Ofuonye dissertation most downloaded in January 2010

ECE Postdoctoral Research Associate Benedict Ofuonye’s dissertation, “Advanced Process Development for Contacts to AlGaN/GaN High Electron Mobility Transistors (HEMTS),” was the most downloaded thesis or dissertation in January on the Illinois Digital Environment for Access to Learning and Scholarship (IDEALS), a digital forum for publishing works from the University.

“I was surprised,” Ofuonye said. “It was just two and a half weeks when I submitted the thesis, and it was put online. At the same time, I knew most of the work hadn’t been published, so I could see how people’s curiosity was being aroused about it.”

Ofuonye researches the process development of high-electron mobility transistors, or HEMTs, which are the next generation of transistors. Ofuonye has been researching different metallization schemes to enhance speed and power applications.

“This is a new area,” he said. “People don’t know, for example, what is the best metallization to use, unlike in silicon, which they have been doing for the last 50 years. They want to know what kind of metallic source and drain they can put, what kind of gate is the best.” 

Ofuonye described a transistor as the electronic equivalent of a faucet. There is a source, a drain, and a valve, or a gate, that regulates the water.

“What I have done is to engineer the source and the drain contacts, and also the gate contacts, which are all metals, to achieve very good control,” Ofuonye said. He is trying to engineer a very high barrier for the gate, and a low barrier and low resistance for the source and drain contacts.

The cores of these high-speed devices operate at high temperatures. But if the gate cannot withstand the heat, it will fry. Ofuonye’s dissertation focuses on showing how thermally resistant the gate metallizations are. 

“I developed a process to find very good gate metallization that can withstand very high temperature operation,” Ofuonye said.

He has been working with AlGaN/GaN heterostructure, a very stable material. When metals are placed on them to get good ohmic contact resistances, they typically have to be heated at 850 degrees Celsius to alloy the metals.

“In my case, I was able to engineer the process using silicon ion implantation such that I can get very good contact resistance with no high temperature annealing of the source and drain contacts,” he said.

This will save one step in the process and allow it to be used in self-aligned transistors, which allow gate metal to be used to determine the source-drain spacing. Shorter source-drain spacing allows for faster devices.

“The possibility of doing that means that we can now do true self-aligned transistors in that kind of a HEMT, which is usually very difficult,” Ofuonye said.

True self-aligned devices will have much greater speed due to the shorter source-drain spacing. 

“Whenever you show that this can improve the performance, people want to know about it,” Ofuonye said.