformly assess explosion risk; toxic gases typically use ppm or mg/m³.

5. Is oxygen concentration required?
Yes. Sensitivity decreases when oxygen concentration falls below 5–10% VOL.

6. Can preheating time be shortened?
Some low-power designs reduce it to tens of seconds, but classic products still require 1–3 minutes.

7. How is multi-sensor fusion achieved in industrial sites?
Through sensor arrays with pattern recognition algorithms or complementary use of electrochemical and catalytic sensors.

8. How can sensor poisoning be avoided and recovered?
Avoid high concentrations of silicones and lead-containing gases; mild poisoning may be recovered through high-temperature cleaning.

9. What is the typical calibration interval?
Every 6–12 months, adjusted based on environment and drift conditions.

10. What advantages does Nexisense offer in selectivity?
Through catalyst doping, composite materials, and algorithmic filtering, suppression rates for common interfering gases typically exceed 70–90%.

Conclusion

Although semiconductor gas sensors may appear simple in principle, they embody rich surface physics, chemistry, and materials engineering. They safeguard countless industrial sites and households at low cost. Understanding their mechanisms, mastering concentration conversion, and scientifically addressing limitations are key steps from merely “using” to truly “using well.”

Nexisense will continue to drive technological advancement to deliver more accurate and reliable gas monitoring solutions. If your project faces specific gas detection challenges, feel free to reach out—we look forward to providing tailored support and solutions.