What is Knee Voltage of PN- Junction Diode?

The semiconductor diode is formed by bringing the n-type and p-type materials together. When the p- and n- type materials are joined, the electrons and holes in the region combines, and a region is formed  in which carriers are absent. This region of uncovered positive and negative ions is called the depletion region. The depletion region of the PN junction diode when diode is not biased is as given below.

The width of the depletion layer reduces when diode is forward biased, and conversely the width of the depletion layer increase when the diode is reverse biased. The electric field is formed in the depletion region. For conduction of the diode, an external electric field needs to be applied across the diode to overcome the electric field formed in the depletion region.

What is Barrier Potential or Built-in potential?

The potential difference required to move the electrons through the electric field is called the barrier potential. The formula of the barrier potential or built-in voltage is as given below.

From above, it is clear that barrier potential depends on amount of doping, temperature, carrier concentration of undoped semiconductor and type of semiconductor material. 

What is knee voltage?

When the p- and n- junction is joind the barrier potential or built-in potential is formed. Now, if the diode is forward biased ( when positive terminal of the battery is connected to anode and negative terminal is connected to the cathode) the diode will not conduct until unless the battery voltage is more than the barrier potential. The barrier potential of the germanium and silicon diode is 0.3 and 0.7 respectively. For silicon diode, if battery volatge is more than 0.7 the diode strats conducting in forward biased. 

The minimum voltage at which the diode starts conducting heavily and current starts increasing rapidly in forwward biased state, the applied voltage is called knee point voltage or diode cut-in voltage. The concept of knee point voltage of the diode can be further understood with the help of forward characteristics  of the diode.

Diode is forward biased by applying postive and negative voltage  at anode and cathode of the diode respectively. The applied voltage is increased gradually from zero volatge. The silicon diode does not conduct till voltage reach to 0.7 volt. The germanium diode does not conduct till voltage reaches to 0.3 volt. The 0.7 volt and 0.3 volt is the cut-in or knee voltage of the silicon and germanium diode respective. When the applied voltage reaches to cut-in or knee voltage the diode starts heavily conducting.

Now, we will further understand knee point of the diode with help of numerical problem.

Calculate circuit current.

Knee point voltage of diode(Vk) = 0.7 V

Applied Voltage(V)                    = 5.0 V

Current limiting resistor(R)        = 50 Ohms

Circuit rurrent (I_D) = (V - Vk)/R

                            =  (5 - 0.7)/50

                            =  4.3/50

                         I_D = 0.086 A =86 mA