Diode Construction

Part of Semiconductors

A diode is the simplest semiconductor device — a one-way valve for electricity. Building a working diode from raw materials proves your semiconductor fabrication skills and provides essential components for radio receivers, power supplies, and signal processing.

Why Diodes Are the First Semiconductor Device

The diode contains a single PN junction. If you can make a working diode, you understand crystal purification, doping, and junction formation well enough to proceed to transistors (which are essentially two back-to-back diodes). Diodes are also immediately useful:

• Radio detection: Convert radio-frequency signals to audio (crystal radio detectors)
• Rectification: Convert AC to DC for battery charging and power supplies
• Voltage regulation: Zener diodes maintain constant voltage
• Signal protection: Prevent reverse current from damaging equipment
• Logic gates: Simple OR and AND gates from diode networks

Point-Contact Diode (Cat’s Whisker)

The point-contact diode is the oldest and simplest semiconductor device. It requires no doping, no furnace, and no sophisticated equipment — just a semiconductor crystal and a sharpened wire.

How It Works

A fine metal wire pressed against a semiconductor crystal creates a tiny PN junction at the contact point. The pressure and the metal’s work function create a barrier that allows current to flow easily in one direction but blocks it in the reverse direction.

Materials

Component	Material	Source
Crystal	Galena (PbS), germanium, silicon, iron pyrite	Natural minerals or refined material
Cat’s whisker	Phosphor bronze, tungsten, or steel wire, 0.1-0.3mm	Spring wire, guitar string
Base	Metal cup or clip	Scrap metal
Housing	Glass tube or small box	Any insulating container

Building a Galena Point-Contact Diode

1. Select a clean galena crystal with bright, flat cleavage faces
2. Mount it firmly in a metal cup using low-melting solder or conductive epoxy (the cup becomes one terminal)
3. Sharpen a 50mm piece of phosphor bronze wire to a fine point
4. Mount the wire on a spring-loaded adjustable arm
5. Touch the wire point to various spots on the crystal surface
6. Test each spot by measuring forward/reverse resistance with an ohmmeter
7. Find a spot where forward resistance is low (<100 ohms) and reverse resistance is high (>10,000 ohms)
8. Lock the adjustment mechanism to hold the whisker in place

Finding the Sweet Spot

Not every point on the crystal rectifies well. Systematically explore the surface, testing each spot. Crystal boundaries, edges, and certain facets work better than others. Once you find a good spot, do not disturb it — even a slight shift can change performance dramatically.

Performance

Parameter	Galena Diode	Germanium Diode
Forward voltage drop	0.2-0.5V	0.2-0.3V
Reverse breakdown	5-20V	20-100V
Maximum current	1-10 mA	10-50 mA
Frequency response	Good to ~10 MHz	Good to ~100 MHz
Reliability	Poor (vibration-sensitive)	Moderate
Best use	Crystal radio detector	Radio detector, signal diode

Junction Diode (Alloy Method)

The junction diode is a permanent, reliable device made by alloying a dopant metal into a semiconductor wafer. Unlike the point-contact diode, it has a stable, repeatable junction that does not shift when vibrated.

Building a Germanium Alloy Diode

This method is how diodes were manufactured in the 1950s:

Materials

• N-type germanium wafer (phosphorus or antimony doped, ~1 ohm-cm resistivity)
• Indium wire or shot (1-3mm pieces)
• Nickel or copper contact wire
• Glass tube for encapsulation
• Hydrogen or nitrogen gas source

Procedure

1. Prepare the wafer: Cut a 3-5mm square from an n-type germanium crystal, 0.5-1mm thick. Lap and polish both faces. Clean in acid (dilute HCl).

2. Place the indium: Put a tiny indium bead (1-2mm diameter) on the center of one face. The indium will create the p-type region.

3. Alloy: Heat in a hydrogen atmosphere furnace:

   • Ramp to 500 degrees C over 5 minutes
   • Hold at 500-550 degrees C for 3-5 minutes (indium melts at 157 degrees C and dissolves into the germanium)
   • Cool slowly over 15-20 minutes to room temperature

4. Attach contacts:

   • Solder a fine wire to the indium dot (p-type, anode)
   • Solder a wire to the back face of the germanium (n-type, cathode)
   • Use indium or tin solder at low temperature to avoid disturbing the junction

5. Encapsulate: Slide a glass tube over the assembly and seal the ends with epoxy or wax to protect from moisture and contamination.

6. Test: Measure forward and reverse resistance. A good junction shows:

   • Forward resistance: 5-50 ohms (at 0.5V)
   • Reverse resistance: >100,000 ohms (at -5V)
   • Ratio: >2000:1

The Hydrogen Atmosphere

Heating germanium in air causes oxide formation that ruins the junction. Use a flowing hydrogen or nitrogen atmosphere during alloying. A simple setup: pass gas from a tank through a quartz tube containing the wafer assembly, heated by an external coil or flame.

