1. Review of SemiconductorReview of Semiconductor
Physics, PN Junction DiodesPhysics, PN Junction Diodes
and Resistorsand Resistors
Semiconductor fundamentalsSemiconductor fundamentals
DopingDoping
PnPn junctionjunction
The Diode EquationThe Diode Equation
Zener diodeZener diode
LEDLED
ResistorsResistors

2. What Is a Semiconductor?What Is a Semiconductor?
•Many materials, such as most metals, allow electrical current toMany materials, such as most metals, allow electrical current to
flow through themflow through them
•These are known as conductorsThese are known as conductors
•Materials that do not allow electrical current to flow throughMaterials that do not allow electrical current to flow through
them are called insulatorsthem are called insulators
•Pure silicon, the base material of most transistors, is consideredPure silicon, the base material of most transistors, is considered
a semiconductor because its conductivity can be modulated bya semiconductor because its conductivity can be modulated by
the introduction of impuritiesthe introduction of impurities

3. SemiconductorsSemiconductors
A material whose properties are such that it is not quite aA material whose properties are such that it is not quite a
conductor, not quite an insulatorconductor, not quite an insulator
Some common semiconductorsSome common semiconductors
– elementalelemental
» Si - Silicon (most common)
» Ge - Germanium
– compoundcompound
» GaAs - Gallium arsenide
» GaP - Gallium phosphide
» AlAs - Aluminum arsenide
» AlP - Aluminum phosphide
» InP - Indium Phosphide

4. Crystalline SolidsCrystalline Solids
In a crystalline solid, the periodic arrangement of atomsIn a crystalline solid, the periodic arrangement of atoms isis
repeated over the entire crystalrepeated over the entire crystal
Silicon crystalSilicon crystal has ahas a diamond latticediamond lattice

5. Crystalline Nature of SiliconCrystalline Nature of Silicon
Silicon as utilized in integrated circuits is crystalline in natureSilicon as utilized in integrated circuits is crystalline in nature
As with all crystalline material, silicon consists of a repeatingAs with all crystalline material, silicon consists of a repeating
basic unit structure called abasic unit structure called a unit cellunit cell
For silicon, the unit cell consists of an atom surrounded by fourFor silicon, the unit cell consists of an atom surrounded by four
equidistant nearestequidistant nearest neighborsneighbors which lie at the corners of thewhich lie at the corners of the
tetrahedrontetrahedron

6. What’s so special about Silicon?What’s so special about Silicon?
Cheap and abundant
Amazing mechanical, chemical and
electronic properties
The material is very well-known to
mankind
SiO2: sand, glass
Si is column IV of the
periodic table
Similar to the carbon
(C) and the
germanium (Ge)
Has 3s² and 3p²
valence electrons

7. Nature of Intrinsic SiliconNature of Intrinsic Silicon
Silicon that is free of doping impurities is calledSilicon that is free of doping impurities is called
intrinsicintrinsic
Silicon has a valence of 4 and forms covalentSilicon has a valence of 4 and forms covalent
bonds with four otherbonds with four other neighboringneighboring silicon atomssilicon atoms

8. Semiconductor Crystalline StructureSemiconductor Crystalline Structure
Semiconductors have a regularSemiconductors have a regular
crystalline structurecrystalline structure
– for monocrystal, extendsfor monocrystal, extends
through entire structurethrough entire structure
– for polycrystal, structure isfor polycrystal, structure is
interrupted at irregularinterrupted at irregular
boundariesboundaries
Monocrystal has uniform 3-Monocrystal has uniform 3-
dimensional structuredimensional structure
Atoms occupy fixed positionsAtoms occupy fixed positions
relative to one another, butrelative to one another, but
are in constant vibration aboutare in constant vibration about
equilibriumequilibrium

9. Semiconductor Crystalline StructureSemiconductor Crystalline Structure
Silicon atoms have 4Silicon atoms have 4
electrons in outer shellelectrons in outer shell
– inner electrons are veryinner electrons are very
closely bound to atomclosely bound to atom
These electrons are sharedThese electrons are shared
with neighbor atoms onwith neighbor atoms on
both sides to “fill” the shellboth sides to “fill” the shell
– resulting structure isresulting structure is
very stablevery stable
– electrons are fairlyelectrons are fairly
tightly boundtightly bound
» no “loose” electrons
– at room temperature, ifat room temperature, if
battery applied, verybattery applied, very
little electric currentlittle electric current
flowsflows

