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editorialview by Dr Richard Stevenson, Editor A really radical device


AT THE HEART OF EVERY ISSUE OF this magazine lie a collection of features detailing breakthroughs at the chip level. All these articles describe some aspect of novelty, which is more radical in some cases than others.


Breakthroughs that are important, but not revolutionary, would include improvements to existing device architectures. This might be the insertion of a superior photonic


structure into an LED that leads to an increase in extraction efficiency, a new gate stack that cuts the density of interface traps in a III-V MOSFET, or a different style of field plate that trims the leakage current in a GaN HEMT.


More radical is the creation of a new device. It might be formed by building an entirely new structure, or it might result from merging two devices into something that exceeds the sum of the parts.


In this issue, we have a contribution in this latter vein from John Dallesasse and Kanuo Chen from the University of Illinois at Urbana-Champaign. This duo has combined a quantum cascade laser (QCL) and a transistor to create a novel source that can produce emission from the mid infrared through to terahertz frequencies.


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Their three-terminal hybrid improves on the capability of the QCL, a promising device for gas sensing. With the QCL, changing the operating voltage shifts the emission wavelength, making it possible to conduct a frequency sweep through the absorption peak of gas molecules. However, when the operating voltage of the transistor is changed, it alters the injection of carriers into the active region, and thus the output power of the laser.


Turning to the transistor-injected QCL eliminates this variation in output power, because it is then possible to independently control the injected current and the voltage across the active region. What’s more, by dithering the base-collector bias voltage at a fixed emitter-base bias, the laser’s frequency can be modulated, aiding the detection of chemical species.


But will this device be used for gas sensing on a grand scale, or will it be confined to academia? Well, signs are good for commercialisation. Its design is similar to a HBT – the main differences are the addition of optical confinement layers on the top and bottom of the device and the inclusion of a cascade region. This means that it should be possible to make the device in a GaAs IC foundry. So maybe, just maybe, this device will be more than just radical – it will be commonplace too.


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