Silicon Alloy Diode

Silicon diodes are more temperature-stable but harder to make:

1. Use an n-type silicon wafer
2. Use aluminum as the p-type dopant (melts at 660 degrees C)
3. Alloy at 700-800 degrees C for 5-10 minutes
4. Cool slowly
5. Silicon diodes have a higher forward voltage drop (~0.6V) but work at higher temperatures

Diode Characteristics

The I-V Curve

A diode’s behavior is defined by its current-voltage curve:

Condition	Voltage	Current	Behavior
Reverse bias	Negative	~0 (microamps)	Blocks current
Zero bias	0V	0	No current
Below threshold	0 to 0.2-0.6V	Very small	Beginning to conduct
Forward bias	Above threshold	Increases rapidly	Conducts freely
Reverse breakdown	Very negative	Sudden large current	Destructive (unless Zener)

Key Parameters

Parameter	What It Means	Typical Values
Forward voltage (Vf)	Voltage needed to conduct	0.2V (Ge), 0.6V (Si)
Reverse leakage	Current in blocking direction	1-100 microamps
Breakdown voltage	Maximum reverse voltage	20-1000V
Maximum forward current	Current before overheating	10 mA - 10 A

Applications in Rebuilding

Crystal Radio Detector

Replace the finicky cat’s whisker with a permanent alloy junction diode:

1. Connect the diode in place of the crystal detector in the radio circuit
2. No adjustment needed — it rectifies consistently
3. Forward voltage drop of germanium (0.2V) is better for this application than silicon (0.6V)

AC-to-DC Rectifier

Build a full-wave bridge rectifier from four diodes:

1. Arrange four diodes in a diamond (bridge) configuration
2. Connect AC input to two opposite corners
3. Take DC output from the other two corners
4. Add a smoothing capacitor (electrolytic, 100-1000 microfarads) across the output
5. Result: smooth DC suitable for charging batteries or powering equipment

Voltage Clamp

Protect sensitive circuits from voltage spikes:

1. Connect a diode from the signal line to the power supply (cathode to supply)
2. Connect another diode from ground to the signal line (cathode to signal)
3. Any voltage spike above the supply or below ground is clamped by the diodes

Testing and Quality Control

Forward Voltage Test

1. Connect the diode in series with a 1K resistor and a 3V battery
2. Measure the voltage across the diode
3. Germanium: should read 0.15-0.30V
4. Silicon: should read 0.55-0.70V
5. If it reads 0V, the diode is shorted. If it reads battery voltage, the diode is open.

Reverse Leakage Test

1. Reverse the diode and measure current (or voltage across a large resistor)
2. Good germanium diode: <50 microamps at 5V reverse
3. Good silicon diode: <1 microamp at 5V reverse
4. High leakage indicates contamination or crystal defects

Quick Test with a Multimeter

Most multimeters have a diode test mode that reads forward voltage directly. This is the fastest way to verify diode operation. Good diodes read 0.15-0.30V (Ge) or 0.55-0.70V (Si) in forward and “OL” (overload/open) in reverse.

Common Mistakes

1. Overheating during alloying: Exceeding the correct temperature or holding too long causes the dopant to diffuse too deep, shorting through the wafer. Monitor temperature carefully.
2. Contaminated surfaces: Fingerprints, oxide, or dirt on the wafer prevent proper alloying. Clean with acid immediately before heating, and handle with tweezers only.
3. Too large an indium bead: An oversized dopant pellet creates a junction area too large for the wafer, reducing reverse breakdown voltage. Start with 1mm beads and increase only if needed.
4. Alloying in air: Oxygen creates insulating oxide layers that prevent junction formation. Always use hydrogen or nitrogen atmosphere.
5. Rough soldering of lead wires: Heat from soldering can melt or damage the junction. Use low-temperature indium solder and apply heat briefly, away from the junction area.

Summary

Diode Construction -- At a Glance

• Point-contact (cat’s whisker): Sharpened wire on galena or germanium crystal; simplest to build, least reliable, works for crystal radios
• Alloy junction: Indium bead melted into n-type germanium at 500 degrees C in hydrogen atmosphere; permanent, reliable, reproducible
• Germanium diodes have lower forward voltage (0.2V) — better for low-signal applications like radio detection
• Silicon diodes have higher forward voltage (0.6V) but work at higher temperatures and have lower leakage
• A forward/reverse resistance ratio of >2000:1 indicates a good junction
• Four diodes in a bridge circuit convert AC to DC for power supply applications