Cell Phone Radios Could Take Another Giant Leap Towards Miniaturization

New acoustic devices may point to ways to make wireless technology even more compact

4 min read
Two interlaced comb-like regions sit on either side of a plain rectangular region is connected to two traces
An image taken by scanning electron microscopy shows details of the amplifier.
Matt Eichenfield

Modern cell phones are packed with radios to send and receive phone calls, text messages and high-speed data. In turn, each relies on a mix of technologies—for instance, the filters used to extract the desired signal from the welter of radio waves hitting the antenna are typically so-called acoustic devices based on piezoelectric crystals, whereas the amplifiers and mixers are semiconductor electronics, and circulators and isolators are often gyromagnetic components.

Now, an acoustic type of amplifier has been developed that is more than 10 times smaller than comparable electronic versions, potentially suggesting a novel strategy to miniaturize wireless technology, according to a new study.

If you can avoid switching between electronic and acoustic components (they are called acoustic because they rely on mechanical motion within the material that makes up the component, but they often operate at radio, not audio, frequencies) you can save a lot of space compared to the current situation: Since today’s radios are all “based on fundamentally different materials and technologies, although some of them can go on the same chip, often they cannot,” says study senior author Matt Eichenfield, a physicist at Sandia National Laboratories in Albuquerque, New Mexico.

“So you often have a bunch of different chips that you’ve made as small as can be made that are integrated together on a circuitboard, but you still need extra area to handle and integrate them together,” Eichenfield says. “If you could make all these different components just a single technology, you’d just need one chip, and the size of that chip could be far smaller than anything integrated on a circuitboard.”

The researchers noted “there is essentially no way to reproduce the performance of the piezoelectric filters currently in radio-frequency signal processors,” Eichenfield says. “In a modern cell phone, there are at least 50 of these filters.” Assuming these components always have to be in place, Eichenfield and his colleagues began exploring whether they can make acoustic versions of electronic radio components.

Now, using concepts essentially abandoned for almost 50 years, the scientists have created the smallest and best-performing acoustic amplifiers yet. They have also created the first acoustic circulator, one far smaller than its typical counterparts.

“There was great work done on acoustic amplifiers in the late ‘60s and early ‘70s, but they came up with their ideas long before there were technological tools to really make them useful.”

Whereas a conventional amplifier uses electric power to amplify the strength of an electric signal, an acoustic amplifier uses electric power to increase the strength of an acoustic signal. Previous research explored making acoustic radio-frequency amplifiers decades ago, but the limits of past manufacturing technologies meant these devices performed too poorly to be useful. Boosting a 300-megahertz signal by a factor of 100 with the old acoustic amplifiers required devices at least 1 centimeter long, 2,000 volts, more than 3 watts of power, and also generated lots of heat.

Prior work tried enhancing acoustic devices using layers of semiconductor materials. However, to work well, these films had to be very thin and very high quality, and back then researchers were only capable of one or the other.

“There was great work done on acoustic amplifiers in the late ‘60s and early ‘70s, but they came up with their ideas long before there were technological tools to really make them useful,” Eichenfield says.

Using modern fabrication techniques, Eichenfield and his colleagues have created acoustic amplifiers using semiconducting indium gallium arsenide layers only 83 layers of atoms thick, roughly 1,000 times thinner than a human hair. These devices can boost a 300-megahertz acoustic signal by a factor of 100 while requiring only 500 microns of length, 36 volts and 20 milliwatts of power, and generating minimal heat.

The scientists created an acoustic amplifier for 276-megahertz frequency signals 450 by 250 microns in size. For 1.9-gigahertz frequency signals, the kind that carries much of modern cell phone traffic, the amplifier would need to be only 65 by 40 microns large, more than 10 times smaller than comparable electronic amplifiers.

The scientists also created the first acoustic circulator, a radio component that separates transmitted and received signals. “Circulators aren’t even used in most wireless technology unless the system itself is big,” Eichenfield says. “Gyromagnetic circulators are huge—they’re often the single biggest component in the entire radio-frequency signal processing chain.”

An acousto-electric chip, top, produced at Sandia National Laboratories includes a radio-frequency amplifier, circulator and filter. This acousto-electric chip includes a radio-frequency amplifier, circulator and filter.BRET LATTER

Whereas a conventional 2.45-gigahertz-frequency circulator might be 23.5 millimeters in diameter and 10 millimeters thick, a comparable acoustic circulator would be about 85 by 56 microns in length and width and maybe 500 microns thick, or about 91,000 times smaller in area and 1.8 million times smaller in volume, Eichenfield says.

“Most commercial wireless technology will not use circulators because of their size, but they would love to if they could, because they can double the available bandwidth, and it allows systems to use the same frequencies for transmission and reception,” Eichenfield says.

Still, “the big win is not necessarily just in a direct comparison between components,” Eichenfield says. Ultimately, “we want to do everything on just one chip using one technology.”

The scientists are now working to decrease the power consumption of the acoustic amplifiers—Eichenfield suggests they could reduce it a hundredfold “through a combination of improved materials properties and geometric optimization.” They also want to create acoustic versions of other radio components, such as mixers, oscillators and phase shifters, and see how well they perform.

It could take a while before these acoustic radio components make it to the market, “but I think we could start to see them in specialty applications where size is paramount,” Eichenfield says. “Maybe over time, the technology will get so good, it will become favorable to carry out entire radio-frequency signal processing domains via acoustics.”

The scientists detailed their findings online May 13 in the journal Nature Communications.

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