Course: From Basic Science to Web Hosting

Module 02 — Electronics

What Is a Diode?

Difficulty: Beginner
Prerequisites: Lesson 009 — What Is a Semiconductor?
Estimated time: 25 minutes

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1. Learning Objectives

After this lesson, you should understand:

• What a diode is
• How a PN junction works
• What the depletion region is
• Forward bias and reverse bias
• How a diode controls current
• The diode I–V characteristic
• Rectification
• Common types of diodes
• Why diodes are important in power supplies and computers

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2. Start With the PN Junction

From the previous lesson:

P-type semiconductor
        │
        │
N-type semiconductor

When P-type and N-type semiconductor regions are joined, we obtain a:

PN Junction

The PN junction is the fundamental structure behind an ordinary semiconductor diode.

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3. What Happens at the Junction?

The P side has many holes as majority carriers.

The N side has many electrons as majority carriers.

Near the boundary:

P-side              N-side

holes  → ← electrons
          │
          │
       Junction

Electrons and holes recombine near the junction.

This leaves behind charged dopant ions that are relatively immobile.

A region with very few mobile carriers forms:

Depletion Region

P-type       Depletion       N-type
███████      ░░░░░░░░░      ███████
███████      ░░░░░░░░░      ███████

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4. The Depletion Region

The depletion region contains very few mobile charge carriers compared with the surrounding regions.

Because of the fixed ionized dopants, an internal electric field develops.

Conceptually:

Carrier diffusion
       ↓
Charge separation
       ↓
Electric field
       ↓
Potential barrier

This barrier affects the movement of carriers across the junction.

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5. What Is a Diode?

A diode is a semiconductor device designed to provide strongly asymmetric current-voltage behavior.

In simple terms:

A diode allows current much more readily in one direction than the other, within its normal operating range.

The schematic symbol is:

──────|>|──────

The exact symbol orientation indicates the diode’s polarity.

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6. Anode and Cathode

A diode has two terminals:

Anode
  │
  ▼
 |>| 
  │
  ▼
Cathode

The cathode is traditionally marked by a line on the physical diode package.

A common mnemonic is:

Cathode = K

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7. Forward Bias

Connect the diode so that the P side is at a higher potential than the N side.

Conceptually:

+ ─── Anode |>| Cathode ─── −

This is called:

Forward Bias

The applied voltage reduces the effective barrier and allows substantial current to flow once the device reaches its conducting region.

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8. Reverse Bias

Reverse the polarity:

− ─── Anode |>| Cathode ─── +

This is:

Reverse Bias

The depletion region becomes wider and the diode normally carries only a very small leakage current.

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9. Forward vs Reverse Bias

Condition	Typical behavior
Forward bias	Significant current can flow
Reverse bias	Very small leakage current
Reverse breakdown	Large reverse current can occur if properly limited

So:

Forward bias
     ↓
Conducting region

Reverse bias
     ↓
Blocking region

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10. Important Correction

A diode is not a perfect one-way valve.

A real diode:

• Has a nonzero forward voltage
• Has leakage current
• Has temperature dependence
• Has finite switching speed
• Can experience reverse breakdown

Therefore:

Real diode ≠ ideal diode

The ideal diode is a useful simplified model.

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11. Forward Voltage

For a common silicon diode, significant forward current often occurs around the order of:

~0.6–0.7 V

under typical operating conditions.

But this is not a fixed universal value.

The actual voltage depends on:

Current
Temperature
Diode type
Device construction

For example, a Schottky diode typically has a lower forward voltage than a conventional silicon PN-junction diode at comparable conditions.

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12. The Diode I–V Curve

A simplified diode characteristic looks like:

Current
  ↑
  │             /
  │           /
  │         /
  │_______/
  │
  └────────────────→ Voltage
          Forward

In reverse bias:

Current
  ↑
  │
  │
──┼───────────────→ Voltage
  │
  │  small leakage

At sufficiently large reverse voltage, breakdown can occur.

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13. The Diode Equation

An idealized PN-junction diode can be modeled using the Shockley diode equation:

I = Iₛ [e^(V/(nVₜ)) − 1]

where:

I   = diode current
Iₛ  = saturation current
V   = diode voltage
n   = ideality factor
Vₜ  = thermal voltage

You don’t need to calculate this yet.

The important idea is:

Forward voltage ↑
        ↓
Current can increase very rapidly

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14. Why Does a Diode Conduct?

At the microscopic level, the applied electric field changes the carrier distributions and the potential barrier at the PN junction.

