Alloy Junction

Part of The Transistor

Fabricating transistors by alloying metal dopant dots into semiconductor wafers at elevated temperature.

Why This Matters

The alloy junction technique was the first reliable method for mass-producing bipolar transistors. Before it, the point-contact transistor (two wires on a germanium chip) was fragile, inconsistent, and prone to sudden failure. The alloy junction transistor, introduced around 1952, had a well-defined internal structure, could be reproducibly manufactured, and was stable enough for commercial radio and hearing aid production by 1954.

For a rebuilding civilization attempting to fabricate transistors, alloy junction is the starting method. It requires a temperature-controlled furnace, reducing gas (hydrogen or forming gas), pure semiconductor wafers, and dopant metal (indium for germanium PNP). No photolithography, no gas-phase chemistry, no vacuum systems. The process is mechanically straightforward and diagnostically accessible: if a device fails, the failure mode can usually be seen by inspecting the device.

Understanding alloy junction at the physical level — what happens during heating and cooling, why uniformity matters, how to diagnose and fix problems — is the foundation for all subsequent transistor fabrication work.

The Alloy Process: Step by Step

Materials:

• N-type germanium wafers: 0.5-1.0 mm thick, 5-10 mm diameter. Resistivity 0.5-5 Ω·cm for transistor base material.
• Indium metal: 99.99% or better purity. Indium is soft, silvery, and melts at 156°C. Available as sheet, wire, or granules.
• Quartz or alumina boats for holding wafers during heating.
• Tube furnace with temperature controller, capable of 500-600°C.
• Reducing gas supply: hydrogen (from electrolysis cell or zinc-acid generator) or forming gas (5% H2 in N2).

Wafer preparation:

1. Saw wafers from the zone-refined germanium ingot using a fine diamond saw or a slow, controlled abrasive cut.
2. Lap both faces flat on a glass plate with 400-grit silicon carbide abrasive slurry, then 1000-grit, then 4000-grit. Final surface should be smooth and flat.
3. Chemical-mechanical polish: briefly polish on a cloth with 0.05 µm alumina or colloidal silica slurry to remove saw damage.
4. Etch in 10% HCl for 30 seconds to remove surface oxide. Rinse in distilled water. Dry in nitrogen stream.
5. Immediately proceed to indium placement — the cleaned surface oxidizes in minutes.

Indium dot preparation:

• Cut indium sheet into small squares, approximately 0.5 × 0.5 × 0.2 mm. Uniformity improves device matching.
• Weigh on a milligram balance; aim for 0.2-0.5 mg per dot. Record weights.
• Handle with clean tweezers only.

Assembly:

1. Place germanium wafer in quartz boat.
2. Place one indium dot precisely centered on each face. The dots must be well aligned when viewed from above — misalignment means unequal emitter-collector geometry.
3. Load boats into furnace tube.

Alloying:

1. Start reducing gas flow (5-10 standard cm³/minute). Allow 5 minutes to purge air from tube.
2. Ramp furnace to 540°C at 5°C/minute.
3. Hold at 540°C for 3 minutes. The indium melts at 156°C and spreads slightly. At 540°C the indium-germanium binary alloy melts, dissolving germanium into the indium melt to form an indium-germanium liquid alloy at the contact interface.
4. Cool at 2°C/minute down to 200°C, then allow to cool naturally. Controlled cooling during solidification is critical: too fast causes cracking; correct rate produces a recrystallized p-type germanium zone with good crystalline quality.
5. Continue reducing gas flow until below 200°C to prevent oxidation during cooling.

Inspection: The alloyed dots should be shiny, silvery, and well-bonded to the germanium surface. Dull, porous, or irregular dots indicate oxidation or insufficient wetting. Dots that lifted off the surface indicate poor initial contact or contamination. Reject any wafer where a dot is missing or clearly defective.

Junction Depth and Its Control

The depth to which the indium alloy penetrates into the germanium base determines the base width of the finished transistor. Too shallow: both alloy dots do not get close enough, base is too thick, gain is low. Too deep: dots penetrate all the way through the wafer and short emitter to collector.

Alloy depth depends on:

• Temperature: Higher temperature increases germanium solubility in the indium melt, increasing penetration. The relationship is approximately exponential.
• Time at temperature: Longer soak time increases depth. Usually the dominant control.
• Dot size (mass): Larger dot has more indium to fill before reaching germanium solubility limit, so the dot can dissolve more germanium and penetrate deeper.

For a 0.5 mm wafer, target alloy depth 150-200 µm from each face, leaving 100-200 µm of original n-type germanium as base.

Cross-section check: Sacrifice one device per batch. Embed in epoxy. Section perpendicular to the alloyed faces. Polish to expose the cross-section. Etch with dilute HF + HNO3 (1:10) to reveal the junction boundary (etching rate differs slightly between the alloyed p+ region and the n-type base). Measure alloy depth under a 10-100× microscope. Adjust temperature, time, or dot size based on measurements.

Contact Formation

The finished transistor requires three electrical contacts: one to each indium button (emitter and collector) and one to the n-type germanium base.

Emitter and collector contacts: Solder a wire lead directly to each indium button. Use low-temperature indium solder (51% indium, 49% bismuth, melting point 85°C) or tin-indium alloy solder. This prevents exceeding the alloy junction formation temperature during contact attachment. Regular tin-lead solder (melting at 183°C) can also be used carefully with rapid soldering technique.

Base contact: The germanium wafer edge or a dedicated region on the base face. Options:

• Solder with indium solder to the bare germanium surface. Requires clean, lightly etched surface. Adhesion can be poor.
• Gold-germanium alloy contact: evaporate gold onto the base contact area, then briefly heat to form a gold-germanium eutectic (melting point 361°C, below the alloy junction temperature). Gold makes an excellent low-resistance contact to n-type germanium.
• Pressure contact: a thin spring wire pressed against the wafer edge. Less reliable long-term but simpler to implement.

All contacts must have lower resistance than desired for base resistance. Base resistance causes voltage drop that reduces effective base-emitter bias at high currents, reducing gain. Target base contact resistance < 10 Ω.

Typical Performance Characteristics

Alloy junction transistors from a well-controlled process exhibit:

DC current gain (hFE): 20-200 for PNP germanium alloy transistors. Gain variation across a batch: factor of 3-5 is typical with good process control. High gain requires thin base (below 100 µm) and long minority carrier lifetime (> 50 µs in the base).

Collector-emitter voltage (BVCEO): Typically 15-40V for germanium alloy. Determined by collector junction doping and depletion zone breakdown physics.

Maximum collector current (IC max): Set by power dissipation (heat removal from small die) and contact current capacity. Typical signal transistors: 10-100 mA. Power transistors with larger die: 100 mA - 2 A.

Maximum frequency (fT): Determined by base transit time and junction capacitances. Alloy junction transistors typically operate well up to 1-5 MHz, usable to ~20 MHz with careful circuit design. Beyond this, the base transit time limits gain. High-frequency alloy transistors with 25 µm base width can reach 50-100 MHz.

Temperature sensitivity: Germanium’s 0.67 eV band gap generates significant thermal carriers. Above 70°C, thermal leakage becomes unacceptable for most circuits. At 85°C, leakage is often 100× the room-temperature value.

These limitations — frequency, temperature, gain uniformity — drove the transistor industry toward silicon diffusion and then planar processes. The alloy junction transistor is the starting point; systematic understanding of its limitations points toward the next fabrication improvements.