INDUSTRY SENSORS
batteries. Running off just one of them, this type of sensor, which incorporates an antimonide-based light emitter and photodetector, can take a reading every 2 minutes for 10 years thanks to its vastly superior efficiency over the incumbent technology.
The conventional sensors, which are under threat from those produced by GSS, typically combine a tungsten filament lamp for a light source with a pyroelectric or thermopile detector. The bulb, which consumes 100 mW or more, provides a broadband source that excites a CO2
asymmetric
stretching vibrational mode. By monitoring the amount of light absorbed by this mode that occurs
at 4.26 µm, it is possible to determine the CO2 level in the air.
GSS’ sensor works on the same operating principal. However, the LED is far more frugal than the bulb, drawing just 3.3 mW, and this solid-state source takes far less time to deliver a stable output.
“With a light bulb, you might have to wait a minute for it to stabilise, but with an LED it stabilises almost instantaneously,” explains the CEO of GSS, Des Gibson. Slashing the stabilisation time holds the key to the vast improvement in the key figure of merit, the energy per measurement. “In our case, it’s millijoules, whereas in a standard thermal source, it’s hundreds-to-thousands of millijoules. So it’s a radical improvement.”
The tremendous energy saving is not the only area where the antimonide-based device
outscores the incumbent. The established CO2 sensor requires optical filtering, but thanks to the relatively narrow emission and detection profiles of the emitter and detector, the GSS design is filter- free. “This reduces cost, because these filters can be expensive,” explains Gibson, who adds that they are also restrictive, requiring parallel light to work effectively. “We can use a neat, folded design that wouldn’t necessarily be amenable to a filter-based approach.”
Multiple markets
In addition to the sensor market for monitoring air quality in buildings, which is valued at £100 million, GSS is targeting other markets. They include the automotive sector that could be worth even
more. “Within cars, CO2 monitoring is coming in as an anti-drowsiness system,” says Gibson. “That’s linked to the air- conditioning system: It will automatically control the in-cabin CO2
going to fall asleep.” Opportunities also exist for CO2
sensors in horticulture. It is possible to accelerate plant
growth by up to 40 percent by optimising CO2 concentration, humidity, temperature and light level. What’s more, CO2
sensors can help divers
stay underwater for longer by switching to re- breathing systems. They were first used in the military by Special Forces, but are now being used for recreation. “You need to be able to monitor CO2
very accurately, because it’s a chemical scrubber system, so you need to make sure that you are removing the CO2
as you recycle,”
explains Gibson, who reveals that GSS sells quite extensively into that market.
Sensors for all these applications are assembled at the company’s facilities in Cumbernauld, a few miles northeast of Glasgow. But production begins at GSS’ facility at the West of Scotland Science Park, Glasgow, where III-V epiwafers are grown in a Veeco GEN 3 MBE reactor on 4-inch GaAs substrates, before they are dispatched to a local foundry, Compound Semiconductor Global, where they are processed to GSS specifications. Another subcontractor then dices these wafers, mounts LED and photodiode chips on bridgeboards and wire bonds them, before shipping these assemblies back to GSS. Here, all the components are brought together – including injection-molded plastic optics from China – and a portfolio of sensors are put together (see box, “The GSS range of sensors” for details of various products).
“Another piece of intellectual property is the calibration process,” explains Gibson. “We calibrate the assembly for temperature and CO2
The core technology of GSS is the combination of mid-infrared LED and photodiode
. We 100 percent test and we download the calibration data into the firmware, so each sensor has its own unique calibration.”
level to a point where the driver isn’t
Getting going Following its founding in 2006 with funding from by Tweed Renaissance Investment Capital and The Scottish Co-Investment Fund, GSS worked with various partners to develop its product. The UK has a very strong track
August / September 2013
www.compoundsemiconductor.net 31
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