Diodes are the simplest components in the semiconductor family. Their most notable characteristic is unidirectional conductivity, allowing current to flow in only one direction—typically from the anode to the cathode. This article covers the classification, naming conventions, and key characteristics of common diodes, serving as a foundational lesson for analog circuits.
### 1. Basics of Diodes
**1.1 Classification of Diodes**
There are various types of diodes, categorized based on materials, structure, and applications. Based on the semiconductor material used, they can be divided into germanium (Ge) and silicon (Si) diodes. Structurally, they can be point-contact, surface-contact, or planar diodes. Functionally, they include detector diodes, rectifier diodes, Zener diodes, switching diodes, Schottky diodes, LEDs, and more.
**1.2 Diode Naming Conventions**
In China, the semiconductor device model follows a five-part system: main name, material and polarity, category, serial number, and specification. In Japan, the naming convention includes numbers for device type, registration symbol, material/polarity, registration number, and product improvement indicators.
**1.3 Common Diode Characteristics**
- **Rectifier Diodes**: Used to convert AC to DC. They have high junction capacitance, limiting their frequency performance. High-current models are often housed in metal cases for heat dissipation.
- **Switching Diodes**: Designed for fast switching in digital circuits, with short reverse recovery times suitable for high-frequency applications.
- **Zener Diodes**: Used for voltage regulation, operating in reverse breakdown without damage. They maintain a stable voltage across their terminals.
- **Varactor Diodes**: Act as variable capacitors by changing the junction capacitance with applied voltage, commonly used in microwave circuits.
- **TVS Diodes**: Protect circuits from transient overvoltage by clamping the voltage during surges, preventing damage from ESD or lightning strikes.
### 2. Selecting Diode Parameters
When selecting a diode, several parameters must be considered:
- **Rated Forward Current**: The maximum continuous forward current the diode can handle.
- **Maximum Inrush Current**: A short-term peak current, typically 20 times the rated current.
- **Maximum Reverse Voltage**: The highest reverse voltage the diode can withstand before breakdown.
- **Reverse Current**: The leakage current under reverse bias; smaller values indicate better performance.
- **Reverse Recovery Time**: The time it takes for the diode to stop conducting when switching from forward to reverse bias.
- **Maximum Power**: The power rating based on voltage and current.
- **Frequency Characteristics**: Higher frequencies may cause the diode to lose its unidirectional behavior due to junction capacitance.
### 3. Choosing Different Types of Diodes
- **Detector Diodes**: Typically point-contact germanium diodes, chosen for high frequency and low reverse current.
- **Rectifier Diodes**: Usually planar silicon diodes, used in power supplies. Consider factors like current, voltage, and recovery time.
- **Zener Diodes**: Used as voltage references or protection devices. Ensure the stable voltage matches the circuit's requirements.
- **Switching Diodes**: Available in various series, suitable for different switching speeds and applications.
- **Varactor Diodes**: Selected based on operating frequency, voltage, and junction capacitance.
### 4. TVS Diode Selection
Key considerations for TVS diodes include:
- **Minimum Breakdown Voltage (VBR)** and **Breakdown Current (IR)**: VBR is the minimum voltage at which the TVS begins to conduct.
- **Maximum Reverse Leakage Current (ID)** and **Rated Reverse Cut-off Voltage (VWM)**: VWM should be slightly higher than the normal operating voltage.
- **Maximum Clamp Voltage (VC)** and **Peak Pulse Current (IPP)**: VC is the voltage the TVS clamps during a surge.
- **PPM Rated Pulse Power**: Reflects the TVS’s ability to absorb energy from surges.
- **Capacitance (C)**: Important for data interface circuits where signal integrity matters.
### 5. Difference Between Schottky and Ordinary Diodes
Schottky diodes have lower forward voltage drops (around 0.3–0.5V) and faster reverse recovery times (10–40ns), making them ideal for high-frequency applications. However, they have lower reverse voltage ratings (usually <150V). Ordinary silicon diodes have higher voltage ratings but slower recovery times, making them unsuitable for high-frequency use.
### 6. Difference Between TVS and ESD Protection Diodes
While both protect against voltage surges, TVS diodes are designed for high-energy events like lightning strikes, while ESD protection diodes target electrostatic discharges. TVS diodes generally have higher capacitance compared to ESD diodes, which are optimized for low capacitance in sensitive circuits.
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