A magnified image of a silicon wafer. (Image courtesy of Jack Spades, Flickr).
In recent years, transistors based on gallium nitride or GaN have been used in power converters inside computers and smartphones, as well as wireless transmitters ranging from millimeter to microwave frequencies. But engineers have never been able to craft these transistors using a complementary metal-oxide-semiconductor or CMOS process – the basis of transistors used in modern chips.
Now scientists have developed a field-effect transistor based on GaN CMOS, paving the way for integrated circuits that could replace silicon in wireless amplifiers, power converters, and other products. The new circuits could be harnessed in power switches that are more efficient, reliable in harsh environments, and smaller than components based on silicon.
Based out of HRL Laboratories, a research institute funded by General Motors and Boeing, the researchers were able to fabricate a chip with an enhancement-mode n-channel MOSFET and a p-channel MOSFET on the same GaN wafer. The results were published last month in the journal IEEE Electron Device Letters.
Rongming Chu, the project’s lead researcher, said that putting “power switches and their driving circuitry on the same chip is the ultimate approach to minimizing the parasitic inductance” that represents the biggest obstacle to GaN CMOS.
In earlier attempts to develop the technology, engineers had to intentionally slow down the switching speed of GaN transistors. This was done in order to control chip-to-chip parasitic inductance, which causes voltage instabilities that limit performance.
Transistors based on GaN can operate at higher temperatures and voltages than gallium-arsenide transistors, which have long been used in microwave power amplifiers. GaN transistors are also finding their way into power converters, as well as cellular base stations and antennas. Lockheed Martin, for instance, recently infused its latest radar technology with GaN transmitters.
CMOS technologies have also been plugged into transceivers for antenna and radar systems. Early last year, Panasonic partnered with imec, a microelectronics research center, to develop silicon CMOS for millimeter wave radars used in automated vehicle safety systems. One of the major benefits of CMOS, the researchers said, is its intrinsically low manufacturing cost.
Until recently, however, researchers struggled to create GaN CMOS transistors for integrated power conversion. Some considered the task impossible, according to Chu.
The problem had always been how to go about making the GaN p-channel transistors and integrating them with n-channel transistors. These transistors are the channels that control the flow of electrons through the integrated circuit.
Using a process called selective area epitaxy, the HRL researchers squeezed both of these power transistors on the same chip. They were then able to fabricate a functional inverter chip from the GaN CMOS technology.