Modern Semiconductor Devices for Integrated Circuits Chapter 4. PN … PN and Metal-Semiconductor...

Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits

Light Penetration Depth-Direct-Gap and Indirect-Gap Semiconductors

1.24( ) ( )

hcPhoton energy eV m

Photons with energy less than Eg are not absorbed

by the semiconductor. Photons with energy larger

than Eg are absorbed but some photons may travel

a considerable distance in the semiconductor

before being absorbed.

Light intensity( )x

x e :

1:

absorption coefficient

penetration depth

1The thickness of solar cell :

• The Si or Ge solar cell must be thick ( > 50 μm)

: due to low α

• The GaAs or InP solar cell is thin (~ 1 μm)

: due to high α

low

high

Related to the specific energy band structure

in order to capture nearly all the photons.


The E–k diagrams

direct-gap semiconductor (GaAs, InP, GaN)

indirect-gap semiconductor (Si, Ge)

large α

small α

Light absorption is inefficient because assistance by phonons is required

to satisfy k-conservation.

Light absorption is efficient because k-conservation (momentum conservation)

is satisfied between electron and hole.


Short-Circuit Current and Open-Circuit Voltage

A P+N solar cell under the short-circuit condition (V = 0)

The excess carrier concentration profile. Effectively only

the carriers generated within x < Lp can diffuse to the

junction and contribute to the short-circuit current.

2 2

2 2 20 p

p p p

d p p d p p GD G

dx dx L D

With generation rate of electron and hole pairs, G, when light shines on the semiconductor.

Assume that the P+ region is very thin and that all the electron-hole

pairs are generated in the N region at a uniform rate of G pairs s-1cm-3.

Boundary conditions

under the short-circuit

2

2

2

0 ( )

(0) 0

( ) constant

p p

p

d p Gp L G

dx D

p

p

/( ) (1 )p

x L

pp x G e

The solution becomes,


/( ) px Lpp p p

p

Ddp xJ qD q Ge

dx L

(0)

( )

sc p p

p p

I AJ AqL G

L or

- Only the holes generated within a distance Lp from the junction are

collected by the PN junction and contribute to the short-circuit current.

- The carriers generated farther from the junction are lost to recombination.

Free of defects and impurities that serves as recombination centers are required

for indirect-gap materials.

Light does not penetrate deep and all carriers are generated in a narrow region

for direct-gap semiconductors, 2

/, ( 1)i p qV kT

p

d p

n DTotal diode current I Aq e AqL G

N L

By setting I = 0, the open-circuit voltage, Voc, can be obtained assuming/

1.ocqV kT

e

2ln( / )oc p d ikT

V GN nq

• Large is good for the solar cell current for indirect semiconductor.( )p pL or

• does not have to large for direct-gap semiconductors.( )p pL or

2

1d

i

N andn

• Solar cell should be fairly heavily doped.

• Operating temperature should be low.


light illumination

Output Power

ocV

Operating point

(maximum ). I V

( )sc ocOutput power I V FF fill factor

max.0.75

sc oc

I VFF

I V

Optical concentrator to focus sunlight

on a solar cell can raise G and improve both Isc and Voc

reduces the solar cell area and cell cost.

increases cell efficiency.

• A larger Eg reduces ni2 exponentially.

Voc increases linearly with Eg .

• If Eg is too large, the material would not absorb the photon in a large long

wavelength (red and infrared) portion of the solar spectrum and Isc drops.

• The best solar cell efficiency (~ 24 %) is obtained with Eg values of 1.2 ~ 1.9 eV.

Commercial rooftop silicon solar-cell panels have conversion efficiency of 15 ~ 20 %.

Tandem solar cells can achieve very high ( > 30 %) energy conversion efficiency.

Importance of Band-Gap Energy


AlGaAs window layer on GaAs passivates the surface states andthereby increases the photogeneration efficiency

A heterojunction solar cell between two different bandgapsemiconductors (GaAs and AlGaAs)

(a)

n p

GaAsAlGaAs

2 eV

Ec

Ev

Ec

Ev

(b)

1.4 eV

A tandem cell. Cell 1 has a wider bandgap and absorbs energetic

photons with h > Eg1. Cell 2 absorbs photons that pass cell 1 and

have h > Eg2.

np

Cell 1 (Eg1

)

n p

Cell 2 (Eg2

< Eg1

)


LightOxide

n

p Le

Inverted pyramid textured surface substantially reduces reflectionlosses and increases absorption probability in the device


Light-Emitting Diodes and Solid-State Lighting

[Band gap and lattice constants of Ⅲ-Nitrides]

• Forward-biased diode

Minority carrier recombination near by SCR.