10. Conduction in Crystal LatticesConduction in Crystal Lattices
Semiconductors (Si and Ge) have 4 electrons in their outer shellSemiconductors (Si and Ge) have 4 electrons in their outer shell
– 2 in the s subshell2 in the s subshell
– 2 in the p subshell2 in the p subshell
As the distance between atoms decreases the discrete subshellsAs the distance between atoms decreases the discrete subshells
spread out into bandsspread out into bands
As the distance decreases further, the bands overlap and thenAs the distance decreases further, the bands overlap and then
separateseparate
– the subshell model doesn’t hold anymore, and the electronsthe subshell model doesn’t hold anymore, and the electrons
can be thought of as being part of the crystal, not part of thecan be thought of as being part of the crystal, not part of the
atomatom
– 4 possible electrons in the lower band (4 possible electrons in the lower band (valence bandvalence band))
– 4 possible electrons in the upper band (4 possible electrons in the upper band (conduction bandconduction band))

11. Energy Bands in SemiconductorsEnergy Bands in Semiconductors
The spaceThe space
between thebetween the
bands is thebands is the
energy gapenergy gap, or, or
forbidden bandforbidden band

12. Insulators, SemiconductorsInsulators, Semiconductors,, and Metalsand Metals
This separation of the valence and conduction bands determinesThis separation of the valence and conduction bands determines
the electrical properties of the materialthe electrical properties of the material
InsulatorsInsulators have a large energy gaphave a large energy gap
– electrons can’t jump from valence to conduction bandselectrons can’t jump from valence to conduction bands
– no current flowsno current flows
ConductorsConductors (metals) have a very small (or nonexistent) energy gap(metals) have a very small (or nonexistent) energy gap
– electrons easily jump to conduction bands due to thermalelectrons easily jump to conduction bands due to thermal
excitationexcitation
– current flows easilycurrent flows easily
SemiconductorsSemiconductors have a moderate energy gaphave a moderate energy gap
– only a few electrons can jump to the conduction bandonly a few electrons can jump to the conduction band
» leaving “holes”
– only a little current can flowonly a little current can flow

13. Insulators, Semiconductors, and MetalsInsulators, Semiconductors, and Metals
(continued)(continued)
Conduction
Band
Valence
Band
Conductor Semiconductor Insulator

14. Hole - Electron PairsHole - Electron Pairs
Sometimes thermal energy is enough to cause an electron toSometimes thermal energy is enough to cause an electron to
jump from the valence band to the conduction bandjump from the valence band to the conduction band
– produces a hole - electron pairproduces a hole - electron pair
Electrons also “fall” back out of the conduction band into theElectrons also “fall” back out of the conduction band into the
valence band, combining with a holevalence band, combining with a hole
pair elimination
hole electron
pair creation

15. Improving Conduction by DopingImproving Conduction by Doping
To make semiconductors better conductors, add impuritiesTo make semiconductors better conductors, add impurities
(dopants) to contribute extra electrons or extra holes(dopants) to contribute extra electrons or extra holes
– elements with 5 outer electrons contribute an extra electron toelements with 5 outer electrons contribute an extra electron to
the lattice (the lattice (donordonor dopant)dopant)
– elements with 3 outer electrons accept an electron from theelements with 3 outer electrons accept an electron from the
silicon (silicon (acceptoracceptor dopant)dopant)

16. Improving Conduction by DopingImproving Conduction by Doping
(cont.)(cont.)Phosphorus and arsenic arePhosphorus and arsenic are
donor dopantsdonor dopants
– if phosphorus isif phosphorus is
introduced into the siliconintroduced into the silicon
lattice, there is an extralattice, there is an extra
electron “free” to moveelectron “free” to move
around and contribute toaround and contribute to
electric currentelectric current
» very loosely bound to
atom and can easily jump
to conduction band
– producesproduces n typen type siliconsilicon
» sometimes use + symbol
to indicate heavier
doping, so n+ silicon
– phosphorus becomesphosphorus becomes
positive ion after giving uppositive ion after giving up
electronelectron

17. Improving Conduction by DopingImproving Conduction by Doping
(cont.)(cont.)
Boron has 3 electrons in its outerBoron has 3 electrons in its outer
shell, so it contributes a hole if itshell, so it contributes a hole if it
displaces a silicon atomdisplaces a silicon atom
– boron is anboron is an acceptoracceptor dopantdopant
– yieldsyields p typep type siliconsilicon
– boron becomes negative ionboron becomes negative ion
after accepting an electronafter accepting an electron

