Beginner’s Guide to Moore’s Law
Any digital circuit will always have one of two states: ON or OFF, 1 or 0. And digital circuits work by switching between these states to perform calculations, send information and learn about other connected devices. So it makes sense for digital circuits to have switches to easily transition between states, amplify signals and read the switch states from other circuits.
Before the 1950’s, this was done using vacuum tubes. They were huge evacuated tubes that used electron emissions from a filament or plate to control the flow of current. Not all tubes were evacuated though, some used gases and smaller ones used photosensitive materials and magnetic fields to control the flow of electrons. But, they all had a few things in common: they were expensive, consumed a lot of power and gave off too much heat. They were very unreliable too, and required a great deal of maintenance. And because they were huge, there was a limit to how small “computers” could be made simply because there was a limit to how small vacuum tubes could be manufactured.
This prompted researchers at Bell Labs to find an alternative. We needed a device that was cheap, consumed less or no power and did not heat up. But most importantly, the device had to be easy to manufacture, have fast switching speeds and be small. The researchers: John Bardeen and Walter Brattain led by William Shockley, invented the transistor in 1947.
The transistor was not only small, but it had less resistance and no moving parts(hence little losses), and it was reliable and gave off little to no heat.
Pretty soon, the whole world was crazy over transistors! Everyone was working on them and new developments in transistor performance, size and reliability were happening almost every month.
Jack Kilby, from Texas Instruments, was one of the first to show the world how to put many of these transistors on a single wafer (piece of Silicon). In 1959, he patented the first IC or Integrated Circuit (although his was made using a Germanium wafer). By the 1960’s, transistors were getting ever smaller, and we were manufacturing complicated ICs and building faster and smaller “computers”.
About this time, Gordon Moore, co-founder of Fairchild Semiconductor and Intel published a paper, stating that the number of components per Integrated Circuit would double every year for the next decade. In 1975, he reviewed his forecast and said that the number of components would now double every two years. This came to be known as Moore’s Law.
And it has proved accurate over several decades. More importantly, Moore’s Law has been a very critical part of manufacture and design of chips. Researchers in AMD and Intel have long set goals and targets based on Moore’s Law. Computers too have gotten smaller and faster due to advancements in chip designs forced by Moore’s Law.
More than a prediction, Moore’s Law has become a goal, a standard, so to speak, that manufacturers aim to achieve.
To put it into perspective, one of the first semiconductor process (the length from the Source to Drain in a MOSFET), in 1971 was 10 um (or 100,000 times smaller than a meter). In 2001, it was 130 nm nearly 80 times smaller than it was 1971!
As of 2017, the smallest transistor process is 10 nm! The diameter of a human hair is 100 um or nearly 10,000 larger than how small transistors are today!
Note: The definition of a process has slightly changed now and today it is defined as the size of the gate length or the half pitch between two identical components.
But there is a problem!
With such small scale, comes more technical challenges. Overcoming these challenges not only needs a lot of time and research, but a lot of money and investments too. And as such, Moore’s Law is actually slowing down, and people fear that it might soon stop!
It took Intel about two and a half years to go from a 22 nm process in 2012 to 14 nm in 2014 and three years to go from there to 10 nm in 2017.
And that makes sense, as Moore’s Law is not really a law, but a prediction, or a forecast. And while it has been a goal that manufacturers have sought to achieve and keep, it has become increasingly difficult to do so. Perhaps it is now time to do another revision of the law, or maybe even discard it all together and sought out a more realistic alternative.
Moore himself has been quoted in 2015 saying that, “I see Moore’s law dying here in the next decade or so”.
So what are some of these problems?
Quantum Tunneling: As we get smaller and smaller, quantum properties and effects come into the picture. Due to the quantum nature of subatomic particles, electrons, and atoms to behave in different, sometimes unintuitive ways.
As we reduce the size of a transistor, the size of its depletion layer also decreases. The Depletion layer is important as that is what stops the flow of electrons.
Researchers have calculated that a transistor smaller than 5 nm will not be able to stop the flow of electrons due to tunneling of electrons in its depletion region. Due to tunneling, the electrons will not perceive the depletion region and it will ‘tunnel’ through it as if it did not exist. And a transistor that cannot stop the flow of electrons is pretty useless.
