Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
Yes. Knee voltage, threshold voltage and cut-in voltage are three names for the same value. It is the forward voltage at which the V-I curve bends sharply (like a knee) and the current starts to increase quickly. NCERT calls it the threshold or cut-in voltage.
The knee voltage depends on the material's built-in potential barrier, which depends on the band gap. Silicon has a larger band gap (about 1.1 eV) than germanium (about 0.7 eV), so silicon needs a larger forward voltage (about 0.7 V) to start conducting, while germanium needs only about 0.2 V.
A very tiny current flows even below the knee, but it is so small that we treat it as almost zero. Real conduction begins only after the applied forward voltage crosses the knee voltage. Then the current rises almost exponentially for a small increase in voltage.
They are closely related but not identical in definition. The potential barrier is the built-in voltage across the depletion region of an unbiased junction (about 0.3 V for Ge, 0.7 V for Si). The knee voltage is the external forward voltage you must apply to overcome that barrier and start strong conduction. In numerical NEET problems, both are used with values around 0.7 V (Si) and 0.2-0.3 V (Ge).
Below the knee, the forward voltage is too small to push majority carriers across the depletion region, so current stays low and the curve looks flat. Once the voltage overcomes the barrier, carriers flood across and current rises almost exponentially, giving the sharp bend called the knee.
When temperature rises, the barrier potential decreases, so the knee voltage becomes slightly smaller. For silicon it drops by roughly 2 mV for every 1 degree Celsius rise. This is why a hot diode conducts at a slightly lower forward voltage.
Two statements are given below: A. When the forward bias voltage across a p-n junction diode increases above a certain threshold voltage, the diode current increases significantly. B. This current is called reverse saturation current. Choose the correct answer.
Consider the following statements and identify the correct answer. A. A zener diode is connected in reverse bias when used as a voltage regulator. B. The potential barrier of a p-n junction lies between 0.1 V to 0.3 V.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the forward voltage at which the diode starts conducting strongly, where the V-I curve bends sharply like a knee (0.2 V for Ge, 0.7 V for Si).
Silicon has a higher knee voltage but a much smaller reverse (leakage) current and works well at higher temperatures, making silicon diodes more reliable, so silicon is preferred in most modern devices.
It is a forward-bias quantity. It is the forward voltage needed to overcome the junction barrier and start conduction.
Threshold voltage and cut-in voltage are the common alternative names; NCERT uses 'threshold or cut-in voltage'.
Assume the diode conducts only when the forward voltage is at least the knee voltage. When conducting, take a constant drop of 0.7 V (Si) or 0.2-0.3 V (Ge) across the diode and subtract it before applying Ohm's law to the rest of the circuit.
