New sorts of transistors can eke out a few more iterations of Moore’s law, but they will get increasingly expensive

THANKS to the exponential power of Moore’s law, the electronic components that run modern computers vastly outnumber all the leaves on the Earth’s trees. Chris Mack, a chipmaking expert, working from a previous estimate by VLSI Research, an analysis firm, reckons that perhaps 400 billion billion (4x1020) transistors were churned out in 2015 alone. That works out at about 13 trillion a second. At the same time they have become unimaginably small: millions could fit on the full stop at the end of this sentence.

A transistor is a sort of switch. To turn it on, a voltage is applied to its gate, which allows the current to flow through the channel between the transistor’s source and drain (see first diagram). When no current flows, the transistor is off. The on-off states represent the 1s and 0s that are the fundamental language of computers.

The silicon from which these switches are made is a semiconductor, meaning that its electrical properties are halfway between those of a conductor (in which current can flow easily) and an insulator (in which it cannot). The electrical characteristics of a semiconductor can be tweaked, either by a process called “doping”, in which the material is spiced with atoms of other elements, such as arsenic or boron, or by the application of an electrical field.

In a silicon transistor, the channel will be doped with one material and the source and drain with another. Doping alters the amount of energy required for any charge to flow through a semiconductor, so where two differently doped materials abut each other, current cannot flow. But when the device is switched on, the electric field from the gate generates a thin, conductive bridge within the channel which completes the circuit, allowing current to flow through.

For a long time that basic design worked better and better as transistors became ever smaller. But at truly tiny scales it begins to break down. In modern transistors the source and drain are very close together, of the order of 20nm. That causes the channel to leak, with a residual current flowing even when the device is meant to be off, wasting power and generating unwanted heat.

Heat from this and other sources causes serious problems. Many modern chips must either run below their maximum speeds or even periodically switch parts of themselves off to avoid overheating, which limits their performance.

Chipmakers are trying various methods to avoid this. One of them, called strained silicon, which was introduced by Intel in 2004, involves stretching the atoms of the silicon crystal further apart than normal, which lubricates the passage of charge carriers through the channel, reducing the heat generated.

In another technique, first adopted in 2007, metal oxides are used to combat the effects of tunnelling, a quantum phenomenon in which particles (such as electrons) on one side of a seemingly impermeable barrier turn up on the other side without ever passing through the intervening space. Developing more such esoteric techniques may allow chipmakers to go on shrinking transistors for a little longer, but not much.

The 3D effect

Beyond that, two broad changes will be needed. First, the design of the transistor will have to be changed radically. Second, the industry will have to find a replacement for silicon, the electrical properties of which have already been pushed to their limits.

One solution to the problem of leaking current is to redesign the channel and the gate. Conventionally, transistors have been flat, but in 2012 Intel added a third dimension to its products. To enable it to build chips with features just 22nm apart, it switched to transistors known as “finFET”, which feature a channel that sticks up from the surface of the chip. The gate is then wrapped around the channel’s three exposed sides (see second diagram), which gives it much better control over what takes place inside the channel. These new transistors are trickier to make, but they switch 37% faster than old ones of the same size and consume only half as much power.

The next logical step, says Mr Snir of Argonne National Laboratory, is “gate-all-around” transistors, in which the channel is surrounded by its gate on all four sides. That offers maximum control, but it adds extra steps to the manufacturing process, since the gate must now be built in multiple sections. Big chipmakers such as Samsung have said that it might take gate-all-around transistors to build chips with features 5nm apart, a stage that Samsung and other makers expect to be reached by the early 2020s.

Beyond that, more exotic solutions may be needed. One idea is to take advantage of the quantum tunnelling that is such an annoyance for conventional transistors, and that will only get worse as transistors shrink further. It is possible, by applying electrical fields, to control the rate at which tunnelling happens. A low rate of leakage would correspond to a 0; a high rate to a 1. The first experimental tunnelling transistor was demonstrated by a team at IBM in 2004. Since then researchers have been working to commercialise them.

In 2015 a team led by Kaustav Banerjee, of the University of California, reported in Nature that they had built a tunnelling transistor with a working voltage of just 0.1, far below the 0.7V of devices now in use, which means much less heat. But there is more work to be done before tunnelling transistors become viable, says Greg Yeric of ARM, a British designer of microchips: for now they do not yet switch on and off quickly enough to allow them to be used for fast chips. Jim Greer and his colleagues at Ireland’s Tyndall Institute are working on another idea. Their device, called a junctionless nanowire transistor (JNT), aims to help with another problem of building at tiny scales: getting the doping right. “These days you’re talking about [doping] a very small amount of silicon indeed. You’ll soon be at the point where even one or two misplaced dopant atoms could drastically alter the behaviour of your transistor,” says Dr Greer.

