Diode

Basic Concepts

A diode is a unidirectional device based on a PN junction, allowing current to flow only from the Anode to the Cathode.

Symbol:  ──▸├──
         Anode Cathode (Triangle points in the direction of allowed current)

Operation: Forward conduction, Reverse cutoff
           P-region connected to +, N-region connected to - → Conduction
           P-region connected to -, N-region connected to + → Cutoff

I-V Curve

  I (mA)
  ↑
  │        ┌────── Forward Conduction (>Vf)
  │        │
  │       /
  │      /
  │     /
──┼────/──────────→ V
  │   /
  │  /
Reverse │         Vf ≈ 0.7V (Si)
Breakdown│         Vf ≈ 0.3V (Schottky)
          │         Vf ≈ 1.8~3.3V (LED)

Shockley Equation

I = Is × (e^(V/(n·VT)) - 1)

Is: Reverse saturation current (~nA~μA)
VT: Thermal voltage ≈ 26mV (at 300K)
n:  Ideality factor 1~2

Three Operating Regions

Common Types

Rectifier Diode

Application: AC to DC rectification
Characteristics: High current (1A~100A+), High voltage rating (50V~1kV+)
Vf ≈ 0.7~1.2V
Reverse recovery time trr is relatively long (μs range)

Typical Models: 1N4007 (1A/1000V), 1N5408 (3A/1000V)

Fast Recovery Diode

trr: Tens to hundreds of ns
Used in switching power supplies, high-frequency rectification
Typical: UF4007, FR107

Schottky Diode

Structure: Metal-Semiconductor contact (NOT a PN junction!)
Vf ≈ 0.15~0.45V  (Low voltage drop!)
trr is extremely short (almost zero)
Drawback: High reverse leakage current, Low voltage rating (<200V)

Applications: Switching power supply freewheeling, ORing, Solar bypass
Typical: 1N5819 (1A/40V), SS34 (3A/40V)

Zener Diode

Operates in the reverse breakdown region to provide a stable reference voltage

Symbol: ──▸├──  (Cathode marking is slightly different)

Vz: 2.4V ~ 200V
Working Principle:
  Vz < 5V: Zener breakdown (Negative temperature coefficient)
  Vz > 5V: Avalanche breakdown (Positive temperature coefficient)
  Vz ≈ 5V: Balance of both, minimum temperature coefficient (Most stable)

Typical Applications: Parallel voltage regulator, Voltage reference, ESD protection
Typical Models: BZX55 series, 1N47xx series

TVS Diode (Transient Voltage Suppression)

Specifically designed to absorb surge/ESD pulses
Response speed: ps range
Clamps transient voltage to protect downstream circuits

Typical: ESD protection for USB/HDMI interfaces, etc.

Light Emitting Diode (LED)

Vf: Red ≈ 1.8V, Green ≈ 2.2V, Blue/White ≈ 3.0~3.3V
Current: Typical 5~20mA (for indication)

Must use a series current-limiting resistor!
R = (Vsupply - Vf) / I_LED

Example: Driving a Red LED at 10mA with 5V
R = (5-1.8)/0.01 = 320Ω → Select 330Ω

Typical Applications

Half-Wave Rectification

      ┌──▸├──┐
AC ───┤      ├── Vout (Pulsating DC)
      └──────┘

Only the positive half-cycle passes through; low efficiency (40.6%)

Full-Wave Bridge Rectification

      ┌──────┐
      │  ┌─▸├─▸├─┐
AC ───┼──┤      ├── Vout
      │  └─▸├─▸├─┘
      └──────┘

Uses 4 diodes, utilizing both half-cycles
Efficiency: 81.2% (without filter capacitor)

Flyback Diode (Freewheeling Diode)

When a relay/motor coil is turned off:
  Coil current cannot change abruptly → Generates high voltage spike
  Reverse-parallel diode provides a path for the current
  
      ┌──▸├──┐
      │      │
    ┌─┴─┐    │
    │Coil│    │
    └─┬─┘    │
      │      │
      └──────┘

A "freewheeling" diode is essential! Otherwise, the MOSFET/Transistor will burn out.

Clamping / Limiting

Uses diodes to limit the signal to a specific voltage range.
Commonly used for ADC input protection: Diodes conduct when signal > Vcc+0.3V or < -0.3V.

Selection Guidelines

Keywords: Diode, PN Junction, Vf, Rectification, Schottky, Zener, TVS, LED, Freewheeling