Diode Core Knowledge Explained
A diode is a basic electronic device made from semiconductor materials (such as silicon, germanium, or selenium), consisting of a PN junction plus electrode leads and package housing. It has two electrodes: the anode (positive terminal) and the cathode (negative terminal).
Basic Working Principle
The core characteristic of a diode is unidirectional conductivity:
| Voltage Type | Diode State | Current Condition |
|---|
| Forward Voltage | Conducting | Current flows from anode to cathode |
| Reverse Voltage | Cut-off | Only minimal leakage current |
When the forward voltage exceeds the threshold voltage (approximately 0.7V for silicon diodes, 0.3V for germanium diodes), the diode conducts. When the reverse voltage exceeds the breakdown voltage, the diode may be damaged.
Comprehensive Diode Classification
Classification by Structure and Application
| Type | Characteristics | Main Applications |
|---|
| Point Contact Diode | Small PN junction area, good high-frequency performance | High-frequency detection circuits, switching circuits |
| Junction Diode | Large PN junction area, high current capacity | Power rectification circuits |
| Planar Diode | Controllable PN junction area | Digital circuits (small area), power rectification (large area) |
| Zener Diode | Operates in the reverse breakdown region | Voltage regulation, reference voltage sources |
| Light Emitting Diode (LED) | Converts electrical energy to light | Indicator lights, displays, and lighting |
| Photodiode | Converts light signals to electrical signals | Light detection, photoelectric control, optical communication |
Special Function Diodes
- Varactor Diodes: Capacitance varies with reverse voltage, used in tuning circuits
- Schottky Diodes: Low forward voltage drop, used in high-speed switching applications
- Tunnel Diodes: Negative resistance characteristics, used in microwave oscillation circuits
Diode Core Characteristics and Parameters
Current-Voltage Characteristic Curve
The electrical characteristics of a diode can be fully described by its I-V characteristic curve:
Forward Characteristics:
Zero voltage → Dead zone (almost no current) → Threshold voltage → Conduction region (sharp current increase)
Reverse Characteristics:
Small reverse voltage → Saturation region (minimal reverse current) → Breakdown voltage → Breakdown region (sharp current increase)
Key Performance Parameters Table
| Parameter | Description | Influencing Factors |
|---|
| Maximum Rectifier Current | Maximum average current allowed long-term | PN junction area, heat dissipation conditions |
| Reverse Breakdown Voltage | Minimum reverse voltage causing breakdown | Material doping concentration, structural design |
| Reverse Saturation Current | Minimal leakage current under reverse bias | Temperature, material purity |
| Junction Capacitance | Capacitance effect formed by the PN junction | Operating frequency, junction area |
| Reverse Recovery Time | Time required to switch from conduction to complete cutoff | Switching speed, material characteristics |
Diode Testing and Identification Methods
Polarity Identification Techniques
- Appearance Mark Identification
- End with a triangle arrow indicates the positive terminal
- Color dot/ring marks: Usually, the end with a color dot is positive, the end with a color ring is negative
- Length differentiation: Longer lead is usually positive
- Multimeter Testing Method
- Measurement showing smaller resistance: Black probe connects to positive terminal
- Diode test mode on digital multimeter: Red probe connects to the positive terminal when the voltage drop is displayed
Performance Testing Essentials
- Normal Diode: Small forward resistance, large reverse resistance
- Damage Judgment: Both directions show small resistance (short circuit) or both show large resistance (open circuit)
- Zener Diode Testing: Requires a special circuit to test the regulated voltage
Detailed Practical Diode Applications
1. Rectifier Circuit Applications
Convert alternating current to direct current, serving as core components in power adapters, chargers, and other devices.
2. Voltage Regulation and Protection
Utilizes the reverse breakdown characteristics of Zener diodes to provide stable voltage references and overvoltage protection for circuits.
3. Signal Processing Functions
- Detection Circuits: Extract original information from modulated signals
- Limiting Circuits: Restrict signal amplitude to prevent overload
- Clamping Circuits: Fix signal level positions
4. Switching and Digital Circuits
Function as electronic switches to implement logic functions with fast response speed and long lifespan.
5. Photoelectric Application Fields
- LED Lighting: Energy-efficient, long-life, environmentally friendly light sources
- Photoelectric Detection: Convert light signals to electrical signals
- Optical Isolation: Achieve electrical isolation between circuits
Selection and Usage Considerations
Selection Considerations
- Current Capacity: Choose an appropriate maximum rectifier current based on circuit requirements
- Voltage Rating: Reverse working voltage should be higher than the maximum possible reverse voltage in the circuit
- Frequency Characteristics: Select types with small junction capacitance for high-frequency circuits
- Temperature Range: Consider the impact of the operating environment temperature on performance
Usage Precautions
- Polarity must not be reversed, as this may cause circuit failure or device damage
- Power diodes require attention to heat dissipation issues
- Control temperature and time during soldering to prevent thermal damage
- Electrostatic-sensitive models require anti-static measures