Forward bias:

Barrier reduced
      ↓
Carrier injection
      ↓
Current increases

Reverse bias:

Barrier increased
      ↓
Majority-carrier transport suppressed
      ↓
Small leakage current

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15. Rectification

One of the most important uses of a diode is:

Rectification

Converting an alternating waveform into a unidirectional/pulsating output.

For example:

AC input

   / \      / \
  /   \    /   \
 /     \  /     \
/       \/       \

A diode can block one polarity and pass the other.

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16. Half-Wave Rectifier

A simple circuit:

AC ───|>|──── Load

Output:

   / \       / \
  /   \     /   \
_/     \___/     \___

Only one half of the waveform is passed.

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17. Full-Wave Rectifier

A bridge rectifier uses four diodes:

       D1       D2
 AC ──|>|──┬──|<|── AC
          │
          Load
          │
 AC ──|<|──┴──|>|── AC
       D3       D4

The circuit routes both halves of the AC waveform so that the load receives the same polarity.

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18. Capacitor + Rectifier

Now connect our previous lesson.

AC
 ↓
Rectifier
 ↓
Pulsating DC
 ↓
Capacitor
 ↓
Smoother DC

This is a fundamental power-supply concept.

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19. Power Supply

A simplified power supply can look like:

AC input
   ↓
Transformer / converter
   ↓
Rectifier
   ↓
Capacitor
   ↓
Regulator
   ↓
DC output
   ↓
Electronic circuit

Your computer, router, server, and networking equipment all depend on power-conversion systems.

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20. Diodes in Computer Hardware

Diodes are used in many applications:

Rectification
Voltage protection
Signal detection
Switching
Clamping
ESD protection
Voltage regulation
Power management

They are also incorporated into integrated circuits.

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21. Zener Diode

A Zener diode is designed to operate in reverse breakdown under controlled conditions.

Conceptually:

Reverse voltage
      ↓
Controlled breakdown
      ↓
Voltage regulation/reference

This can be useful for voltage regulation and reference circuits.

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22. LED

An LED is a:

Light-Emitting Diode

When appropriately forward biased, it emits light.

Conceptually:

Electrical energy
      ↓
Semiconductor
      ↓
Electron-hole recombination
      ↓
Photon emission
      ↓
Light

LEDs are used in:

Displays
Indicators
Lighting
Optical communication

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23. Photodiode

A photodiode is designed to detect light.

Light
 ↓
Photodiode
 ↓
Electrical signal

This is important in optical communication.

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24. Fiber-Optic Communication

This connects directly to web hosting.

A simplified fiber communication system:

Computer
   ↓
Electrical signal
   ↓
Optical transmitter
   ↓
Light
   ↓
Fiber
   ↓
Photodetector
   ↓
Electrical signal
   ↓
Computer

Photodiodes are commonly used at the receiving side of optical communication systems.

Therefore:

Semiconductor physics
      ↓
Photodiode
      ↓
Fiber communication
      ↓
Internet
      ↓
Web hosting

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25. Diode → Transistor

This is the most important transition.

A diode uses a PN junction.

A transistor uses semiconductor structures to provide much more powerful control of electrical signals.

PN junction
    ↓
Diode
    ↓
Multiple semiconductor regions
    ↓
Transistor

A transistor can be used as:

Switch
Amplifier
Signal-control element

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26. From Transistor to CPU

Now our complete chain becomes:

Silicon
 ↓
Doping
 ↓
P-type / N-type
 ↓
PN junction
 ↓
Diode
 ↓
Transistor
 ↓
Logic gate
 ↓
Digital circuit
 ↓
CPU
 ↓
Computer

Then:

Computer
 ↓
Operating System
 ↓
Network Interface
 ↓
Networking
 ↓
Internet
 ↓
Web Server
 ↓
Web Hosting

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27. Quick Check

What is a diode?

A semiconductor device with strongly asymmetric current-voltage behavior.

What are its two terminals?

Anode
Cathode

What is forward bias?

Applying polarity that reduces the PN-junction barrier and allows substantial current.

What is reverse bias?

Applying the opposite polarity, normally producing only small leakage current until breakdown.

What is rectification?

Converting an alternating waveform into a unidirectional/pulsating output.

What is an LED?

A light-emitting diode.

What is a photodiode?

A semiconductor device designed to detect light.

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Next Lesson

Lesson 011 — What Is a Transistor?

This is one of the most important lessons in the entire course.

We will go from:

Semiconductor
      ↓
P-type
      ↓
N-type
      ↓
PN junction
      ↓
Transistor
      ↓
Switch
      ↓
0 and 1
      ↓
Logic gates
      ↓
Computer

Then we will begin understanding how billions of transistors can become a CPU, which eventually brings us all the way to the server running your WordPress and web-hosting infrastructure.