### Correct Option: D. Minority carriers ### Detailed Explanation: A reverse-biased junction diode is a semiconductor device that is designed to allow current to flow in one direction (forward bias) while blocking it in the opposite direction (reverse bias). In reverse bias, the p-n junction is connected such that the p-type material is connected to the negative terminal of the power supply and the n-type material is connected to the positive terminal. This setup widens the depletion region and prevents the majority carriers from crossing the junction. #### Step-by-Step Breakdown: 1. **Understanding the Junction Diode**: - A diode consists of two types of semiconductor materials: p-type (which has an abundance of holes, or positive charge carriers) and n-type (which has an abundance of electrons, or negative charge carriers). - In a p-n junction, the p-type and n-type materials are brought together, creating a depletion region where electrons and holes recombine, leaving behind charged ions. 2. **Reverse Bias Condition**: - When a diode is reverse-biased, the external voltage increases the width of the depletion region. This means that the barrier for charge carriers to cross the junction becomes larger. - Majority carriers (holes in p-type and electrons in n-type) are pushed away from the junction, preventing them from contributing to current flow. 3. **Current Flow in Reverse Bias**: - Despite the majority carriers being pushed away, there is still a small amount of current that can flow due to the presence of minority carriers. - Minority carriers are the less abundant charge carriers in a semiconductor. In a p-type material, the minority carriers are electrons, and in an n-type material, the minority carriers are holes. - In reverse bias, the minority carriers can still move across the junction. For example, electrons from the p-type region can drift into the n-type region, and holes from the n-type region can drift into the p-type region. 4. **Resulting Current**: - The current that flows in a reverse-biased diode is primarily due to these minority carriers. This current is typically very small and is known as reverse saturation current. - The reverse saturation current is temperature-dependent and increases with temperature due to increased thermal generation of electron-hole pairs. ### Why Other Options Are Incorrect: - **Option A: Positive holes alone**: - This option is incorrect because, in reverse bias, holes (the majority carriers in p-type material) are pushed away from the junction and do not contribute to current flow. - **Option B: Majority carriers**: - This option is incorrect because majority carriers are not able to cross the junction in reverse bias. They are repelled away from the junction, thus not contributing to the current. - **Option C: Electrons alone**: - This option is misleading. While electrons are indeed involved in the current flow, they are minority carriers in the p-type material. The statement does not account for the fact that holes (the minority carriers in n-type) also contribute to the current, albeit to a lesser extent. ### Summary of Key Points: - In a reverse-biased junction diode, current flows due to minority carriers (electrons in p-type and holes in n-type). - Majority carriers are repelled from the junction and do not contribute to current flow in reverse bias. - The current in reverse bias is typically very small and is known as reverse saturation current. - Understanding the behavior of minority carriers is crucial for analyzing diode operation in reverse bias. This thorough understanding of how current flows in a reverse-biased junction diode is essential for mastering semiconductor physics and preparing for related exam questions.