INTRODUCTION
SEMICONDUCTOR PHYSICS
INTRODUCTION:
SEMICONDUCTOR PHYSICS
• Silicon bond model: electrons and holes
• Generation and recombination
• Thermal equilibrium
• Intrinsic semiconductor
• Extrinsic semiconductor
Silicon Bond Model:
Electrons and Holes
Si is in column IV
of periodic table
Silicon Bond Model:
Electrons and Holes
• Electronic structure of Si atom:
– 10 core electrons (tightly bound)
– 4 valence electrons (loosely bound, responsible
for most chemical properties)
• Other semiconductors:
– Ge, C (diamond form), SiGe
– GaAs, InP, InGaAs, ZnSe, CdTe (on average, 4
valenceelectrons per atom)
Silicon Bond Model:
Electrons and Holes
• Silicon crystal structure
– Silicon is a crystalline material
• Long range atomic arrangement
– Diamond lattice:
• Atoms tetrahedrally bonded by sharing valence
electrons (covalent bonding)
– Each atom shares 8 electrons
• In low energy and stable situation
– Si atomic density: 5 x 1022 cm-3
• Simple “flattened model of Si crystal:
Silicon Bond Model:
Electrons and Holes
• At 0K
– All bonds satisfied  all valence electrons engaged in
bonding
– No free electrons
• At finite temperature
–
–
–
–
Finite thermal energy
Some bonds are broken
“free” electrons (mobile negative charge, -1.6x 10-19 C)
“free” holes (mobile positive charge , 1.6x 10-19 C)
Silicon Bond Model:
Electrons and Holes
Silicon Bond Model:
Electrons and Holes
“Free” electrons & holes are called carriers
– Mobile charged particles
– “electron” means free electron
– not concerned with bonding electrons or core
electrons
n = (free) electron concentration [cm -3]
p = hole concentration [cm -3]
Generation and Recombination
• Generation = break up of covalent bond to form
electron and hole
– Requires energy from thermal or optical sources (or
other external sources)
• Recombination = formation of bond by bringing
together electron and hole
– Releases energy in thermal or optical form
– A recombination event requires 1 electron + 1 hole
Generation & recombination most likely at surfaces
where periodic crystalline is broken.
Thermal Equilibrium
• Thermal Equilibrium = steady state + absence
of external energy sources
• Important consequence:
– In thermal equilibrium and for a given
semiconductor, np product is a constant that
depends only on temperature!
Intrinsic Semiconductor
• When a bond breaks, an electron and a hole are
produced:
n0 = p0
Also:
n0p0 = ni 2
Then:
n0 = p0 = ni
ni = intrinsic carrier concentration [cm -3 ]
In Si at 300K (room temperature): ni = 1x 1010 cm -3
Extrinsic Semiconductor
• Doping
– introduction of foreign atoms to engineer
semiconductor electrical properties
• Donors
– Introduce electron to the semiconductor
– For Si, group-V atoms with 5 valence electrons (As, P,
Sb)
– 4 electrons of donor atom participate in bonding
– 5th electron is easy to release
– Donor site become positively charged
Periodic Table
Source: http://www.chemicool.com/
Valence Electrons
• The valence electrons are the electrons in the
last shell or energy level of an atom.
• The valence electrons increase in number as
you go across a period
• The number of valence electrons stays the
same as you go up or down a group, but they
increase as you go from left to right across the
periodic table
Extrinsic Semiconductor: Donor
• Nd = donor concentration [cm -3 ]
• If Nd << ni , doping irrelevant
– Intrinsic semiconductor  n0 = p0 = ni
• If Nd >> ni , doping controls carrier
concentrations
– Extrinsic semiconductor  n0 = Nd , p0 = ni 2 /Nd
Note:
n0 >> p0  n-type semiconductor
Extrinsic Semiconductor: Donor
group-V atoms with
5 valence electrons
Extrinsic Semiconductor: Acceptor
• Acceptors
– Introduce holes to the semiconductor
– For Si, group-III atoms with 3 valence electrons (B)
– 3 electrons used in bonding to neighboring Si
atoms
– 1 bonding site “unsatisfied”
• Easy to accept neighboring bonding electron to
complete all bonds
• At room temperature, each acceptor releases 1 hole
that is available to conduction
– Acceptor site become negatively charged
Extrinsic Semiconductor: Acceptor
• Na = acceptor concentration [cm -3 ]
• If Na << ni , doping irrelevant
– Intrinsic semiconductor  n0 = p0 = ni
• If Na >> ni , doping controls carrier
concentrations
– Extrinsic semiconductor  p0 = Na , n0 = ni 2 /Na
Note:
p0 >> n0  p-type semiconductor
Extrinsic Semiconductor: Acceptor
group-III atoms with 3
valence electrons
Extrinsic Semiconductor
Pentavalent impurities
- impurity atoms with 5 valence
electrons
- produce n-type semiconductors by
contributing extra electrons
Trivalent impurities
- impurity atoms with 3 valence electrons
- produce p-type semiconductors by
producing a "hole" or electron deficiency
Extrinsic Semiconductor
P-type & N-type Semiconductor
Summary
• In a semiconductor, there are two types of “carriers”:
electrons and holes
• In thermal equilibrium and for a given semiconductor
n0p0 is a constant that only depends on temperature:
n0p0 = ni 2
• For Si at room temperature:
ni = 1x 1010 cm -3
• Intrinsic semiconductor: pure semiconductor
n0 = p0 = ni
Summary
• Carrier concentration can be engineered by addition
of “dopants” (selected foreign atoms):
– Pentavalent impurities (P, As, Sb)  n-type semiconductor:
n0 = Nd , p0 = ni 2 /Nd
– Trivalent impurities (B, Al, Ga)  p-type semiconductor:
p0 = Na , n0 = ni 2 /Na
Video Links from Youtube
AMD MICROPROCESSOR
http://www.youtube.com/watch?v=-GQmtITMdas&feature=related
From Sand to Silicon
http://www.youtube.com/watch?v=Q5paWn7bFg4
CH IP MANUFACTURING PROCESS
http://www.youtube.com/watch?v=9rCyu8B0tYs&feature=related
Semiconductor Electronics Theory Lesson 1 Segment 1 - Semiconductor Atoms
http://www.youtube.com/watch?v=qkjCe0r5-cw
Introduction to Semiconductor Materials (2)
http://www.youtube.com/watch?v=AgkQrCeJF1Y&NR=1
Semiconductors Theory 1 Segment 2A - Doped Silicon Crystal
http://www.youtube.com/watch?v=U8daujO20nM&feature=related