# Exact (quantitative) definition of "knee" and/or "cut-in" voltage?

Sort of a 2-part question:

1) I know they are used interchangeably but are the "knee" and "cut-in" voltages of a diode actually the exact same thing? I had thought that the "cut-in" voltage was where the diode first started to curve and the "knee" voltage was a little further along where the diode begins to turn linear (ie: .5V & .7V respectively in the following curve)

2) Is there a quantitative definition for either? (ie: equal to built-in voltage, voltage @ ____mA, resistance < ____ohms, x-int of linear region of forward voltage, etc)?

Plotting the IV curve of a 1n4001 diode myself I get the x-int of the linear forward voltage region (.702 V) to be right on the commonly accepted value of .7V for si diodes but this doesn't seem like the best way to define it as it would depend on the bulk resistance of the diode...

• Duplicate of physics.stackexchange.com/q/177910 - not easy to find necessarily on search. Good first question, just to bad its here already... Commented Nov 23, 2015 at 15:06
• I disagree. The duplicate (nice answer by the way) explains the mathematics behind the curve; it does not give a definition of the difference between "knee" and "cut-in" voltage. I vote to keep this open. Commented Nov 23, 2015 at 19:50
• @NickAlexeev and others: Would Electrical Engineering be a better home for this question? Commented Nov 23, 2015 at 20:29
• Thanks for the link Jon, definitely lots of good info. While it doesn't address the specific terminology my question is about, I am leaning to believe that "turn-on", "knee", "forward", and "on" voltages are all synonymous and defined as: somewhere around the sharpest part of the curve. Commented Nov 25, 2015 at 7:07
• I would encourage leaving this Q up as it is likely a common question anyone first encountering this terminology is likely to ask. I was just hoping to find a more specific answer as I would like to characterize the turn-on voltage for some self-fabricated (far from ideal) diodes as part of a fab-lab course. Commented Nov 25, 2015 at 7:19