Hacker News Reader

Comment by unnouinceput | Hacker News Reader

Quote": Instead of describing the motion of constantly moving electrons, it is easier to study the motion of the hole. If the electrons move to the right, the hole moves to the left—and this movement follows certain physical rules, just like the movement of an ordinary particle."

That's precisely how my physics professor in 11th grade explained to us how PNP transistors works, more then 25 years ago

View on HN

Yep. That's the part we all know and is in the article only as background information.

It is also known that and electron and a hole can be bound and form an "exciton" that is stable enough to be called a quasiparticle. It's somewhat similar to positronium. [1]

The novel part of the article is that (in some materials?) two electrons and two holes can bound together and form something that is stable enough to be called a quasiparticle.

[1] Why nobody call positronium a quasiparticle?

You may ask, Why does nobody call atoms quasi-particles?

The original quasi-particle is the phonon.

Ultimately, it comes down to convenience. It's a zoo down there. Physicists have to impose some order to be able to say anything about what goes on.

Quasiparticles need a substrate to live in, they usually are localised collective excitations of the particles that make up a solid that behave like a particle.

The positronium is a bound state of two real particles.

I know all analogies, especially those explaining complicated physics to 11th graders, have to oversimplify some aspects of a phenomenon, but this one always rubbed me the wrong way. The hole moves with the electrons, if they move right, the hole also moves right.

The crucial point is that the electrons at the top of a valence band have effective negative mass, i.e. they move opposite to an applied force. Of course their true mass hasn't changed, this is simply a consequence of the fact that we're considering a system of uncountably many electrons moving in unison. This means a full band carries no current (which is the situation in isolators), but if a band has some electrons missing at the top, these holes will be transported in the opposite direction of the usual electron current, which happens to be the correct direction for positive charges. Hence, we can talk about positive charge carries.

A better analogy would be a bubble of air underwater in a river. It moves with the flow of the water, not against it. I know it doesn't really matter, but I feel an analogy must at least have a core of truth.

Bubbles in the shape of water.

Loved reading your first line’s first phrase as the sentence “I know all analogies.”

The technically correct sentence would have been "I know, all analogies..."

But yeah, this one was fun and I read it like you did at first as well!

"I know it doesn't really matter..."

Au contraire.

Analogies, mental models, metaphors help us make predictions. Better is better.

"The difference between the almost right word and the right word is really a large matter. 'tis the difference between the lightning bug and the lightning." ― Mark Twain

The hole only moves with the electrons if the electrons are moving in unison. If they move one at a time, into the hole left by the previous electron, then the hole moves the opposite way. E.g. say a, b, c, ... are electrons:

  1. abcde
  2. abcd e
  3. abc de
  4. ab cde
  5. a bcde
  6.  abcde

(I don't know much chemistry or electronics, but that's how I've always understood the analogy.)

That is exactly the analogy I have a problem with, it is variously explained with cars in parking lots, people on seats, .... It is wrong however, the electrons do not move one at a time, they move in unison opposite to the normal flow of current.

Apart from theoretical considerations, we know it has to be like this because the voltage induced by the Hall effect flips for these kinds of semiconductors, and since there are no positively charged free particles in solids, this has to mean the electrons are now moving in the other direction, giving a complete illusion of a positive charge carrier.

Did your professor explain why an electron moves from one atom to another within a crystal to fill a hole? As in what triggers that "move" or is it a constant reshuffling of electrons within a crystal trying to fill holes in vain because simultaneously a new hole is created?

> it is easier to study the motion of the hole.

Easier to study because electronics notation accidentally settled upon the arrow going from + to - instead of the direction electrons move, or easier for a deeper physical reason?

Goes back a lot further than 25 years too.

It was the explanation I got in an after-school electronics program in 1975.

Reply on news.ycombinator.com