1

Properties Related to Band Theory

History of Semiconductors Conductivity Intrinsic and Extrinsic Semiconductors Direct & Indirect Bandgap Semiconductors p-n Junction Photoelectricity Nanoscaled Semiconductor

Semiconductors Historical Timelines

Semiconductor Products are Proliferating

LANs WANs

Routers Hubs

Switches

Workstations Interne

t Servers

Video Games

Voice Over IP

Digital Cameras

Wireless Handset

s PDAs

PCs Storage Systems

Set-Top Boxes

Internet Browsers

Scanners

Digital Copiers

Internet

The Next Big Thing…A Lot of Little Things

2

$27.0

$12.8

0

5

10

15

20

25

30

2000 2005

Billio

ns

US

$

China Semiconductor Consumption

China to grow faster than the world at CAGR (Compound Annual Growth Rate) of 17% from 20012005, reaching $27B in 2005

China currently produces only 1 of every 4 chips it consumes

The Electronics Ecosystem

Materials

Semiconductor Equipment

Semiconductors

Electronic End Equipment

SEMI MEMBERSHIP

$990B

2001

Estimate 2004

$21B

$28B

$139B

$879B

$28B

$46B

$218B

Three Types of Solid Materials Based on Electrical Conductivity

1084-4-8-12-16-20 101010 10 10 10 10

glass

diamond

fused silica

silicon

germanium

iron

copper

insulators semiconductors metals

0-2410

Conductivity ( -1cm-1 )

isolator

= alloy increasing resistivity

resistivity below Tc = 0 !!

decreasing resistivity

Temperature Dependence of the Electrical Conductivity of Metals and Semiconductors

(Isolators)

3

Electrical Conductivity

Conductivity of metals decreases with temperature as atomic vibrations scatter free electrons.

Conductivity of semiconductors increases with temperature as the number of carriers increases.

Creation of Carriers in Intrinsic Semiconductors by Thermal Excitation

Thermally induced electrical conductivity

T=0 K Conduction band empty Valence band completely filled No electrical conductivity T>>0 K The thermal energy is responsible for

the promotion of electrons to the conduction band.

Creation of electronhole pairs: carriers electrical conductivity

Experimental Observation: Conductivity of Semiconductors

Semiconductor block connected to the terminals of a battery

No conductivity observed at low or room temperature or in the dark.

When we increase the temperature or expose the semiconductor to light, we observe that it starts conduction.

Semiconductors

Intrinsic Semiconductors: If a semiconductor crystal contains no impurities, the only charge carriers present are thus produced by thermal breakdown of the covalent bonds. The conducting properties are thus characteristic of the pure semiconductor. Such a crystal is termed an intrinsic semiconductor. Extrinsic Semiconductors: If a semiconductor crystal contains n-type or p-type impurities, the conducting properties are chiefly due to the impurities. Such a crystal is termed an extrinsic semiconductor.

4

Conductivity of Intrinsic Semiconductors

•The valence band of semiconductors is completely filled. However, the band gap between the valence and conduction bands is small, and electrons can be promoted to the conduction band. •In semiconductors, only the electrons promoted to the conduction band and the holes created in the valence band will be carriers. •The smaller the gap, the easier to promote electrons to the conduction band. •At the same temperature, smaller gap semiconductors will show a larger conductivity. •The higher the temperature, the larger the number of carriers. •Conductivity increases with temperature

TK2

E

B

g

Ce

the Response of Equilibrium to Temperature

The van’t Hoff equation

12

0

12

2

o

P

0

T/1T/1

R/HKlnKln

RT

H

dT

Klnd

R

H

)T/1(d

)K(lnd

Temperature Effects

Intrinsic semiconductors

Concentration of holes and free electrons increase with temperature. Because increasing thermal energy will excite more e- across the band gap.

Ge has a greater charge concentration than Si. Because Ge has a smaller band gap than Si (0.67 vs 1.11)

Carrier Mobility

The intrinsic carrier mobility is defined as the drift velocity per unit electric field.

Similar to metals, charge carriers in semiconductors lose mobility with increasing dopant concentration.

E

VD

5

Carrier Mobility

Temperature also affects carrier mobility. Note regardless of dopant concentration, high temperatures reduce mobility.