18. EpitaxialEpitaxial
Growth ofGrowth of
SiliconSilicon
EpitaxyEpitaxy grows silicon on top ofgrows silicon on top of
existing siliconexisting silicon
– uses chemical vaporuses chemical vapor
depositiondeposition
– new silicon has samenew silicon has same
crystal structure ascrystal structure as
originaloriginal
Silicon is placed in chamber atSilicon is placed in chamber at
high temperaturehigh temperature
– 12001200 oo
C (2150C (2150 oo
F)F)
Appropriate gases are fed intoAppropriate gases are fed into
the chamberthe chamber
– other gases addother gases add
impurities to the miximpurities to the mix
Can grow n type, then switch toCan grow n type, then switch to
p type very quicklyp type very quickly

19. Diffusion of DopantsDiffusion of Dopants
It is also possible to introduceIt is also possible to introduce
dopants into silicon by heatingdopants into silicon by heating
them so theythem so they diffusediffuse into theinto the
siliconsilicon
– no new silicon is addedno new silicon is added
– high heat causes diffusionhigh heat causes diffusion
Can be done with constantCan be done with constant
concentration in atmosphereconcentration in atmosphere
– close to straight lineclose to straight line
concentration gradientconcentration gradient
Or with constant number of atomsOr with constant number of atoms
per unit areaper unit area
– predepositionpredeposition
– bell-shaped gradientbell-shaped gradient
Diffusion causes spreading ofDiffusion causes spreading of
doped areasdoped areas
top
side

20. Diffusion of Dopants (continued)Diffusion of Dopants (continued)
Concentration of dopant in
surrounding atmosphere kept
constant per unit volume
Dopant deposited on
surface - constant
amount per unit area

21. Ion Implantation of DopantsIon Implantation of Dopants
One way to reduce the spreading found with diffusion is to use ionOne way to reduce the spreading found with diffusion is to use ion
implantationimplantation
– also gives better uniformity of dopantalso gives better uniformity of dopant
– yields faster devicesyields faster devices
– lower temperature processlower temperature process
Ions are accelerated from 5 Kev to 10 Mev and directed at siliconIons are accelerated from 5 Kev to 10 Mev and directed at silicon
– higher energy gives greater depth penetrationhigher energy gives greater depth penetration
– total dose is measured by fluxtotal dose is measured by flux
» number of ions per cm2
» typically 1012
per cm2
- 1016
per cm2
Flux is over entire surface of siliconFlux is over entire surface of silicon
– use masks to cover areas where implantation is not wanteduse masks to cover areas where implantation is not wanted
Heat afterward to work into crystal latticeHeat afterward to work into crystal lattice

22. Hole and Electron ConcentrationsHole and Electron Concentrations
To produce reasonable levels of conduction doesn’tTo produce reasonable levels of conduction doesn’t
require much dopingrequire much doping
– silicon has about 5 x 10silicon has about 5 x 102222
atoms/cmatoms/cm33
– typical dopant levels are about 10typical dopant levels are about 101515
atoms/cmatoms/cm33
In undoped (intrinsic) silicon, the number of holes andIn undoped (intrinsic) silicon, the number of holes and
number of free electrons is equal, and their productnumber of free electrons is equal, and their product
equals a constantequals a constant
– actually, nactually, nii increases with increasing temperatureincreases with increasing temperature
This equation holds true for doped silicon as well, soThis equation holds true for doped silicon as well, so
increasing the number of free electrons decreases theincreasing the number of free electrons decreases the
number of holesnumber of holes
np = ni
2

23. INTRINSIC (PURE) SILICONINTRINSIC (PURE) SILICON
At 0 Kelvin Silicon
density is 5*10²³
particles/cm³
Silicon has 4 valence
electrons, it covalently bonds
with four adjacent atoms in
the crystal lattice
Higher temperatures create
free charge carriers.
A “hole” is created in the
absence of an electron.
At 23C there are 10¹º
particles/cm³ of free carriers

24. DOPINGDOPING
The N in N-type stands for negative.
A column V ion is inserted.
The extra valence electron is free to
move about the lattice
There are two types of doping
N-type and P-type.
The P in P-type stands for positive.
A column III ion is inserted.
Electrons from the surrounding
Silicon move to fill the “hole.”

