WHY does silicon bond this way?
Silicon has atomic number 14, electron configuration 1s22s22p63s23p2. Its valence shell (n=3) holds 4 valence electrons. A full outer shell wants 8 electrons (the octet). Silicon is exactly halfway: it can neither easily give away 4 electrons (too much energy) nor grab 4 (too much energy). So the cheapest route to a full shell is to share — each atom shares one electron with each of 4 neighbours, giving each atom access to 4+4=8 electrons.
Deriving the intrinsic idea from scratch: if a bond needs Eg to break, then the number of broken bonds (free electrons) rises steeply with temperature, following Boltzmann statistics:
ni∝e−Eg/(2kBT)Why the 2? Breaking one bond creates a pair (electron + hole), and the energy Eg is split across the two carrier populations in the exponent — this is derived by minimising the free energy of the electron-hole system (mass-action law np=ni2).
What is silicon's coordination number in the crystal?
4 (tetrahedral, one bond to each neighbour)
Why does silicon bond covalently rather than ionically?
With 4 valence electrons, giving away or grabbing 4 costs too much energy; sharing is cheapest to reach an octet.
How many electrons fill each Si atom's outer shell in the crystal?
8 (its own 4 + 4 shared from neighbours)
What is the band gap of silicon at 300 K?
≈ 1.12 eV
Why is silicon an insulator at 0 K?
All valence electrons are locked in covalent bonds; no free carriers exist.
What is created when a covalent bond breaks in silicon?
A free electron AND a hole (an electron–hole pair)
What is a hole and does it carry current?
A missing bonded electron; it behaves as a mobile positive charge and does carry current.
Temperature dependence of intrinsic carrier concentration?
ni∝e−Eg/(2kBT) — rises steeply with T.
Why is diamond an insulator but silicon a semiconductor despite identical bonding?
Diamond's much larger band gap (~5.5 eV) makes bond-breaking essentially impossible at room temperature.
What lattice structure does crystalline silicon form?
Diamond cubic, with 109.5° tetrahedral bond angles.
Recall Feynman: explain to a 12-year-old
Imagine 4 friends and you each have one hand free. To feel safe (a "full team of 8 hands around you"), you hold hands with 4 neighbours — you give one hand, they give one hand, and everyone feels complete. That hand-holding is the covalent bond. When it's cold, everyone holds tight and nobody can move → no electricity. When it gets hot, some hands let go, and those freed hands (electrons) can run around → now electricity flows. The empty spot left behind (a hole) also seems to "move" as friends shuffle to fill it. That in-between behaviour — locked when cold, free when warm — is what makes silicon a semiconductor, perfect for switches in computer chips.
Silicon ka atom group IV ka hai, matlab uske outer shell mein sirf 4 valence electrons hote hain. Ab stable hone ke liye har atom ko 8 electrons chahiye (octet). To silicon na to 4 electron de sakta hai aasani se, na hi 4 grab kar sakta hai — dono mein bahut energy lagti hai. Isliye woh smart raasta chunta hai: electrons ko share karta hai. Har Si atom apne 4 padosiyon ke saath ek-ek electron pair share karta hai, jisse har atom ko effectively 8 electrons feel hote hain. Yehi covalent bonding hai, aur yeh atoms ko ek tetrahedral (109.5°) diamond-cubic crystal mein baandhta hai.
Ab hardware ke liye important baat: 0 K (bilkul thanda) par saare electrons bonds mein locked hote hain, koi free electron nahi — isliye pure silicon ek insulator jaisa behave karta hai. Lekin jaise hi thoda heat do (ya doping karo), kuch bonds toot jaate hain. Ek electron free ho jata hai aur peeche ek hole (missing electron) chhod jata hai jo positive charge ki tarah move karta hai. Isi liye silicon "semiconductor" kehlata hai — beech ka banda, na pura conductor na pura insulator.
Kitni aasani se bond tootega yeh band gapEg≈1.12 eV decide karta hai. Formula ni∝e−Eg/2kBT batata hai ki temperature badhaane par free carriers exponentially badhte hain. Compare karo diamond se — wahi bonding, lekin Eg≈5.5 eV, isliye diamond insulator reh jata hai. Yaad rakho: 4 padosi share karte hain, 8 electrons feel hote hain, aur gap chhota hai — bas yehi silicon ki poori kahaani hai jo chips ko possible banati hai.
