PN Junctions

January 3, 2013 10:09:06 PM CST

Well.. that was quite a gap between posts. Life was life-ing at me.

Where were we? ah yes: the PN junction..

PN junctions:

When you put a chunk of P-type semiconductor right next to a chunk of N-type semiconductor, things get interesting. The N-type material has lots of extra electrons floating around, and the P-type material has lots of extra holes.

Those conditions are ripe for recombination.

As electrons from the N-type material fall into holes in the P-type material, the silicon lattice becomes happy, but as I said last time, in a doped silicon lattice, someone is always unhappy. When the silicon lattice is balanced (each silicon atom sharing a pair of electrons with its four nearest neighbors), the dopants end up with extra electrons or holes.

Now, while electrons and holes can move easily from one atom to another, dopant atoms are pretty much locked in place by the silicon atoms around them. When dopants pick up extra electrons or holes, they become 'fixed charges'.. just the thing we need to create an electrical field.

Since recombination puts electrons on dopants in the P-type material and holes on the dopants in the N-type material, the resulting electrical field tries to push the carriers back to where they started.

The field can't push all the carriers back to their native material though.. that would eliminate all the fixed charges that make the field happen in the first place. What we end up with is a situation where carriers cross the boundary between the P-type and N-type materials to recombine, but then stay 'stuck' to the boundary (called the 'PN junction') by the electrical field they just created.

Eventually, all the holes and electrons near the PN junction recombine, creating a region called the 'depletion zone'.

The depletion zone:

'Depletion' means there's no room for free carriers to exist in that chunk of material.. all the silicon atoms are sharing a pair of electrons with their nearest neighbors, and all the unbalanced charges are localized around dopant atoms.

Current can't flow through a depletion zone because there are to free carriers to .. well .. carry it.

As long as carriers can get around or through the depletion zone to recombine with carriers on the other side, they will. Once they do, they'll get trapped near the PN junction, expanding the depletion zone and making it harder for the next carrier to get through.

The depletion zone stops growing when carriers just can't get across it any more.

Reverse saturation current:

Technically, that last statement is only true at absolute zero.

At temperatures above absolute zero, there's always a chance that thermal energy will knock an electron in the depletion zone up into the conduction band, creating an intrinsic electron/hole pair. When that happens, the electrical field pushes the electron toward the N-type material and the hole toward the P-type material.

When those carriers get to the far edge of the depletion zone they'll recombine with carriers from the non-depleted material (called the 'bulk' semiconductor), but they won't make the depletion zone any wider. The creation of the intrinsic electron/hole pair makes the depletion zone slightly weaker, and when the carriers come out the edges it goes back to its regular strength.

From the outside, it looks like the depletion zone spontaneously got weak enough to let a pair of carriers through, then hardened back up again.

That kind of thing happens a few thousand times per second at room temperature, so it looks like a very small amount of current (a few femtoamperes) can still flow through the depletion zone. That's called the 'reverse saturation current', and it plays a major role in the way diodes work.

The term 'saturation' is the one-sided version of 'depletion'. A region of doped semiconductor is 'saturated' when all its dopants hold fixed charges. You can make that happen outside a depletion zone by applying an external electrical field, but that leads to a discussion of mosfets, not diodes.

The term 'reverse' means the current flows through the diode backwards.

Diode navigation:

The terms 'forward' and 'reverse' refer to the way carriers move in relation to the PN junction:

  • Electrons that move from N-type material toward P-type material are going forward
  • Electrons that move from P-type material toward N-type material are going backward
  • Holes that move from P-type material toward N-type material are going forward
  • Holes that move from N-type material toward P-type material are going backward

Recombination pulls carriers forward across the PN junction, the electrical field created by fixed charges in the depletion zone pushes them backwards.

That backwards electrical field in the depletion zone also plays a major role in how diodes work, and I'll go into the details of that next time.

(hopefully that won't take another few months)

Random brain cookies:

He had occasional flashes of silence that made his conversation perfectly delightful. -- Sydney Smith