Semiconductors

Semiconductors

Molecular orbitals • In diamond, there is a large energy gap b/w the filled and empty MO’s

Molecular orbitals • This means that diamond is an insulator • e- have to be excited • silicon is similar but the bands are closer

Therefore... • Silicon is a semiconductor-can conduct e-, but not many

To make it better... • You can enhance the conductivity of silicon by “doping” it • replace some Si atoms with As (one more e-)

Doping • The extra e- become available for conducting • they are easily excited to the conducting bands

n-type semiconductor • Substance whose conductivity is increased by doping it with atoms having more valence e-’s

Another kind of doping • Replace the silicon by doping the crystal with an element that has one less valence e- • B

doping • This creates electron “holes” and e-’s have to jump to fill the hole • p-type semiconductor

p-n junction • Putting two semiconductors together- makes great source of AC • p 468

Molecular Solids Revisited briefly

Molecular Solids • Molecules occupy lattice positions • strong covalent bonds within, but weak forces in between

Molecular Solids • Many have no dipole • intermolecular forces are LDF • size up, LDF up

Molecular Solids • When there are dipoles, IM forces are greater • with H-bonding, even greater

Ionic solids Really quickly...

Ionic solids • Stable, held together by strong, electrostatic forces b/w oppositely charged ions