# Logarithmic IV graphs of diodes

## Summary:

If we plot logI against V for a diode we get a straight line, but what does that mean?
Hello there,
I've been working through a task (that doesn't have an answer sheet or explanation) in which we plot I against V for three different diodes. Each has a different threshold voltage and displays the usual charcteristic curve. The final question is this:
"It is suggested that the behviour of a diode is logarithmic. Plot a graph of I against V using a base 10 logarithmic scale for the current axis. Discuss whether your graph supports this suggestion."

So, if you do this for the data given you get three straight lines with similar gradients. So I'd say 'Yes, this behaviour appears to be logarithmic'.

But, what does this mean? What is the physical significance? What else can we look at that 'grows logarithmically'? How does this compare to exponential growth, perhaps?

Any help or suggestions are welcome :)
~Owls

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## Answers and Replies

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berkeman
Mentor
Summary:: If we plot logI against V for a diode we get a straight line, but what does that mean?

Hello there,
I've been working through a task (that doesn't have an answer sheet or explanation) in which we plot I against V for three different diodes. Each has a different threshold voltage and displays the usual charcteristic curve. The final question is this:
"It is suggested that the behviour of a diode is logarithmic. Plot a graph of I against V using a base 10 logarithmic scale for the current axis. Discuss whether your graph supports this suggestion."

So, if you do this for the data given you get three straight lines with similar gradients. So I'd say 'Yes, this behaviour appears to be logarithmic'.

But, what does this mean? What is the physical significance? What else can we look at that 'grows logarithmically'? How does this compare to exponential growth, perhaps?

Any help or suggestions are welcome :)
~Owls
Welcome to PhysicsForums.

Are you familiar with the Diode Equation? It's fundamental to the operation of semiconductor junctions (like your diode):
https://en.wikipedia.org/wiki/Shockley_diode_equation

sophiecentaur
Science Advisor
Gold Member
Summary:: If we plot logI against V for a diode we get a straight line, but what does that mean?

But, what does this mean? What is the physical significance? What else can we look at that 'grows logarithmically'? How does this compare to exponential growth, perhaps?
I'm not sure about "significance". The relationship between two variables happens to be logarithmic. That is no more significant than the square law relationship between distance and time under constant acceleration. Log or exponential relationship is just the result of doing what's allowed in any equations - the same thing to each side.

Last edited:
bobob
Gold Member
Summary:: If we plot logI against V for a diode we get a straight line, but what does that mean?
It means that if you plot the log of an exponential on a log scale, you get a straight line.
But, what does this mean? What is the physical significance? What else can we look at that 'grows logarithmically'? How does this compare to exponential growth, perhaps?
A derivation of the diode equation would help with that question.