Flashcards in Electronic Band Structure Deck (12):
What do network solids have instead of molecular orbitals
Define the valence and
Band of occupied orbitals
Define the conduction band
Band of unoccupied orbitals
Define band gap
Energy gap between valence and and conduction band. The band gap is the minimum energy a network wiki must absorb to promote an electron from valence band to conduction band
What appearance qualities does a band gap determine?
Small band gap (e.g. Silicon) absorbs all visible light - appears black
Big band gap (e.g. Diamond) absorbs no visible light. Appears transparent
Relate conductivity to band gap
In order for an electron to conduct electricity it must have access to an unoccupied energy level.
The valence band of diamond is full so electrons in the valence band cannot move as there are no nearby empty orbitals
In order for diamond to conduct electricity it must absorb high enough energy to promote electrons from the valence band to the conduction band
Account for the differences in allotropes, graphite and diamond
Graphite is soft solids with tiny band gap. Absorbs all light - black. Highly conducting.
Diamond - big band gap - absorbs no light - transparent - hard solid and insulator
Describe the differences between insulators, intrinsic semiconductors and metal
Insulator: large band gap - electrons cannot be promoted to conduction band
Intrinsic semiconductor: elections can be promoted from valence to conduction upon heating
Metal: no band gap
Identify the 2 different categories of semiconductors
Intrinsic semiconductor is pure and has small band gap - electrons can be promoted to conduction band leaving holes in the valence band
Extrinsic semiconductors are doped - atoms are substituted (with impurities) to increase conductivity.
Define and draw n-type doping
Element with more electrons is added to semiconductive element with less electrons. Extra electrons reside in donor levels just below the conduction band. As the material is heated these electrons are promoted to the conduction band and vastly outnumber any holes in the valence band
Describe and draw a pic of p-type doping
P type doping - there are fewer electrons and more positive charge carriers. This is achieved by substituting with an element tot the left on the periodic table which has fewer electrons.
The electron poor atoms generate acceptor levels just above the valence band. Valence band electrons are promoted to the acceptor levels leaving fm holes in the valence band. I'm a P type semiconductor the major conduction is due to holes