Junction Diode

Part of The Transistor

A junction diode is the simplest semiconductor device — a single PN junction that conducts electricity in one direction only, making it the foundation for rectifiers, signal detectors, and the emitter-base junction of every bipolar transistor.

Why This Matters

The junction diode is both a practical device in its own right and the conceptual building block of transistors. A transistor is essentially two junction diodes connected back-to-back, sharing a common middle layer. Before you can understand how a transistor amplifies or switches, you must understand what a single junction does and why.

Diodes have direct, immediate applications in rebuilding civilization: rectifying AC power to DC for charging batteries and running electronics, detecting radio signals, protecting circuits from reverse voltage, and clipping waveforms. The vacuum tube diode served this role for decades before semiconductor diodes replaced it. Semiconductor junction diodes are simpler, smaller, require no filament power, and can be made with basic chemistry and metallurgy from available materials.

Understanding the junction diode also lets you test transistors intelligently. A bipolar transistor’s base-emitter and base-collector junctions should both behave like diodes. If they do not, the transistor has failed.

Physics of the Junction

When p-type and n-type semiconductor are fused together, three things happen at the boundary:

1. Diffusion: Electrons from the n-side diffuse into the p-side (attracted by the abundance of holes). Holes from the p-side diffuse into the n-side. This is driven purely by concentration difference.

2. Depletion: As electrons and holes cross and recombine, the region near the boundary becomes depleted of free carriers. Left behind are immobile charged ions: positive donor ions on the n-side, negative acceptor ions on the p-side.

3. Built-in field: These fixed charges create an electric field pointing from n to p — opposing further diffusion. Equilibrium is reached when the drift current (caused by the field) exactly cancels the diffusion current.

The depletion region is typically 0.1–1 micron wide and acts as an insulating layer under zero bias.

Forward and Reverse Characteristics

Forward Bias (Conducting)

Apply positive voltage to the p-side (anode), negative to the n-side (cathode). This external field opposes the built-in field, shrinks the depletion region, and allows current to flow.

Current vs. voltage (Shockley equation):

I = I₀ × (e^(V / 0.026) − 1)

Where I₀ is the reverse saturation current (very small) and 0.026 V is the thermal voltage at room temperature.

Practical values:

• Germanium: Starts conducting visibly at ~0.2 V, full conduction at ~0.3 V
• Silicon: Starts at ~0.5 V, full conduction at ~0.6–0.7 V

Once past the threshold, current rises steeply. A 60 mV increase in voltage increases current approximately 10-fold. This means most of the forward voltage drop appears across the threshold region — once conducting, the diode maintains nearly constant voltage regardless of current (useful as a reference voltage).

Reverse Bias (Blocking)

Apply positive voltage to the cathode, negative to the anode. This extends the depletion region, reinforcing the built-in field. Very little current flows — only the tiny reverse saturation current I₀:

Type	I₀ at 25°C
Small germanium diode	~1–10 µA
Small silicon diode	~1–100 nA

Silicon is superior here: 100–10,000× less leakage current than germanium at room temperature.

Reverse breakdown: If reverse voltage is increased beyond the breakdown voltage (typically 20–1,000 V depending on device), current suddenly increases catastrophically. In ordinary diodes this destroys the device. In Zener diodes, breakdown is controlled and used deliberately for voltage regulation.

Building a Junction Diode

Method 1: Alloying (Point-Contact and Alloy Junction)

The earliest practical method, used for the first germanium transistors:

1. Start with n-type germanium wafer (lightly phosphorus-doped during crystal growth)
2. Place a small indium pellet (1–2 mm diameter) on the surface
3. Heat to 500–550°C in a nitrogen or hydrogen atmosphere for 2–5 minutes
4. The indium melts (melting point 157°C) and dissolves into the germanium surface
5. Cool slowly over 20–30 minutes
6. The resolidified region is p-type (indium is a Group III acceptor)
7. The boundary between indium-alloyed p-region and n-type bulk is a PN junction

Testing the result:

• Measure resistance in both directions with an ohmmeter
• Forward (anode+, cathode−): should be 10–500 Ω
• Reverse (anode−, cathode+): should be >100 kΩ, ideally >1 MΩ
• Ratio >1000:1 indicates a good junction

Method 2: Point-Contact Diode

The simplest possible semiconductor diode — press a sharpened metal wire against a semiconductor surface:

1. Take a fragment of n-type germanium crystal
2. Sharpen a piece of phosphor-bronze or tungsten wire to a fine point
3. Press the point against the cleaned germanium surface with light pressure
4. The contact forms a tiny rectifying junction through a combination of metal-semiconductor work function difference and pressure

Point-contact diodes are fragile but work at very high frequencies because the contact area is tiny (minimizing junction capacitance). The original microwave radar detectors in WWII used point-contact germanium diodes.

Using Junction Diodes in Practice

Half-Wave Rectifier

Converts AC to pulsed DC — the simplest power supply building block:

1. Connect one diode in series with a load resistor
2. AC input on one side, load resistor from the other side to the circuit return
3. The diode passes only positive half-cycles; negative half-cycles are blocked
4. Output is pulsed DC — add a large capacitor across the load to smooth it

Full-Wave Bridge Rectifier

Uses four diodes to conduct during both halves of the AC cycle:

1. Arrange four diodes in a diamond (bridge) configuration
2. AC input connects to the two side corners
3. DC output taken from top and bottom corners
4. During each half-cycle, two diodes conduct in series

Output ripple at twice the input frequency — much easier to filter than half-wave.

Signal Detector

A diode can detect (demodulate) an AM radio signal:

1. High-frequency RF signal arrives at the antenna, runs through a tuned LC circuit
2. Diode rectifies the RF, allowing only positive peaks through
3. Small capacitor (10–100 pF) filters out the RF carrier, leaving audio envelope
4. Result is audio-frequency signal that drives headphones or an amplifier

Germanium diodes are preferred for this application because their lower threshold voltage (~0.2 V) allows detection of weak signals. Silicon diodes require at least 0.5 V to conduct, cutting off weak signals entirely.

Summary

Junction Diode — At a Glance

• A junction diode is a single PN junction: conducts when forward-biased, blocks when reverse-biased
• Forward threshold: ~0.2–0.3 V for germanium, ~0.6–0.7 V for silicon
• Reverse leakage: silicon has 100–10,000× less than germanium — prefer silicon for power applications
• Alloy-junction diodes can be made by melting indium pellets onto n-type germanium
• Test with ohmmeter: forward resistance 10–500 Ω, reverse >100 kΩ; ratio >1000:1 is good
• Applications: AC rectification, signal detection (AM radio), reverse polarity protection
• Every transistor contains two diode junctions — understanding one explains the other