Semiconductor Diode Project

7/29/2019 Semiconductor Diode Project

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Semiconductor Physics

Semiconductor fundamentals

Doping

Pn junction

The Diode Equation Zener diode

LED


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What Is a Semiconductor?

Many materials, such as most metals, allow electrical current to

flow through them

These are known as conductors

Materials that do not allow electrical current to flow through

them are called insulatorsPure silicon, the base material of most transistors, is considered

a semiconductor because its conductivity can be modulated by

the introduction of impurities


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Semiconductors

A material whose properties are such that it is not quite aconductor, not quite an insulator

Some common semiconductors

elemental

Si - Silicon (most common) Ge - Germanium

compound

GaAs - Gallium arsenide

GaP - Gallium phosphide

AlAs - Aluminum arsenide

AlP - Aluminum phosphide

InP - Indium Phosphide


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Crystalline Solids

In a crystalline solid, the periodic arrangement of atoms is

repeated over the entire crystal

Silicon crystal has a diamond lattice


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Crystalline Nature of Silicon

Silicon as utilized in integrated circuits is crystalline in nature

As with all crystalline material, silicon consists of a repeating

basic unit structure called a unit cel l

For silicon, the unit cell consists of an atom surrounded by fourequidistant nearest neighbors which lie at the corners of the

tetrahedron


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Whats so special about Silicon?

Cheap and abundantAmazing 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 3s and 3p

valence electrons


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Nature of Intrinsic Silicon

Silicon that is free of doping impurities is called

intrinsic

Silicon has a valence of 4 and forms covalent

bonds with four other neighboring silicon atoms


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Semiconductor Crystalline Structure Semiconductors have a regular

crystalline structure

for monocrystal, extends

through entire structure

for polycrystal, structure is

interrupted at irregularboundaries

Monocrystal has uniform 3-

dimensional structure

Atoms occupy fixed positions

relative to one another, but

are in constant vibration about

equilibrium


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Semiconductor Crystalline Structure

Silicon atoms have 4electrons in outer shell

inner electrons are veryclosely bound to atom

These electrons are shared

with neighbor atoms onboth sides to fill the shell

resulting structure isvery stable

electrons are fairly

tightly bound no loose electrons

at room temperature, ifbattery applied, verylittle electric currentflows


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Conduction in Crystal Lattices

Semiconductors (Si and Ge) have 4 electrons in their outer shell

2 in the s subshell

2 in the p subshell

As the distance between atoms decreases the discrete subshells

spread out into bands As the distance decreases further, the bands overlap and then

separate

the subshell model doesnt hold anymore, and the electronscan be thought of as being part of the crystal, not part of the

atom4 possible electrons in the lower band (valence band)

4 possible electrons in the upper band (conduction band)


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Energy Bands in Semiconductors

The space

between the

bands is the

energy gap, or

forbidden band


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Insulators, Semiconductors, and Metals

This separation of the valence and conduction bands determinesthe electrical properties of the material

Insulators have a large energy gap electrons cant jump from valence to conduction bandsno current flows

Conductors (metals) have a very small (or nonexistent) energy gapelectrons easily jump to conduction bands due to thermal

excitationcurrent flows easily

Semiconductors have a moderate energy gap

only a few electrons can jump to the conduction band leaving holesonly a little current can flow


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Insulators, Semiconductors, and Metals

(continued)

Conduction

Band

Valence

Band

Conductor Semiconductor Insulator


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Hole - Electron Pairs

Sometimes thermal energy is enough to cause an electron tojump from the valence band to the conduction band

produces a hole - electron pair Electrons also fall back out of the conduction band into the

valence band, combining with a hole

pair elimination

hole electron

pair creation


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Improving Conduction by Doping

To make semiconductors better conductors, add impurities(dopants) to contribute extra electrons or extra holes

elements with 5 outer electrons contribute an extra electron to

the lattice (donordopant)

elements with 3 outer electrons accept an electron from the

silicon (acceptordopant)


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(cont.) Phosphorus and arsenic are

donor dopants if phosphorus is

introduced into the siliconlattice, there is an extraelectron free to movearound and contribute toelectric current

very loosely bound toatom and can easily jumpto conduction band

produces n type silicon sometimes use + symbol

to indicate heavierdoping, so n+ silicon

phosphorus becomespositive ion after giving upelectron

i C i i


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(cont.)

Boron has 3 electrons in its outershell, so it contributes a hole if itdisplaces a silicon atom

boron is an acceptordopant

yieldsp type silicon

boron becomes negative ionafter accepting an electron

E i i l


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Epitaxial

Growth of

Silicon Epitaxy grows silicon on top of

existing silicon

uses chemical vapordeposition

new silicon has samecrystal structure asoriginal

Silicon is placed in chamber athigh temperature

1200 o C (2150 o F) Appropriate gases are fed into

the chamber other gases add

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

p type very quickly


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Diffusion of Dopants It is also possible to introduce

dopants into silicon by heatingthem so they diffuse into thesilicon

no new silicon is added high heat causes diffusion

Can be done with constantconcentration in atmosphere

close to straight lineconcentration gradient

Or with constant number of atomsper unit area

predepositionbell-shaped gradient

Diffusion causes spreading ofdoped areas

top

side


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Diffusion of Dopants (continued)

