Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Semiconductor diodes
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Semiconductor diodes
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Subject: Basic Electronics
Class: Senior Secondary 1
Term: 3rd Term
Week: 9
Theme: Semi Conductor Devices
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Explain the concept of diodes Differentiate between the types of diodes. State the operational principles of Semiconductor diodes State the rating of diode State the applications of the different types of diodes. Cconstruct simple circuits using semiconductor diodes.
A diode is a two-terminal electronic component that permits electric current to flow primarily in one direction while largely blocking current flow in the opposite direction. It functions much like a one-way valve in a water pipe, allowing water to flow through only when pressure is applied from one specific direction.
Diode Symbol: The standard schematic symbol for a general-purpose diode consists of a triangle (pointing in the direction of conventional current flow, from positive to negative) with a bar across its apex. The triangular part represents the Anode (A), which is the positive terminal. The bar part represents the Cathode (K), which is the negative terminal. (Teacher should draw this symbol clearly on the board, labeling Anode and Cathode)*. Most modern diodes are made from semiconductor materials, primarily Silicon (Si) or Germanium (Ge). The unique properties of diodes stem from the formation of a P-N junction.
Semiconductors: These are materials (like Silicon, widely available for electronics manufacturing) whose electrical conductivity lies between that of conductors (e.g., copper) and insulators (e.g., rubber). Their conductivity can be significantly altered by adding impurities.
Doping: This is the process of intentionally adding specific impurities to a pure semiconductor material to change its electrical properties.
P-type Semiconductor: Created by doping a pure semiconductor (e.g., Silicon) with trivalent impurity atoms (e.g., Boron). These impurities have fewer valence electrons than Silicon, leading to "holes" (vacancies where an electron should be). These holes act as positive charge carriers.
N-type Semiconductor: Created by doping a pure semiconductor with pentavalent impurity atoms (e.g., Phosphorus). These impurities have more valence electrons than Silicon, leading to "free electrons" which act as negative charge carriers.
P-N Junction Formation: When a P-type semiconductor material is physically joined with an N-type semiconductor material, a P-N junction is formed.
Depletion Region: At the junction, free electrons from the N-side diffuse across to the P-side and combine with holes. This movement creates a region around the junction that is "depleted" of mobile charge carriers (both electrons and holes). The N-side of this region becomes positively charged (due to immobile donor ions), and the P-side becomes negatively charged (due to immobile acceptor ions).
Potential Barrier: The accumulated positive and negative charges across the depletion region create an internal electric field, which acts as a "potential barrier" or "barrier voltage." This barrier opposes further diffusion of charge carriers. For Silicon diodes, this barrier voltage is typically around 0.7 Volts (V), and for Germanium diodes, it is about 0.3 V at room temperature. The behaviour of a semiconductor diode depends on how it is connected to an external voltage source (biasing). 2.3.
1. Forward Bias: Connection: The external voltage source's positive terminal is connected to the Anode (P-side) and its negative terminal to the Cathode (N-side).
Effect: The external voltage opposes the diode's internal potential barrier. When the applied forward voltage exceeds the barrier voltage (e.g., 0.7V for Silicon), the depletion region narrows significantly. This allows majority charge carriers (holes from P-side, electrons from N-side) to easily cross the junction. A large electric current then flows through the diode.
I-V Characteristic: In a forward-biased diode, current remains very small until the applied voltage reaches the barrier voltage (knee voltage or cut-in voltage). Beyond this point, even a small increase in voltage causes a sharp, exponential increase in current. 2.3.
2. Reverse Bias: Connection: The external voltage source's negative terminal is connected to the Anode (P-side) and its positive terminal to the Cathode (N-side).
Effect: The external voltage adds to the diode's internal potential barrier. This causes the depletion region to widen, effectively blocking the flow of majority charge carriers. Only a very small current, called reverse leakage current (due to thermally generated minority carriers), flows. This current is usually in the microampere (μA) or nanoampere (nA) range and is often negligible for practical purposes.
Reverse Breakdown: If the reverse bias voltage is increased to a sufficiently high level (known as the Reverse Breakdown Voltage or Peak Inverse Voltage - PIV), the diode's P-N junction will break down. This leads to a sudden and very large reverse current, potentially damaging or destroying the diode. This breakdown phenomenon is utilized intentionally in Zener diodes for voltage regulation. Different types of diodes are manufactured with specific characteristics for various applications. 2.4.
1. Rectifier Diodes (General Purpose Diodes): Function: Primarily used for rectification, which is the process of converting alternating current (AC) into pulsating direct current (DC).