Energy is released in the form of a photon.

• Different energy gap different colors

g

cE h h E


LED Materials and Structures

g

cE h h E

Direct-gap semiconductor such as GaN

Radiative recombination

via a deep level

via an Auger

process

• Recombination mechanisms: two significant fundamental process

radiative recombination ; photon

non-radiative recombination ; phonon or lattice vibration


• Internal Quantum Efficiency, :

the percentage of radiative recombination events compared with the total

number of recombination events

q int: Injection efficiency

: radiative recombination efficiency

q

Device Efficiency

int

# / .

# / .

of photons emitted from active region s

of electrons injecteded to LED s

Rpn

n

JJJ

J

defined by the ratio of electron current to total current

Injection Efficiency, :

int


1

1 1nrr r

q

r nr nr rr nr

R

R R

- The fraction of recombination that is radiative

- The radiative efficiency will increase if the possible nonradiaive process can be

eliminated

SRH recombination, Auger recombination, and recombination via surface states

Radiative Recombination Efficiency, :q

- quantified the conversion efficiency of electrical energy into an emitted external

optical energy.

- incorporates the internal quantum efficiency and the subsequent efficiency of

photon extraction from device.

IV

opticalPoutexternal

)(

• Overall Device Efficiency (Conversion Efficiency), :

into ext ext q

o

• External Quantum Efficiency, : ext


g

cE h h E

• Wavelength:

1.24 1.24( )

( )gLED wavelenrth m

photon energy E eV


Energy band diagram showing

the quantum well.

A red LED with sloped sides for

better light extraction

Carrier loss in heterostructures:

Carrier Confinement in Quantum Well

Fn C C V Fp VE E E or E E E


Solid-State Lighting: White LED

• Refers to space lighting using LEDs in lieu of traditional light sources such as incandescent

light bulbs and fluorescent.

• About 25 % of the world electricity usage is attributable to lighting.

Improving the energy efficiency of light can significantly reduce energy consumption and

greenhouse gas emission.

Lumen (lm): A measure of the visible light energy,

normalized to the sensitivity of the human eye at different wavelength.

Example) 100 W incandescent light bulb produces 1,700 lm with luminous efficacy

of 17 lm/W


Typical Fabrication Methods for White LED

YAG-GaN LED ZnSe Homoepitaxy Three-component LED

InGaN LED + YAG

fluorescent (형광체)II-VI LED structure on n-

ZnSe subtrate doped with I,

Cl, Br, Al, Ga, Al

- red LED : AlGaAs, GaAsP

- green-yellow LED : GaP

- blue LED : SiC, InGaN



• Narrow bandwidth

• Varying the size or the composition

of QDs

• High quantum yield (QY)

• Solution Process

Quantum Dot LED Thin film Passivation for OLED

• Need to new structure and material

for thin film passivation

• MgF2 & Zn material: Short distance

among the ions & good optical

transmittance

Flexible substrateITO

Organic LayerCathode

Thin Film

Passivation

Organic LED (OLED)


LED Applications

traffic lights, indicator lights, back light source for LCD, lighting, and so on


Blue LED

Shuji Nakamura moved from Nichia to UCSB.

Breakthrough in Blue LED

What Nakamura did was to figure out how to grow the crystal so that

it would have the n and p semiconductor structure that would create

"quantum wells" for the electrons at the junction.

One key thing he did to create the wells was to add indium to the

gallium nitride crystal. Without the indium, the gallium nitride crystal

produces a higher frequency ultraviolet light, which is not visible. The

addition of indium results in lowering the frequency of the emitted

photons to visible blue, but the indium also creates the quantum well

effect, so that electrons falling into the passing holes first fall into the

well and therefore collect electron mass before being injected into the

holes. That massing in the well creates a more vigorous injection.


