What is the primary cause of junction capacitance in a p-n diode?

Explanation:

Junction Capacitance in a p-n Diode

Definition: Junction capacitance, also known as depletion capacitance, is a property of a p-n junction that arises due to the separation of charges in the depletion region. It is a significant parameter in understanding the electrical behavior of diodes, especially under varying voltage conditions.

Correct Option: The correct answer is:

Option 3: Change in depletion region width with applied voltage.

Explanation:

In a p-n diode, the depletion region is formed at the junction of the p-type and n-type materials. This region is devoid of free charge carriers (electrons and holes) because they combine near the junction, leaving behind immobile ions. This creates an electric field across the depletion region. The capacitance associated with this region is termed as junction capacitance or depletion capacitance.

The junction capacitance is defined as:

C = ε × A / W

Where:

• C: Junction capacitance
• ε: Permittivity of the semiconductor material
• A: Cross-sectional area of the junction
• W: Width of the depletion region

The width of the depletion region (W) is dependent on the applied voltage across the diode. When a reverse bias voltage is applied, the depletion region widens, increasing W and thereby decreasing the capacitance (C). Conversely, when a forward bias voltage is applied, the depletion region narrows, reducing W and increasing the capacitance (C).

Thus, the primary cause of junction capacitance in a p-n diode is the change in the width of the depletion region (W) with the applied voltage. This behavior is crucial in applications like varactor diodes, where the junction capacitance is intentionally varied by changing the applied voltage.

Important Information:

Let us analyze the other options to understand why they are incorrect:

Option 1: Variation in minority carrier concentration with applied voltage

This option is incorrect because the minority carrier concentration does not directly contribute to the junction capacitance. Junction capacitance is primarily related to the immobile charges in the depletion region and the change in its width, not to the behavior of minority carriers.

Option 2: Thermal generation of carriers in the depletion region

This option is incorrect because thermal generation of carriers primarily affects the leakage current in a diode under reverse bias. While it influences the diode's overall behavior, it is not the cause of junction capacitance.

Option 4: Variation in majority carrier concentration with applied voltage

This option is also incorrect because majority carriers are not directly involved in the formation of the depletion region or its associated capacitance. The majority carriers are swept away from the junction, creating the depletion region, but their concentration does not vary significantly with the applied voltage in a way that influences capacitance.

Conclusion:

The correct answer is Option 3 because the junction capacitance arises from the change in the width of the depletion region with the applied voltage. This is a fundamental property of the p-n junction and plays a significant role in the behavior of diodes, especially in high-frequency and voltage-variable applications. Understanding this concept is essential for analyzing and designing circuits involving diodes, such as rectifiers, varactors, and RF circuits.