At What Point Does Transistor Gate Length Stop Getting Smaller?

When I started doing IC design back in 1978 we had 6,000 nm channel gate lengths, and today you can buy a smart phone with 16 nm or 14 nm technology, although the gate lengths in those phones are more like 34 nm. The International Technology Roadmap for Semiconductors (ITRS) makes predictions about emerging trends in our industry and they just released a chart showing transistor gate length stopping its typical shrinking trend in the year 2021:

Illustration: Erik Vrielink
Source: IEEE Spectrum

Notice how in just the two years from 2013 to 2015 that the ITRS increased their pessimism on the economics of ever shrinking transistor gate lengths. Does this mean that it’s impossible to build transistors with gate lengths shorter than 10nm? No, but it costs so much that you have to question why do it.

If it simply costs too much money to get smaller than 10 nm channel length, then what are semiconductor manufacturers going to be doing? There are lots of ideas, like adding 3D fabrication to add more transistor density or even changing the transistor orientation to vertical. Groups like the Semiconductor Industry Association (SIA) will collaborate with the Semiconductor Research Corporation (SRC) to list research priorities that could be used by industry or government programs. There’s even an IEEE initiative called Rebooting Computing that could provide some direction for how semiconductor technology can continue to add value.

The semiconductor roadmap from ITRS started back in 1998 and it really helped the equipment manufacturers focus on achieving milestones for the industry. Our industry had some 19 companies developing leading-edge fabs in 2001, however today we only have the big four: Intel, TSMC, Samsung and GLOBALFOUNDRIES. You won’t find these four competitors sharing much together about their detailed technology challenges and directions, but these companies do drive their equipment and material suppliers.

NAND Flash chips are a leading user of 3D structures as a means to increased density, and Samsung announced a 256 Gb 3D NAND flash memory in April 2016 that uses 48 memory layers.

FinFET transistors have been used for several years now, starting with Intel’s 22 nm TriGate devices where the transistor gate has three sides of a horizontal, fin-shaped channel where current is controlled. The ITRS roadmap predicts that a different type of transistor will surpass FinFET by using a lateral, gate-all-around device. Beyond the lateral device, the report predicts vertical transistors with pillars or nanowires that stand up on end. Even the silicon material used in the channel region will change to use III and V column materials like silicon germanium.

Smaller transistor sizes have not always been accompanied by faster chip performance in the same percentage expected because the wires used to connect the transistors have now become the dominant factor in determining speed. The IEEE even has their own roadmap called the International Roadmap for Devices and Systems (IRDS).

There’s an October event, the 1st International Reboot Computing Conference, and IRDS will be having a meeting to continue their roadmap efforts.

I’ve witnessed first-hand in my lifetime the transition from Bipolar to NMOS, NMOS to CMOS, and planar CMOS to FinFET, so look forward to the continuing saga of semiconductor creativity that battles to extend Moore’s Law to 10 nm gate lengths.