Electron

Electrons and holes are the two kinds of charge carrier that actually move inside a semiconductor — electrons are free negative charges, and holes are the absence of an electron in a spot that normally has one, which behaves exactly like a positive charge moving the opposite way. Every transistor, diode, and sensor on a chip works by controlling where these two carriers go and how many of each are available in a given region of silicon at a given moment.

An electron is a genuine particle that can wander through the silicon crystal lattice when it has enough energy to escape its bonded position. In pure silicon at room temperature, nearly every electron stays locked into a covalent bond holding two silicon atoms together, and the material conducts almost nothing. Add a donor dopant like phosphorus, which has one more outer electron than silicon, and that extra electron isn't needed for bonding — it sits loosely attached, easily knocked free by ordinary thermal energy, and from then on it's free to drift through the lattice carrying negative charge wherever an electric field pushes it. This donor-doped material is called n-type, and electrons are its majority carrier.

A hole isn't a particle at all — it's a vacancy, and vacancies move in a way that's mathematically identical to a positive particle moving. Dope silicon instead with a trivalent acceptor like boron, which has one fewer outer electron than silicon needs for a complete bond, and one bonding position is left empty. A neighboring electron can hop into that empty position to fill it, which leaves ITS old position empty instead — and from the outside, it looks exactly like the empty spot itself physically moved in the opposite direction the electron actually traveled. Track that empty spot's position over time, assign it a positive charge and an effective mass, and every equation of motion that works for a real charged particle works for a hole too. This acceptor-doped material is called p-type, and holes are its majority carrier.

Both carrier types exist simultaneously in every piece of silicon — what changes with doping is which one dominates. Even heavily doped n-type silicon still has a small population of holes, and vice versa; these are called minority carriers, and although there are far fewer of them, they matter enormously in device physics. A diode's reverse-leakage current, a solar cell's photocurrent, and a bipolar transistor's entire operating principle all run on minority-carrier behavior that a simpler majority-carrier picture would miss entirely.

Electrons and holes don't move at the same speed under the same electric field, and that asymmetry shapes real transistor design. Electron mobility in silicon is roughly two to three times higher than hole mobility, because electrons sit in a conduction-band energy structure that lets them accelerate more freely than holes do in the valence band. This is the direct reason PMOS transistors (which conduct via holes) are built physically wider than equivalent NMOS transistors (which conduct via electrons) to deliver matching drive current — and it's part of why strain engineering techniques that specifically boost hole mobility, like compressively-strained silicon-germanium source/drain regions, became such a high-value fix once node scaling alone stopped being enough.

When an electron and a hole meet, they annihilate each other in a process called recombination, releasing the energy difference as heat or, in the right material, as light. This is the physical basis of the LED: a direct-bandgap semiconductor engineered so that electron-hole recombination releases its energy as a photon instead of heat. Run the same basic physics backward — shine light on a semiconductor and knock electrons loose, creating electron-hole pairs faster than they'd otherwise recombine — and you get a solar cell or a photodiode. Nearly every optoelectronic device on a chip is just electron-hole generation and recombination, engineered to happen at a controlled rate, in a controlled place, for a controlled purpose.