Concentration of dopant insurrounding atmosphere kept

constant per unit volume

Dopant deposited on

surface - constantamount per unit area


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Ion Implantation of Dopants

One way to reduce the spreading found with diffusion is to use ionimplantation also gives better uniformity of dopant yields faster devices lower temperature process

Ions are accelerated from 5 Kev to 10 Mev and directed at silicon higher energy gives greater depth penetration total dose is measured by flux

number of ions per cm2 typically 1012 per cm2 - 1016 per cm2

Flux is over entire surface of silicon use masks to cover areas where implantation is not wanted

Heat afterward to work into crystal lattice


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Hole and Electron Concentrations

To produce reasonable levels of conduction doesntrequire much doping

silicon has about 5 x 1022 atoms/cm3

typical dopant levels are about 1015 atoms/cm3

In undoped (intrinsic) silicon, the number of holes andnumber of free electrons is equal, and their productequals a constant

actually, ni increases with increasing temperature

This equation holds true for doped silicon as well, soincreasing the number of free electrons decreases the

number of holes

np = ni2


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INTRINSIC (PURE) SILICON

At 0 Kelvin Silicon

density is 5*10 particles/cm

Silicon has 4 valence

electrons, it covalently bonds

with four adjacent atoms in

the crystal latticeHigher temperatures create

free charge carriers.

A hole is created in theabsence of an electron.

At 23C there are 10

particles/cm of free carriers


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DOPING

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.


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Energy-band Diagram

A very important concept in the study of semiconductors is theenergy-band diagram

It is used to represent the range of energy a valence electron can

have

For semiconductors the electrons can have any one value of acontinuous range of energy levels while they occupy the valence

shell of the atom

That band of energy levels is called the valence band

Within the same valence shell, but at a slightly higher energylevel, is yet another band of continuously variable, allowed energy

levels

This is the conduction band


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Band Gap

Between the valence and the conduction band is a range of energy

levels where there are no allowed states for an electron

This is the band gap

In silicon at room temperature [in electron volts]: Electron voltis an atomic measurement unit, 1 eV energy is

necessary to decrease of the potential of the electron with 1 V.

EG

E eVG

11.

1eV 1.602 10 joule19


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Impurities

Silicon crystal in pure form isgood insulator - all electrons are

bonded to silicon atom

Replacement of Si atoms can alter

electrical properties ofsemiconductor

Group number - indicates number

of electrons in valence level (Si -

Group IV)


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Impurities

Replace Si atom in crystal with Group V atom

substitution of 5 electrons for 4 electrons in outer shell

extra 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 atom

substitution of 3 electrons for 4 electrons

extra 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


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COUNTER DOPING

Insert more than onetype of Ion

The extra electron and

the extra hole cancel out


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A LITTLE MATH

n= number of free electrons

p=number of holes

ni=number of electrons in intrinsic silicon=10/cm

pi-number of holes in intrinsic silicon= 10/cm

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 10 free

electrons (similar for p-type)


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P-N Junction

Also known as a diode

One of the basics of semiconductor technology -

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

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

mobile charges (electrons) in n-type


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P-N Junction

Region of charges left behind (dopants fixed in crystallattice)

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

charge)

Group IV in n-type (one more proton than Si - positivecharge)

Region is totally depleted of mobile charges - depletion

region

Electric field forms due to fixed charges in the depletion

region

Depletion region has high resistance due to lack of mobile

charges


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THE P-N JUNCTION


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The Junction

The potential or voltage acrossthe silicon changes in the depletion

region and goes from + in the n

region toin the p region


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Biasing the P-N Diode

Forward BiasApplies - voltage

to the n region

and + voltage to

the p region

CURRENT!

Reverse BiasApplies + voltage to

n region andvoltage to p region

NO CURRENT

THINK OF THE

DIODE AS A

SWITCH


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P-N JunctionReverse Bias

positive voltage placed on n-type material

electrons in n-type move closer to positive terminal, holes

in p-type move closer to negative terminal

width of depletion region increases

allowed current is essentially zero (small drift current)


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P-N JunctionForward Bias

positive voltage placed on p-type material

holes in p-type move away from positive terminal, electrons in n-

type move further from negative terminal

depletion region becomes smaller - resistance of device decreases

voltage increased until critical voltage is reached, depletion region

disappears, current can flow freely


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P-N Junction - V-I characteristics

Voltage-Current relationship for a p-n junction (diode)


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Current-Voltage Characteristics

THE IDEAL DIODE

Positive voltage yields

finite current

Negative voltage yields

zero current

REAL DIODE


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The Ideal Diode Equation

I IqV

kT

whereI diode current with reverse bias

q coulomb the electronic ch e

keV

K

Boltzmann s cons t

0

0

19

5

1

1602 10

8 62 10

exp ,

. , arg

. , ' tan


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Semiconductor diode - opened region

The p-side is the cathode, the n-side is the anode

The dropped voltage, VD is measured from the cathode

to the anode

Opened: VD VF:

VD = VF

ID = circuit limited, in our model the VD cannot exceed VF


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Semiconductor diode - cut-off region

Cut-off: 0 < VD < VF:

ID 0 mA


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Semiconductor diode - closed region

Closed: VF < VD 0:

VD is determined by the circuit, ID = 0 mA

Typical values of VF: 0.5 0.7 V


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Zener Effect

Zener break down: VD


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LED

Light emitting diode, made from GaAs

VF=1.6 V

IF >= 6 mA


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