# What Is a Semiconductor?

5 minutes

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♪ Out loud ♪♪

Here I have a circuit with a battery, light bulb, and gap. If I fill this gap with a metal, the light comes on. If I fill this gap with glass, the light stays off. You probably know that metal is an electric conductor, and glass is an insulator. What happens when I fill this gap with a silicon wafer? The light stays off, so maybe silicon's an insulator. But what if I heat it up? Thank you. It lights up. The silicon is insulating at room temperature, but conducts electricity when it's very hot. It's a semiconductor whose conductivity changes based on the environment. This special ability makes semiconductors the perfect brains for electronic devices. Circuits of small semiconductor switches, called transistors, are at the heart of computer chips, enabling them to do math and run programs. Semiconductors have enabled electronics to become smaller, faster, and more reliable. But what is it exactly about these semiconductors that allow them to either conduct or insulate? In a single atom, electrons can occupy specific energy levels. When multiple atoms bond, the electrons are shared between them. Because the atoms are interacting, the energy levels shift around. In a solid, trillions of atoms interact with each other. Their individual energy levels smear into energy bands. For a material to conduct, electrons jump from lower energy states to higher ones. The spacing of levels and how they're filled with electrons determines if the material is a conductor, insulator, or semiconductor. A huge gap between the energy levels hinder electrons from jumping to higher levels, so current can't flow, and it's an insulator, like this glass. Metals have no gap at all. Electrons move easily to higher energy levels. Current can flow. Semiconductors fall somewhere in the middle. They have a medium-size band gap. Technically, I could make this glass conduct electricity if I added enough energy, through heat, to push the electrons into a higher band. But that amount of heat would either melt or break the glass before it actually conducts. This is true of most insulators. The amount of energy needed to make them conduct is too high. In a semiconductor, the small band gap allows electrons to jump into the higher energy band. The amount of heat we apply determines how many electrons jump into the higher band and how much current flows. Heat isn't the only way to change conductivity in a semiconductor. We can use light, electric currents, and in a computer, electric fields. As I've said, computers are made up of transistors that switch between conducting and insulating. Computers use electric fields because heat is slow and would burn too much energy. We can turn this wafer into a computer chip by printing a circuit of transistors on it using a process called photolithography. In this photo room, we cover the wafer with a light-sensitive material and expose it to light that's shined through a patterned mask. The wafer's developed like film in photography, leaving behind a pattern that becomes the circuit. Printing the transistors at once lets you make circuits that are smaller and cheaper than if you built them from individual parts. Transistors make up the logic elements, the memory components, and the communication modules that let computers talk to each other. With semiconductors, you can cheaply add transistors to almost any device-- from spaceships, to servers, to maybe even your toaster. Semiconductors have enabled the technological revolution, the Internet, the computer, and the cell phone. No semiconductors, no information age. I'm Jamie, and thanks for watching Science Out Loud. Check out our other videos, including mine on how computers compute. Check on out our website for more information. The end. Ugh! I'm just like-- Accessibility provided by the U.S. Department of Education.

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Semiconductors are in everything from cell phones to rockets. But what exactly are they, and what makes them so special? Find out from an electrical engineer. Part of the "Science Out Loud" series.

## Media Details

Runtime: 5 minutes

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