Question

Direction for Assertion - Reason type questions (A) If both Assertion and Reason are true and reason is the correct explanation of assertion. (B) If both Assertion and Reason are true but Reason is not the correct explanation of assertion. (C) If Assertion is true but Reason is false (D) If both assertion and reason are false A: Photodiode are operated in reverse bias \(\mathrm{R}\) : In reverse bias fractional change in minority charge carrier is more (A) a (B) \(b\) (C) c (D) \(\mathrm{d}\)

Short answer

\( \boxed{A} \) If both Assertion and Reason are true and reason is the correct explanation of assertion.

Step-by-step solution

A photodiode is a semiconductor device that converts light into an electrical current. Photodiodes are operated in reverse bias, which means that the voltage applied across the diode is in the opposite direction to the flow of the majority charge carriers. This voltage helps the photodiode to better detect and respond to incident light. #Step 2: Understanding Minority Charge Carriers and Reverse Bias#

In a reverse-biased junction, the electric field is enhanced, and the flow of minority carriers increases significantly. This behavior leads to an increase in the sensitivity of the photodiode to detect and generate current under the presence of light. #Step 3: Evaluating the Assertion and Reason#

The assertion (A) states that photodiodes are operated in reverse bias, which is true since the photodiode operates with enhanced sensitivity under reverse bias. The reason (R) states that in reverse bias, the fractional change in minority charge carrier is more. This is also true because the electric field is increased, and the flow of minority carriers significantly contributes to generating current in the presence of light. #Step 4: Determining the relationship between the Assertion and Reason#

Since both the Assertion and Reason are true, and the Reason provides a correct explanation for why photodiodes are operated in reverse bias, we can determine that the correct option for this problem is: (A) If both Assertion and Reason are true and reason is the correct explanation of assertion.

Key concepts

Reverse Bias

When a photodiode is reverse biased, it means that the voltage applied across the diode is in the opposite direction to the flow of the usual current. This setup increases the electrical resistance and minimizes the typical current flow through the photodiode.
In the reverse bias condition, the potential barrier of the diode is increased. As a result, the electric field across the junction becomes stronger. This stronger electric field is crucial for the operation of devices like photodiodes.

• Reverse bias is important because it allows the photodiode to respond more effectively to light.
• The reversed voltage prevents the majority of current carriers from moving freely, thereby allowing the diode to be more sensitive to light.
• This sensitivity makes the photodiode useful in applications where detection of light is necessary.

Using reverse bias also reduces the capacitive effect of the diode, which can improve the speed at which the device can operate. All these points make reverse bias a key concept in understanding photodiode functionality.

Minority Charge Carriers

In semiconductor physics, there are two types of charge carriers: majority and minority. Majority carriers are the ones that carry the most charge current in a semiconductor, while minority carriers are present in fewer quantities.
In a reverse-biased photodiode, the minority charge carriers play a pivotal role. Here, electrons in a p-type semiconductor and holes in an n-type semiconductor serve as the minority carriers. An increase in the electric field, due to the reverse biasing, significantly accelerates the minority carriers across the junction.

• This acceleration increases the chances of these carriers contributing to electric current when light photons strike the photodiode.
• The fractional change, or the movement, of these minority carriers under reverse bias is substantially greater compared to the majority carriers.
• Because of this behavior, photodiodes become more effective in detecting light.

Understanding how minority carriers function and their contribution to current is essential for harnessing the full capabilities of photodiodes.

Semiconductors

A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. It's the backbone of electronic devices and forms the basis of modern electronics such as photodiodes.
Semiconductors like silicon or germanium possess properties that allow them to be modified to improve their conductivity. The process is known as doping and involves adding impurities to control the number of charge carriers.

• In semiconductors, charge carriers can be electrons (negative) or holes (positive).
• Doping can create n-type or p-type semiconductors, depending on whether the material is meant to increase electron or hole availability.
• Semiconductors used in photodiodes are specifically designed to respond to light by generating minority charge carriers.

By manipulating semiconductor properties, engineers design photodiodes that are specifically tailored for various light detection applications.

Electric Field

The electric field in a semiconductor is crucial for the functioning of devices like photodiodes, especially when they are in reverse bias. An electric field is a force field that can influence charged particles like electrons and holes, affecting their movement across the semiconductor material.
In a reverse-biased photodiode, the electric field created is responsible for separating charge carriers.

• The reverse-biasing forms a depletion region where the electric field is most intense.
• This field aids in sweeping minority carriers across the junction efficiently.
• Thus, when light photons generate electron-hole pairs, the electric field immediately directs these to the respective collecting terminals of the photodiode.

This behavior is fundamental for photodiodes to quickly and accurately detect changes in light intensity and convert them into electrical signals effectively. Understanding the role of the electric field provides deeper insight into the operation and optimization of photodiodes.