日`청색LED발명가` 인센티브 2만엔→200억엔“20만원이 2000억원으로.”세계적발명품인청색발광다이오드(LED)를개발한기술자에게 2만엔(20만원)의포상금만지급했던일본기업이그 100만배인 200억엔(2000억원)을지급하라는법원판결을받았다. 최근일본에서는그동안회사측에귀속하는것으로알려진직무발명(업무수행과관련된발명)의대가를직원에게돌려야한다는판결이잇따라커다란사회적논란이되고있다.발명자개인의연구의욕을고취시킨다는환영의견이있는가하면, 회사측부담을키워국제경쟁력을해친다는반론도만만찮다.일본도쿄지방법원은 30일청색 LED의 개발주역인나카무라슈지(中村修二) 미캘리포니아대교수가전직장니치아(日亞)화학을상대로낸특허소송에서청구액전액인 200억엔을지급하라고판결했다.더욱이재판부는회사측이얻은이익을 1208억엔으로보고이중 50%인 604억엔이원고의몫이지만 200억엔만청구했기때문에이를전액인정하는것이라고밝혔다.청색 LED는 휴대전화, 대형디지털스크린에장착되는필수장비로현재세계휴대전화의 90%가니치아화학제품을쓰고있다.

문화일보 2004년 1월 31일자

일본의에디슨 "나카무라슈지" 박사“일본에반기를든세계적인 LED 과학자, 나카무라슈지박사”그는꿈의광원인청색LED와청색자외선레이저기술등을개발, 평판모니터· 옥외전광게시판·휴대폰백라이트·교통신호등·차세대DVD 저장기술등의다양한분야에커다란영향을끼치며세계과학계로부터‘일본의에디슨’으로불리고있는인물이다.그의기술덕분에일본열도의변방인시코구지방의조그만중소화학업체에불과했던니치아화학은단숨에연간 10억달러규모의매출을올리는스타벤처기업으로발돋움할수있었다. 하지만문제는나카무라박사가이런엄청난공헌을했음에도불구하고회사로부터는어떠한보상도받지못했다는것이다. 일본전자산업계에서기술개발자란거대조직에속한일개샐러리맨에지나지않으며그가개발한것은당연히회사에귀속된다는집단주의관행이뿌리깊게자리잡고있었기때문이다.나카무라가특히기술개발과그후회사측이보여준태도에분노하고있다. 니치아경영진은당초나카무라가청색LED 기술을개발하겠다고제안했을때기술적가능성이없고비용만축낸다는이유로연구중단을명령했었다. 이때문에젊고확신에찬개발자였던나카무라는회사측을속이고비밀리청색LED 연구를진행해야했으며마침내개발에성공하고서도회사가모르게회사이름으로특허를내는황당한일(!)까지저질러야했다.뒤늦게이같은사실을알게된회사측은나카무라의기술이당초자신들의예상과달리‘황금알을낳는사업’이될수있다는것을발견하고는명령불복종을문제삼지않는대신이특허로막대한로열티수입을벌어들이기시작했다. 회사는이과정에서직원이개발한것은당연히회사의소유라는관행에따라나카무라에게어떤보상도주지않았으며그런자신들의처사를결코부당하다고생각하지않았다.이는미국과극명하게대조되는것이다. 미국기업들은과학자들과엔지니어들의창의적인기술로돈을벌었을경우개발자들에게로열티나스톡옵션등으로수익을나누어주어그들의기술개발의욕을고취하고있다. 이는실리콘밸리를중심으로한미국 IT산업의기술경쟁력의밑바탕이되고있다. 결국지난 2000년초이같은일본의열악한풍토에염증을느낀나카무라는“일본은사랑하지만일본식시스템에실망했다”는유명한말을남기고미국으로떠나고말았다. 당시일본언론들은이를두고미·일사이의’두뇌유출’의상징적사건으로대서특필했었다. 그후나카무라는미국캘리포니아대학에서교수로활동하면서일본에있는선후배와동료들이자신과같은전철을밟지않도록니치아측과법적싸움을시작했다.

2002-09-24 종합 / 디지털타임스


Diode Lasers

Light Amplification

Laser: Light Amplification by Stimulated Emission of Radiation

Under population inversion, light (wave amplitude)

is amplified in the semiconductor.

Three types of light–electron

interactions

• Absorption

• Spontaneous Emission

• Stimulated Emission

Normally, light is absorbed in the semiconductor

Light Amplification

population

inversion

Modern Semiconductor Devices for Integrated Circuits Chapter 4. PN … PN and Metal-Semiconductor...

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