So it took me a little bit to understand this, but I want to make sure I have a few things right.

First of all, when a Crystal Structure with One side N-Doped, One Side P-Doped are in the same crystal lattice, the Electrons from the N-side are going to jump over to the P-side at the Junction where they meet.

When they jump, this creates a Positive charge on the N-Side (Since electrons are leaving) and a Negative Charge on the P-Side (Since Electrons are being added) And these 2 sides were "Neutral" before.

My few questions is in regards to this. (Lets pretend no voltage is applied)

  1. The Depletion region makes sense, but the "equilibrium" with the depletion region: is that because the Negative charge over on the P-Side near the junction is opposing any more electrons from coming over? If so....how come more "holes" aren't moving into the negatively created region on the P-side...like I guess how come the negatively created region isn't jumping all over the place on the P-side, or for that matter, why aren't more extra electrons just jumping into the positive side created on the N-doped part near the junction.

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Like Look at the + Side in the "Space charged region".....why aren't the electrons jumping from the N-doped neutral region? Seems like the Free electrons should want to be attracted to the + side of the space charged region. Or is it just because the electric field from the negative side near the P-doped region near the junction is pushing them away?

  1. Since the concept of "holes" just confuses me more, and I like to just think of it as electron movement (Which is what it is anyways right?).....what exactly happens when the depletion region expands due to reverse bias? I understand Forward Bias....the Electrons are being "FORCED" over the Opposing Electric field created by the Depletion region. But when it's reverse bias what is "really" happening? Obviously "holes" aren't getting pulled out since that makes no sense(or is it that actually electrons are getting "shoved" to the other side by the + terminal), Is the depletion region "basically" expanding one way sorta, like towards the P-doped side. Since the electrons are getting pulled out of the P-doped side it's just going to create bigger gap....but why aren't electrons from the other side just jumping right back into the gap? Or is it because the Negative charged region is getting filled up more and more towards the P-doped side, which opposes any more electrons from coming over?

Sorry for the long questions, just "Holes" moving really whacks it up for me since the way I imagine it is just electrons flowing, and that makes the most sense to me!

  • $\begingroup$ One thing that helps me "understand" holes moving, is that I visualize the game of Chinese checkers. A marble is the electron and the holes on the board represent the holes. When a marble moves to a new hole, it leaves a hole behind. So as an electron "jumps" to a new hole (lets say from right to left), it leaves a hole behind, giving the appearance that the hole moved from left to right, but only the electron actually moved! $\endgroup$
    – Guill
    Commented Mar 30, 2014 at 23:43
  • $\begingroup$ @Guill why holes can't move form the p-side to the n-side. I can't see this form the energy band diagram. If you have p-region on the left and n-region on the right then electrons would have to climb a potential barrier going from n to p (move uphill). But the valance gap is higher on the p side than on the n side. So it looks to me that holes will decrease their energy going 'down hill' to the n side. Why is this understanding wrong? Of course there is an electric field, but just by looking at the curve of the bands I am getting a bit confused. $\endgroup$ Commented Oct 18, 2016 at 11:00

1 Answer 1


Your first question- Why aren't electrons being attracted by the positive charge region?

Any free charge will move in response to an electric field created by some charge distribution. So it's important to see the electric field in the region.

Well, the first thing you should do is find out the where the electric fields exist and where they don't. Electric field exists only in the depletion region, not in the 'neutral' p and n regions.

But then you ask why? The answer is clear if you know Gauss's law. It says the divergence of electric field is proportional to the net charge density. In the neutral regions there are no net (uncompensated) charges (Is a p-type neutral or charged? It's neutral. Because of each hole there's an acceptor ion that is negatively charged). On the other hand in the depletion region there are uncompensated positive and negative charges, and this creates the electric field.

To your second question, again, invoke the electric field: When you apply a voltage that increases the built in field, then you must have a larger number of uncompensated charges to support that field. That's why it expands in reverse bias and thins in forward bias.

In reality, you see, at equilibrium, electron and hole movement DOESN'T cease. There are still some electrons which are diffusing to the p-side, but an equal number is coming to the n side because of drift---electrons 'near' the depletion region on the p-side being sweeped by the electric field to the n-side. Similar things hold for holes.

Try reading Semiconductor Fundamentals by R F Pierret. It's a very good book for beginners.

  • $\begingroup$ On your last part, u say movement doesn't cease...you speak of back and forth movement correct? You are saying that near the points of the depletion region electrons are still going back and forth due to the electric field....but not like across and into the regions (cause if so.....why would we call it a depletion region?) $\endgroup$
    – user20118
    Commented Feb 2, 2013 at 3:49
  • 1
    $\begingroup$ Okay, let me just explain that last point a bit more. Electron and hole transport occurs because of two mechanisms- one is DIFFUSION, which occurs because of difference in concentration in space. The other is DRIFT-electron/hole movement under application of electric field. $\endgroup$
    – user20507
    Commented Feb 3, 2013 at 3:53
  • 1
    $\begingroup$ (Contd.)Electrons, due to diffusion, move across the depletion(dep for now) reg into the p- reg. Electrons on the p-side near the edge of dep reg, due to drift, move across the dep region into the n-reg. They don't oscillate back and forth in the dep region. They just cancel numerically. (What 'near' means will be clear when study further- just take it for granted now that this happens. Otherwise you'll confuse yourself.) $\endgroup$
    – user20507
    Commented Feb 3, 2013 at 4:04

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