What is knee voltage of transistor?

The knee refers to the shape of the curve when plotting voltage vs current in forward biased semiconductor junctions.

Knee voltage, or threshold voltage, varies with device construction, doping level, and operating temperature among other parameters. This is due to the bandgap at the PN junctions of the diode. Therefore, even when forward biased, little current will flow until the diodes threshold voltage is exceeded.

Threshold voltages for common silicon diodes is approximately 0.7V, and for Schottky devices near 0.3V. Germanium based diodes exhibit lower voltage thresholds than silicon based diodes, but are less common due to other limitations.

All diodes also exhibit a reverse bias breakdown. Engineers normally avoid operation small signal and rectifier diodes in this mode by selecting a device with a reverse voltage specification much higher than expected voltages. However; zener diodes are specially constructed and doped to produce a relatively low reverse breakdown voltage which is commonly used in shunt regulators and overvoltage protection circuits. Various values of zener voltage are available.