25. Energy-band DiagramEnergy-band Diagram
A very important concept in the study of semiconductors is theA very important concept in the study of semiconductors is the
energy-band diagramenergy-band diagram
It is used to represent the range of energy a valence electron canIt is used to represent the range of energy a valence electron can
havehave
For semiconductors the electrons can have any one value of aFor semiconductors the electrons can have any one value of a
continuous range of energy levels while they occupy the valencecontinuous range of energy levels while they occupy the valence
shell of the atomshell of the atom
– That band of energy levels is called theThat band of energy levels is called the valence bandvalence band
Within the same valence shell, but at a slightly higher energyWithin the same valence shell, but at a slightly higher energy
level, is yet another band of continuously variable, allowed energylevel, is yet another band of continuously variable, allowed energy
levelslevels
– This is theThis is the conduction bandconduction band

26. Band GapBand Gap
Between the valence and the conduction band is a range of energyBetween the valence and the conduction band is a range of energy
levels where there are no allowed states for an electronlevels where there are no allowed states for an electron
This is the band gapThis is the band gap
In silicon at room temperature [in electron volts]:In silicon at room temperature [in electron volts]:
Electron voltElectron volt is an atomic measurement unit, 1 eV energy isis an atomic measurement unit, 1 eV energy is
necessary to decrease of the potential of the electron with 1 V.necessary to decrease of the potential of the electron with 1 V.
EG
E eVG = 11.
1eV 1.602 10 joule19
= × −

27. ImpuritiesImpurities
Silicon crystal in pure form isSilicon crystal in pure form is
good insulator - all electrons aregood insulator - all electrons are
bonded to silicon atombonded to silicon atom
Replacement of Si atoms can alterReplacement of Si atoms can alter
electrical properties ofelectrical properties of
semiconductorsemiconductor
Group number - indicates numberGroup number - indicates number
of electrons in valence level (Si -of electrons in valence level (Si -
Group IV)Group IV)

28. ImpuritiesImpurities
Replace Si atom in crystal with Group V atomReplace Si atom in crystal with Group V atom
– substitution of 5 electrons for 4 electrons in outer shellsubstitution of 5 electrons for 4 electrons in outer shell
– extra electron not needed for crystal bonding structureextra electron not needed for crystal bonding structure
» can move to other areas of semiconductor
» current flows more easily - resistivity decreases
» many extra electrons --> “donor” or n-type material
Replace Si atom with Group III atomReplace Si atom with Group III atom
– substitution of 3 electrons for 4 electronssubstitution of 3 electrons for 4 electrons
– extra electron now needed for crystal bonding structureextra electron now needed for crystal bonding structure
» “hole” created (missing electron)
» hole can move to other areas of semiconductor if electrons continually
fill holes
» again, current flows more easily - resistivity decreases
» electrons needed --> “acceptor” or p-type material

29. COUNTER DOPINGCOUNTER DOPING
Insert more than one
type of Ion
The extra electron and
the extra hole cancel out

30. A LITTLE MATHA LITTLE MATH
n= number of free electrons
p=number of holes
ni=number of electrons in intrinsic silicon=10¹º/cm³
pi-number of holes in intrinsic silicon= 10¹º/cm³
Mobile negative charge = -1.6*10-19
Coulombs
Mobile positive charge = 1.6*10-19
Coulombs
At thermal equilibrium (no applied voltage) n*p=(ni)2
(room temperature approximation)
The substrate is called n-type when it has more than 10¹º free
electrons (similar for p-type)

31. P-N JunctionP-N Junction
Also known as a diodeAlso known as a diode
One of the basics of semiconductor technology -One of the basics of semiconductor technology -
Created by placing n-type and p-type material in closeCreated by placing n-type and p-type material in close
contactcontact
Diffusion - mobile charges (holes) in p-type combine withDiffusion - mobile charges (holes) in p-type combine with
mobile charges (electrons) in n-typemobile charges (electrons) in n-type

32. P-N JunctionP-N Junction
Region of charges left behind (dopants fixed in crystalRegion of charges left behind (dopants fixed in crystal
lattice)lattice)
– Group III in p-type (one less proton than Si- negativeGroup III in p-type (one less proton than Si- negative
charge)charge)
– Group IV in n-type (one more proton than Si - positiveGroup IV in n-type (one more proton than Si - positive
charge)charge)
Region is totally depleted of mobile charges - “depletionRegion is totally depleted of mobile charges - “depletion
region”region”
– Electric field forms due to fixed charges in the depletionElectric field forms due to fixed charges in the depletion
regionregion
– Depletion region has high resistance due to lack of mobileDepletion region has high resistance due to lack of mobile
chargescharges