Delocalized Bonding Model

energy

Conduction band

Valence band

electrons

holes

Bonding Picture of Silicon

Delocalized bonding picture

mobile holes:acid species electrons:basic species

Semiconductors and Acid-Base Analogy

Chemical Equilibrium in Solution H2O H++OH-

Kw=[H+][OH-] [H+]1014 ions/cm3

Chemical Equilibrium in Solid

Si(crystal)h++e-

K=[h+][e-]=p•n

[h+]1.5x1010cm-3

6

Donor States: ntype Semiconductors

If an atom in the lattice is substituted by an atom of a different element with more valence electrons, once the impurity is accommodated to the lattice and the new bonds are formed, there will be a remaining negative charge.

Example: Pentavalent Sb impurity in a silicon crystal (tetrahedrally coordinated)

Extrinsic Semiconductors:

Valence Band

Donor States: ntype Semiconductors

Acceptor States: ptype Semiconductors

If an atom in the lattice is substituted by an atom of a different element with less valence electrons, once the impurity is accommodated to the lattice and the new bonds are formed, there will be a remaining positive charge.

Example: Trivalent boron (B) impurity in a silicon crystal (tetrahedrally coordinated)

Extrinsic Semiconductors:

Valence Band

Acceptor States: ptype Semiconductors

7

Intrinsic vs. Extrinsic

Valence Band

Conduction Band

Intrinsic Extrinsic (doped)

Conduction Band

Valence Band

h+

e-

Conduction Band

Valence Band

ntype ptype

Silicon Si:P Si:Al

ptype semiconductors Addition of acceptor states

ntype semiconductors Addition of donor states

Examples: P, As, or Sb impurities in Si or Ge.

Examples: B, Al, Ga, or In impurities in Si or Ge.

Extrinsic (Doped) Semiconductors

We can enhance the electrical properties of a semiconductor by adding impurities to it. The addition of impurities is called doping and the doped semiconductor is called extrinsic.

Example: the addition of 1 Boron atom every 105 Silicon atoms enhance the conductivity of Silicon by a factor of 103 at room temperature.

Extrinsic semiconductors are the basic materials in the electronics technology. Great importance in current technology: lasers, solar cells, rectifiers, transistors, ...

Band Diagram (n and p type)

Electrons can jump to Al atom

Electrons can jump from P atom to Conduction Band

n type

p type

Acceptor level in Band Gap

Ea

Donor Level in Band Gap

Ed

Temperature Effects

undoped

Extrinsic ntype Semiconductors Low Temperatures Thermal energy is insufficient to excite electrons from the donor state Intermediate Temperatures e’s from donor state are excited into the conduction band. e concentration equal to dopant concentration. High Temperatures Enough thermal energy to excite an effective amount of valence e’s into the conduction band

ptypes behave similarly with temperature

8

Fermi Level

Ef

metal

Ef

undoped semiconductor

Ef

Ef

ptype semiconductor ntype semiconductor

Ed Ea

The pH of aqueous solutions and the Fermi level in semiconductors play analogous roles in determining the extent of ionization in the two media.

Analogy Between pH and Fermi Level (Ef)

Extent of Ionization: Weak Acid Acceptor Analogy

When pH = pKa

[HA] = [A]

Acidbase system Semiconductor

When Ef ~ Ea, [A] ~ [A]

Ea = acceptor energy level

aa

a

KlogpK

]HA[

]A][H[K

AHHA

AhA

Energy Levels for Impurities in Silicon

Donors

Acceptors

e

h+

shallow

shallow

deep

9

Interaction of Light and Electrons

absorption

spontaneous

emission

stimulated

emission

Optical Properties of Semiconductors

longest wavelength absorption to promote e corresponds to Eg

Eg is energy between “HOMO” of valence band and “LUMO” of conduction band

Eg

Absorption Emission

Eg

Band Gap Energy and Color

.

