MAHENDRA SINGHMAHENDRA SINGH

PGT PHYSCIS PGT PHYSCIS KV,NO 1,AFS,JODHPURKV,NO 1,AFS,JODHPUR JODHPUR, RAJASTHANJODHPUR, RAJASTHAN

Semiconductor PhysicsSemiconductor Physics

Semiconductor fundamentalsSemiconductor fundamentals DopingDoping PnPn junction junction The Diode EquationThe Diode Equation Zener diodeZener diode LEDLED

What Is a Semiconductor?What Is a Semiconductor?

                                                                                                              

                                                                                           

•Many materials, such as most metals, allow electrical current to Many 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 through Materials 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 considered Pure silicon, the base material of most transistors, is considered a semiconductor because its conductivity can be modulated by a semiconductor because its conductivity can be modulated by the introduction of impuritiesthe introduction of impurities

SemiconductorsSemiconductors

A material whose properties are such that it is not quite a A 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

Crystalline SolidsCrystalline Solids

In a crystalline solid, the periodic arrangement of atomsIn a crystalline solid, the periodic arrangement of atoms is is repeated over the entire crystalrepeated over the entire crystal

Silicon crystal Silicon crystal has a has a diamond latticediamond lattice

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 repeating As with all crystalline material, silicon consists of a repeating

basic unit structure called a basic unit structure called a unit cellunit cell For silicon, the unit cell consists of an atom surrounded by four For silicon, the unit cell consists of an atom surrounded by four

equidistant nearest equidistant nearest neighborsneighbors which lie at the corners of the which lie at the corners of the tetrahedrontetrahedron

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

Nature of Intrinsic SiliconNature of Intrinsic Silicon

Silicon that is free of doping impurities is called Silicon that is free of doping impurities is called intrinsicintrinsic

Silicon has a valence of 4 and forms covalent Silicon has a valence of 4 and forms covalent bonds with four otherbonds with four other neighboring neighboring silicon atomssilicon atoms

Semiconductor Crystalline StructureSemiconductor Crystalline Structure Semiconductors have a regular Semiconductors have a regular

crystalline structurecrystalline structure

– for monocrystal, extends for monocrystal, extends through entire structurethrough entire structure

– for polycrystal, structure is for polycrystal, structure is interrupted at irregular interrupted at irregular boundariesboundaries

Monocrystal has uniform 3-Monocrystal has uniform 3-dimensional structuredimensional structure

Atoms occupy fixed positions Atoms occupy fixed positions relative to one another, butrelative to one another, butare in constant vibration about are in constant vibration about equilibriumequilibrium

Semiconductor Crystalline StructureSemiconductor Crystalline Structure Silicon atoms have 4 Silicon atoms have 4

electrons in outer shellelectrons in outer shell– inner electrons are very inner electrons are very

closely bound to atomclosely bound to atom These electrons are shared These electrons are shared

with neighbor atoms on with neighbor atoms on both sides to “fill” the shellboth sides to “fill” the shell

– resulting structure is resulting structure is very stablevery stable

– electrons are fairly electrons are fairly tightly boundtightly bound

» no “loose” electrons– at room temperature, if at room temperature, if

battery applied, very battery applied, very little electric current little electric current flowsflows

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 subshells As the distance between atoms decreases the discrete subshells spread out into bandsspread out into bands

As the distance decreases further, the bands overlap and then As the distance decreases further, the bands overlap and then separateseparate

– the subshell model doesn’t hold anymore, and the electrons the subshell model doesn’t hold anymore, and the electrons can be thought of as being part of the crystal, not part of the can 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))

Energy Bands in SemiconductorsEnergy Bands in Semiconductors

The space The space between the between the bands is the bands is the energy gapenergy gap, or , or forbidden bandforbidden band

Insulators, SemiconductorsInsulators, Semiconductors,, and Metals and Metals

This separation of the valence and conduction bands determines This separation of the valence and conduction bands determines the electrical properties of the materialthe electrical properties of the material

InsulatorsInsulators have a large energy gap have 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 thermal electrons easily jump to conduction bands due to thermal

excitationexcitation– current flows easilycurrent flows easily

SemiconductorsSemiconductors have a moderate energy gap have 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

