The forward voltage Vf of a diode

I want to know about the forward voltage $$\V_{f}\$$ parameter of a diode device, given an I-V charateristic curve and in ambient temperature. What is the criteria to define the $$\V_{f}\$$ for any diode? How can I define it if I work with this current expression $$\i_D = I_s (\exp^{v_D/V_{th}}-1)\$$? Sometimes I see the value of this parameter like $$\V_{f}=0.7\ \mbox{V}\$$ for some type of diode, but how can I calculate given any $$\i_D\$$ expression?

• Welcome to EE.SE. By Von I assume you mean the forward voltage drop. By criteria I assume you mean ambient temperature and forward current. Give us a few clues please. – Sparky256 Sep 23 '18 at 0:54
• Hi @Sparky256 , thanks for your time, and sorry for my english. I edited the post. I mean about, given a I-V curve of the diode, how can I define the $V_{on}$? What is the criteria to build this parameter? – Alan Mejia Sep 23 '18 at 2:34
• Read this answer: electronics.stackexchange.com/a/120612/27943 – nidhin Oct 19 '18 at 18:09