Question

What is the difference between knee voltage and breakdown voltage ?

Answer 1

PN junction is a diode formed by p-type and n-type semiconductors.P-type are doped with group 13 elements and has excess of holes whereas n-type are doped with group 15 elements and has excess of electrons. When voltage is applied through it current flows through PN junction.

Complete answer:
PN junction when biased with $p$-type connected to positive terminal and $n$-type to negative terminal is Forward bias. When $p$-type to negative and $n$-type to positive terminal then the PN junction is reversed biased. .After biasing the electrons from n type diffuse through junction toward $p$ and holes from p diffuse to $n$ region.

Due to this neutrality of $n$ and $p$ type is distributed with formation of a layer of negative ions near p region and positive ions near n region, these layers are called depletion layers formed in PN junction diodes.

Knee Voltage	Breakdown Voltage
It is the voltage at which the currents starts flowing or increases in forward biased PN junction.	It is the reverse voltage at which the breakdown of junction takes place and reverse current rapidly increases.
Width of the depletion layer decreases.	Width of the depletion layer increases.
For Ge $0.3\,V$ and for Si $0.7\,V$.	For Ge $25\,V$ and for Si $35\,V$

Note: A diode can conduct current in one direction and it is a two terminal device. Diodes are used to transform AC current to DC current and also it protects the circuit by regulating the voltage. Zener diode is a special type of PN junction diode which is made to work in reverse biased condition, it is also used as voltage regulator.