Complete Guide to Diodes

This guide covers everything about diodes: types, characteristics, applications, calculations, rectifiers, LEDs, Zener diodes, and advanced semiconductor uses.

What is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow primarily in one direction. It has a low resistance in the forward direction and a very high resistance in the reverse direction. Diodes are widely used for rectification, voltage regulation, signal modulation, and protection.

  • Forward-biased: allows current
  • Reverse-biased: blocks current
  • Two terminals: Anode (+) and Cathode (-)
  • Forward voltage drop: typically 0.7V for silicon diodes, 0.3V for germanium

Diode Symbol and Polarity

The schematic symbol for a diode is a triangle pointing to a line. The triangle represents the anode, and the line represents the cathode. Correct polarity is essential for proper operation.

// Forward bias: V_anode > V_cathode → Conducts// //Reverse bias: V_anode < V_cathode → Blocks current//

Diode Characteristics

The diode’s V-I curve shows its behavior:

  • Forward region: current increases exponentially after the threshold voltage
  • Reverse region: small leakage current until breakdown
  • Breakdown region: current rises rapidly in Zener or avalanche diodes

Types of Diodes

There are many types of diodes for different applications:

  • Standard Silicon Diodes: General-purpose rectification
  • Schottky Diodes: Low forward voltage drop, fast switching
  • Zener Diodes: Voltage regulation, breakdown-based operation
  • LEDs (Light Emitting Diodes): Emit light when forward-biased
  • Photodiodes: Generate current in response to light
  • Varactors: Voltage-controlled capacitance for RF tuning
  • Tunnel Diodes: Quantum tunneling effect for high-speed switching
  • Diode Arrays: Multiple diodes in one package for complex applications

Forward Bias Operation

When the anode is more positive than the cathode, the diode conducts. The current increases exponentially after the threshold voltage.

// Forward current calculation Vf = Forward voltage drop (0.7V for Si) I = (V_supply - Vf) / R

Reverse Bias Operation

When the anode is less positive than the cathode, the diode blocks current. Only a very small leakage current flows.

// Reverse leakage current IR ≈ nA to µA depending on diode type

Zener Diodes

Zener diodes operate in reverse breakdown to maintain a constant voltage.

// Zener voltage regulation Vz = Constant voltage I = (V_supply - Vz) / R_series

Rectifier Circuits

Diodes are used to convert AC to DC:

  • Half-wave rectifier: single diode, simple but inefficient
  • Full-wave rectifier: two diodes (center-tap) or four diodes (bridge), better efficiency
// Half-wave rectifier Vdc_avg = V_peak / π // Full-wave rectifier Vdc_avg = 2 × V_peak / π

Diode Applications

  • Rectification in power supplies
  • Voltage regulation with Zener diodes
  • Signal clipping and clamping circuits
  • LED indicators and displays
  • Reverse polarity protection
  • Voltage multipliers
  • RF detection and demodulation

LEDs (Light Emitting Diodes)

LEDs emit light when forward biased. Choose appropriate series resistor to limit current.

// Series resistor for LED R = (V_supply - Vf) / I_led Example: V_supply=5V, Vf=2V, I_led=20mA R = (5-2)/0.02 = 150Ω

Photodiodes

Photodiodes generate current proportional to light intensity. Can be used in reverse bias for fast response.

Schottky Diodes

Low forward voltage (~0.3V), fast switching. Used in switching power supplies and RF circuits.

Varactors (Tuning Diodes)

Used in RF circuits for voltage-controlled capacitance. The capacitance decreases as reverse bias increases.

Diode Circuits Examples

// Example: Bridge rectifier AC_input = 12V RMS Vdc_output = 12 × √2 - 2×0.7 ≈ 15V Load resistor = 1kΩ I_load = Vdc / R_load = 15 / 1000 ≈ 15mA

Protection Circuits

  • Reverse polarity protection with single diode or MOSFET-based solutions
  • Flyback diodes across inductive loads to prevent voltage spikes
  • Transient voltage suppression (TVS) diodes for surge protection

Advanced Topics

  • Temperature effects on forward voltage and leakage current
  • High-speed switching diodes for digital circuits
  • Diode capacitance effects at high frequencies
  • LED color and wavelength determined by semiconductor material

Diode Formulas and Calculations

// Forward current: I = Is × (e^(Vf / (nVt)) - 1) Is = saturation current n = ideality factor (1-2) Vt = thermal voltage ≈ 26mV at 300K // Zener current: Iz = (V_supply - Vz) / R_series // LED resistor: R = (V_supply - Vf) / I_led

Diode Applications in Circuits

Half-wave, full-wave, and bridge rectifiers convert AC to DC. Calculations include RMS, peak voltage, and load current.

Zener diodes maintain constant voltage across loads. Series resistor selection critical for proper current.

Diodes shape signals by limiting voltage above or below thresholds. Used in waveform shaping and logic circuits.

Indicators, display matrices, automotive and general illumination, RGB control, PWM dimming circuits.

Light detection, solar cells, optocouplers, and automatic brightness circuits.

Diode Types Quick Reference

Silicon Diode
0.7V drop
Germanium Diode
0.3V drop
Schottky Diode
0.3V drop, fast switching
Zener Diode
Voltage regulation
LED
Light emission
Photodiode
Light detection
Tunnel Diode
High-speed, quantum
Varactor
Variable capacitance

Choosing Diodes for Projects

Power Rectification
1N400x series silicon diodes
Fast Switching
1N4148, Schottky diodes
Voltage Regulation
Zener diodes 3.3V–24V
Indicator LEDs
3mm–5mm standard LEDs
Photodetection
BPW34, S1223 photodiodes