I want to determine the threshold voltage of a diode from the graph of iv characteristic diode. From the point of view of the data analysis I do not know how to do to find a specific value. I have read the advice of graphing the logarithm of the relationship between the current passing through the diode and the reverse saturation current. I get a straight line from which I do not know what parameter could be useful to me. I'm stuck and I do not know what to do.
The threshold voltage is the minimum voltage needed to create a conducting path along the diode. As a real diode is never completely off, one can define the threshold voltage at the point where the diode starts conducting in the exponential fashion. So there are different ways to define the actual threshold. (It is also heavily dependent on temperature.)
Here is one way to get a value for the threshold voltage:
If you increase the voltage applied to the diode and measure the current flowing along the diode, you can use this data to determine the threshold voltage by plotting the sqrt of the current. I did this some time ago from simulation data for a nmos device, so actually this curve represents the diode characteristic, in which you are interested in.
In the Appendix of the book CMOS Analog Circuit Design from P. E. Allen and D. R. Holberg you find this procedure in more detail.
There is no real "threshold" because the diode starts conducting already for voltages far below the well-known magic value of 0.7 volts (as you can see in the first diagram). According to the most common specification, you could draw a tangent to the linear part of the characteristic. This line (tangent) will cross the horizontal axis at a value (app. 0.7 volts) which very often is called "threshold".