Terahertz triumph

As Northrop Grumman and DARPA smash electronics speed records, project leaders reveal this is only the beginning. Compound Semiconductor reports.

Terahertz circuits open the door to high resolution security imaging, faster communication networks and more.

Late last month, Northrop Grumman and DARPA set a world record with a breathtakingly fast terahertz integrated circuit amplifier, opening the door to extra-sensitive spectrometers, incredibly high resolution imaging and high capacity data networks.

The amplifier uses ten, 25 nm InP HEMTs in series to produce an overall gain of 10 dB at 1 THz, and 9 dB at 1.03 THz. These power gain figures massively outstrip those in state-of-the-art ICs, and according to the Northrop Grumman-DARPA team, this is only the beginning.

"The team has made an amazing breakthrough by demonstrating an amplifier at 1 THz," says Dev Palmer, program manager of DARPA's THz Electronics program. "The theoretical maximum operating frequency of this technology is somewhere between 2 and 3 THz, so there's plenty of room at the top."

Terahertz is the slice of the electromagnetic spectrum that lies between infrared light and radio waves, from 300 GHz to 3 THz. Called the sub-millimetre wave frequency band, the wavelengths here are shorter than 1mm and able to penetrate many materials without using ionising radiation.

The chief aim of the DARPA-funded Terahertz Electronics program has been to fabricate all the components, from the subcomponents of an exciter to the parts for a receiver, needed to make a terahertz radio. To date, Northrup Grumman, has achieved just this at 700 GHz and 850 GHz and is set to repeat history at a terahertz.

"A decade ago, there was no consensus in the scientific community whether an integrated circuit operating at one terahertz was technologically possible," says Bill Deal, program manager of Terahertz Electronics at Northrop Grumman "But, as an interdisciplinary team of scientists and engineers, [we have] worked together in scaling all facets of MMIC technology to enable this result."

The next crucial step for the team is to increase the gain of the amplifier IC from 10 db to more than 15dB. As Deal explains: "We want to increase the gain to a level where it makes sense to package the component, and then it becomes a very easy-to-use laboratory component."

"To do this we need to get our gain above 15 dB, and we hope to achieve this in a matter of months," he adds.

And of course, scaling the electronics to boost gain and speed is as important as ever. Using MBE, Deal and colleagues grow the InP-based transistor's epitaxial layers, then using electron beam lithography to fabricate a T-gate to form the transistor.

"Scaling the gate length is really critical," says Deal. "It reduces the capacitances and increases the maximum frequency of oscillation of the transistor, allowing the device to have gain at higher frequencies."

And while the gate length of a III-V device to 25 nm is small - today's standard CMOS processes have only just hit 14 nm - future gate sizes could shrink further yet.

"We do have some experiments aimed at shrinking the gate to as low as 20nm," says Deal. "But a lot of this is very experimental and and other parts of the device have to be scaled at the same time to improve performance."

DARPA's Palmer has no qualms about the project's success, highlighting: "Northrop Grumman has demonstrated all components at 700 and 850 GHz, so I think it's well within their capability to build the same at 1 THz."

DARPA's terahertz monolithic integrated circuit is the first solid-state amplifier demonstrating gain above 1 THz (1012 GHz). [DARPA]

In the meantime, the Terahertz team is also working on fabrication. In Deal's words, 'it's a very sophisticated process', but the researchers have fabricated the amplifier IC across several runs, each comprising batches of typically six wafers. Statistical analyses are underway as transistor performances are monitored and improved.

Looking to the future, Deal reckons the early adopters of the technology will be other researchers. "I actually see scientists using it first for applications such as atmospheric sensing and radio astronomy," he says. "These researchers are pushing the limits on the types of measurements they can do."

Deal says the project has also received interest from businesses looking to establish large bandwidth, short range data connections between data farms using radio links instead of fibre optics cable.

However, the key applications are high resolution radar imaging and spectroscopy. "For high resolution imaging, image resolution increases as wavelength shrinks," explains Palmer. "And we have many molecules, including industrial chemicals, with distinct signatures in this frequency band for rotational spectroscopy."

And the breakthrough also brings good news for high data rate communications. "Even very small fractional bandwidths at terahertz frequencies are many gigahertz wide," adds Palmer. "So you can get high data rates without resorting to complex modulation schemes."