semiconductor physics mine aa

8/11/2019 Semiconductor Physics Mine Aa

1/30


2/30

Why Semiconductors?

Information

acquisition(sensors)

Image, sound,

temperature,pressure,

Informationprocessing

(Amps, A/D,processors,tranceivers)

Information

processing(tranceivers,processors, )

Displays

Information transmission(wires, busses, cables, optical fibers, or just air!)

Brains and muscles of thesystem are made ofsemiconductors

Metals & dielectrics are used astransmission media

Why?

Image, sound,



3/30

Whats common for all the core components?

Light, sound,


sensorVoltage,current

input

o u t p

u t

AV in V in

V in V out

V out

V in

V out

Modulation of some physical quantity (output) by some others

Some kind of gain, conversion ratio, sensitivity, etc


4/30

Example: Field-Effect Transistors (FETs)Semiconductor vs Metal

V in V out

V in V out

FETs are building blocks.

S D

G

Schematic illustration of a FET

For SiO2

dielectric, breakdown field E b

~ 10 7 V/cm. No matter how thick it is, the maximum inducedcarrier area density is r 0 E b/q = 2 10 13 /cm 2.

For a 1 m thick Si channel,ni = 1.45 10 10 /cm 3,

the background carrier area density isni 10 4 cm = 1.45 10 6 /cm 2.

In principle, the area carrier density, and thereforethe channel conductance, can be modulated by 7orders of mag!!!

For Al, n = 1.8 10 23 /cm 3. Even for 1 nm thin (monolayers!) Al, the background carrierarea density is 1.8 10 16 /cm 2. The conductance can only be modulated by 0.1%!!!

What are semiconductors, anyway???


5/30

What Is a Semiconductor?

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

These are known as conductorsMaterials that do not allow electrical current to flow through them are calledinsulators

Pure silicon, the base material of most transistors, is considered asemiconductor because its conductivity can be modulated by the introductionof impurities


6/30

Semiconductors

A material whose properties are such that it isnot quite a conductor, 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


7/30

Semiconductors: Si, Ge, and Compound (III-V, II-VI)

Four valenceelectronsCovalent bonding:no free electrons at

0K

P-type dopants

N-type dopants

Review: materials and devices(after El-Kareh, Streetman & Kano)

Dopants have to be compatible with processing (ex. slow

diffusion through oxide) to have high solubility in Si


8/30

Silicon Crystal Structure

Diamond lattice (Si, Ge, GaAs)

Two interpenetrating FCCstructures shifted by a/4 inall three directions

All atoms in both FCCs

Atoms inside one FCC come fromthe second lattice

Diamond covalent bonding

(100) Si for devices(111) Si not used - oxide charges

Plummer


9/30

Semiconductor CrystallineStructure

Silicon atoms have 4 electronsin outer shell inner electrons are very

closely bound to atom These electrons are shared

with neighbor atoms on bothsides to fill the shell

resulting structure is verystable

electrons are fairly tightlybound

no loose electrons at room temperature, ifbattery applied, very littleelectric current flows


10/30


11/30

Energy Bands inSemiconductors

The spacebetween thebands is theenergy gap , or

forbidden band


12/30

The quantized atomic energy

levels broaden into energybands when the atomscombine to form a solid.

N = number of atoms in the solid

Pauli Exclusion Principle:only 1 electron with a givenset of quantum numbers canbe in a state (2 can be thereif they have opposite spin).

Energy Band Model


13/30

Energy Band Structure In Solid State


14/30

Simplified energy-band diagram illustratingthe increase in the band-width with increasingprinciple quantum number.

Energy Levels in Periodic SolidState (crystalline materials)

Band Structure in Crystals


15/30

Shallow donors and acceptors in silicon

Compensated semiconductor

Two Level Band Structure

Metals

Semiconductors

Insulators


16/30

Insulators, Semiconductors, andMetals

This separation of the valence and conduction bandsdetermines the electrical properties of the material

Insulators have a large energy gap electrons cant jump from valence to conduction bands

no current flows Conductors (metals) have a very small (or nonexistent)

energy gap electrons easily jump to conduction bands due to thermal

excitation current flows easily

Semiconductors have a moderate energy gap only a few electrons can jump to the conduction band

leaving holes only a little current can flow


17/30

Semiconductors

The materials whose electrical conductivity lies between those

of conductors and insulators, are known as semiconductors.

Silicon 1.1 eVGermanium 0.7 eVCadmium Sulphide 2.4 eV

Silicon is the most widely used semiconductor.

Semiconductors have negative temperature coefficients of

resistance, i.e. as temperature increases resistivity deceases


18/30

Energy Band Diagram

Conductionelectrons


19/30

Energy Band Diagram

Forbidden energy band is smallfor semiconductors.

Less energy is required forelectron to move from valence

to conduction band. A vacancy (hole) remains whenan electron leaves the valenceband.