Characteristics: Designed to handle significant forward current and withstand high reverse voltages (PIV). They are not intended to operate in the reverse breakdown region.
Examples: The 1N400x series (e.g., 1N4001, 1N4007) are common general-purpose rectifier diodes.
Nigerian Context: Found in almost every electronic device's power supply (e.g., phone chargers, radio power adapters, television power boards) to convert the AC power from the grid (NEPA) into the DC power required by the device. 2.4.
2. Zener Diodes: Function: Designed to operate reliably in the reverse breakdown region. Their primary use is for voltage regulation (maintaining a constant voltage across a load) and providing a stable voltage reference.
Characteristics: When reverse-biased to its specific Zener voltage, it maintains a nearly constant voltage across its terminals, even if the current through it changes within limits.
Symbol: Similar to a standard diode symbol, but with 'Z' shaped bends at the cathode bar.
Nigerian Context: Used in voltage stabilizers common in homes and businesses to protect sensitive electronics (e.g., computers, refrigerators, medical equipment in hospitals) from power fluctuations and surges that are common in many areas. 2.4.
3. Light Emitting Diodes (LEDs): Function: Emit light when forward biased. They convert electrical energy directly into light energy (electroluminescence).
Characteristics: Low power consumption, long lifespan, available in various colours (red, green, blue, white, etc.). Requires a current-limiting resistor in series to prevent damage. Forward voltage drop is typically higher than Silicon diodes (1.5V to 3.5V depending on colour).
Symbol: Standard diode symbol with two arrows pointing outwards (representing light emission).
Nigerian Context: Ubiquitous as indicator lights on electronic gadgets (TVs, radios, power banks), in modern streetlights and traffic lights (e.g., in major cities like Lagos, Abuja, Port Harcourt), torchlights, decorative lighting during festive periods, and displays (e.g., seven-segment displays on digital clocks). 2.4.
4. Photodiodes: Function: Convert light energy into electrical current. They are designed to operate in reverse bias or zero bias. When light strikes the P-N junction, it generates electron-hole pairs, increasing the reverse current.
Characteristics: Their resistance decreases with increasing light intensity.
Symbol: Standard diode symbol with two arrows pointing inwards (representing light reception).
Nigerian Context: Used in light sensors for automatic streetlights (which switch on at dusk and off at dawn), remote controls (e.g., for TVs and decoders), optical communication systems, and as the fundamental light-sensing component in solar cells (though solar cells are optimized for power generation). 2.4.
5. Schottky Diodes: Function: Known for very fast switching speeds and a low forward voltage drop (typically 0.15V to 0.45V). They are used in high-frequency applications.
Characteristics: Formed from a metal-semiconductor junction (instead of P-N). They have virtually no reverse recovery time, making them faster than P-N junction diodes.
Nigerian Context: Found in high-frequency power supplies (e.g., switched-mode power supplies in computers, modern inverters, solar charge controllers) where efficiency and rapid switching are critical.
Connecting the topic to real-life situations helps students appreciate its relevance. Mobile Phone Chargers and Power Supplies (Rectification): Almost all electronic devices, including mobile phones, laptops, and radios, operate on Direct Current (DC).
However, the electricity supplied by the national grid (NEPA) or generators is Alternating Current (AC). Diodes (specifically rectifier diodes) are the core components in the power adapters that convert this AC supply into the usable DC required by these devices. Without diodes, these essential gadgets would not function from our wall sockets.
Integration:* Discuss the importance of stable power for electronic devices in a country with occasional power fluctuations. LED Lighting and Displays (Light Emitting Diodes): LEDs have revolutionized lighting technology in Nigeria, evident in modern streetlights being installed across cities like Lagos and Abuja, traffic lights, home lighting bulbs (replacing incandescent and fluorescent lamps), and various indicator lights on appliances (e.g., your TV's power light, car dashboard indicators). They are energy-efficient, long-lasting, and durable, making them ideal for a range of applications from essential infrastructure to domestic use.
Integration:* Talk about energy saving benefits and environmental impact of LEDs compared to older lighting technologies. Voltage Stabilization and Protection (Zener Diodes): In many parts of Nigeria, power supply can be unstable, experiencing surges and drops. Sensitive electronic equipment (e.g., computers, medical diagnostic machines in hospitals, sophisticated industrial controls) can be damaged by such fluctuations. Zener diodes are used in voltage stabilizers and power supply units to ensure a constant, regulated voltage is supplied to these devices, protecting them from damage due to input voltage variations.
Integration:* Discuss the economic implications of equipment damage due to unstable power and how electronics help mitigate this.