How Are Quasiparticles Separate From Particles?

**SPEAKER_1** (0:00)
No one goes to Hank's for his spreadsheets. They go for a darn good pizza. Lately, though, the shop's been quiet, so Hank decides to bring back the $1 slice. He asks Copilot in Microsoft Excel to look at his sales and costs and help him see if he can afford it. Copilot shows Hank where the money's going and which little extras make the dollar slice work. Now Hank says, I'll line out the door. Hank makes the pizza. Copilot handles the spreadsheets. Learn more at m365copilot.com/work.

**SPEAKER_2** (0:30)
You said this place was steps from the water.

**SPEAKER_3** (0:32)
We just haven't found the steps yet.

**SPEAKER_4** (0:35)
How much did we save?

**SPEAKER_7** (0:37)
Enough.

**SPEAKER_6** (0:39)
Enough to get lost?

**SPEAKER_7** (0:41)
Or you could book a stay with Hilton.

**SPEAKER_6** (0:43)
Welcome to your oceanfront room. Just steps from the water.

**SPEAKER_7** (0:46)
The Hilton sale is on now. Book on hilton.com or the Hilton app and save up to 20% to get the stay you expected. When you want savings, not surprises, it matters where you stay.
Hilton, for this day.

**Matt O'Dowd** (1:00)
Thank you to Magellan TV for supporting PBS. The device you're watching this video on is best understood by thinking about positive and negative charges moving around a circuit of diodes and transistors, but the only elementary particle actually flowing in that circuit are the negatively charged electrons. And yet those flowing positive charges are there, in the form of a particle you may have never heard of. Your device works because of Quasi-Particles, a class of strange emergent behaviors of nature that enable our most important technologies and are behind some of the weirdest phenomena that we have ever encountered.
Let's begin our discussion of quasi-particles by talking about the particular quasi-particle that lets me talk to you about quasi-particles right now.
Now, electrons, which are regular particles, are pushed around inside electrical circuits, but that's only half the story. In the semiconductors that make up transistors, diodes and solar cells, the pushing around of a quasi-particle is equally important. Let's look at the material that is central to all modern electronics, silicon.
The silicon atom has four electrons in the outer or valence shell. Atoms are most stable with full valence shells, which means eight electrons. That's why silicon likes to form covalent bonds with four other silicons, and each of those with another four forming this tetrahedral crystal lattice. We're going to depict this as a square grid in 2D to save on animation costs. The valence electrons are now locked in place by the now full valence energy level. But they can still get bumped up to higher energy states, say by thermal vibrations, or in the case of solar cells, by a photon. At which point the electron is free to move from atom to atom, for example if pulled by a voltage applied across the silicon. Meanwhile, the gap left by this electron allows some movement in the valence shell. A neighboring electron can move there, and its neighbor can fill the new gap, etc.
It looks like the hole moves around, and under a voltage the hole moves in the opposite direction to the flow of electrons. This is our first quasi-particle. It's an electron hole. It has an effective positive charge due to the charge of the nucleus not being properly cancelled by electrons in that location. It even has an effective positive mass.
We can model it as though it's a real particle. To see how this quasi-particle is more than just a way of looking at things, consider perhaps the simplest semiconductor device, the diode.
Diodes allow our current to travel in one direction but not the other. They consist of two layers of silicon. On one side there's an excess of valence electrons, and on the other a deficit. This is achieved by doping, contaminating each layer by a different element. On one side we sprinkle the silicon ladders with a tiny number of atoms that have five rather than four valence electrons. Phosphorus is a popular choice. Those extra electrons are more free to move around because they aren't part of the crystal bonds. This is an N-type semiconductor. N because the flowing charge is negative. The other side is doped with atoms that have three valence electrons, frequently boron. Now electrons on that side can move a little bit. They can shuffle to fill the gaps in the underfilled valence shells. So we have flowing positively charged electron holes and a P-type semiconductor. In a diode, N and P types are fused together. At the P-N junction, extra electrons in the N-type diffuse into the gaps in the P-type. So we end up with a region where all valence shells are filled, so charge can't flow.