Question

what is the ideal value of knee voltage for the diode used.

Answer 1

The knee refer to the shape of the curve when the plotting voltage vs current in forward biased semiconductor junction. Knee voltage , or threshold voltage, varies with the device construction, doping level, and operating temperature among other parameter.This is due to the bandgap at the PN junctions of the diode.Therefore, even forward biased, little current will follow until the diodes threshold voltage is exceeded.Threshold voltage for common silicon diodes is approximately 0.7 V, and for Schottky devices near 0.3 V.Germanium based diodes exhibit lower voltage threshold than silicon based diodes, but are less common due to other limitation.All diodes also exhibits a reverse bias breakdown. Engineers normally avoid operation small signal and rectifier diodes in this mode by selection a device with a reverse voltage specification much higher than expect voltages. However; zener diodes are specially construted and doped to produce a reletively low reverse breakdown voltage which is commonly used in shunt regulators and overvoltage protection circuits. various values of zener voltage are available

Answer 2

in short
ideal value of knee voltage IS 0.7 V