# Diode's internal resistance

#### buzaiandras

Joined Jul 18, 2011
54
Hello all,

I am trying to correctly learn about diodes and I have some questions related to them.
Does a diode, a non-linear device, have internal resistance?
In case it has it is so small that it can be neglected?
Is the voltage drop of the diode caused by by it's internal resistance or by its semiconductor structure?

Thank you ,

Buzai Andras

#### kubeek

Joined Sep 20, 2005
5,758
Hello all,

I am trying to correctly learn about diodes and I have some questions related to them.
Does a diode, a non-linear device, have internal resistance?
In case it has it is so small that it can be neglected?
Is the voltage drop of the diode caused by by it's internal resistance or by its semiconductor structure?

Thank you ,

Buzai Andras
Yes.
Yes at low currents.
Semicondoctor structure.
As you increase the current, the slope of voltage increase after that point is the internal resistance.

#### crutschow

Joined Mar 14, 2008
27,755
A semiconductor diode has a logarithmic relationship between voltage and current for any current above it's leakage current to amps. Added to that is a small ohmic resistance. This resistance is smaller for high current diodes and is one of the main differences between small and large diodes.

If you plot a diode voltage versus current with current on a logarithmic scale it will be a straight line except for the deviation cause by the ohmic resistance.

Joined Dec 26, 2010
2,148
The effect of the ohmic resistance can sometimes be seen in curves shown in device data-sheets.

http://www.fairchildsemi.com/ds/1N/1N914.pdf

In this 1N914 data sheet, Figures 3 & 4 on Page 2 show the voltage-current characteristic following a logarithmic slope up to 10mA. In Figure 5 on Page 3, the curve can be seen curving upwards at higher currents. This becomes evident as the current approaches 100mA. You might like to estimate how much ohmic resistance would be required to explain this deviation.

Thank you all for clarifying this 