What is Knee Voltage of PN-Junction Diode

A PN junction diode is a non-linear component and it consists of two junctions namely P-junction & N-junction where the majority and minority charge carriers are present like electrons and holes. It is also known as a semiconductor diode or PN junction diode. This diode includes two terminals namely anode and cathode, where a p-type semiconductor is an anode (positive voltage) and N-type semiconductors are cathode (negative voltage). The flow of current in the diode is in one direction only because it opposes in another direction with high resistance. This article gives an overview of what is the knee voltage of a diode and its characteristics.

What is Knee Voltage?

In the forward characteristics of a diode once the voltage is applied then the junction starts increasing rapidly. It is known as knee voltage and an alternate name of this is cut in voltage.

In forward characteristics of the diode, if we notice the graphical representation, conduction begins to increase rapidly is looks like or leg but technically it is known as Cut in voltage which is discussed below.

PN Junction Diode Characteristics

The PN junction diode VI characteristics are simply a curve between the flow of current in the diode and the applied voltage across the two terminals of the diode. The characteristics of diode separated into two divisions like forwarding characteristics and reverse characteristics.


Forward Characteristic

The diode arrangement in forwarding bias is shown below. By using this circuit forward bias characteristics can be obtained. The forward bias connection can be done by connecting p junction to the positive terminal and N junction to a negative terminal of the battery. In this arrangement, the majority charge carriers are holes & minority charge carriers are electrons.

When the PN junction diode is connected in forwarding bias using a battery, where the P junction is connected to a +ve terminal of the battery & the N junction is connected to the –ve terminal of the battery. This arrangement is called forward biasing of a diode. In this kind of arrangement, the diode works as a short circuit due to its less forward resistance within the range of ohms. It means the flow of current is very easy in this bias.


In the above characteristics of a diode, when the voltage across the diode increases then the current will be increased. If we notice in the graph, the diode current is extremely small up to a set point. The voltage is cross toward the barrier potential, the diode current raises quickly and diode performs greatly. This barrier voltage at which the flow of current will increase is known as knee voltage. The knee voltage value for ‘Si’ diode is 0.7 volts & for ‘Ge’ diode it will be 0.3

Knee Voltage Formula

The knee point voltage of ct can be calculated using the following formula.

Vkp = K * If/CTR x (RCT + RL + RR)


K = Constant, usually taken as 2.0

Vkp = The least Knee Point Voltage

If = Utmost fault current at the position in Amperes

CTR = CT Ratio

RCT = Secondary winding resistance of CT in Ohms

RL = Two-way lead resistance in Ohms

RR = Relay Burden in Ohms

Knee Voltage of Zener Diode

In the forward bias of Zener diode, once the voltage at the anode terminal is superior to the knee voltage (threshold voltage) on the cathode, then it conducts current. The current flows from the anode to the cathode. Even though, this diode is considered as a short circuit when it is in forwarding bias & an open circuit when it is in reverse bias. Actuality, when a diode is forward bias, then it performs as much current when the exterior circuitry commands, & changes its interior resistance so the voltage drop across it is constantly 0.7 V.

Difference between Knee Voltage and Breakdown Voltage

The main difference between knee-voltage and breakdown voltage includes the following.

The forward voltage at which the flow of current during the PN Junction begins increasing quickly is known as knee voltage. This voltage is also known as cut-in voltage. This voltage is the least reverse voltage at which the PN Junction can behave without harm to the current.

It is the position within the forward bias of the curve wherever conduction begins to raise quickly of a PN-Junction Diode.

The diode’s breakdown voltage can be defined as the least reverse voltage which is used to make the diode perform in reverse. Breakdown voltage is a factor of a diode that describes the highest reverse voltage. This voltage can be used without affecting an exponential rise in the current of the diode.

Knee Voltage of Silicon and Germanium

The knee voltage value for silicon & germanium includes the following.

Silicon (Si) diode is 0.7 V

Germanium (Ge) diode is 0.3 V

Knee Voltage of LED

Once the light-emitting diode is connected to an external voltage in the forward bias, the potential barrier height across the PN junction will be decreased. This exact voltage is known as the knee voltage of the LED. When this voltage is achieved, then the flow of current may raise however the voltage does not vary.

Thus, this is all about knee voltage and the difference between breakdown voltage. We hope that you have got a better understanding of this concept. Furthermore any queries regarding any technical information please give your feedback by commenting in the comment section below. Here is a question for you, how to find knee voltage from the graph?