Bandgap e

nerg

y (e

V)

Color thatcorresponds toband gap energy

Apparent colorof material(unabsorbed light)

4

3

2

1

red

yellow

greenblue

violet

colorless

black

yellow

orange

ultraviolet

infrared

red

Semiconductor Glossary

Direct Bandgap Semiconductor: semiconductor in which the bottom of the conduction band and the top of the valence band occur at the momentum k=0; in this case, energy released during bandtoband electron recombination with a hole is converted primarily into radiation (radiant recombination); wavelength of emitted radiation is determined by the energy gap of semiconductor. e.g. GaAs, InP, etc. Indirect Bandgap Semiconductor: semiconductor in which bottom of the conduction band does not occur at effective momentum k=0, i.e. is shifted with respect to the top of the valence band which occurs at k=0; energy released during electron recombination with a hole is converted primarily into phonon; e.g. Si, Ge, GaP.

10

An important property of direct semiconductors is that electrons may easily drop from the conduction band to the valence band by emitting a photon

This process is known as electron-hole recombination since the electron drops to occupy a hole state in the valence band the energy of the photon emitted by the semiconductor is determined by the size of its energy gap recombination is therefore analogous to the level transitions that occur in atomic systems

Bandstructure in Three Dimensions

E

k

PHOTON

Electron-hole Recombination in a direct semiconductor such as GaAs

An electron drops from the conduction band to the valence band and its excess energy is emitted in the form of a photon

Note that in the figure shown here the initial and final wavevector states are the same … this is an important property of direct semiconductors

In indirect semiconductors, the bottom of the conduction band and the top of the valence band occur at different points in kspace

An electron cannot therefore drop from the conduction band to the valence band just by emitting a photon since this would violate momentum conservation instead the electron must simultaneously emit a photon and exchange momentum with the crystal lattice the probability of this double process occurring is very small, so indirect semiconductors turn out to be much poorer emitters of light than direct ones

E

k

PHOTON

Electron-hole recombination in an indirect Semiconductor

In order to conserve energy and momentum, an electron must drop to the valence band by emitting a photon and exchanging momentum with the crystal

Because this process has a low probability, indirect semiconductors such as Si or Ge cannot be used in optoelectronic applications as light emitters

Bandstructure in Three Dimensions

E

k

PHOTON

The opposite process to recombination is electronhole generation in which an electron is excited from the valence band into the conduction band by absorbing a photon

Since this process also must conserve momentum the electron is excited into a state with the same kvalue as the initial valenceband state Both direct and indirect semiconductors may therefore be used as photodetectors to detect electromagnetic radiation The absorption of these materials strongly increases once the photon energy exceeds the direct band gap

Bandstructure in Three Dimensions

absorption of light by direct (left) and indirect (right) semiconductors

E

k

PHOTON

Luminescence

11

Solid line: direct bandgap materials Dotted line: indirect bandgap materials

Matched system to reduce the strain effect and epitaxial growth defects!

What's Luminescence?

The spontaneous emission of light upon electronic excitation is called luminescence.

Absorption and Luminance pn Junction

What happens if we bring a ptype semiconductor in contact with a ntype semiconductor?

Electrons close to the junction diffuse across the junction into the ptype region. Holes are filled by recombination.

Equilibrium is established resulting in a potential difference.

If the two regions are connected in a circuit a variety of applications are possible.

p n

- - - -

e

+ + + +

h+

12

Biasing the pn Junction

Biasing introduction of a voltage into the circuit containing the pn junction.

Forward bias negative voltage is applied to ntype side. Decreases energy barrier for electrons and holes to flow through the junction.

Reverse bias positive voltage applied to ntype side. Raises energy barrier for current flow.

p n

V + —

e

“Majority Carrier” and Current Flow in ptype Silicon

p Type Silicon + -

Hole Flow

Current Flow

“Majority Carrier” and Current Flow in ntype Silicon

Electron Flow

n Type Silicon + -

Current Flow

the pn Junction

p n 0 Volts

Hole Diffusion

Electron Diffusion

Holes and Electrons “Recombine” at the Junction

13

A Depletion Zone (D) and a Barrier Field Forms at the pn Junction

The Barrier Field Opposes Further Diffusion

(Equilibrium Condition)

p ++ n 0 Volts

Hole (+) Diffusion

Electron () Diffusion

D

Barrier Field

Donor Ions

Acceptor Ions

“Forward Bias” of a pn Junction

•Applied voltage reduces the barrier field •Holes and electrons are “pushed” toward the junction and the depletion zone shrinks in size •Carriers are swept across the junction and the depletion zone •There is a net carrier flow in both the p and n sides = current flow!

