New technology could reduce the cost of auto radar
Just as auto-makers are rolling out futuristic, radar-guided safety systems in their top-of-the-line models, College of Engineering researchers have made an advance that could help extend radar to all cars.
Systems that employ radar to detect an impending crash and then warn the driver or apply the ideal brake pressure to avoid collision are the latest thing in car safety. But the semiconductor materials used to manufacture radar are very expensive, limiting these systems so far to pricey luxury cars, says Electrical and Computer Engineering Assistant Professor Zhenqiang (Jack) Ma.
A team led by Ma has now devised a way to make auto radar based on the common semiconductor silicon — a less expensive and more robust material. Silicon is also more manufacturing-friendly than the materials in today's radar systems and the processes for creating silicon-based devices are much better established.
“Silicon provides high-level integration — it's a property of the material,” says Ma. “On silicon chips you can integrate microprocessors and many other things, including these devices for radar. So, all parts of the system can be fabricated together and the cost of radar manufacturing should go down.”
Like certain wireless surveillance and remote sensing systems, car radar operates at extremely high frequencies. For these applications, Ma's group studies advanced silicon devices, known as heterojunction bipolar transistors (HBTs), which act as signal amplifiers. To get HBTs to operate at radar frequencies, engineers make these devices very small. And to achieve the high power levels radar requires, many HBTs must be joined together in an array.
Doing so leads to problems, however. For one, the electrical lines that connect transistors operating at high frequencies must be very long. Consequently, arrayed transistors function at much lower frequencies than they normally do individually, often dropping below radar frequencies when the device reaches sufficient power.
Moreover, multiple transistors generate large amounts of heat, which dissipates unevenly and creates hotspots in the array. Hotter transistors in the center of the array steal electrical current from their cooler neighbors at the edges, eventually hogging so much current that those at the edges stop working. And the usual means to fix this problem doesn't work at high frequency.
Ma's group recently found that operating HBTs in what's termed a common-base mode enables these devices to function efficiently at both higher frequencies and higher power levels than HBTs working in the more typical common-emitter mode. The switch to the common-base mode — and the boost in power it provides — means fewer transistors and shorter electrical connections can be used, says Ma.
In addition, to overcome the “current-hogging” problem, Ma's group turned to a particular way to arrange transistors, known as a cascode. The cascode is not new, says Ma, but the unique cascode configuration his group has designed and tested is. In it, HBTs are hooked to another type of transistor, known as a field effect transisor (FET), in a special way. Ma calls the device an FET-disciplined bipolar transistor.
Although they're still refining the cascode arrangement, Ma and his team have found it achieves what less expensive, silicon-based radar needs to become reality — high power at high frequencies — without causing the current-hogging or severe loss of frequency seen in previous designs.
“I believe in the future this will be the best configuration of devices to get the best performance,” Ma says. He is patenting the technology through the Wisconsin Alumni Research Foundation, the patenting and licensing organization for UW-Madison.