By Jonny Lupsha, Wondrium Staff Writer
Semiconductors are so deeply ingrained as a part of society that we use them every day. They’re used in smartphones, refrigerators, weapons systems, and more. They may be the biggest invention of the 20th century.
Humanity uses such a large amount of semiconductors that a global shortage of them has many industries worried. When the novel coronavirus pandemic struck in 2020, car manufacturers lowered their production levels, decreasing the need to supply semiconductors to auto makers, but demand from companies making work-from-home electronics like laptops soared. As the auto industry recovered, a shortage loomed.
The current presidential administration in the White House held a meeting for officials to discuss the growing concern about a semiconductor shortage. Since semiconductors have been integrated into the everyday products of so many aspects of our lives, a shortage could spell trouble. In his video series Physics in Your Life, Dr. Richard Wolfson, Professor of Physics at Middlebury College, explained why transistors—one of the most common semiconductors on Earth—could be the greatest invention of the 20th century. Much of a transistor revolves around a component called a PN junction.
Nature Abhors a Vacuum
Dr. Wolfson explained that the transistor is at the heart of all major electronics. “It’s the single element that does the real work of modern semiconductor-based electronics,” he said. But in order to understand how transistors work, we first need to understand the process of diffusion.
“Materials expand to fill a vacuum,” Dr. Wolfson said. “If I have a region where there is a high concentration of something—air molecules, perfume molecules, electrons, whatever—that high concentration tends, over time, to spread out and even out the concentration.
“Diffusion is a process where a high concentration of particles tend to spread out into regions of low concentration.”
The PN Junction
Keeping this in mind, semiconductor physics involve two types of semiconductors: P-type, in which the majority of charge carriers are positive, or “holes”; and N-type, in which the majority of charge carriers are negative, or electrons. On their own, P-type materials and N-type materials have no net charge. However, what happens if they’re put together in a junction—known as a PN junction? Dr. Wolfson said to imagine you have a piece of P-type material on one side and N-type material on the other.
“When I put this structure together, when I first assemble a PN junction, holes are going to diffuse […] from the P side of the junction to the N side of the junction,” Dr. Wolfson said. “Electrons are going to do the opposite. Electrons will diffuse from the N side of the junction, where they are in high concentration, to the P side of the junction.”
As holes and electrons cross over, they can recombine and remove two charge carriers from the system, making the hole vanish while leaving the electron, which is now part of the bonding structure. It also depletes much of the PN junction from having any free charges—charges that are free to move around—therefore making it a poor conductor of electricity.
But Wait; There’s More!
“There is still an excess of electrons, and so the edge of the P-type material near the edge of the junction in fact has a net negative charge,” Dr. Wolfson said. “Similarly, holes that are fused into the N-type material, bringing with them positive charge, we have combined with electrons, so that the holes are not there anymore.
“However, the dearth of electrons that they represent is still there as an imbalance.”
This distribution of material sets up an electric field in the region, which almost instantly blocks further diffusion from happening. However, if we hooked up the positive end of a battery to the P-type material—which now has a negative charge, after diffusion—and the negative end to the N-type material, good things happen.
“The depletion region around the junction fills up with free carriers that are free to carry current, and the whole block of PN-type material becomes an electrical conductor,” Dr. Wolfson said. “A PN junction fundamentally blocks the flow of electric current in one direction, and allows the flow of electric current in the other direction. They are essentially one-way valves of electricity.”
Wrapping It All Up
PN junctions may be the most complicated part of a transistor, but now that they’ve been demystified, the rest of a transistor is far simpler by comparison.
“Almost all of our electronic devices require direct current, steady current that flows in one direction, for their power sources,” Dr. Wolfson said. “The fundamental device that turns alternating current to direct current is the PN junction diode. It’s called a diode because it has two elements to it—just two places where you connect wires to it, the ends of the P- and N-type materials.”
While a diode has two places to connect electricity, a transistor has three. The third is a control electrode. What makes a transistor so valuable is that it allows one electric circuit to control another. According to Dr. Wolfson, that is the key element to computing, stereo amplifiers, and almost every other electronic device.