p + n + Volts

Volts

Current

“Reverse Bias” of a pn Junction

p +++ n Volts

D + Volts Current

•Applied voltage adds to the barrier field •Holes and electrons are “pulled” toward the terminals, increasing the size of the depletion zone. •The depletion zone becomes, in effect, an insulator for majority carriers. •Only a very small current can flow, due to a small number of minority carriers randomly crossing D (= reverse saturation current)

Forward Bias Holes and free electrons flow together and recombine at the junction. Current flows. Reverse Bias Holes and free electrons flow away from each other. The center of the diode quickly becomes a dead zone with no charge carriers. Current is reduced.

14

Energydiagram of pn Junction

When ptype and ntype semiconductors touch, the Fermi levels do not align until equilibrium is reached.

Applications of pn Junction Diode

Rectifier

Photodetectors

solar cells

LEDs

diode lasers

Optoelectronics

Why call it pn Junction as a Diode?

PN junction

Simple Application: Rectifier

One of the most important uses of a diode is rectification. The normal p-n junction diode is wellsuited for this purpose as it conducts very heavily when forward biased (lowresistance direction) and only slightly when reverse biased (high resistance direction). If we place this diode in series with a source of ac power, the diode will be forward and reverse biased every cycle. Since in this situation current flows more easily in one direction than the other, rectification is accomplished.

15

pn Rectifying Junction

A diode’s properties can be seen when the voltage is examined

Optoelectronics

In optoelectronic applications of semiconductor devices, the basic idea is that the device is used to either detect or to emit electromagnetic radiation

In detection, the incident light is converted into a

measurable electrical signal by exploiting internal carrier processes within the device. Examples of such devices include photodetectors and solar cells

In emission, on the other hand, the internal

processes allow the conversion of an electrical signal into detectable light and examples of such devices include LEDs and lasers

Photodetector

Photodetector converts optical energy into electrical energy, thus making possible data reading in the optical storage systems, such as CD or DVD drives. Modern photodetectors are typically semiconductor photodiodes.

socalled "reverse bias pn photodiode" with the carriers flowing away from the pn junction thus creates a depletion region. There is very little current flowing through this junction until the light illuminates the surface of the photodiode. Then, the absorbed photons create pairs of electrons and holes mostly in the depletion area. Those new carriers move quickly in opposite directions, and moving electrons create current in the external circuit.

LED (LightEmitting Diodes)

LEDs are pn junction devices constructed of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP). Silicon and germanium are not suitable because those junctions produce heat and no appreciable IR or visible light.

The junction in an LED is forward biased and when electrons cross the junction from the n- to the p-type material, the electronhole recombination process produces some photons in the IR or visible in a process called electroluminescence. An exposed semiconductor surface can then emit light.

16

Lightemitting diodes (LEDs) operate in the opposite manner to photodetectors by exploiting the enhanced diffusion of carriers that occurs across the depletion region in forwardbiased junctions in direct semiconductors, the additional carriers recombine through bandtoband processes giving rise to the emission of light from the junction

PHOTON

PHOTON

PHOTON

when the p-n junction is forward biased, large numbers of electrons and holes diffuse across the depletion region

in a direct semiconductor, these carriers may recombine by emitting photons

the photon flux increases as the forwardbias voltage and so the corresponding diffusion current is increased

LED

With forward bias, electrons reaching the ptype side can recombine with the abundant holes and emit light according to the energy difference. Holes reaching the ntype side can recombine with the abundant electrons emitting light.

Color of luminescence is controlled by the composition of the solid solutions in the semiconductors.

A great advantage of semiconductor optoelectronic devices is that they can be fabricated in a highly compact manner and can even be incorporated into integrated circuits

semiconductor

lens

In contrast to photodetectors, an important requirement for light emitting diodes is that they be fabricated from a direct semiconductor

While gallium arsenide is a direct semiconductor, its energy gap (1.42 eV) corresponds to a photon wavelength (870 nm) that lies outside of the visible spectrum

For display applications, it is therefore necessary to use alloys of GaAs which allow access to photon frequencies in the visible range of the spectrum.

