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+---- Thread: Semiconductors & Diodes — The Foundation of Modern Electronics (/showthread.php?tid=358)
Semiconductors & Diodes — The Foundation of Modern Electronics - Leejohnston - 11-17-2025
Thread 3 — Semiconductors & Diodes
How Silicon Became the Heart of Every Modern Device
All modern electronics are built on one thing:
semiconductors.
They give us:
• diodes
• LEDs
• transistors
• microchips
• processors
• sensors
This thread introduces the essential concepts behind semiconductor physics and the simplest semiconductor device — the diode.
1. What Is a Semiconductor?
A semiconductor is a material whose conductivity is between a conductor (like metal) and an insulator (like glass).
Common semiconductor materials:
• silicon (most popular)
• germanium
• gallium arsenide (high-speed electronics)
Why semiconductors matter:
• we can control their conductivity
• adding impurities (doping) changes their behaviour
• they enable switching, amplification, and logic
2. Doping — How We Control Semiconductors
Doping = adding tiny amounts of other atoms to silicon.
Two types:
• N-type (negative carriers)
Contains extra electrons.
(Electrons = charge carriers)
• P-type (positive carriers)
Contains “holes” — places where electrons are missing.
(Holes act like positive charge carriers)
When you join P-type and N-type together…
…you get a **PN junction**.
This is the heart of diodes.
3. The PN Junction — The Engine Inside Every Diode
At the boundary between P and N type materials:
• electrons and holes combine
• this creates a “depletion zone”
• no free charges can move
• acts like an electrical barrier
Behaviour depends on which way you connect it.
4. Forward Bias — Diode ON
Connect:
• positive to P
• negative to N
The depletion zone shrinks.
Current flows.
This is how a diode “turns on”.
Forward voltage drop:
• ~0.7 V for silicon
• ~0.3 V for germanium
• ~2–3 V for LEDs
5. Reverse Bias — Diode OFF
Connect:
• positive to N
• negative to P
The depletion zone grows.
Current stops.
This is why diodes are used for:
• blocking reverse current
• protecting circuits
• rectifying AC to DC
6. Diode I–V Curve (Behaviour Graph)
A diode is NOT a linear resistor.
Its behaviour curve:
• almost no current until ~0.7 V
• after that -> exponential rise
• in reverse → almost zero current (until breakdown)
This special behaviour is why diodes are crucial for control and power conversion.
7. Real Uses of Diodes
• Rectifiers (AC → DC)
Used in power supplies.
• Protection diodes
Protect sensitive electronics from reverse polarity.
• Clipping & clamping circuits
Shape signals in audio & radio systems.
• Voltage regulation (Zener diodes)
Maintain a stable voltage.
• Light emission (LEDs)
When electrons recombine → light is produced.
• Fast switching
In computers, microcontrollers, and digital logic.
8. Special Types of Diodes
• Zener Diode
Operates in reverse breakdown safely.
Used for voltage regulation.
• Schottky Diode
Low voltage drop (~0.2–0.3 V).
Used in high-efficiency power electronics.
• LED (Light Emitting Diode)
Emits light when forward biased.
Colours depend on band gap energy.
• Photodiode
Generates current when light hits it.
Used in sensors and solar cells.
• Varactor Diode
Voltage-controlled capacitor.
Used in radio tuning circuits.
9. Simple Practical Example — Half-Wave Rectifier
AC input → diode → resistor → output DC-like waveform
Diagram (ASCII-safe):
AC ~~~>|~~~ R ~~~~~~~ +DC
|
GND
Uses:
• low-power power supplies
• signal demodulation
• battery chargers
10. What Comes Next?
You now understand:
• semiconductor basics
• PN junction
• why diodes work
• real-world diode applications
This sets the stage for the next major leap:
Thread 4 — Transistors: BJTs & MOSFETs (the basis of all modern computing)
Transistors give us:
• amplifiers
• switches
• digital logic
• CPUs
• microcontrollers
• GPUs
Everything we rely on today starts with understanding diodes — and now you do.
End of Thread — Semiconductors & Diodes