What is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow primarily in one direction only. It acts like a one-way valve for electrical current.

It is made by joining a P-type semiconductor with an N-type semiconductor, creating a P-N junction at their interface.

Diodes are one of the most fundamental components in modern electronics — found in power supplies, radio receivers, LEDs, and solar cells.

P-Type: Holes (⊕) N-Type: Electrons (⊖) Anode (+) → Cathode (−) Forward Voltage ≈ 0.7V (Si)

Circuit Symbol

Anode (+) Cathode (−) Current → P N

The triangle points in the direction of conventional current flow (P→N in forward bias).

P-N Junction Structure

P-Type Majority: Holes ⊕ + + + + + N-Type Majority: Electrons ⊖ − − − − − Depletion

When P and N semiconductors are joined, electrons from the N-side diffuse to fill holes in the P-side, creating a neutral Depletion Region (space charge region) with no free carriers.

How a Diode is Made

Base Material: Pure silicon (Si) or germanium (Ge) — semiconductor atoms with 4 valence electrons arranged in a crystal lattice.

P-Type Doping: Adding a trivalent impurity (Boron, Gallium) with only 3 valence electrons creates "holes" — positive charge carriers. Each dopant atom creates one hole in the lattice.

N-Type Doping: Adding a pentavalent impurity (Phosphorus, Arsenic) with 5 valence electrons donates a free electron per atom. These free electrons are the charge carriers.

Junction Formation: A single crystal is grown with P-type on one side and N-type on the other, or one region is diffused into the other using heat and dopant gases.

Diode Type
Voltage Source
-15V (Reverse) +15V (Forward)
View Mode
Series Resistance (Load)
10 Ω 10 kΩ
Temperature
No Bias
Measurements
Supply Voltage (Vs)
0.000 V
Diode Voltage (VD)
0.000 V
Diode Current (ID)
0.000 mA
Power Dissipation
0.000 mW
Depletion Width
Normal
Bias State
Zero Bias
Diode Info
Shockley Equation
I = Is(eV/nVT - 1)

1. Semiconductor Fundamentals

A semiconductor is a material with electrical conductivity between that of a conductor (like copper) and an insulator (like glass). Silicon (Si) and Germanium (Ge) are the most common examples.

In pure (intrinsic) silicon, each atom forms 4 covalent bonds with neighboring atoms. At absolute zero, all electrons are bound and silicon is an insulator. At room temperature, thermal energy breaks some bonds, creating electron-hole pairs.

The key to making a diode is doping — deliberately introducing impurity atoms to create excess charge carriers:

P-Type Semiconductor

Doped with trivalent atoms (B, Ga, In) having 3 valence electrons. Each dopant creates a "hole" — a missing electron that acts as a positive charge carrier.

  • Majority carriers: Holes (⊕)
  • Minority carriers: Electrons (⊖)
  • Acceptor dopants
  • Net charge: Neutral

N-Type Semiconductor

Doped with pentavalent atoms (P, As, Sb) having 5 valence electrons. The extra electron is loosely bound and free to move.

  • Majority carriers: Electrons (⊖)
  • Minority carriers: Holes (⊕)
  • Donor dopants
  • Net charge: Neutral

2. The P-N Junction

When P-type and N-type materials are joined, a P-N junction forms at the interface. This junction is the heart of the diode.

Diffusion: Free electrons from the N-side diffuse across the junction to fill holes in the P-side. This is driven by the concentration gradient — carriers move from high to low concentration.

Built-in Electric Field: As electrons move from N to P, they leave behind positively charged donor ions in N. The holes they fill create negatively charged acceptor ions in P. This creates an electric field pointing from N→P (opposing further diffusion).

Depletion Region: The area around the junction is depleted of free carriers. It acts as an insulating barrier with a built-in potential (≈0.7V for Si, ≈0.3V for Ge). Equilibrium is reached when the drift current (due to the electric field) exactly balances the diffusion current.

3. Biasing the Diode

Applying an external voltage to the diode changes the width of the depletion region and controls current flow:

✅ Forward Bias

Connect +ve to Anode (P) and −ve to Cathode (N).

  • External field opposes built-in field
  • Depletion region narrows
  • At ~0.7V (Si), barrier is overcome
  • Large current flows exponentially
  • Diode acts like a low-resistance wire

🚫 Reverse Bias

Connect −ve to Anode (P) and +ve to Cathode (N).

  • External field reinforces built-in field
  • Depletion region widens
  • Only tiny leakage current (nA–μA)
  • Diode acts like a high-resistance open circuit
  • At breakdown voltage: large reverse current

4. Shockley Diode Equation

The ideal diode current is described by the Shockley Equation (also called the ideal diode equation), derived by William Shockley in 1949:

ID = Is × (eVD / (n × VT) − 1)

Where:
• Is = Reverse saturation current (~10⁻¹² A for Si)
• VD = Voltage across diode
• n = Ideality factor (1 for ideal, 1–2 for real)
• VT = Thermal voltage = kT/q ≈ 26 mV at 300K
• k = Boltzmann's constant = 1.38×10⁻²³ J/K
• q = Electron charge = 1.6×10⁻¹⁹ C

The exponential nature means current roughly doubles for every ~18mV increase in forward voltage (at room temperature). This makes the diode highly non-linear.

5. Types of Diodes

💡

LED

Emits light via electron-hole recombination (electroluminescence)

⚡

Zener Diode

Designed to operate in reverse breakdown for voltage regulation

🌞

Photodiode

Converts light into current; operates in reverse bias (photovoltaic mode)

📡

Schottky Diode

Metal-semiconductor junction; very fast switching, low forward drop (~0.3V)

🔄

Rectifier Diode

High-current diode for AC-to-DC power conversion

📻

Tunnel Diode

Quantum tunneling; used in high-frequency oscillators

6. Real-World Applications

Rectification: Converting AC to DC — the most common diode application. A bridge rectifier (4 diodes) converts full-wave AC to pulsating DC used in nearly every power supply.

Clipping & Clamping: Diodes limit (clip) signal voltages or shift (clamp) signal levels in waveform shaping circuits.

Logic Gates: Diode-Resistor Logic (DRL) was the first transistor-era logic implementation — diodes naturally implement AND/OR gates.

Overvoltage Protection: Reverse-biased diodes (transient voltage suppressors) clamp voltage spikes to protect sensitive components.

Signal Demodulation: In AM radio, a diode extracts the audio signal from the modulated carrier wave — the envelope detector.

Solar Cells: A large-area photodiode that generates current when illuminated, exploiting the photovoltaic effect.