Insulators, Semiconductors, and Metals Insulators, Semiconductors, and Metals (continued)(continued)

Conduction Band

Valence Band

Conductor Semiconductor Insulator

Hole - Electron PairsHole - Electron Pairs Sometimes thermal energy is enough to cause an electron to Sometimes 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 the Electrons 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

Improving Conduction by DopingImproving Conduction by Doping

To make semiconductors better conductors, add impurities To 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 to elements 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 the elements with 3 outer electrons accept an electron from the silicon (silicon (acceptoracceptor dopant) dopant)

Improving Conduction by Doping Improving Conduction by Doping (cont.)(cont.) Phosphorus and arsenic are Phosphorus and arsenic are

donor dopantsdonor dopants– if phosphorus is if phosphorus is

introduced into the silicon introduced into the silicon lattice, there is an extra lattice, there is an extra electron “free” to move electron “free” to move around and contribute to around and contribute to electric currentelectric current

» very loosely bound to atom and can easily jump to conduction band

– produces produces n type n type siliconsilicon» sometimes use + symbol

to indicate heavier doping, so n+ silicon

– phosphorus becomes phosphorus becomes positive ion after giving up positive ion after giving up electronelectron

Improving Conduction by Doping Improving Conduction by Doping (cont.)(cont.)

Boron has 3 electrons in its outer Boron has 3 electrons in its outer shell, so it contributes a hole if it shell, so it contributes a hole if it displaces a silicon atomdisplaces a silicon atom– boron is an boron is an acceptoracceptor dopant dopant– yields yields p type p type siliconsilicon– boron becomes negative ion boron becomes negative ion

after accepting an electronafter accepting an electron

Epitaxial Epitaxial Growth of Growth of

SiliconSilicon EpitaxyEpitaxy grows silicon on top of grows silicon on top of

existing siliconexisting silicon– uses chemical vapor uses chemical vapor

depositiondeposition– new silicon has same new silicon has same

crystal structure as crystal structure as originaloriginal

Silicon is placed in chamber at Silicon is placed in chamber at high temperaturehigh temperature– 1200 1200 oo C (2150 C (2150 oo F) F)

Appropriate gases are fed into Appropriate gases are fed into the chamberthe chamber– other gases add other gases add

impurities to the miximpurities to the mix Can grow n type, then switch to Can grow n type, then switch to

p type very quicklyp type very quickly

Diffusion of DopantsDiffusion of Dopants It is also possible to introduce It is also possible to introduce

dopants into silicon by heating dopants into silicon by heating them so they them so they diffusediffuse into the into the siliconsilicon– no new silicon is addedno new silicon is added– high heat causes diffusionhigh heat causes diffusion

Can be done with constant Can be done with constant concentration in atmosphereconcentration in atmosphere– close to straight line close to straight line

concentration gradientconcentration gradient Or with constant number of atoms Or with constant number of atoms

per unit areaper unit area– predepositionpredeposition– bell-shaped gradientbell-shaped gradient

Diffusion causes spreading of Diffusion causes spreading of doped areasdoped areas

top

side

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

Ion Implantation of DopantsIon Implantation of Dopants One way to reduce the spreading found with diffusion is to use ion One 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

Hole and Electron ConcentrationsHole and Electron Concentrations To produce reasonable levels of conduction doesn’t To produce reasonable levels of conduction doesn’t

require much dopingrequire much doping– silicon has about 5 x 10silicon has about 5 x 102222 atoms/cm atoms/cm33

– typical dopant levels are about 10typical dopant levels are about 101515 atoms/cm atoms/cm33

In undoped (intrinsic) silicon, the number of holes and In undoped (intrinsic) silicon, the number of holes and number of free electrons is equal, and their product number of free electrons is equal, and their product equals a constantequals a constant– actually, nactually, nii increases with increasing temperature increases with increasing temperature

This equation holds true for doped silicon as well, so This equation holds true for doped silicon as well, so increasing the number of free electrons decreases the increasing the number of free electrons decreases the number of holesnumber of holes

np = ni2

INTRINSIC (PURE) SILICONINTRINSIC (PURE) SILICON

At 0 Kelvin Silicon density is 5*10²³

particles/cm³Silicon 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

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.”

