How Gas Sensitive Semiconductor (GSS) Technology Can Help You Produce Highly Accurate Data

Our expertise in heated metal oxide sensors can be traced back to fundamental work by scientists at the Atomic Energy Research Centre at Harwell in the United Kingdom in the 1980s, led by Professor David Williams. David went on to build a center of excellence in gas sensor research at University College of London in the mid-1990s and was joined by New Zealander Dr. Geoff Henshaw. In 2000 these two co-founded Aeroqual on the back of their deep knowledge of the science of HMOS gas sensing.

HMOS sensors are based on the property that certain metal oxides exhibit a change in electrical resistance in the presence of a target gas. This resistance change is caused by a loss or gain of surface oxygen species via reaction with the target gas. The surface oxygen species act as electron trap states in the material and their concentration therefore controls the resistivity of the material. If the oxide is an n-type, a reducing gas such as CO results in loss of surface oxygen, an increase in conduction band electrons, and a decrease in resistance.  An oxidizing gas such as O3 causes the reverse behavior. For p-type metal oxides, the resistance change to a target gas is opposite to n-type. There is a well-defined relationship between gas concentration and change in electrical resistance which leads to a measurement of the target gas concentration. Since the gas reaction which causes the change in electrical resistance in the metal oxide occurs at the surface, material parameters such as surface area and porosity contribute to overall gas sensitivity.

To optimize a GSS sensor for a specific gas, many elements are controlled:

Mastery of all of the above is required to perform at the highest levels. This explains why many companies that make HMOS sensors struggle to match the performance demonstrated by Aeroqual’s GSS sensors.

Aeroqual GSS sensors consist of a highly porous metal oxide sensor layers which are deposited onto alumina chips. The proprietary oxide materials are formulated and deposited using cleanroom manufacturing practices.  Alumina was selected as the base ceramic due to its high thermal and electrical stability. Gold electrodes for resistance measurement are photo-etched at the oxide/chip interface (see diagram below). A platinum micro-heater is also deposited on the underside of the chip to heat the sensor layer to the correct temperature for the target gas. The platinum heater offers very precise temperature control. The composition and microstructure of the oxide, its thickness, and its operating temperature are optimized to maximize selectivity.

The method of operation of a HMOS sensor is also crucial in achieving high performance. GSS technology includes precise tuning of the sensor temperature and sample flow to maintain sensor stability and reduce cross-sensitivities (a significant issue for other HMOS sensors). GSS packaging design incorporates low off-gassing materials for ppb level gas measurement and flow control elements.

Aeroqual’s GSS technology is the culmination of more than 25 years of materials research perfecting compositions and optimizing sensor driver algorithms. It is this commitment to understanding and applying the underlying scientific principles that has enabled Aeroqual products to achieve near reference accuracy, low detection limits (ppb levels for O₃, VOC, NO₂, and NO_x), high reliability and wide applicability at an affordable price.

Air quality monitoring is changing toward greatly increased spatial density of measurement and this is driving growing demand for high-performance sensors.  To meet this challenge Aeroqual GSS technology is evolving toward lower power, higher performance, and longer-lasting designs that can be manufactured more easily in greater volumes.  We will continue to achieve this through in-house innovation and productive partnerships.

If you have any questions about our products or air quality in general, please get in touch!