Atoms & Electrons
Atomic structure, electron shells, and valence electrons in semiconductors
Atomic Structure Basics
Atomic Structure Basics
Every atom consists of a nucleus (protons + neutrons) surrounded by electrons in discrete energy levels (shells). For semiconductor physics, the electrons are what matter most:
- Core electrons: Tightly bound to the nucleus, don't participate in bonding or conduction.
- Valence electrons: The outermost electrons that determine chemical and electrical behavior.
Silicon (element 14) has the electron configuration: 1s² 2s² 2p⁶ 3s² 3p². This gives it 4 valence electrons in its outer shell — the key to its semiconductor behavior.
Key Concept: The Rule of Four
Silicon's 4 valence electrons allow it to form exactly 4 covalent bonds with neighboring atoms, creating a stable tetrahedral structure. This is the same bonding pattern as carbon in diamond — which is why silicon's crystal structure is called "diamond cubic."
Covalent Bonding in Silicon
Covalent Bonding in Silicon
In pure crystalline silicon, each atom shares its 4 valence electrons with 4 neighboring silicon atoms, forming covalent bonds. At absolute zero (0 K), all electrons are locked in bonds and silicon acts as an insulator.
At room temperature (~300 K), thermal energy breaks some bonds, freeing electrons to conduct. This is why semiconductor conductivity increases with temperature — more thermal energy = more broken bonds = more free carriers.
Analogy: The Dance Floor
Imagine dancers (electrons) holding hands in pairs (bonds). At low temperature, everyone is paired up and standing still. As the music gets louder (temperature rises), some pairs break apart and individual dancers start moving around the floor (conducting electricity).
Knowledge Check
Knowledge Check
1 / 2How many valence electrons does silicon have?