Diode Types: Functionality, Applications, and Symbols

Diodes are fundamental components in electronics, each engineered for specific tasks. Their ability to allow current flow in only one direction makes them vital for tasks like rectification, signal modulation, and voltage regulation. This guide explores a variety of diode types, detailing their symbols, functionalities, and applications.

Introduction: What is a Diode?

• A diode is an electronic device comprised of two electrodes: an anode and a cathode.
• Represented by a specific symbol, it allows current to flow in one direction only (forward direction), where the anode potential is higher than the cathode. This is known as the forward-biased state.

alt text: Diode symbol

alt text: Diode characteristic

• Reversing the voltage polarity puts the diode in a reverse-biased state, where it attempts to block current flow within its rated limits.
• Diodes find use in switches, rectifiers, regulators, voltage multipliers, and clipping/clamping circuits.
• The cathode is often marked by a band, indicating the lead from which conventional current flows out in the forward-biased mode.
• Diodes have identification codes using American and conventional systems.
  • American System: Starts with “1N” followed by a serial number.
  • Conventional System: Two letters at the start; the first indicates the semiconductor material (A = Germanium, B = Silicon), and the second indicates application (A = Signal Diode, Y = Rectifier Diode, Z = Zener Diode). For instance, “AA119” signifies a germanium signal diode.

Types of Diodes

Here’s a look at some commonly used diodes, including their functions, characteristics, applications, and symbols.

p-n Junction Diode

The p-n diode consists of a p-n junction with connections to both the p-side (anode) and n-side (cathode). Key electrical specifications include forward current and maximum reverse voltage.

alt text: p-n diode fabrication structure

Function: P-n junction diodes are primarily used as rectifiers, converting AC to DC in power supplies. Silicon is generally preferred over germanium due to its lower reverse current and higher breakdown voltage.

Zener Diode

Unlike standard diodes, Zener diodes are designed to operate in the breakdown region without damage, provided a resistor limits current.

alt text: zener diode as voltage regulator

Primarily functions as a rectifier diode.

Point-Contact Diode

Constructed by pressing a gold or tungsten wire tip onto a pellet of n-type germanium, a small p-type region is formed around the tip by passing a brief current, creating a p-n junction.

alt text: point contact diode

These diodes are used as signal diodes, detecting radio frequency signals due to their very low capacitance. Suitable for high-frequency signal detection due to the tiny junction area. Germanium is preferred for signal diodes due to its lower “turn-on” voltage compared to silicon.

Varactor Diode

Designed with a specific capacitance range (e.g., 2 to 10 pF), the capacitance value is adjusted by varying the reverse voltage, which alters the depletion layer width.

alt text: varactor diode

Used in tuning circuits for TV and VHF radio receivers, enabling automatic frequency control (AFC).

Gunn Diode

Made from n-type gallium arsenide sandwiched between metal electrodes, it exhibits negative resistance, making it suitable for oscillators.

alt text: Gunn diode

Applications:

• Microwave oscillators
• Pump sources in parametric amplifiers
• Police radars and CW Doppler radars

Tunnel Diode

• Heavily doped p-n junction diode (impurity concentration of 1 part in 10^3 compared to 1 part in 10^8 in a typical p-n junction diode).
• Fabricated from germanium, gallium arsenide (GaAs), and gallium antimonide.
• Also known as an Esaki diode.
• Very small depletion layer width (about 100 Å).

alt text: Tunnel diode characteristic and symbol

Exhibits negative resistance, making it useful in oscillators and microwave amplifiers.

PIN Diode

Features an insulating layer between the P and N type materials, differing from the normal PN junction diode structure.

alt text: P-I-N diode structure

Applications: RF delay lines/phase shifters, amplitude modulators, and switches.

Schottky Diode

Constructed on a thin silicon (n+ type) substrate, the metal-semiconductor junction is formed by depositing metal over SiO2.

alt text: schottky diode symbol

alt text: schottky diode and equivalent circuit

A low-voltage diode widely used in RF mixers and as rectifiers in power electronics.

IMPATT Diode

• Full name: Impact Ionization Avalanche Transit Time
• Working operation: Avalanche Multiplication
• Frequency range: 4 GHz to 200 GHz

alt text: Impatt diode structure

Used as amplifiers and oscillators.

TRAPATT Diode

• Full name: Trapped Plasma Avalanche Triggered Transit
• Working operation: Plasma Avalanche
• Frequency range: 1 to 10 GHz

alt text: Trapatt diode structure

Used as oscillators.

BARITT Diode

• Full name: Barrier Injection Transit Time
• Working operation: Thermionic emission
• Frequency range: 4 GHz to 8 GHz

alt text: Baritt diode structure

Used as local oscillators in electronic circuits.

Step Recovery Diode

Heavily doped at the ends, with doping levels decreasing towards the junction.

Applications:

• Parametric amplifiers or pulse generators in microwave circuits.
• Generating extremely fast rise-time pulses.
• Frequency comb generation, harmonic frequency multipliers, and samplers.
• Charge-controlled switches.

Light Emitting Diode (LED)

A junction diode made from a semiconductor compound, like gallium arsenide phosphide.

alt text: LED diode

A cheap and convenient light source.

Laser Diode

Produces an intense beam of monochromatic, coherent, and collimated light or infrared radiation.

alt text: laser diode

Used in optical fiber systems, typically made of gallium arsenide phosphide.

Photodiode

A photodetector that converts light into current or voltage.

alt text: Photodiode

Applications: Fire alarms, counting systems, and automatic control systems; also used as fast counters and in light meters.

Conclusion

Understanding the various types of diodes and their applications is crucial in electronics. Choosing the right diode is essential for optimizing circuit performance in rectification, voltage regulation, or signal modulation tasks. With this knowledge, you can make informed decisions and enhance your electronic projects.