ALLOY COLOR

GaAs0.6P0.4Ga RED

As0.35P0.65:N ORANGE-RED

GaAs0.14P0.86:N YELLOW

GaP:N GREEN

GaP:ZnO RED

AlGaAs RED

AlInGaP ORANGE

AlInGaP YELLOW

AlInGaP GREEN

SiC BLUE

GaN BLUE

color characteristics of commercial LEDs

changes in alloy composition are exploited to modify the energy band gap or to introduce impurity levels that mediate photon emission in indirect semiconductors

the development of the blue GaN LED (from 1994) now allows the possibility of manufacturing full color LED-based displays

17

Band Gap Engineering in Semiconductors: Solid Solutions

Trends in cubic unit cell lattice parameter and Eg as a function of the composition x for the solid solution ternary semiconductor AlxGa1-xAs:

Band gap engineering enables a range of optical and electronic devices to be fabricated.

The roughly linear dependence of the physical properties on composition is known as Vegard’s law and proves that the distribution of the Al and Ga is random :

P(AlxGa1-xAs) = xP(AlAs) + (1x)P(GaAs) P = physical property Any physical property is the atomic fraction

weighted average of the two end members.

Semiconductor Heterostructures In addition to alloying, fabricating artificial structures with

tailored optical and electronic properties has been possible using crystal growth techniques, such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). These techniques allow monolayer control in the chemical composition of the growing crystal. When two different semiconductors are grown into a single structure, the structure is called heterostructure. One such structure is superlattice in which two (or more) semiconductors A and B are grown alternately with thickness dA and dB,respectively.

GaAsAlGaAs superlattice. On the left is a sequence of nearly thirty different layers, while on the right the individual atomic resolution is indicated.

In1-xGaxN bandgap (room temp)

Define Emission Color via Band Engineering

InGaN growth 780C 760C 720C 690C 630C

Indium (%) ~5% ~10% ~20% ~30% ~35%

ΔΕ(eV) 3.18 2.95 2.64 2.38 2.14

Emission (nm) 390 420 470 520 580

FWHM (nm) 7 27 30 48 61

)x1(x43.1)x1(77.0x42.3)x(EG

Wu, et al, Superlattices Microstruct., 2003,34, 63

Electrically Driven SQW Nanowire Multicolor LEDs

—— Tuning emission colors from ultraviolet to visible —— Can be assembled and individually addressed

Lieber*, Nano Lett., 2005, 5, 2287

18

What is a solar cell?

A solar cell is a kind of semiconductor device that takes advantage of the photovoltaic effect, in which electricity is produced when the semiconductor's p-n junction is irradiated.

Solar Cell (Photovoltaic)

If light of sufficient energy strikes the semiconductor, electrons are promoted into the high energy state and move toward the ntype semiconductor. Holes are also generated and move toward the ptype semiconductor.

This creates an electric current.

Magnitude of current depends on intensity of light.

p n

e-

h+

h

V

Electron Flow in a Solar Cell

ntype ptype

h

e-

h+

How A Solar Cell Works

When sunlight strikes a solar cell, only certain bands (or wavelengths) of light will cause electrons to move within the semiconductor, thereby producing electric current. The energy "band gap" of the semiconductor determines the ideal portion of the light spectrum that will create this effect. To allow it all to happen, the semiconductor layers must be constructed so as to produce an electric field (shown as the layer above).

19

Amorphous Silicon Solar Cells

Amorphous silicon solar cells are cells containing noncrystalline silicon, which are produced using semiconductor techniques. Amorphous silicon solar cells are mostly used as power sources for devices requiring little electricity or as modulated light sensors. They are common in pocket calculators, watches, light detectors for cameras, and television and car navigator screens.

Solar Vehicle Project

“Spirit of Canberra” II solar vehicle (Australian)

Amorphous Silicon and Solar Cell House

Self-supplying Solar Cell House in Germany

Water + primary energy sources Hydrogen + oxygen water

Clean Energy by Means of Advanced Materials

20

Transistor

Collector (n) Emitter (n)

Base (p)

Both pn junctions are reversed biased

First transistor 1947 John Bardeen, Walter Brattain, William Shockley.