33. THE P-N JUNCTIONTHE P-N JUNCTION

34. The JunctionThe Junction

The “potential” or voltage across
the silicon changes in the depletion
region and goes from + in the n
region to – in the p region

35. Biasing the P-N DiodeBiasing the P-N Diode
Forward Bias
Applies - voltage
to the n region
and + voltage to
the p region
CURRENT!
Reverse Bias
Applies + voltage to
n region and –
voltage to p region
NO CURRENT
THINK OF THE
DIODE AS A
SWITCH

36. P-N JunctionP-N Junction – Reverse Bias– Reverse Bias
positive voltage placed on n-type materialpositive voltage placed on n-type material
electrons in n-type move closer to positive terminal, holeselectrons in n-type move closer to positive terminal, holes
in p-type move closer to negative terminalin p-type move closer to negative terminal
width of depletion region increaseswidth of depletion region increases
allowed current is essentially zero (small “drift” current)allowed current is essentially zero (small “drift” current)

37. P-N JunctionP-N Junction – Forward Bias– Forward Bias
positive voltage placed on p-type materialpositive voltage placed on p-type material
holes in p-type move away from positive terminal, electrons in n-holes in p-type move away from positive terminal, electrons in n-
type move further from negative terminaltype move further from negative terminal
depletion region becomes smaller - resistance of device decreasesdepletion region becomes smaller - resistance of device decreases
voltage increased until critical voltage is reached, depletion regionvoltage increased until critical voltage is reached, depletion region
disappears, current can flow freelydisappears, current can flow freely

38. P-N Junction - V-I characteristicsP-N Junction - V-I characteristics
Voltage-Current relationship for a p-n junction (diode)Voltage-Current relationship for a p-n junction (diode)

39. Current-Voltage CharacteristicsCurrent-Voltage Characteristics
THE IDEAL DIODE
Positive voltage yields
finite current
Negative voltage yields
zero current
REAL DIODE

40. The Ideal Diode EquationThe Ideal Diode Equation
I I
qV
kT
where
I diode current with reverse bias
q coulomb the electronic ch e
k
eV
K
Boltzmann s cons t
=





 −






=
= ×
= ×
−
−
0
0
19
5
1
1602 10
8 62 10
exp ,
. , arg
. , ' tan

41. Semiconductor diode - opened regionSemiconductor diode - opened region
The p-side is the cathode, the n-side is the anodeThe p-side is the cathode, the n-side is the anode
The dropped voltage, VThe dropped voltage, VDD is measured from the cathode tois measured from the cathode to
the anodethe anode
Opened: VOpened: VDD ≥≥ VVFF::
VVDD == VVFF
IIDD = circuit limited, in our model the V= circuit limited, in our model the VDD cannot exceed Vcannot exceed VFF

42. Semiconductor diode - cut-off regionSemiconductor diode - cut-off region
Cut-off: 0Cut-off: 0 << VVDD << VVFF::
IIDD ≅≅ 00 mAmA

43. Semiconductor diode - closed regionSemiconductor diode - closed region
Closed: VClosed: VFF << VVDD ≤≤ 0:0:
– VVDD is determined by the circuit, Iis determined by the circuit, IDD == 00 mAmA
Typical values of VTypical values of VFF: 0.5 ¸ 0.7 V: 0.5 ¸ 0.7 V

44. Zener EffectZener Effect
Zener break down: VZener break down: VDD <= V<= VZZ::
VVDD = V= VZZ, I, IDD is determined by the circuit.is determined by the circuit.
In case of standard diode the typical values of the breakIn case of standard diode the typical values of the break
down voltage Vdown voltage VZZ of the Zener effect -20 ... -100 Vof the Zener effect -20 ... -100 V
Zener diodeZener diode
– Utilization of the Zener effectUtilization of the Zener effect
– Typical break down values of VTypical break down values of VZZ : -4.5 ... -15 V: -4.5 ... -15 V

45. LEDLED
Light emitting diode, made from GaAsLight emitting diode, made from GaAs
– VVFF=1.6 V=1.6 V
– IIFF >= 6 mA>= 6 mA

46. Resistor in an Integrated CircuitResistor in an Integrated Circuit