Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Diodes
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Diodes
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Subject: Radio Television And Repairs
Class: Senior Secondary 2
Term: 1st Term
Week: 1
Theme: Basic Electricity
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Identify typesof diodes and draw the irgraphicsymbols. Use multimeterto determinethe anode and cathode of adevice and alsoits condition.
This section provides a detailed explanation of diodes, their operation, types, and practical testing procedures using a multimeter. 2.
1. Introduction to Diodes A diode is a two-terminal electronic component that primarily conducts current in one direction (unidirectional current flow). It acts like a one-way valve for electricity. The two terminals are called the anode (positive terminal) and the cathode (negative terminal). Most diodes are made from semiconductor materials like silicon (Si) or germanium (Ge). 2.
2. P-N Junction Formation The core of a diode is a P-N junction, formed by joining two types of semiconductor materials: P-type material: Doped with "acceptor" impurities (e.g., boron) creating an abundance of "holes" (absence of electrons) which act as positive charge carriers.
N-type material: Doped with "donor" impurities (e.g., phosphorus) creating an abundance of free electrons which act as negative charge carriers. When P-type and N-type materials are joined, electrons from the N-side diffuse into the P-side and holes from the P-side diffuse into the N-side. This recombination creates a region near the junction devoid of free charge carriers, called the depletion region or depletion layer. This region acts as an insulator, preventing current flow unless an external voltage is applied. 2.
3. Diode Biasing Biasing refers to applying an external voltage across the diode to control its operation. 2.3.
1. Forward Bias: Condition: The positive terminal of the voltage source is connected to the anode (P-side), and the negative terminal is connected to the cathode (N-side).
Effect: The applied voltage pushes holes towards the junction from the P-side and electrons towards the junction from the N-side. This reduces the width of the depletion region, eventually eliminating it.
Current Flow: Once the external voltage overcomes the inherent potential barrier of the depletion region (approximately 0.7V for silicon diodes and 0.3V for germanium diodes), the diode conducts current freely. This voltage is known as the forward voltage drop (Vf).
Analogy: A gate that opens when pushed hard enough in one direction. 2.3.
2. Reverse Bias: Condition: The negative terminal of the voltage source is connected to the anode (P-side), and the positive terminal is connected to the cathode (N-side).
Effect: The applied voltage pulls holes away from the junction on the P-side and electrons away from the junction on the N-side. This increases the width of the depletion region.
Current Flow: The diode blocks current flow. Only a very small leakage current (reverse saturation current) flows due to minority carriers, which is usually negligible.
Reverse Breakdown: If the reverse voltage is increased sufficiently, a point called the reverse breakdown voltage (Vbr) is reached. At this point, the diode's depletion region breaks down, and a large reverse current flows, potentially destroying the diode if current is not limited. (Zener diodes are designed to operate safely in this breakdown region).
Analogy: A gate that is pushed in the wrong direction, making it seal tighter. 2.
4. Types of Diodes and Their Graphic Symbols | Diode Type | Description Basic Electricity Diodes Term: 1st Term Week: 4 ---