Instead, he and his colleagues propose to build their JNTs, just 3nm across, out of one sort of uniformly doped silicon. Normally that would result in a wire rather than a switch: a device that is uniformly conductive and cannot be turned off. But at these tiny scales the electrical influence of the gate penetrates right through the wire, so the gate alone can prevent current flowing when the transistor is switched off.

Whereas a conventional transistor works by building an electrical bridge between a source and a drain that are otherwise insulated, Dr Greer’s device works the other way: more like a hose in which the gate acts to stop the current from flowing. “This is true nanotechnology,” he says. “Our device only works at these sorts of scales. The big advantage is you don’t have to worry about manufacturing these fiddly junctions.”

Material difference

Chipmakers are also experimenting with materials beyond silicon. Last year a research alliance including Samsung, Global Foundries, IBM and State University New York unveiled a microchip made with components 7nm apart, a technology that is not expected to be in consumers’ hands until 2018 at the earliest. It used the same finFET design as the present generation of chips, with slight modifications, but although most of the device was built from the usual silicon, around half of its transistors had channels made from a silicon-germanium (SiGe) alloy.

This was chosen because it is, in some ways, a better conductor than silicon. Once again, that means lower power usage and allows the transistor to switch on and off more quickly, boosting the speed of the chip. But it is not a panacea, says Heike Riel, the director of the physical-sciences department at IBM Research. Modern chips are built from two types of transistor. One is designed to conduct electrons, which carry a negative charge. The other sort is designed to conduct “holes”, which are places in a semiconductor that might contain electrons but happen not to; these, as it turns out, behave as if they were positively charged electrons. And although SiGe excels at transporting holes, it is rather less good at moving electrons than silicon is.

Future paths to higher performance along these lines will probably require both SiGe and another compound that moves electrons even better than silicon. The materials with the most favourable electrical properties are alloys of elements such as indium, gallium and arsenide, collectively known as III-V materials after their location in the periodic table.

The trouble is that these materials do not mix easily with silicon. The spacing between the atoms in their crystal lattices is different from that in silicon, so adding a layer of them to the silicon substrate from which all chips are made causes stress that can have the effect of cracking the chip.

The best-known alternative is graphene, a single-atom-thick (and hence two-dimensional) form of carbon. Graphene conducts electrons and holes very well. The difficulty is making it stop. Researchers have tried to get around this by doping, squashing or squeezing graphene, or applying electric fields to change its electrical properties. Some progress has been made: the University of Manchester reported a working graphene transistor in 2008; a team led by Guanxiong Liu at the University of California built devices using a property of the material called “negative resistance” in 2013. But the main impact of graphene, says Dr Yeric, has been to spur interest in other two-dimensional materials. “Graphene sort of unlocked the box,” he says. “Now we’re looking at things like sheets of molybdenum disulphide, or black phosphorous, or phosphorous-boron compounds.” Crucially, all of those, like silicon, can easily be switched on and off.

If everything goes according to plan, says Dr Yeric, novel transistor designs and new materials might keep things ticking along for another five or six years, by which time the transistors may be 5nm apart. But beyond that “we’re running out of ways to stave off the need for something really radical.”

His favoured candidate for that is something called “spintronics”. Whereas electronics uses the charge of an electron to represent information, spintronics uses “spin”, another intrinsic property of electrons that is related to the concept of rotational energy an object possesses. Usefully, spin comes in two varieties, up and down, which can be used to represent 1 and 0. And the computing industry has some experience with spintronics already: it is used in hard drives, for instance.

Research into spintronic transistors has been going on for more than 15 years, but none has yet made it into production. Appealingly, the voltage needed to drive them is tiny: 10-20 millivolts, hundreds of times lower than for a conventional transistor, which would solve the heat problem at a stroke. But that brings design problems of its own, says Dr Yeric. With such minute voltages, distinguishing a 1 or a 0 from electrical noise becomes tricky.

“It’s relatively easy to build a fancy new transistor in the lab,” says Linley Gwennap, the analyst. “But in order to replace what we’re doing today, you need to be able to put billions on a chip, at a reasonable cost, with high reliability and almost no defects. I hate to say never, but it is very difficult.” That makes it all the more important to pursue other ways of making better computers.