Silicon for cheaper radar

Scientists at the University of Wisconsin-Madison have found a way to make radar guided car safety systems cheaper.

Radar systems can be fitted to cars to detect an impending crash and then warn the driver or apply the brakes, but the semiconductor materials used to manufacture radar are very expensive, according to Electrical and Computer Engineering Assistant Professor Zhenqiang Ma.

A team led by Ma has now devised a way to make car radar systems based on silicon, which is less expensive and more robust. Silicon is also easier to manufacture than the materials in today's radar systems as the processes for creating silicon-based devices are much better established.

Car radar operates at extremely high frequencies. The team used silicon devices known as heterojunction bipolar transistors (HBTs), which act as signal amplifiers. To get HBTs to operate at radar frequencies, the devices were made very small, and to achieve the high power levels radar requires, many HBTs were joined together in an array.

However, using an array can cause transistors to function at much lower frequencies than they normally do individually, often dropping below radar frequencies when the device reaches sufficient power. It can also create a lot of heat in the centre of the array, causing current hogging from outer components.

Ma's group recently found that operating HBTs in a common-base mode enables the 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.

To overcome the “current-hogging” problem, the group used a specific way to arrange transistors, known as a cascode. HBTs are hooked to a field effect transistor (FET), in a particular way. Ma calls the device an FET-disciplined bipolar transistor.

When the arrangement is perfected, the research could lead to less expensive, silicon-based car radar without causing the current-hogging or severe loss of frequency seen in previous designs.