A whirlwind tour of the materials that make up everything we will discuss. Each one's role in electronics ultimately traces back to one of the physics ideas above.
6.1 Conductors
- Copper (Cu). The default for wiring everything from PCB traces to power lines. High conductivity, ductile, oxidation-resistant (forms a passivating layer). Resistivity: 1.7 × 10⁻⁸ Ω·m.
- Aluminum. Lighter and cheaper than copper, used in long-distance power transmission and (historically) IC interconnect. Modern fabs switched to copper around 2000 because its lower resistance enables faster chips.
- Gold. Used for wire bonds inside chip packages and for connector platings. Does not oxidize.
- Silver. Highest conductivity of any metal, but expensive and tarnishes. Used in some high-frequency RF components and in MLCC inner electrodes.
- Tungsten. High melting point, used as filaments inside chip vias and in old incandescent bulbs.
6.2 Insulators
- Silicon dioxide (SiO₂). Glass. The classic gate dielectric of MOSFETs for fifty years. At thicknesses below 1.5 nm, it leaks too much (tunneling) and was replaced by high-k dielectrics around 2007.
- Hafnium oxide (HfO₂). A modern "high-k" dielectric — about 6× the permittivity of SiO₂ — used as the gate dielectric in cutting-edge transistors. Allows a thicker physical layer for the same electrical effect, dramatically reducing tunneling.
- FR-4. Glass-fiber-reinforced epoxy. The brown-green material of nearly every consumer-grade PCB. Dielectric constant about 4.5, dielectric loss tangent low enough for digital signaling but bad above ~1 GHz.
- Polyimide (Kapton). Yellow flexible film used in flex circuits and high-temperature insulation.
- Air, vacuum. Best insulator we have, but mechanically inconvenient.
6.3 Semiconductors
- Silicon (Si). The dominant material of all modern electronics. Indirect-bandgap (1.12 eV); cheap; abundant (sand); tolerates high temperatures and well-understood fabrication.
- Germanium (Ge). First mass-produced semiconductor, displaced by silicon for being unstable at higher temperatures. Now used in some high-speed RF transistors and silicon-germanium heterojunction transistors (HBTs).
- Gallium arsenide (GaAs). Direct band-gap (1.42 eV), high mobility — used in RF amplifiers, satellite communications, fiber-optic lasers.
- Indium phosphide (InP). Highest-mobility semiconductor in mass production; used in 100+ Gbps optical transceivers.
- Gallium nitride (GaN). Wide band-gap (3.4 eV); used in high-voltage, high-efficiency power transistors. The reason your USB-C charger is half the size of a 2010 charger.
- Silicon carbide (SiC). Wide band-gap (3.3 eV); used in EV powertrains, high-power motor drives.
6.4 Magnetic materials
- Iron, nickel, cobalt and their alloys. Strong ferromagnetism — atomic magnetic moments line up over macroscopic regions. Used in transformer cores, motor windings, hard-disk platters.
- Ferrites. Ceramic magnetic materials (e.g., MnZn, NiZn). Lossier than iron at low frequency, but at high frequency (kHz to MHz) they are better because they have very low eddy-current losses (high resistivity). Used in switch-mode power supplies, RF transformers, and the suppression beads on USB cables.
- Permalloy. A nickel-iron alloy with very high magnetic permeability — used in magnetic shielding (around CRT yokes, sensitive transformers).
Hard disks deserve a special mention: each bit is a tiny magnetic domain in a thin film of cobalt-platinum on the spinning platter, with the magnetic orientation either north-up or north-down. A read head (a magnetoresistive sensor) flies a few nanometers above the platter and senses the orientation. Hard disks are the largest commercially deployed application of quantum mechanics in the 21st century — modern read heads use giant magnetoresistance (GMR) discovered in the 1980s, for which Albert Fert and Peter Grünberg won the 2007 Nobel Prize.
6.5 Superconductors
Below a critical temperature (, varying from less than 1 K up to about 130 K depending on the material), certain materials lose all electrical resistance. Superconductors are not just "very good conductors" — they are exactly zero resistance, with bizarre additional properties (perfect diamagnetism, magnetic flux quantization).
- Niobium-titanium (NbTi). Superconducts below about 9 K. Used in MRI machine magnets — every hospital MRI is a superconducting magnet kept cold by liquid helium.
- Yttrium-barium-copper-oxide (YBCO). A "high-temperature" superconductor, K, kept cold with cheaper liquid nitrogen. Used in some research applications and high-energy-physics experiments.
- Aluminum and niobium thin films are used to build Josephson junctions — the building blocks of superconducting quantum computer qubits (IBM, Google, Rigetti). These run below 20 millikelvin in dilution refrigerators.
- SQUIDs (Superconducting Quantum Interference Devices). The most sensitive magnetometers ever built — sensitive enough to measure brain magnetic activity (magnetoencephalography). Also used in some hardware-security labs to detect minute fields from running chips.
6.6 Nanomaterials and exotica
- Carbon nanotubes. Rolled-up graphene sheets. Conductive or semiconducting depending on how they are rolled. Possible future transistor channels at the nanometer scale.
- Graphene. Single-atom-thick sheet of carbon. Highest mobility of any known material. Most uses today are in composites and conductive coatings; transistors made from graphene are still research.
- Liquid crystals. Ordered liquids whose molecular orientation can be controlled by an electric field, modulating optical transmission. Every LCD screen — laptops, watches, kindle, calculator — uses this.
- Photoresists. Photo-sensitive polymers used in lithography. Expose to light, the chemistry changes; develop, you have a pattern. The industrial throughput of these is in the billions of square meters per year.