Diodes

1. Definition

1.1 Diode

A diode is a two-terminal semiconductor device that allows current to flow in one direction only, blocking it in the opposite direction.
It is used for rectification, protection, and signal control in electronic circuits.

What is a semiconductor A semiconductor is a material whose ability to conduct electricity falls between that of a conductor (like copper) and an insulator (like glass).

types of diodes

Diodes symbols

diode symbol

Cathode Vs. Anode

In an LED, the cathode is the negative terminal (typically the shorter lead and the side with a flat edge on the casing), while the anode is the positive terminal (typically the longer lead). In other diodes, the cathode is usually marked with a band, and the anode is the opposite end.

cathode marking symbol

Polarity Marking in Different Diodes

cathode marking symbol

1.2 Forward and Reverse Bias

  • Forward bias: current flows when the anode is at a higher voltage than the cathode, and the voltage exceeds the diode’s forward voltage drop (typically 0.7 V for silicon, 0.3 V for germanium).
  • Reverse bias: current is blocked when the cathode is at a higher voltage than the anode, up to the diode’s breakdown voltage.

2. Features

  • Forward Voltage Drop (Vf): voltage needed for the diode to conduct (e.g., 0.7 V for silicon, ~0.2–0.3 V for Schottky).
  • Maximum Forward Current (If): maximum continuous current the diode can handle.
  • Reverse Breakdown Voltage (Vbr): voltage at which the diode will conduct in reverse.
  • Reverse Leakage Current: small current that flows under reverse bias.
  • Recovery Time: important in high-speed switching applications.
  • Package Type: through-hole (DO-41, DO-35) or SMD.

4. How to Use

Safety Note

Connecting a diode in reverse beyond its breakdown voltage can permanently damage it (except in Zener diodes, where breakdown is intended).

4.1. Identify the diode type and polarity

  • Polarity: the cathode is usually marked with a band; the other lead is the anode.
  • Common types: Rectifier, Schottky, Zener, Light Emitting Diode (LED), Photodiode.

diode polarity marking

4.2. Choose the right diode

  • Application: rectification, voltage regulation, signal detection, light emission, etc.
  • Current rating (must be higher than circuit current).
  • Reverse voltage rating (must be higher than maximum reverse voltage in circuit).
  • Speed: for high-frequency circuits, use fast recovery or Schottky diodes.

4.3. Connect it correctly

  • Series connection: to allow current in one direction only.
  • Parallel (reverse) connection: for polarity protection.
  • Observe correct polarity ; anode to positive, cathode to negative (unless using a Zener in reverse mode).

4.4. Test and verify

  • Use the diode test mode on a multimeter to measure forward voltage drop.
  • Check that it blocks current in reverse direction under normal conditions.
  • For LEDs, test with low current to avoid damage.

4.5. Install in circuit

  • On breadboard: insert leads into holes, observing polarity.
  • On PCB: solder into the correct orientation according to PCB silkscreen markings.

5. When to Use

  • Rectification: converting AC to DC in power supplies. → standard rectifier diodes (1N4007, etc.)
  • Reverse Polarity Protection: prevent damage from wrong power connection. → general-purpose or Schottky diodes
  • Voltage Regulation: Zener diodes for fixed voltage reference. → Zener diodes
  • Signal Demodulation: in AM/FM radio circuits. → small-signal diodes (1N4148, etc.)
  • Switching: in high-speed logic and RF circuits. → fast-recovery, Schottky, or PIN diodes
  • Light Emission/Detection: LEDs and photodiodes. → LEDs and photodiodes

6. Video Explanation

video coming soon