What are semiconductors?
Semiconductors power the chips in every computer. But how exactly are they made, and how do they work?
Semiconductors power the chips in every computer. But how exactly are they made, and how do they work?
Computers have revolutionized the world, from the earliest mainframes through to the powerful machines we all have in our pockets today. The chips that power these computers use tens of thousands of tiny transistors, which have shrunk from the size of a fingernail to about the size of a single strand of DNA over the decades.
But how do transistors work? How have these devices transformed the way that we live? The answer lies in the sand.
In the physical world, there are two extremes when it comes to materials and electricity; they’re either conductors or insulators. A conductor is a material that allows a charge, or electric current, to flow through it, whereas an insulator does not allow a charge to easily pass through. Perhaps unsurprisingly, a material that is a semiconductor sits between these two extremes. For some semiconductor materials, their ability to conduct a charge increases as the temperature drops. The conductivity of other materials is affected by magnetic fields, impurities in the materials, or even exposing them to light. The process of adding impurities to materials to make them better conductors is called doping.
By sending a charge through a semiconducting material, you can create signals with a set of binary states, sending a charge or not sending a charge. This proved to be extremely useful for a range of electronics applications, such as early radio, telephony, and amplifiers, but proved most transformative with the creation of transistors.
At their most basic level, transistors are essentially semiconducting materials with several connecting points to an electric circuit. Applying a current to a pair of these connections in the circuit affects the semiconductor’s ability to conduct current. The flowing or restricted current are essentially on and off signals that are interpreted in digital systems as 1s and 0s. And in modern devices, those on and off signals can switch more than 5 billion times per second, or 5 gigahertz. Combining transistors in different ways forms circuits, which are designed to perform different math operations.
Early computers were essentially just used as powerful calculators, running equations too great for a person to comprehend on their own. Before we had digital computers, we used to refer to the people who did sums for a living, like those calculating launch trajectories for early NASA missions, as computers. But around the turn of the last century, people started to realize that you could represent anything in terms of the digital sums that computers could run. Interpreting images, sound, movement, color, time, and many other concepts as massive strings of 1s and 0s eventually led to the vision of the computer as we know it today.
While there are several materials that can be semiconductors — including arsenic, boron, carbon, germanium, selenium, and sulfur — if you’re reading this at the beach or on a smartphone the most common material for transistors is likely right in front of you. Silicon (which is usually found in sand and glass panes, like the one on your phone) is seen as an ideal semiconductor. It’s ubiquitous, inexpensive, easy to work with, and stable in many situations. It’s actually the eighth-most common element (by mass) in the entire universe.
And on Earth, silicon truly is everywhere. Silica, or silicon dioxide, is a compound of two of the most abundant elements on earth. It covers our beaches and fills mines around the world. When small amounts of impurities like boron or phosphorus are added to pure silicon, it becomes an excellent semiconductor. And although there are other materials that are naturally better semiconductors, silicon’s comparably high melting point (around 1,414°C) and ability to form strong bonds with other elements make it well-suited to be used in transistors — especially given its natural abundance. Silicon was also chosen as the go-to material to fabricate transistors because a single crystal silicon is one of the purest materials on earth.
The process for turning silicon into the semiconducting material used in computer chips involves running the raw silicon material through a superhot arc furnace with other materials like carbon and carbon dioxide, which creates a rod of silicon. This is then crystallized through a process called the Czochralski method (named after the Polish scientist who discovered it) where a small crystal of silicon is pulled slowly up from a pool of molten silicon. The end result is a large, tubular object with a roughly conical top. This is silicon that’s now ready to be sliced into wafers and turned into chips. It’s also the reason that although most computer chips are rectangular, the wafers they’re produced on are circular, as they’re cut from the silicon that has grown in a tube shape.
Once the silicon is in wafer form, it’s ready to be turned into chips. There are many processes for making chips, but one of the state-of-the-art patterning methods is called extreme ultraviolet lithography (or EUV lithography) to make the smallest features.
The semiconductor industry uses high-powered lasers to print circuit designs into a photosensitive layer that is coated onto a silicon wafer, which then creates a relief pattern that can be etched into the wafer. This process is repeated dozens of times to create the complex transistor and wiring designs of today’s chips. The lithography machines in use today, such as those at IBM Research’s Albany Nanotech Complex, use lasers that emit light below the ultraviolet portion of the electromagnetic spectrum. IBM Research is using these lithography machines to design the computer chips of the future. In 2021, IBM Research unveiled the world’s first 2 nanometer node chip, still the smallest in the world today, and using these same machines, researchers are working on pathways to even smaller devices that will power the computers of tomorrow. That’s just part of the vast array of work IBM Research carries out into the future of semiconductors.
And none of that would be possible without the unique physics of semiconductors and the pieces of rock that generations of scientists have managed to turn into machines that have transformed the way we live, play, and work.