Why Don't We Add Potentials in a Forward-Biased PN Junction?

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Discussion Overview

The discussion revolves around the behavior of a forward-biased PN junction diode when connected to a battery. Participants explore the concept of potential addition in this context, questioning why the junction potential is not simply added to the battery potential, similar to connecting two batteries in series. The scope includes conceptual understanding and technical reasoning related to semiconductor physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that when a battery is connected to a PN junction diode, the potentials should be added, similar to connecting two batteries in series.
  • Another participant argues that the situation becomes complex once the diode is connected to a circuit and current begins to flow, indicating that the behavior of the depletion layer is crucial.
  • Some participants mention that the opposing potential of the diode generally leads to a subtraction of potentials rather than an addition.
  • There are claims that the forward biasing of the diode alters the potential landscape, allowing charge carriers to move across the junction more freely.
  • One participant expresses confusion regarding the application of electric fields and potentials in the context of forward biasing, noting discrepancies in explanations provided in videos.
  • Links to external resources are shared for further clarification on the topic.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the potentials should be added or subtracted in the context of a forward-biased PN junction. Multiple competing views and interpretations of the behavior of the diode under forward bias remain present throughout the discussion.

Contextual Notes

Some participants express uncertainty regarding the definitions of potential and electric fields in the context of the diode, and there are references to videos that may not fully clarify the concepts discussed. The discussion reflects a range of assumptions and interpretations that are not resolved.

Who May Find This Useful

This discussion may be useful for students and enthusiasts interested in semiconductor physics, particularly those seeking to understand the behavior of PN junctions under different biasing conditions.

Topsykret
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If two batteries are connected such that positive terminal of one is in contact with negative terminal of other then we add the potentials so the potential will then be their sum.

Similarly if a battery is connected to a pn junction diode such that positive terminal is connected to negative side of junction potential (ie.connected to p type) then why don't we add their potentials? Isn't junction potential similar to the second battery as in the first case?

Please explain it at level of a high school student.
 
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Topsykret said:
why don't we add their potentials
Who says we don't ? Certainly in the case the other side of the diode is hanging in the air, https://www.electronics-tutorials.ws/diode/diode_2.html is just fine. But things only become interesting once that other side is connected to something and current starts flowing. If the depletion layer thickens, the diode blocks and if it gets thinner, it conducts. Read on, McDuff !
 
BvU said:
Who says we don't ? Certainly in the case the other side of the diode is hanging in the air, https://www.electronics-tutorials.ws/diode/diode_2.html is just fine. But things only become interesting once that other side is connected to something and current starts flowing. If the depletion layer thickens, the diode blocks and if it gets thinner, it conducts. Read on, McDuff !

But since it is the opposing potential generally we subtract them.
 

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https://www.electronics-tutorials.ws/diode/diode4.gifyou sure about that ? :wink:
 
BvU said:
https://www.electronics-tutorials.ws/diode/diode4.gifyou sure about that ? :wink:
if we forward bias it, +ve terminal of battery gets - potential of diode and -ve of battery gets +ve potential of diode.(-,+) of battery(-,+), this doesn't seem to be a barrier to the battery.
 
Nothing happens until the other side of the diode is also connected.
Check the current starts flowing video.
 
Topsykret said:
if we forward bias it, +ve terminal of battery gets - potential of diode and -ve of battery gets +ve potential of diode.(-,+) of battery(-,+), this doesn't seem to be a barrier to the battery.
 

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Topsykret said:
if we forward bias it, ... -ve of battery gets +ve potential of diode.(-,+) of battery(-,+)
and a lot of things happen ! The loose pn junction was steady state thanks to this small potential difference. That is now reduced and inverted by the battery so charge starts hopping over the pn boundary with gusto. The boundary goes thinner and etc.

See the video (are you such a quick viewer ?)
 
BvU said:
Nothing happens until the other side of the diode is also connected.
Check the current starts flowing video.
 

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  • #10
Any further questions ? Methinks you understand how it works, right ?
 
  • #11
BvU said:
Any further questions ? Methinks you understand how it works, right ?
please check new reply. :)
 
  • #12
I only see a yes.png. Does that mean: yes I understand ?
 
  • #13
BvU said:
I only see a yes.png. Does that mean: yes I understand ?
That means, in the 1st video she said we have E(field) from n side to p side. We apply larger E( from battery) against it (during forward biasing).
In 2nd video, she uses +ve potential at p and -ve potential at n side. No reason provided. Back to original question.
As I said earlier barrier potential is +ve on p side.
(Since last 10 hours I am stuck on this.)
 
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  • #14
Topsykret said:
That means, in the 1st video she said we have E(field) from n side to p side. We apply larger E( from battery) against it (during forward biasing).
In 2nd video, she uses +ve potential at p and -ve potential at n side. No reason provided. Back to original question.
As I said earlier barrier potential is +ve on p side. (reverse of potential used in your mage earlier)
(Since last 10 hours I am stuck on this.)
1 https://electronics.stackexchange.c...tion-region-which-side-is-at-higher-potential
2 https://electronics.stackexchange.c...e-a-battery?noredirect=1#comment909858_375761
3 https://electronics.stackexchange.com/questions/256295/pn-junction-under-forward-bias/256303

related and helpful links
 
  • #15
See Wikipedia:
Valance and condition bands.
 

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