Energy-band DiagramEnergy-band Diagram A very important concept in the study of semiconductors is the A 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 can It is used to represent the range of energy a valence electron can

havehave For semiconductors the electrons can have any one value of a For semiconductors the electrons can have any one value of a

continuous range of energy levels while they occupy the valence continuous range of energy levels while they occupy the valence shell of the atomshell of the atom– That band of energy levels is called the That band of energy levels is called the valence bandvalence band

Within the same valence shell, but at a slightly higher energy Within the same valence shell, but at a slightly higher energy level, is yet another band of continuously variable, allowed energy level, is yet another band of continuously variable, allowed energy levelslevels– This is the This is the conduction bandconduction band

Band GapBand Gap

Between the valence and the conduction band is a range of energy Between 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 volt Electron volt is an atomic measurement unit, 1 eV energy is is 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

ImpuritiesImpurities Silicon crystal in pure form is Silicon crystal in pure form is

good insulator - all electrons are good insulator - all electrons are bonded to silicon atombonded to silicon atom

Replacement of Si atoms can alter Replacement of Si atoms can alter electrical properties of electrical properties of semiconductorsemiconductor

Group number - indicates number Group number - indicates number of electrons in valence level (Si - of electrons in valence level (Si - Group IV)Group IV)

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 electrons substitution 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

COUNTER DOPINGCOUNTER DOPING

Insert more than one type of Ion

The extra electron and the extra hole cancel out

A LITTLE MATH A 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)

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 close Created by placing n-type and p-type material in close

contactcontact Diffusion - mobile charges (holes) in p-type combine with Diffusion - mobile charges (holes) in p-type combine with

mobile charges (electrons) in n-typemobile charges (electrons) in n-type

P-N JunctionP-N Junction Region of charges left behind (dopants fixed in crystal Region of charges left behind (dopants fixed in crystal

lattice)lattice)– Group III in p-type (one less proton than Si- negative Group III in p-type (one less proton than Si- negative

charge)charge)

– Group IV in n-type (one more proton than Si - positive Group IV in n-type (one more proton than Si - positive charge)charge)

Region is totally depleted of mobile charges - “depletion Region is totally depleted of mobile charges - “depletion region”region”– Electric field forms due to fixed charges in the depletion Electric field forms due to fixed charges in the depletion

regionregion

– Depletion region has high resistance due to lack of mobile Depletion region has high resistance due to lack of mobile chargescharges

THE P-N JUNCTIONTHE P-N JUNCTION

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

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

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, holes electrons 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)

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 region voltage increased until critical voltage is reached, depletion region

disappears, current can flow freelydisappears, current can flow freely

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)

Current-Voltage CharacteristicsCurrent-Voltage Characteristics

THE IDEAL DIODE

Positive voltage yields finite current

Negative voltage yields zero current

REAL DIODE

The Ideal Diode EquationThe Ideal Diode Equation

I IqV

kT

where

I diode current with reverse bias

q coulomb the electronic ch e

keV

KBoltzmann s cons t

0

0

19

5

1

1602 10

8 62 10

exp ,

. , arg

. , ' tan

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 is measured from the cathode

to the anodeto the anode

Opened: VOpened: VDD V VFF::

VVDD == VVFF

IIDD = circuit limited, in our model the V= circuit limited, in our model the VDD cannot exceed V cannot exceed VFF

Semiconductor diode - cut-off regionSemiconductor diode - cut-off region

Cut-off: 0Cut-off: 0 << VVDD << VVFF::

IIDD 00 mAmA

Semiconductor diode - closed regionSemiconductor diode - closed region

Closed: VClosed: VFF < < VVDD 0:0:– VVDD is determined by the circuit, I is determined by the circuit, IDD == 00 mAmA

Typical values of VTypical values of VFF: 0.5 ¸ 0.7 V: 0.5 ¸ 0.7 V

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 break In case of standard diode the typical values of the break down voltage Vdown voltage VZZ of the Zener effect -20 ... -100 V of 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

LEDLED

Light emitting diode, made from GaAsLight emitting diode, made from GaAs

– VVFF=1.6 V=1.6 V

– IIFF >= 6 mA >= 6 mA