29P Electron Isolated copper Atom Conductor Valence orbit has only one Electron and is loosely bound to core Core

Isolated silicon atom Electron Semiconductor Valence orbit has four electrons 14P r1r1 r2r2 r3r3 Center of core r1r1 r2r2 r3r3 Energy Energy levels in a single atom Electrons in the same orbit has same energy

A silicon crystal is formed by zillions of silicon atoms

14P Covalent Bond Electron Silicon crystal An electron shared by two neighboring atoms to form a covalent bond. This way an atom can have a stable structure with eight valence band electrons.

14P Energy bands Electron(in conduction band) Hole(in valence band) In a crystal, electrons in the same orbit do not have the same energy and thus form energy bands 1 st band 2 nd band Valence band Conduction band Higher band higher energy

14P Electron Hole Thermal energy produces free electron and hole pair (in valence band) (in conduction band)

14P Electron Hole Recombination of free electron and hole (in valence band) (in conduction band)

A B CD E F Free Electron (in conduction band) Hole/electron flow through a semiconductor The hole moves A-B-C-D-E-F (pseudo movement) The electron moves F-E-D-C-B-A Hole 14P (in valence band)

Intrinsic and extrinsic semiconductor Intrinsic = pure Extrinsic = impure or doped

Doping Doping means mixing a pure semiconductor with impurities to increase its electrical conductivity Can be done in two ways: Increasing the number of electrons by mixing pentavalent elements such as phosphorous, arsenic, antimony (means adding donor impurities) Increasing the number of holes by mixing trivalent elements such as aluminum, boron, gallium (means adding acceptor impurities)

15P 14P Free Electron N-type semiconductor Phosphorous atom Has many free electrons in conduction band and few holes In valence band

13P 14P P-type semiconductor Aluminum atom Hole Has few free electrons in conduction band and many holes In valence band

Majority and minority carriers Electrons are Majority carriers in N-type semiconductor Minority carriers in P-type semiconductor Holes are Majority carriers in P-type semiconductor Minority carriers in N-type semiconductor

A diode is formed by putting a N-type and P-type of semiconductor together N typeP type Note: Both N and P-type of materials are electrically neutral Anode Cathode P-N Junction

P typeN type Migration of holes from P to N And electrons from N to P causes a formation of depletion layer This gives rise to barrier potential(E γ ) preventing further migration of holes and electrons Anode Cathode

Energy P N Depletion layer Energy bands in a unbiased diode Conduction band Valence band

P typeN type Forward Biased diode R VBVB AnodeCathode +- VγVγ

Energy P N Smaller depletion layer Energy bands of a forward biased diode Conduction band Valence band

Forward Biased diode The diode behaves like a ‘ON’ switch in this mode Resistance R and diode’s body resistance limits the current through the diode V B has to overcome V γ in order for the diode to conduct

P typeN type Reverse biased diode Larger depletion layer Anode Cathode VBVB

Energy P N Larger Depletion layer Valence band Conduction band Energy bands in a reverse biased diode

Reverse Biased diode The diode behaves like a ‘OFF’ switch in this mode If we continue to increase reverse voltage V B breakdown voltage of the diode is reached Once breakdown voltage is reached diode conducts heavily causing its destruction

Breakdown Diode breakdown is caused by thermally generated electrons in the depletion region When the reverse voltage across diode reaches breakdown voltage these electrons will get sufficient energy to collide and dislodge other electrons The number of high energy electrons increases in geometric progression leading to an avalanche effect causing heavy current and ultimately destruction of diode