Point-Contact Diode

Part of Semiconductors

The simplest semiconductor rectifier — a fine wire pressed against semiconductor crystal.

Why This Matters

The point-contact diode is the most primitive functional semiconductor device and historically the first to be deployed at scale. Radio operators in World War I used “cat’s whisker” detectors — a fine wire touching a galena (lead sulfide) or silicon crystal — to demodulate AM radio signals. No doping, no junctions, no furnace required. Just a fine wire, a crystal, and mechanical pressure.

For a rebuilding civilization, the point-contact diode represents an immediately achievable milestone. If you have access to a piece of semiconductor material — not even purified, just naturally occurring galena or carborundum (silicon carbide) — and a fine metal wire, you can make a diode. It detects radio signals, rectifies AC to DC, and demonstrates semiconductor behavior before any sophisticated fabrication is available.

Understanding why it works — and why junction diodes are superior — teaches the fundamentals of semiconductor-metal contacts and the Schottky barrier concept. This knowledge extends to modern Schottky diodes, which are faster-switching than junction diodes and important in high-frequency circuits.

How the Point-Contact Diode Works

When a metal wire touches a semiconductor, charge transfers between them until their Fermi levels equalize (reaching electrochemical equilibrium). The resulting charge redistribution creates a thin depletion zone and potential barrier at the metal-semiconductor interface — a Schottky barrier.

The Schottky barrier behaves like a p-n junction: forward bias lowers the barrier and allows current to flow; reverse bias raises the barrier and blocks current. The mechanism is slightly different (majority carrier transport rather than minority carrier injection), but the electrical behavior — rectification — is similar.

Schottky diode advantages over p-n junction:

• Speed: No minority carrier storage. P-n junction diodes store minority carriers in the quasi-neutral regions; when reverse bias is suddenly applied, these must be swept out (reverse recovery time). Schottky diodes have no stored minority carriers — they switch in picoseconds rather than nanoseconds. This is why Schottky diodes are preferred in high-frequency circuits.
• Lower forward voltage: Metal-semiconductor barriers can be made with lower built-in potential than p-n silicon junctions. A Schottky silicon diode can conduct at 0.2-0.4V (versus 0.6-0.7V for p-n junction).

Point-contact disadvantages:

• Fragile: the mechanical contact can move or fail.
• High series resistance: small contact area limits current capacity.
• Noisy: mechanical instability generates noise — a problem for sensitive signal detection.
• Variable: different spots on the same crystal have different characteristics.

Historical Cat’s Whisker Construction

The original detector for crystal radios:

Crystal: Galena (lead sulfide, PbS) is the classic choice. Also works: pyrite (FeS2), carborundum (SiC), silicon, germanium, and several other naturally occurring minerals. Galena is found in lead ore deposits worldwide and is easily identified by its bright metallic luster and cubic cleavage.

Wire: Phosphor bronze or beryllium copper, 0.05-0.1 mm diameter. The wire must be hard and springy enough to maintain consistent pressure on the crystal. Tungsten wire (from burned-out light bulb filaments) also works. The wire is sharpened to a fine point by electrochemical etching or careful grinding.

Mounting: The crystal is secured in a small metal cup or clamp. The wire is mounted in an adjustable holder that allows the contact point to be moved across the crystal surface to find a responsive spot. This is the “cat’s whisker” — probing for the best spot on the crystal.

Sensitivity adjustment: Not all crystal surfaces are equally rectifying. The operator searches by moving the whisker until the detected radio signal is loudest. Sensitive spots may only be a fraction of a millimeter across. The contact must not be disturbed once found — crystals in active radios were carried gently and re-adjusted after any movement.

Circuit: Series with the antenna-tuned circuit and headphones. The crystal detector and capacitor form a simple envelope detector. No battery required — this is the crystal radio, powered entirely by the received radio signal.

Galena Crystal Preparation

For optimal performance:

Cleaving: Galena cleaves perfectly along cubic planes. Strike the crystal with a sharp blade at an edge to expose a fresh, clean surface. The fresh surface is bright silver-metallic. Work quickly — the exposed surface begins to oxidize within hours.

Cleaning: Briefly dip in dilute HCl (10%) to remove surface oxide. Rinse in distilled water. Dry by blotting, not rubbing. Handle only by edges.

Testing: Probe different areas of the freshly cleaved surface. The best detector spots are often near crystal face centers, away from inclusions and surface defects visible as dull patches or discolorations. Mark good spots with a tiny scratch at the crystal edge for future reference.

Reconditioning: If a galena crystal stops detecting, cleave a new surface. Crystals slowly lose sensitivity as the surface oxidizes and contamination accumulates.

Making a Simple Schottky Diode

A more reproducible point-contact diode can be made with purified semiconductor and a formed wire contact:

Forming: Apply a brief electrical pulse through the contact point. A current of 50-100 mA for 1-10 milliseconds heats the contact locally, alloying the wire slightly into the semiconductor and creating a small, reproducible junction. This “forming” process is what made early transistors: the point-contact transistor used two formed contacts on the same germanium chip.

Wire materials and work function: The work function of the wire metal determines whether the contact is rectifying (Schottky) or ohmic (non-rectifying). For n-type germanium: platinum, gold, and tungsten form rectifying contacts. Indium and aluminum tend to form ohmic contacts. For n-type silicon: platinum and gold are rectifying; aluminum is ohmic. The difference is whether the metal’s work function is greater or less than the semiconductor’s.

Stabilizing the contact: After forming, encapsulate the wire-crystal assembly in wax or resin to prevent mechanical movement. The encapsulant should not touch the contact area — keep it to the wire mount and crystal body.

Performance specification after forming: Forward voltage at 1 mA should be 0.2-0.4V for germanium Schottky; 0.3-0.5V for silicon. Reverse leakage at 5V should be less than 100 µA. Breakdown voltage depends on semiconductor doping — for lightly doped material, 10-30V is typical.

Applications in Early Rebuilding

Crystal radio: Most immediately useful application. A galena detector and a coil-capacitor tuned circuit can receive AM broadcasts from hundreds of kilometers away with no power supply. A good outdoor antenna (10-20 meters of wire elevated 5-10 meters) and good earth ground improve range dramatically. Headphones or a sensitive earphone complete the system. Build this before building any active circuit — it requires no battery and no other components.

RF signal detection: Point-contact diodes are superior to junction diodes for microwave and very high frequency (> 100 MHz) detection because of their low capacitance (tiny contact area). If building any radio frequency equipment, use Schottky or point-contact diodes at the detector stage.

Voltage doubler for low signals: Two diodes in a Cockcroft-Walton configuration can double the voltage of a weak rectified signal. Useful for power supply from low-voltage sources.

Polarity protection: Even a crude point-contact diode in series with sensitive electronics prevents damage from reversed power supply connections. Its forward drop (0.2-0.3V) is acceptable in many circuits.

Document every crystal batch, every wire type tested, every forming parameter tried. The point-contact device is the beginning of systematic semiconductor work, and the records from this stage inform the progression to junction diodes and transistors.