1947

Pictorial History of Transistors How a Transistor Works

• The transistor can function as: – An insulator – A conductor

• The transistor's ability to fluctuate between these two states that enables to switch or amplify.

• The transistor has many applications, but only two basic functions: switching and modulation (amplification).

• In the simplest sense, the transistor works like a dimmer. – With a push the knob of the dimmer, the light comes on

and off. You have a switch. Rotate the knob back and forth, and the light grows brighter, dimmer, brighter, dimmer. Then you have a modulator.

• Both the dimmer and the transistor can control current flow. • Both can act as a switch and as a modulator/amplifier. • The important difference is that the “hand” operating the

transistor is millions of times faster.

21

MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) MOSFET transistor: consists of two small islands of

p-type semiconductor created within n-type silicon substrate. Islands are connected by narrow p-type channel.

Metal contacts are made to islands (source and drain), one more contact (gate) is separated from channel by a thin (< 10 nm) insulating oxide layer.

Gate serves the function of the base in a junction transistor (the electric field induced by the gate controls the current through the transistor)

MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor)

Voltage applied from source encourages carriers (holes in the case shown below) to flow from the source to the drain through the narrow channel.

Width (and hence resistance) of channel is controlled by intermediate gate voltage. For example, if positive voltage is applied to the gate, most of the holes are repelled from the channel and conductivity is decreasing.

Current flowing from the source to the drain is therefore modulated by the gate voltage (amplification and switching)

p n n

gate

electrode

Potential as seen

by electrons

When Vbias > 0

Gate voltage > Vt

e e e e e e

e e e

e e e

e e e

e e e

e e e

p+ p+ p+ p+

p+ p+ p

+ p+

Vbias

Metal-

Oxide-

Semiconductor

Field-

Effect

Transistor

Electron

potential

energy

(negative of

electric

potential)

npn MOSFET (n-FET)

Moderate bias

e e e e e e

e e e

e e e

p+ p+ p+ p+

p n n

gate

electrode

Vbias

e e e

e e e

Strong bias

e e e

e e e

e

e

Very strong bias

Zero bias

Punch-Through

22

p n n

gate

electrode

Vbias

e e e e e e

e e e

e e e

p+ p+ p+ p+

e e e

e e e

e e e

e e e

e

e

p n n

e e e e e e

e e e

e e e

p+ p+ p+ p+

e e e

e e e e e

e

Smaller size & same voltage

higher electric field strengths

easier punch-through

Vbias

Need for Voltage Scaling Nano-Scale MOSFET

• Metal Oxide Semiconductor Field Effect Transistor • Three terminal device

• Source, gate and drain

• Vg controls the conduction from source to drain

• Half thickness of the gate is called “Feature size λ”

• Current feature sizes in production – 90nm (Intel Pentium 5)

• Demonstrated feature sizes up to 20nm (IBM).

Ph

oto

Co

urt

esy:

Fu

jits

u L

ab

s

Challenges

• Difficulties • High electric fields

• Power supply vs. threshold voltage

• Heat dissipation

• Interconnect delays

• Vanishing bulk properties

• Shrinkage of gate oxide layer

• Too many problems to continue miniaturization as physical limits approach

• Proposed solutions are short term

• Open Problems

• Improve lithographic precision (eBeam)

• Explore new materials (GaAs, SiGe, etc.)

• As a long term goal explore new devices

The MOSFET dominates the microelectronic industry (memories, microcomputers, amplifiers, etc.)

Large Si single crystals are grown and purified. Thin circular wafers (“chips”) are then cut from the crystals

Circuit elements are then constructed by selective introduction of specific impurities (diffusion or ion implantation)

A single 8” diameter wafer of silicon can contain as many as 1010 1011 transistors in total

Cost to consumer ~ 0.00001cent each.

Transistors and Microelectronic Devices

23

Sand Silicon Computer Chips Silicon Wafer Preparation

Evaluation

Melting

Preparation

Body growth

Cooldown Ingot removal

Slicing

Lapping

Etching

Heat Treatment

Polishing

Silicon Wafer Preparation

Epitaxial Processing

Cleaning

Inspection

Packing

Silicon Wafer Preparation

24

Four Stages of Semiconductor Manufacturing

raw semiconductor material mined and purified

crystal growth and wafer preparation

the devices or integrated circuits are actually formed in and on the wafer surface. Chip fabrication!

packaging

Why wide bandgap semiconductors?

