Unit
- Atomic anatomy: Every atom is constructed from a dense nucleus containing positively charged protons and neutral neutrons, which are orbited by negatively charged electrons. These electrons occupy quantized energy levels rather than random paths. The atomic number, set by proton count, defines the element's identity and its chemical reactivity.
- Electron shells: Electrons populate discrete shells designated K, L, M, N, with each shell following a maximum occupancy of 2n^2 electrons. The principal quantum number n (1,2,3...) determines energy and size. Inner shells fill first; remaining electrons occupy the valence shell, which dictates bonding capacities for elements like silicon.
- Valence electrons: The valence shell's vacancy or occupancy controls electrical behavior. For silicon, germanium, and carbon, four valence electrons exist. This tetravalent configuration permits covalent sharing with four neighboring atoms, creating a stable diamond cubic lattice that underlies semiconductor device fabrication.
- Energy band theory: In a crystal, atoms in close proximity cause individual energy levels to split into continuous bands. The valence band holds bound electrons; the conduction band permits free movement. The forbidden band gap (Eg) separating these bands directly determines whether a material is conductor, insulator, or semiconductor.
- Conduction mechanics: At temperatures above absolute zero, phonon interactions transfer energy to bound electrons. When an electron receives at least Eg, it transitions to the conduction band, leaving behind a hole in the valence band. Applied electric fields cause electrons and holes to drift, producing current flow.
- Group IV elements: Silicon and germanium sit in group IVA of the periodic table, each contributing four valence electrons. These elements crystallize into a face-centered cubic structure with two atoms per lattice point, giving high atomic density. This arrangement is exceptionally stable and forms the substrate for most integrated circuits.
- Temperature dependence: Intrinsic semiconductors are insulators at 0 Kelvin. Rising temperatures generate electron-hole pairs via thermal excitation. Although this increases conductivity, it also introduces temperature noise and leakage currents, requiring compensation circuits in precise analog designs.
- Band gap magnitude: The band gap energy defines electrical and optical properties. Silicon at 1.12 eV supports most digital logic. Germanium at 0.67 eV has higher carrier mobility but more leakage. Gallium arsenide at 1.43 eV enables high-speed and optoelectronic devices. These values guide material selection for each product.
Process Flow
- Atomic stability: Electrons fill lowest energy shells first in accordance with the Aufbau principle.
- Energy absorption: When a photon or thermal energy matches the band gap, an electron excites from valence to conduction band.
- Electron-hole pair generation: The excited electron leaves a hole in the valence band; both carriers participate in conduction.
- Recombination: Free electrons recombine with holes, releasing energy as light or heat, which decays conductivity.
- Equilibrium restoration: Carrier generation and recombination rates balance at constant temperature, setting baseline conductivity.
Safety Consideration
Hazard: Electrostatic discharge (ESD) can damage sensitive semiconductor materials during handling and characterization.
Mitigation: Use grounded wrist straps, conductive mats, and ionizers when working with wafers and components. Ensure equipment is connected to earth.
Quick Revision
- Atoms consist of a nucleus and electrons in quantized shells.
- Valence electrons determine bonding and conductivity.
- Energy bands and band gap classify materials as metals, insulators, or semiconductors.
- Silicon's 1.12 eV band gap is foundational for IC manufacturing.