---
title: Diode
url: https://doc.liz6.com/en/hardware/03-semiconductor/03-diode
locale: en
area: hardware
tags:
- hardware
- semiconductor
date: 2026-06-30
modified: 2026-07-16
description: 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. --- I-V Curve Shock…
---

# 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
| Region | Condition | Behavior |
|------|------|------|
| Forward | V > Vf (approx. 0.7V) | Conducts, I increases exponentially |
| Reverse | Vbr < V < 0 | Cutoff, only nA~μA leakage current |
| Breakdown | V < Vbr | Reverse current increases sharply |

---

## 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

| Parameter | Description |
|------|------|
| Vrrm (Reverse Voltage Rating) | ≥ 1.5x Maximum Reverse Voltage |
| If (Forward Current) | ≥ 1.5x Maximum Operating Current |
| Vf (Forward Voltage Drop) | Prefer Schottky for low-voltage circuits |
| trr (Reverse Recovery Time) | High frequency → Fast recovery or Schottky |
| Package | High power → DO-201; SMD → SOD-123/SMA |

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