Metal Oxide Semiconductors

This is a brief summary of metal oxide semiconductors (MOS).

Fig 1. Structure of a metal oxide sensor. The sensitive layer on the top reacts with the presence of certain Volatile Organic Compounds (VOCs) and thus sensors change in conductivity. The platinum heater placed on the bottom of the sensor consists of a dissipating resistance that outputs heat. By heating the sensors, a higher VOC specificity is reached. Credit - DOI: 10.1038/s41598-022-17795-8

Metal oxide semiconductors (often abbreviated as MOX) are one of the most widely used technologies in modern chemical sensing. They power many of the “electronic noses” used today in environmental monitoring, food quality control, and increasingly, early-stage disease detection. Their low cost, high sensitivity and small size make them an attractive technology for portable and home-based diagnostics.

What are Metal Oxide Semiconductors

Unlike the silicon MOS structures used in microelectronics, MOX gas sensors are typically made from metal oxides such as tin oxide (SnO₂), zinc oxide (ZnO), titanium dioxide (TiO₂), tungsten oxide (WO₃), indium oxide (In₂O₃) and copper oxide (CuO). These materials are semiconductors capable of interacting with oxygen and volatile organic compounds (VOCs) on their surface.

When oxygen molecules adsorb onto a MOX surface, they withdraw electrons from the material, creating a surface-depleted layer. When a VOC comes into contact with this oxygen layer, it undergoes an oxidation or reduction reaction. This reaction alters the number of electrons available for conduction, causing a measurable change in the material’s electrical resistance. This simple but powerful principle enables the detection of gases at extremely low concentrations.

Metal oxide semiconductors are widely used when designing eNOSEs (electronic Noses) as they are easily available on the market, inexpensive and well performing. They already have applications in medical diagnosis, quality control in the food industry and pharmaceuticals as well as environmental monitoring.

Why are MOX Sensors Popular?

Metal Oxide sensors are widely adopted for many reasons. They are low-cost and easy to manufacture, highly sensitive (often detecting VOCs at parts-per-billion levels), compact, fast and already commercialised with many off the shelf options available.

Other Technologies Used in Electronic Noses

Whilst MOX sensors are the most common choice, other sensing approaches are often used depending on the application.

• Conducting polymers - these are organic polymers that conduct electricity.

• Polymer composites - which are similar but formulated of non-conducting polymers with the addition of conducting material such as carbon black.

• Quartz crystal microbalance (QCM) - measures mass per unit area by measuring the change in frequency of a quartz crystal resonator.

• Surface acoustic waves (SAW) - these are microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon.

• Mass Spectrometers – are the most accurate method for identifying specific VOCs, which are detailed in a previous post.

How they work

MOS surface conductivity changes upon adsorption and then reaction of gases with the already adsorbed oxygen. As this reaction is either an oxidation or reduction (in the case of SnO or ZnO sensors) the concentration of electrons available for conduction gets altered and causes this material to be an optimal semiconductor.

The nose component is by making Volatile Organic Compound (VOC) specific sensors. As the first step, these sensors need to be trained with qualified samples to build a database of reference. Then the instrument can recognize new samples by comparing a volatile compounds fingerprint to those in its database. Allowing for it to perform qualitative or quantitative analysis.

Limitations

High Operating Temperature

A major limitation of MOX sensors is their need for high operating temperatures, typically between 200–400 °C. This requirement arises because oxygen adsorption and reaction kinetics on the sensor surface are highly temperature dependent. To achieve these temperatures, MOX sensors integrate micro-heaters, which increase power consumption and complicate device design. As a result, achieving long battery life, true portability or integration into compact at-home diagnostic devices becomes more challenging. Although promising research is emerging on room temperature MOX materials often enhanced with nanostructures, graphene hybrids or noble-metal doping, most commercial sensors still rely on elevated temperatures to function reliably.

Low Selectivity

While MOX sensors are extremely sensitive, they suffer from inherently low selectivity. Many different volatile organic compounds can produce similar changes in electrical resistance, meaning the sensor struggles to discriminate between individual gases in complex mixtures. This issue is especially problematic in medical diagnostics where overlapping VOC signatures are common. Improving selectivity usually requires arrays of differently doped MOX sensors, catalytic additives, specialised nanostructures or advanced machine-learning algorithms to tease apart subtle signal patterns. Even with these enhancements, achieving molecular specificity comparable to techniques such as SERS or mass spectrometry remains difficult.

Environmental Sensitivity and Drift

MOX sensors are also heavily influenced by external environmental factors. Humidity, ambient temperature, airflow and even long-term sensor ageing can alter baseline resistance and cause signal drift over time. These fluctuations make consistent calibration essential yet challenging, particularly in real world or home environments where conditions vary throughout the day. Drift and environmental sensitivity reduce reliability and complicate longitudinal monitoring, an important requirement for urine based diagnostics where day to day biological variation is already high. Overcoming these issues remains a key research focus for improving the stability of MOX-based sensing systems.

Why MOX Sensors Struggle with Urine Diagnostics

Although MOX sensors have shown strong performance in breath analysis and environmental monitoring, urine presents a far more complex challenge. The urine metabolome is highly variable and strongly influenced by a person’s diet, hydration levels, gut microbiome, medication use, physical activity and daily biological rhythms. This variability makes it difficult to establish stable VOC fingerprints that reliably indicate disease.

In addition, urine samples can contain mixtures of volatile and semi-volatile compounds with overlapping chemical signatures. Which MOX sensors with their inherently low selectivity struggle to distinguish. Environmental factors such as humidity and sensor drift further complicate measurement consistency, making it difficult to achieve the stability and precision required for clinical-grade urine diagnostics.

Conclusions

Despite these limitations, MOX sensors still hold meaningful potential in future at-home diagnostic systems. Their low cost, rapid response and small size make them excellent candidates for early screening tools, environmental health monitoring or integrated components within smart bathroom devices.