Needs: Solidstate amplifiers for:

Broad band wireless communications

Sophisticated controllers for electric grids, Radars

Base stations of future wireless network

Multifunction RF Systems

Military Applications

Requirements:

Ultra high power

High efficiency

High Frequency

Linearity

Manufacturability

Low Cost

High Temperature (300C400C) & Hostile Surrondings Endurance

Silicon Devices can not substain these requirements

Why wide bandgap semiconductors?

Gallium Arsenide GaAs

Indium phosphide InP

Silicon Carbide SiC

Gallium Nitride GaN

GaN will replace GaAs devices and all of its properties comes from electrical and physical characteristics

Important Compound Semiconductors Materials

25

SPEED

GHz POWER W/mm NOISE FIGURE

GaAs 1050 1 Good

InP 100200 0.5 Very Good

SiC 10 0.6 (like Si)

GaN 30 10 Good

Commercial devices available Not commercial devices available

Front-end Devices Semiconductor characteristics

Semiconductor

characteristics

Silicon

Gallium

Arsenide

Indium

Phosphide

Silicon

Carbide

Gallium

Nitride

Bandgap(eV) 1.1 1.42 1.35 3.26 3.49

Electron mobility

(cm2/Vs)

1500 8500 5400 700 1000-2000

Saturated (peak)

electron velocity

(x107cm/s)

1.0(1.0) 1.3 (2.1) 1.0 (2.3) 2.0 (2.0) 1.3 (2.1)

Critical breakdown

field MV/cm

0.3 0.4 0.5 3.0 3.0

Thermal

conductivity

1.5 0.5 0.7 4.5 >1.5

Relative dielectric

constant (er)

11.8 12.8 12.5 10 9

Technology development costs can be amortized over several large electronic and opto-electronic applications, like BLUE & WHITE LED and BLUE LASER.

Cost Advantages

Power density increased

Device variability

Reliability

Complexity

Leakage

Power dissipation limits device density

Transistor will operate near ultimate limits of

size and quality – eventually, no transistor can

be fundamentally better

◄

Transistor problems

26

The Future of transistors

Molecular electronics

Carbon nanotubes transistors

Nanowire transistors

Quantum computing

CMOS devices will add functionality to CMOS non-volatile memory, opto-electronics, sensing….

CMOS technology will address new markets macroelectronics, bio-medical devices, …

Biology may provide inspiration for new technologies bottom-up assembly, human intelligence

Nanocrystal 3 D

Quantum layer 2 D

Quantum wire 1 D

Quantum dot 0 D

Nanoscaled Semiconductor

Quantum Confinement

Trap particles and restrict their motion Quantum confinement produces new material behavior/phenomena “Engineer confinement” control for specific applications Structures

Quantum dots (0D) only confined states, and no freely moving ones

Nanowires (1D) particles travel only along the wire

Quantum wells (2D) confines particles within a thin layer

Variation of the Optical Properties with the Crystal Size

27

Optical Properties Are Directly Dependent on Size

EXAMPLE: Gold and silver nanoparticles Cut to 1000nm the color would be golden color. Cut to 700nm the color would be red.

Cut to 600nm the color would be orange.

The color changes because each color has a specific wavelength.

1.0 1.5 2.0 2.5

1600 1200 800

Wavelength (nm)

Ab

sorb

an

ce (

a.u

.)

Energy (eV)

Ph

oto

lum

ine

sce

nce

5.8 nm

5.0 nm

4.6 nm

4.0 nm

3.3 nm

2.9 nm

2.4 nm

6.4 nm

600

Quantum Confinement in InAs

Nanocrystals

Particle in a box model

E n

e r

g y

r

1Sh 1Ph

1Se

1Pe

1Sh

1Ph

1Se

1Pe

Luminescence from Indirect Gap Semiconductors

It is possible to observe luminescence from indirect gap semiconductors when their crystal size is very small. The origin of this emission is the modification of the electronic structure due to the size, although some theories support some other possible radiative paths in nanocrystals (defects, surface effects,...)

