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

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


For forward-biased heavily doped N+P+

junction, population inversion is

achieved

Fn Fp gE E E

Confinements:

• Carrier Confinement

• Optical Confinement

• Current Confinement

Quantum-well structure has better carrier

and optical confinement.

Population Inversion in Semiconductor

Fewer excess carriers are needed to

achieve population inversion.

Lower threshold current for lasing.


00

2

00

22

d

dnLLn

d

dm

md

dn

nLn 1

0

0

2

00 )1(

2

2

mL

Laser Mode

210 0

0

0

Mode spacing, (1 ) ,2

1 ( )

dn

Ln n d

m and neglecting sign

n 00

2 2,

L Lnm


Optical Feedback

• Electronic oscillator circuit: signal amplification (gain) and feedback

• Laser (optical oscillator): light (optical) amplification and

optical feedback (?)

Mirrorcleaved facet, or

polish the end faces of the laser diode

VCSEL (vertical-cavity surface-emitting laser)

Edge-emitting diode laser with cleaved

mirror surface

R1 R2

R1(DBR: distributed feedback reflector)

R2(DBR: distributed feedback reflector)

Provide distributed feedback

multiple mode

single mode


1 21 exp[ (2 )]exp[ (2 )]f

i

PR R g L L

P

Condition for Laser Oscillation (Laser Threshold)

1 2 1R R G For laser oscillation, the net round-trip gain, G,

at threshold

1 2

1 1, ln( )

2thThreshold gain g

L R R

Diode Laser Applications

• Red diode lasers (GaAs): CD,DVD

• Blue diode lasers (GaN): high density DVD or Blu-ray DVD

• Infrared diode lasers(InGaAsP): fiber-optic communication (1.55μm)

Current (mA)

0

Optical output power

Laser diode

LED

10050

5 mW

10 mW

0 (nm)

LED

Optical Power

Laser

Ith

~0.1 nm

1475 16251550

I

(a) (b)


• To maximize the sensitivity, the depletion

layer should be as wide as possible.

PIN photodiode

Photodiode

• EHP generated in SCR move toward to their

respective majority-carrier regions, due to electric field in SCR.

light-generated current adds up to the thermal

reverse-bias current IS.

• Light also generates EHP in neutral regions

that can diffuse to SCR, contributing to photocurrent.

Reverse –biased PN Photodiode


i-Si n+

p+

SiO2

Electrode

net

eNa

eNd

x

(a)

(b)

Electrode

W

x

E(x)

R

Eo

Iph

h > Eg

W

(c)

(d)

Vr

Vout

E

eh+

W

AC ro

dep

E E r ro

V V

W W or VV Eo

E

( )E x

To maximize the sensitivity, the depletion

layer should be as wide as possible.

PIN photodiode

PIN Photodiode


Avalanche Photodiode

Photo-generated carriers are multiplied by impact ionization as they travel through

the depletion layer and thereby the sensitivity of the detector is increased.


Summary

Photodiode,

Solar cell

LED, LD


Part III: Metal-Semiconductor Junction

Schottky Barriers

Schottky Contacts, Ohmic Contacts

Ideal metal semiconductor contacts ( ( ))M Sfor N type

Mq

EFm

E0

EC

EF

Ei

EV

Sq

e-

e- : very small

EFm

E0

EF

W

1, ( ) [ ( ) ]bi M S B C F FBBuilt in potential E E

q

, ( / )B MSchottky barrier height q

MqSq

Bq biq

0

: [ ]

: [ ], ( )

: [ ]

:

M

S S C F FB

q Metal work function eV

q Semiconductor work function eV q E E

Electron affinity eV

E Vacuum level


:

:

Bn

B

Bp

barrier against electron flowin N type

barrier against hole flow in Ptype

( )M Sfor P type

1( )Bp g ME

q

Aff( / ) inity Ru" le"Bn M q called

( )[ ]:

g F Vbi M S M

E E Enegative

q q q

biq

( )M Sfor N type

What would be happen?

( )M Sfor N type

( )M Sfor P type

Ohmic contact !

1( ) [ ( ) ]bi M S B C F FBE E

q

( / ) ( / )Bn Bp M g M gq q E E


An “ideal” metal–silicon contact.

In a real metal–silicon contact,

there is a dipole at the interface.

( / ) ( / )Bn Bp M Si Si g M gq q E E

Real metal-silicon contacts ( ( ))M Sfor N type

( / )Bn M Si q

Affinity rule suggests that should increase with

increasing by 1 eV for each 1 eV change in .Bn

M M

In real metal- silicon contact, affinity rule is in

qualitative agreement, but not in quantitative

agreement as shown Table 4-4 below.

( / )Bn M Si q Due to high densities of energy

states in the band gap at the

metal-semiconductor interface

Fermi-level pinning

Affinity rule is valid only when

the interface charge is zero.


Fermi-level pinning

Interface states or surface states

• Acceptor-like surface state

: neutral when the state is empty or negative

when the state is filled.

• Donor-like surface state

: neutral when the state is filled or positive

when the state is empty.

Donor-like

Acceptor-like

All energy states above Fermi-level are empty

and all energy states below Fermi-level are

filled, then the net charge at interface is zero

because the approximately upper half of the

states are acceptor-like and lower half of the

states are donor-like.

mostly empty

mostly filled

( / ) 0.6 ~ 0.7Bn M q V

1) If ≈ 4.6 V (EF at surface is

around in the middle of the band

gap), affinity rule is valid.

M

2) If ≠ 4.6 V, there is a dipole at the

interface as shown in real metal-

silicon contact and it prevent

from moving very far from around 0.7 V.

M

Bn

Fermi-level pinning

0.7 0.2( 4.75)Bn MV


a negative voltage (reverse bias) applied to

the metal.

How to determine ?B

( ) ln Cbi M S Bn C F Bn

d

Nq q E E q kT

N

2 ( )s bidep

d

VW

qN

s

dep

C AW

no voltage applied

ϕbi (and hence ϕB) can be extracted from

the C–V data as shown.

2 2

2( )1 bi

d s

V

C qN A


Energy band diagram of a

Schottky contact with a

forward bias V applied

between the metal and the

semiconductor.

Thermionic Emission Theory

Electron concentration at the interface is

(assuming EFn is flat all the way to the peak of the

barrier) 3/ 2

( ) / ( ) /

2

22B Bq V kT q V kTn

C

m kTn N e e

h

It can be shown that the average velocity of

the left traveling electron is

2 /thx nv kT m

2/ /2 / 2 / /

03

41

2B Bq kT q kTqV kT qV kT qV kTn

S M thx

m kJ qnv T e e KT e e J e

h

Only half of the electrons travel

toward the left.

/

0

( ) /

B

B

q kT

q V kT

S M

J e

J e

Determines how many electrons possess

sufficient energy to surpass the peak of the

energy barrier and enter the metal.

22 2

3

4100 /( / K )nqm k

K A cmh

called Richardson constant

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