2.2 Conductors, Semiconductors & Insulators

Classifying Materials by Electron Mobility

Syllabus topic 3.1 continues into how molecular structure decides whether a substance is a conductor, semiconductor, or insulator. For the CAAS SAR-66 Module 3 exam, classification is not trivia—it tells you why a bus bar gets hot, why a loom can short after chafing, and why a line-replaceable unit (LRU) dies after a careless static zap.

The practical test is simple: under a modest electric field at room temperature, do electrons (or other charge carriers) move readily?

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Conductors: Copper and Aluminium on Aircraft

Copper is the workhorse of aircraft electrical systems. Its single valence electron is weakly bound, so a dense free-electron population gives low resistivity, good ductility for crimping, and reliable solderability on many terminations. Pure copper also has high thermal conductivity, which helps dissipate I²R heating in high-current feeders.

Aluminium appears in some power feeders and bus structures where weight saving matters. Aluminium also has three valence electrons available to conduction, but:

• Resistivity is higher than copper for the same cross-section, so aluminium feeders are sized larger for the same current.
• Aluminium forms a tough oxide film that raises contact resistance if joints are not prepared and treated correctly.
• Coefficients of thermal expansion and galvanic behaviour differ from copper—mixed Cu–Al joints need approved hardware and anti-oxidant compounds.

Worked comparison: Suppose a copper feeder of cross-section A carries current I with acceptable voltage drop. An aluminium feeder for the same drop typically needs a larger A because aluminium’s resistivity is roughly 1.6× that of copper. Weight may still favour aluminium on long heavy-current runs, which is why design offices—not random hangar improvisation—specify the metal.

Technician traps:

• Never “upgrade” a wire gauge or metal without engineering authority.
• Never sand aluminium joint faces and leave them untreated; oxide regrows quickly.
• Silver plating on some contacts reduces fretting corrosion; do not abrade plating off.

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Insulators: Glass, Rubber, Plastics, and Ceramics

Insulators have molecular structures that lock valence electrons in place. In glass and ceramics, strong ionic/covalent networks leave almost no free carriers at service voltages. In rubber and polymer wire jackets (PVC, PTFE/Teflon, polyimide, cross-linked polyethylene, and elastomer blends), long molecular chains bind electrons tightly while remaining flexible enough for looms.

Aircraft insulation must survive more than voltage:

• Temperature — engine-bay and high-current bundles need higher temperature ratings.
• Fluids — Skydrol, fuels, oils, and cleaning agents attack some plastics.
• Abrasion and vibration — chafing against structure removes insulation and creates intermittent shorts—the classic “ghost” fault.
• Moisture and contamination — conductive films of dirt and water can bridge insulator surfaces even when the bulk material is sound (surface tracking).

Scenario: A technician finds scorched grommet material where a bundle rubbed a sharp lightening hole. The copper conductor was fine; the insulator failed first. Module 3 language: the molecular structure that blocked free electrons was physically destroyed, exposing a low-resistance metallic path to structure (ground fault).

Dry air is also an insulator—until ionization voltage is reached. That is why connector pin spacing, conformal coating, and moisture seals matter in avionics.

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Semiconductors: Silicon and Germanium

Silicon and germanium each have four valence electrons. In a pure crystal they form a covalent lattice: at absolute zero, essentially no free carriers exist. At room temperature, a small number of electrons gain enough thermal energy to break bonds, leaving mobile electrons and positive holes. Conductivity sits between metals and insulators—and can be engineered.

Doping adds trace impurities:

• Donor impurities (extra valence electrons) create n-type material with electron majority carriers.
• Acceptor impurities create p-type material with hole majority carriers.

Joined p–n regions form diodes, transistors, and integrated circuits that fill modern aircraft: FADEC, radios, EFIS, solid-state power controllers. Germanium was historically important; silicon dominates today because of temperature stability and manufacturing maturity, but exam questions may still mention both.

Heat and light raise semiconductor conductivity further (more carriers). That is useful in sensors and catastrophic in overheated modules. Unlike copper, you cannot treat a semiconductor as a simple ohmic lump; its behaviour is non-linear and polarity-sensitive.

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Aircraft Wiring and Insulation — Putting the Classes Together

A typical aircraft wire is a composite system:

1. Conductor — stranded copper (often tin- or silver-plated) or aluminium for specialised feeders.
2. Primary insulation — polymer chosen for dielectric strength and temperature class.
3. Jacket / braid / shield — mechanical protection and EMI control on signal wires.
4. Termination — crimp or solder designed so the conductor carries current and the insulator keeps it where it belongs.

Maintenance implications aligned with Module 3:

• Measure insulation resistance with approved megohm techniques when the AMM requires it—you are checking whether the insulator still blocks charge.
• Replace, do not “tape forever,” damaged insulation on primary power.
• Keep semiconductors in LRUs protected: grounded wrist straps, ESD mats, and sealed packaging exist because semiconductor junctions are destroyed by tiny static energies that copper bus bars would ignore.

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Quick Decision Tree for Exam Questions

1. 1–3 valence electrons, metallic feel, low resistance? → Conductor (Cu/Al family).
2. 4 valence electrons, doping or diode behaviour mentioned? → Semiconductor (Si/Ge).
3. Glass, rubber, plastic, ceramic, or “prevents current”? → Insulator.
4. Aircraft context about chafing or fluid attack? → Focus on insulation failure, not magic changes to copper’s atomic number.

Remember: the conductor carries the useful current; the insulator defines the safe path; the semiconductor switches and processes signals. Module 3 wants you fluent in all three roles.