# Physics of n-p Junction in a diode

Discussion in 'Electronics Homework Help' started by Greg J., May 23, 2014.

1. ### Greg J.

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Oct 8, 2013
I am trying to understand plain silicon diodes at the physics level. Can someone check my thinking about what happens at the p-n junction?

I would love to use electron flow notation, but my Multisim trial doesn’t support that, so I am trying to avoid labels and the word current (and its direction) in my thinking and in this post and just understand which side of the AC Power Source is an electron source and which is an electron sink. (However, I have to translate to conventional flow thinking and notation when referencing Multisim.)

My understanding at the moment:

The diode schematic symbol’s triangular arrow points to the diode’s cathode side which has the N-plate = semiconductor material doped with excess weakly bound electrons.

The other side of the junction is the diode’s anode side which has the P-plate = semiconductor material doped to have excess holes.

When the diode’s cathode is connected to an electron source, there’s more than enough electrons present in the N-plate, and as a result, there are lots of free electrons and charge flows across the junction normally—from the cathode side to the anode side.

When the diode’s cathode is connected to an electron sink, the sink attracts the weakly bound electrons, pulling them away from the N-P junction, and as a result, there are few free electrons and charge (mostly) does not flow.

2. ### LvW

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Apr 12, 2014
As a starting point you must try to understand what a depletion regon is - and how it originates (diffusion process and corresponding voltage).

10,585
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Nov 17, 2011
4. ### Ratch

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Mar 10, 2013
I have a problem with the explanation of the above tutorial. It states that the forward and reverse current is controlled by the width of the depletion region. I aver that the depletion width does vary with the voltage, but the width does not cause the current change. The real cause of current increase is the lowering of the barrier voltage in the forward direction which promotes more diffusion, and the decrease of current is caused by the rise of the barrier voltage which suppresses diffusion. The depletion area width varies, but is irrelevant, with respect to controlling the diode current.

Ratch

5. ### LvW

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Apr 12, 2014
When the applied voltage changes, then reacts initially the width of the space charge region or the voltage barrier?
Remember: Voltage (potential difference) is the integral over the electric field E - and E is the integral over the space charge.

6. ### Ratch

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Mar 10, 2013

Yes, and what difference does it make? I am saying that the diffusion current across the diode junction is not dependent on the depletion region width like the link says it is. It does not matter if the depletion region width expands or shrinks. The barrier voltage controls the diffusion and therefore the diode current. Manipulating the barrier voltage with an externally applied voltage therefore determines the diode current as Shockley's equation shows.

Ratch

7. ### LvW

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Apr 12, 2014
Yes - You are right, the question was not clear.
It is clear that the barrier voltage is controlling the diffusion - however, one could ask what is the physical effect that controls the barrier voltage? The electric field in this region?
And does this field depend on the space charge or the width of the region where this space charge does exist?
With other words: Who is first? Width or voltage?

8. ### Ratch

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Mar 10, 2013
The barrier voltage is the sum of the "built-in voltage" and the applied voltage. The built-in voltage is determined at the time of manufacture, and depends on the intrinsic concentration of the semiconductor and the concentration of the P and N sides of the diode. https://answers.yahoo.com/question/index?qid=20061108222624AA1XRcK

Assuming the barrier voltage is the sum of the applied voltage and built-in voltage, it would seem that the applied voltage is the only control the circuit has over the barrier voltage.

The space charge is caused by the immobile ions that lost or gained a charge due to diffusion. Fig. a shows this charge. Fig. b shows the electric field throughout the depletion region. The electric field is greatest inside the depletion region where the charges change polarity, and zero outside the depletion region.

Neither is first or last. They both change together.

Ratch

9. ### LvW

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Apr 12, 2014
"Neither is first or last. They both change together."

Ratch - only this last sentence meets the core of my question. All other statements/explanations are known to me.
When they "change together" would it really be wrong to say that the "collector current is controlled by the width of the depletion region" (see your post#4). ?
Let us use a very simple question: Closed loop of a changing voltage source and a resistor. Voltage and current "change together", correct? Neither is first or last.
Nevertheless, one of both is the cause of the other, also correct?
That was the background of my question. Do you now understand?
During calculation I can add two voltages (Quote: "The barrier voltage is the sum of the "built-in voltage" and the applied voltage") - but that´s not the question.
The problem is related to the question of "cause and effect" only.

10. ### Ratch

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Mar 10, 2013
Yes, it would. That proposition is similar to the wrong belief that the base current of a BJT controls the collector current. The base current is an indicator of the collector current, not the cause.

Voltage controls and causes current provided there is a conduction path. You can have voltage without current, but you cannot have current without voltage.

I am not sure that I do.

The cause is the applied voltage across the diode. The effect is the increase/decrease of the diode current. Can you find a equation in semiconductor physics that relates the width of the depletion region to the diode current the way Shockley's equation relates the applied voltage to the diode current? I can explain the physics by saying the lowering/raising of the barrier voltage allows more/less charge carriers to diffuse continuously across the depletion region because the energy needed to cross the barrier is less/more. The width of the depletion region is less than a human hair. Explain how is the width change going to affect the diode current at 0.6 volts.

Ratch

11. ### LvW

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Apr 12, 2014
Your "..proposition is similar to the wrong belief that the base current of a BJT controls the collector current. The base current is an indicator of the collector current, not the cause."

Ratch - you may remember that I am with you regarding this question. Hence, I do not understand the background of this remark.

"Voltage controls and causes current provided there is a conduction path. You can have voltage without current, but you cannot have current without voltage."

I cannot remember to have denied this perception.

"The cause is the applied voltage across the diode. The effect is the increase/decrease of the diode current. Can you find a equation in semiconductor physics that relates the width of the depletion region to the diode current the way Shockley's equation relates the applied voltage to the diode current?

I also cannot remember to have denied the role of the voltage. But the effect of the voltage is twofold - that´s all I want to say: It lowers the barrier and - at the same time - it alters the width of the depletion zone. The diffusion current depends on this width! You cannot deny the influence of this width. There are good reasons to make it as small as it is.
_____________

Anyway, I think it´s better to stop the discussion at this point.
However, it would be interesting to hear from the questioner (Greg) if he has some further specific questions.

LvW

12. ### Ratch

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Mar 10, 2013
I do remember that you agree that Vbe controls the collector current, not the Ib. I used Vbe-Ic as an example of how a correlation of one parameter does not necessarily mean that this parameter controls a different one, I did not say you denied the example I used. As for the depletion region width influencing or partially influencing the diode current, show me a relationship of width vs current with an equation, and explain the physics of how a microscopic physical width change can influence the current. I still aver that the width change is a consequence of the current change and not the cause of current change.

Ratch

13. ### LvW

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Apr 12, 2014
Quote: "...explain the physics of how a microscopic physical width change can influence the current".
One last sentence from my side:
I think, a "microscopic change" of a microscopic dimension can be remarkable.
(By the way: If I remember well, it is the change of the BJT`s depletion layer that is responsible for the well-known Early effect).

14. ### Ratch

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Mar 10, 2013
Why would a 0.0001 unit change of a 0.001 unit change be remarkable?

That is true. However, in a transistor, the base-collector depletion layer is intruding into the already very thin physical base layer, and changing the electrical characteristics of the transistor. Since the length of the N and P sides of a diode are infinitely long compared to the depletion region, than cannot and does not happen in a diode. The effects of depletion region change in a junction diode and a BJT are unrelated.

Ratch