modern semiconductor devices for integrated circuits chapter 4. … modern semiconductor devices for...
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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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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.
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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)
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
• 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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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.
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
Summary
Photodiode,
Solar cell
LED, LD
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
:
:
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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.
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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
Chapter 4. PN and Metal-Semiconductor JunctionsModern Semiconductor Devices for Integrated Circuits
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