Hole acts as a positive chargecarrier.


20/30

Both fu l l and empty bands do no t p a r take in e lec t r ical con duc t ion .

.:: Semiconductor, Insulators, Conductors ::.

Full band

All energy levels areoccupied by electrons

Empty band

All energy levels are empty( no electrons)


21/30

.:: Semiconductor energy bands at low temperature ::.

At low temperatures the valanceband is full , and the conductionband is empty.

Recall that a full band can notconduct, and neither can an emptyband.

At low temperatures, s/cs do notconduct, they behave likeinsulators.

The thermal energy of the electronssitting at the top of the full band ismuch lower than that of the Eg atlow temperatures .

Forbiddenenergy gap [Eg]

Emptyconductionband

Fullvalanceband

E l e c

t r o n e n e r g y


22/30

Conduction Electron :

Assume some kind of energy isprovided to the electron ( valenceelectron ) sitting at the top of thevalance band.

This electron gains energy from theapplied field and it would like tomove into higher energy states.

This electron contributes to theconductivity and this electron iscalled as a conduction electron .

At 0 0K, electron sits at the lowestenergy levels. The valance band isthe highest filled band at zero

kelvin.


Emptyconductionband

Fullvalanceband


23/30

When enough energy is supplied tothe e - sitting at the top of thevalance band, e - can make atransition to the bottom of theconduction band.

When electron makes such atransition it leaves behind a missingelectron state.

This missing electron state is calledas a hole.

Hole behaves as a positive chargecarrier.

Magnitude of its charge is the samewith that of the electron but with anopposite sign.

Semiconductor energy bands at room temperature


Fullvalanceband

Emptyconductionband

+e - +e - +e - +e - energy


24/30

Conclusions ::.

Holes contribute to current in valance band (VB) as e -sare able to create current in conduction band (CB).

Hole is not a free particle. It can only exist within thecrystal. A hole is simply a vacant electron state.

A transition results an equal number of e - in CB andholes in VB. This is an important property of intrinsic, or

undopeds/cs

. For extrinsic , or doped , semiconductorsthis is no longer true.


25/30

Intrinsic Semiconductor

Both silicon and germanium aretetravalent, i.e. each has four

electrons (valence electrons) intheir outermost shell.

Each atom shares its fourvalence electrons with its fourimmediate neighbours, so thateach atom is involved in four

covalent bonds.

A semiconductor, which is in its extremely pure form, is knownas an intrinsic semiconductor. Silicon and germanium are themost widely used intrinsic semiconductors.


26/30

Intrinsic Semiconductor

When the temperature of an intrinsic semiconductor isincreased, beyond room temperature a large number ofelectron-hole pairs are generated.

Since the electron and holes are generated in pairs so,

Free electron concentration (n) = concentration of holes (p)= Intrinsic carrier concentration (n i)


27/30

Extrinsic Semiconductor

Pure semiconductors have negligible conductivity at roomtemperature. To increase the conductivity of intrinsicsemiconductor, some impurity is added. The resultingsemiconductor is called impure or extrinsic semiconductor.

Impurities are added at the rate of ~ one atom per 10 6 to 10 10 semiconductor atoms. The purpose of adding impurity is toincrease either the number of free electrons or holes in a

semiconductor.


28/30

Extrinsic SemiconductorTwo types of impurity atoms are added to the semiconductor

Atoms containing 5valance electrons

(Pentavalent impurity atoms)

Atoms containing 3valance electrons

(Trivalent impurity atoms)e.g. P, As, Sb, Bi e.g. Al, Ga, B, In

N-type semiconductor P-type semiconductor


29/30

N-type Semiconductor

The semiconductors which are obtained by introducing

pentavalent impurity atoms are known as N-typesemiconductors.

Examples are P, Sb, As and Bi. These elements have 5electrons in their valance shell. Out of which 4 electrons willform covalent bonds with the neighbouring atoms and the 5 th electron will be available as a current carrier. The impurity atomis thus known as donor atom.

In N-type semiconductor current flows due to the movement ofelectrons and holes but majority of through electrons. Thuselectrons in a N-type semiconductor are known as majoritycharge carriers while holes as minority charge carriers.


30/30

P-type Semiconductor

The semiconductors which are obtained by introducing trivalentimpurity atoms are known as P-type semiconductors.

Examples are Ga, In, Al and B. These elements have 3electrons in their valance shell which will form covalent bondswith the neighbouring atoms.

In P-type semiconductor current flows due to the movement ofelectrons and holes but majority of through holes. Thus holes ina P-type semiconductor are known as majority charge carrierswhile electrons as minority charge carriers.

The fourth covalent bond will remain incomplete. A vacancy,which exists in the incomplete covalent bond constitute a hole.The impurity atom is thus known as acceptor atom.

semiconductor physics mine aa

Documents