Size of Atom: Moreover, we are now slowly approaching the size of an atom itself and you cannot build a transistor smaller than an atom! The Silicon atom has a diameter of around 1 nm and right now we are manufacturing transistors with gates at about 10 times that size. In a few years, not taking into account quantum effects, we will not be able to go any smaller considering that we are reaching the physical limit of how small something can be.
Heating and Current Effects: What probably worries manufacturers more than the previous two problems is the heating effects that is caused by the small size of transistors. As we go smaller, transistors tend to get more “leaky”, meaning that even in their OFF state, they let some current pass through. This is called the leakage current.
If we take the leakage current to be 100 nA and a CPU to have 100 million transistors, then the leakage current will be 10A. And that will drain a phone battery in minutes.
A higher gate voltage can reduce the amount of leakage current, but that causes more heating effects. Even without considering that, each clock tick itself dissipates a huge amount of heat.
Manufacturers have to play around with these properties and get them just right to prevent these effects. And doing so is proving harder and harder as processes become smaller.
The high leakage current has also lead to the problem of Dark Silicon and Dark Memory. This is where, even though we may have a lot of transistors in a chip, most of them have to be kept OFF to prevent the chip of overheating and melting.
All those OFF transistors are taking space that could be used to place other components. Which begs the question: Do we really need to go smaller, or do we improve our current chip design?
Taking all this into account, Intel’s own CEO and the International Technology Roadmap for Semiconductors has said that 5 nm is probably the best we can do.
5 nm is expected to make its debut in 2021. So what can we expect after that?
One way to figure out what to do is by studying what happened when we reached the end of other similar laws.
Dennard’s Scaling – Dennard Scaling was considered to be a sister law to Moore’s Law. It was formulated by Robert Dennard in 1974 and states that as transistors get smaller, their power density also decreases. This means that as transistors get smaller, the amount of voltage and current needed to operate them will also decrease. This law allowed manufacturers to both reduce the size of transistors and increase clock speeds by huge jumps every iteration.
However, around 2007, Dennard’s Scaling broke down. This is because at smaller sizes, leakage currents cause transistors to heat up and that creates further losses.
You might have noticed that clock speeds have not risen in the last decade, despite the fact that transistors have gotten smaller, and this is due to the breakdown of Dennard Scaling. The high losses at higher clock speeds is also why smartphone chips use a lesser clock speed (usually 1.5 GHz).
Better Pipelining – Moore’s Law can be thought of trying to cut down a forest using an axe. Every two years, we add a new person to help us cut trees using another axe. This helps us cut trees faster. But that is not the only way we can improve our tree cutting performance. We can, for instance, spend time sharpening existing axes. And that is one of the areas that we will focus on more after Moore’s Law ends.
By improving current chip implementations and having better pipelining of instructions, we can improve the performance of our chips. To learn more about chip performance, you can read another article of mine here.
This is more generally called as Koomey’s Law which is still going very strong. Koomey’s Law says that the number of calculations per joule of energy will keep doubling every 1.5 years. And this is expected to keep going up until at least 2048 when Landauer’s principle and simple laws of thermodynamics will stop further improvements. Currently, computers are about 0.00001% efficient by Landauer Limits.
Traditional programming languages like Java, C++, and Python were built to run only on a single device. But as devices get smaller and cheaper, we can run the same program on many chips (concurrently or parallelly) thus further improving performance. In this regard, languages like Go are going to take a more prominent role.
Better Materials – Most importantly, researchers around the world are figuring out newer and innovative ways to build smaller and faster transistors. Materials like Gallium Nitride and Graphene have been shown to have lesser losses at faster switching frequencies.
Quantum Computing – A more likely solution is Quantum Computers. Many companies like D-Wave and Rigetti Computing are working extensively in this field and more importantly, the law for scaling up of Qubits hasn’t even started!
The way we got around Dennard Scaling is by putting more cores in a single chip to improve performance. And I think that we will find some similar alternatives when Moore’s Law breaks down.
Researchers in the UK have already built transistors that are 1 atom thick and 10 atoms wide using Graphene. But the commercial viability of such transistors has not been investigated. Quantum Computing too has shown great promise as they can have more than 1 state at a time (unlike other computers). Anytime we hit a roadblock, we innovate and find alternatives or workarounds for them and I am sure that we will do the same for Moore’s Law too.