Researchers from Purdue and Harvard universities have
created a new type of transistor made from a material that could replace
silicon and have a 3D structure instead of conventional flat computer chips.
The approach could enable engineers to build faster, more
compact, and efficient integrated circuits and lighter laptops that generate
less heat than today’s. The transistors contain tiny nanowires made not of
silicon, like conventional transistors, but from a material called
indium-gallium-arsenide.
The device was created using a so-called
“top-down” method, which is akin to industrial processes to precisely
etch and position components in transistors. Because the approach is compatible
with conventional manufacturing processes, it is promising for adoption by
industry, says Peide “Peter” Ye, a professor of electrical and computer
engineering at Purdue.
A new generation of silicon computer chips, due to debut
in 2012, will contain transistors having a vertical structure instead of a
conventional flat design. However, because silicon has a limited “electron
mobility”—how fast electrons flow—other materials will likely be needed
soon to continue advancing transistors with this 3D approach, Ye says.
Indium-gallium-arsenide is among several promising
semiconductors being studied to replace silicon. Such semiconductors are called
III-V materials because they combine elements from the third and fifth groups
of the periodic table.
“Industry and academia are racing to develop
transistors from the III-V materials,” Ye says. “Here, we have made
the world’s first 3D gate-all-around transistor on much higher-mobility
material than silicon, the indium-gallium-arsenide.”
Findings will be detailed in a paper to be presented
during the International Electron Devices Meeting.
Transistors contain critical components called gates,
which enable the devices to switch on and off and to direct the flow of
electrical current. In today’s chips, the length of these gates is about 45 nm.
However, in 2012 industry will introduce silicon-based 3D transistors having a
gate length of 22 nm.
“Next year if you buy a computer it will have the
22-nm gate length and 3D silicon transistors,” Ye says.
The 3D design is critical because the 22-nm gate lengths
will not work in a flat design.
“Once you shrink gate lengths down to 22 nm on
silicon you have to do more complicated structure design,” Ye says.
“The ideal gate is a necklike, gate-all-around structure so that the gate
surrounds the transistor on all sides.”
The nanowires are coated with a “dielectric,”
which acts as a gate. Engineers are working to develop transistors that use
even smaller gate lengths, 14 nm, by 2015.
However, further size reductions beyond 14 nm and
additional performance improvements are likely not possible using silicon,
meaning new designs and materials will be needed to continue progress, Ye says.
“Nanowires made of III-V alloys will get us to the
10-nm range,” he says.
The new findings confirmed that the device made using a
III-V material has the potential to conduct electrons five times faster than
silicon.
Creating smaller transistors also will require finding a
new type of insulating layer essential for the devices to switch off. As gate
lengths shrink smaller than 14 nm, the silicon dioxide insulator used in
conventional transistors fails to perform properly and is said to
“leak” electrical charge.
One potential solution to this leaking problem is to
replace silicon dioxide with materials that have a higher insulating value, or
“dielectric constant,” such as hafnium dioxide or aluminum oxide.
In the new work, the researchers applied a dielectric
coating made of aluminum oxide using a method called atomic layer deposition.
Because atomic layer deposition is commonly used in industry, the new design
may represent a practical solution to the coming limits of conventional silicon
transistors.
Using atomic layer deposition might enable engineers to
design transistors having thinner oxide and metal layers for the gates,
possibly consuming far less electricity than silicon devices.
“A thinner dielectric layer means speed goes up and
voltage requirements go down,” Ye says.