Quantum Wells

The optical properties of a semiconductor are altered by quantum size effects; at least one of the dimensions of material is on the order of De Broglie’s wavelength of an electron: = h/m; if m ~ eV = ~ a few nm;

28

Superlattices based on

Bi2Te3, Si/Ge, GaAs/AlAs

Ec

Ev

x

E

Quantum well (QW) Barrier

Top View

Nanowire

Al2O3 template

Nanowires based on

Bi, BiSb, Bi2Te3, SiGe Eg(GaN) 3.4 eV

Eg (InGaN) 2.1-3.3 eV

n-GaN | InGaN | p-GaN

An inner shell of smaller bandgap material sandwiched between the nanowire core and outer shell of larger bandgap materials

5.5V 4V

5 m

6V

Lighting Up with Nanowires

Nanowire Double Heterostructures

Lieber*, Nano Lett., 2004, 4, 1975

Nature, June 10, 2004

Outlook for Nanocrystal LEDs Brightens

Victor Klimov and colleagues at Los Alamos National Laboratory assembled their cadmium selenide dots on top of a so-called quantum well, a thin sheet of semiconductor sandwiched between two barrier layers. A quick flash of laser light aimed at the well generates pairs of electrons and positively charged "holes" in the middle layer. Normally the pairs would recombine and emit a photon, but by making the top layer of the well thinner than 30 Angstroms, the researchers forced the recombined pairs to release their energy as a wiggling electric field. This field generated electron-hole pairs in the adjacent dots; these pairs recombine, producing photons.

Important Semiconductor Materials for Optoelectronics

Materials Type Substrate Devices Wavelength range(mm)

Si SiC Ge

GaAs

AlGaAs

GaInP GaAlInP

GaP GaAsP

InP InGaAs

InGaAsP InAlAs

InAlGaAs GaSb/GaAlSb

CdHgTe ZnSe ZnS

IV IV IV

III-V

III-V

III-V III-V III-V III-V III-V III-V III-V III-V III-V II-VI II-VI] II-VI II-VI

Si SiC Ge

GaAS

GaAS

GaAs GaAS GaP GaP InP InP InP InP InP

GaSb CdTe ZnSe ZnS

Detectors, Solar cells Blue LEDs Detectors

LEDs, Lasers, Detectors, Solar Cells, Imagers, Intensifiers

LEDs, Lasers, Solar Cells, Imagers Visible Lasers, LEDs Visible Lasers, LEDs

Visible LEDs Visible LEDs Solar Cells Detectors

Lasers, LEDS Lasers, Detectors Lasers, Detectors Lasers, Detectors

Long wavelength Detectors Short wavelength LEDs Short wavelength LEDs

0.5-1 0.4

1-1.8 0.85

0.67-0.98

0.5-0.7 0.5-0.7 0.5-0.7 0.5-0.7

0.9 1-1.67 1-1.6 1-2.5 1-2.5 2-3.5

3-5 and 8-12 0.4-0.6 0.4-0.6

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Commercial Applications of Optoelectronic Devices

Materials Devices Applications

Remote control TV, etc., video disk players, range-finding, solar energy conversion, optical fiber communication systems (local networks), image intensifiers Space solar cell Optical fiber communications (long-haul and local loop) Optical fiber communications, Military applications, medicine, sensor Displays, control, compact disk players, laser printers/scanners, optical disk memories, laser medicine equipment Solar energy conversions, e.g. watches, calculators, cooling, heating, detectors Detectors Displays, optical disk memories, etc. Infrared imaging, night vision sights, missileseekers, other military applications Commercial applications (R&D stages only)

Detectors, Infrared LEDs and Lasers Solar cell Infrared LEDs, Lasers (1-1.6mm) 1-1.67mm Detectors 1.67-2.4mm Detectors 0.5-0.7mm LEDs and Lasers Detectors and Solar Cells Detectors Blue LEDs Long wavelength detectors/smitters Visible LEDs

GaAs/AlGaAs

InP/InP InP/InGaP

InP/InGaAs

InGaAlAs/InGaAs GaAs/GaInP/

GaInAlP

Si

Ge SiC

GaSb/GaAlSb/InSb

ZnSe/ZnS