What is the minimum size a CPU can achieve?

Throughout the 20th century, inventors developed technologies that became essential to daily life. One of the most significant inventions was the transistor, created in 1947 by engineers at Bell Laboratories. Initially, the transistor was designed to amplify sound for phone lines, replacing the less reliable, bulky, and heat-producing vacuum tubes.

The first transistor, a point-contact transistor, stood half an inch (1.27 cm) tall. Though it was not very powerful, scientists quickly recognized its potential. Soon after, transistors found their way into various electronic devices, and over time, engineers learned how to make them smaller and more efficient.

In 1958, engineers placed two transistors onto a silicon crystal, giving birth to the first integrated circuit [source: Intel]. This breakthrough led to the creation of the microprocessor. If a computer were a human, the microprocessor would be its brain, handling calculations and data processing.

In the 1960s, computer scientist and Intel co-founder Gordon Moore made a notable observation: every 12 months, engineers managed to double the number of transistors on a square inch of silicon. This consistent reduction in transistor size has made possible the creation of devices such as personal computers, smartphones, and mp3 players. Without transistors, we'd still be relying on vacuum tubes and mechanical switches to perform calculations.

Since Moore's observation, the trend of shrinking transistors has persisted, though not as rapidly as Moore initially predicted. Now, the number of transistors doubles every 24 months. This raises an intriguing question: How much smaller can transistors—and CPUs—get? In 1947, a single transistor stood just over one-hundredth of a meter high. By the 2010s, Intel produced microprocessors with transistors only 45 nanometers wide. A nanometer equals one-billionth of a meter!

Intel and other microprocessor manufacturers are already developing the next generation of chips, which will feature transistors just 32 nanometers wide. However, some physicists and engineers believe we may be approaching the fundamental physical limits of transistor size.

Transistor Anatomy

Before diving into the physical limits of transistors, it’s important to understand their components and how they function. Essentially, a transistor acts as a switch made from a special type of material. One way to categorize matter is based on its ability to conduct electricity, classifying it as either conductors, insulators, or semiconductors. Conductors are materials where electrons can flow freely, such as metals. Insulators, on the other hand, consist of atoms that don’t have space for electrons, preventing the flow of electricity. Ceramic and glass are good examples of insulators.

Semiconductors are unique. These materials contain atoms with some available space for electrons, but not enough to conduct electricity in the same way metals do. Silicon is a prime example. Depending on the conditions, silicon can act as a conductor or an insulator. By adjusting these conditions, it’s possible to control the movement of electrons. This concept is the basis for today’s most advanced electronic devices.

Engineers discovered that by doping — adding specific materials — into silicon, they could influence its electrical conductivity. The process starts with a base material called a substrate, which is then doped with either negatively charged or positively charged elements. Negatively charged materials have an excess of electrons, while positively charged materials have an excess of holes — places where electrons can fit. In this example, we’ll consider an n-type transistor, which features a positively charged substrate.

On this foundation, three terminals are established: a source, a drain, and a gate. The gate sits between the source and drain and functions as a barrier allowing voltage to pass into the silicon but preventing it from escaping. The gate is covered by a thin insulating layer called an oxide layer that stops electrons from flowing back through. In this example, the insulator is placed between the gate and the positively charged substrate.

The source and drain in our example are negatively charged terminals. When a positive voltage is applied to the gate, it draws the few free electrons from the positively charged substrate into the gate’s oxide layer, forming an electron channel between the source and drain. If you then apply positive voltage to the drain, electrons flow from the source, through the channel, to the drain. If the gate voltage is removed, the electrons in the substrate no longer stay attracted to the gate, and the channel is interrupted. This means that when there is voltage applied to the gate, the transistor is 'on.' Without voltage, the transistor is 'off.'

Electronics interpret this switching as data in the form of bits and bytes, which is how your computer and other devices process information. However, since electronics rely on the movement of electrons to manage data, they are governed by specific physical laws. We will explore these in the next section.

Transistors on the Nanoscale

Every year, it seems like a journalist writes an article claiming that transistors have reached their smallest size and that Moore's Law is a thing of the past. Yet, engineers continue to find groundbreaking ways to make transistors even smaller, proving the critics wrong. At this point, many writers hesitate to predict the end of Moore's Law.

However, it is true that one day, we will encounter the physical limits of how small traditional transistors can become. Once we reach the nanoscale, we enter the strange realm of quantum mechanics. In this world, matter and energy behave in ways that are counterintuitive. Quantum physics differs greatly from classical physics—observing something at the quantum level itself affects its behavior.

One quantum phenomenon is electron tunneling, which is akin to teleportation. When material is extremely thin—only about one nanometer thick, or roughly 10 atoms—electrons can pass right through it as if the material isn't even there. The electron doesn’t punch a hole in the material; it simply vanishes from one side and reappears on the other. Since gates are designed to control the flow of electrons, this becomes a problem. If electrons can pass through a gate regardless of conditions, there’s no way to regulate their movement. In transistors with leaks, electron flow can’t be controlled, making the processor ineffective or even inoperable.

With companies like Intel developing transistors that are just 32 nanometers wide, it won't be long before the oxide layer is too thin to function effectively as a gate for electrons in traditional transistors. While engineers have faced challenges in shrinking transistors before, they've always found solutions to keep Moore's Law intact. However, this could change once we hit a fundamental physical limitation.

There is a possibility that engineers will discover a way to create an effective insulator even when it’s only one nanometer thick. But even if they achieve this, there may not be much more room to advance with transistors as we currently understand them. Beyond the nanoscale, we enter the atomic scale, where materials are only a few atoms wide.

This doesn't mean transistors will disappear, but it may signal that progress in microprocessor development could slow and plateau. The growth in processing power may no longer follow an exponential trend. Nonetheless, companies are likely to discover ways to enhance the efficiency and performance of microprocessors.

There is also the possibility that microprocessor manufacturers will find a new alternative to traditional transistors. Some are already exploring methods to leverage quantum effects at the nanoscale—essentially turning nano-sized problems into innovative solutions.

It seems that microprocessor manufacturers may only be able to sustain Moore's Law for a few more years. However, if you look back at predictions made decades ago, you’ll find journalists making similar claims. Perhaps engineers view these predictions as a personal challenge to overcome what seems like an impossible barrier.