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TRANSCRIPT
Chapter
11
SEMICONDUCTOR
OPTOELECTRONICS
Semiconductor based optoelectronic devices form an important component of modern information
age. The following �gures provide an overview of important optoelectronic processes and devices.
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
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Immunity to electromagnetic interference
AAAAAAAAAAAAAAAAAANon-interference of two or
more crossed beams
High parallelism
High speed–high bandwidth
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Beam steering for reconfigurable interconnects
Special-function devices
AAAAAAAAAAAAAAAAAAWave nature of light for
special devices
Nonlinear materials
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Photonics-electronics coupling
Can be transmitted without distortion due to electrical storms, etc.
Unlike electrical signals, optical signals can cross each other without distortion
Two-dimensional information can be sent and received
Potential bandwidths for optical communication systems exceed 1013 bits per second
Free space connections allow versatile architecture for information processing
Interference or diffraction of light can be used for special applications
New logic devices can be created
The best of electronics and photonics can be exploited by optoelectronic devices
ADVANTAGES OF OPTICAL DEVICES
Challenges: How does one harness the tremendous potential?
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
InSb PbS Ge Si GaAs CdSe GaP CdS SiC GaN ZnS
GaAs1-yPyHgCdTe
Infrared Red Green Violet Ultraviolet
Orange Yellow Blue
λ (µm)
Human eye repsonse
Eg (eV)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
6.0 3.0 2.0 1.5 1.0 0.9 0.8 0.7 0.6 0.5 0.45 0.4 0.35
CdTe
OPTOELECTRONICS: MOTIVATIONS FROM SYSTEM DEMANDS
DISPLAY APPLICATIONS: Light emitters covering red, green, blueOPTICAL MEMORIES: Short wavelength light emittersCOMMUNICATIONS: Light emitters/detectors operating at low absorption/dispersion
points of optical fibers (1.55 µm, 1.3 µm)
SEMICONDUCTOR BANDGAPS (WAVELENGTHS) AND HUMAN EYE RESPONSE
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
+
CONDUCTION
BAND
Photon
VALENCE
BAND
ABSORPTION PHONON-ASSISTED
ABSORPTION
Photon
Phonon
DIRECT BANDGAP INDIRECT BANDGAP
ωh > Eg
LIGHT ABSORPTION IN SEMICONDUCTORS
• Energy conservation hω > Eg• Momentum conservation
ki = kf for strong processes• Indirect gap materials have weak absorption
102
105
103
104
101.0 1.5 2.0 2.5 3.0
Ge
GaAsCdTe
Si
GaP
PHOTON ENERGY (eV)
AB
SOR
PTIO
N C
OE
FFIC
IEN
T (c
m–1
)
Electron-hole pair generation rate
Pop: Optical intensity (Watts/cm2)
Direct gap materials:
hω, Eg in units of eV
α(hω) ~5.6 x 1041/2hω–Eg
hω( (
GL =αPophω
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
hω
_
+
_
+hω
hω
VR
p+n+i
Signal
W
_
+
EV
Carriers are collected from the depletion region
x=0 x=W
hω
Ec
eVR
P-I-N PHOTODETECTORS
The detector is reverse biased to collect any electron-hole pairs created by light absorption.
Photocurrent:
IL = eAJph(0)[1– exp (–αW)]Jph = Optical photon particle currentW = Depletion regionA = Device area
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
SEMICONDUCTOR LIGHT EMITTERS FOR COMMUNICATION APPLICATIONS
Light emitters for long haul communication systems must emit light at λ = 1.55 µm or λ = 1.3 µm.
• GaAs lasers (λ ~0.88 µm) are used for local area networks where distances to be covered are only a few kilometers.
• InGaAsP lasers are used for long haul communication. Optical pulses travel ~40-50 km and are then separated by repeater lasers. Optical fiber amplifiers are also used to boost the signal.
1.55 µm: Lowest loss point in optical fibers
1.3 µm: Lowest dispersion point in optical fibers
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
100
50
20
10
5
2
1
0.5
0.2
0.1
0.05
–OH absorption peaks
Infrared absorption tail from lattice transitions
Rayleigh scattering
FIB
ER
AT
TE
NU
AT
ION
(dB
km
–1)
WAVELENGTHS, MICRONS
1.55µm loss ~ 0.2 dB/km1.3µm loss ~ 0.5 dB/km
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
LIGHT-EMITTING DIODE: GENERAL PRINICPLES
The LED is a forward biased p-n diode. Electrons (holes) are injected into p-side (n-side) region where they recombine with holes (electrons) to emit photons.
Emitted photon energy ~EgEmitted spectral linewidth ~kBTUpper limit to LED switching time ~1 ns electron-hole recombination time
+
_
Electron injection
Photons willemerge fromthe device
_ _
+Hole injection
p nPhotonsBuried region
TOP LAYER
EFnEFp
+
+ +
_ _
_ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _
+ + + + ++ + + + + + + +
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
LED: ELECTRON-HOLE RECOMBINATION TIME
• Electrons and holes recombine in LEDs via a process called spontaneous recombination.• Energy-momentum conservation rules apply.Recombination rate αfe(k) • fh(k)
fe(k): probability of finding an electron with momentum hkfh(k): probability of finding a hole with momentum hk
CONDUCTION BAND
VALENCE BAND
Eg
Ec
Ev
k
h2k2
2m*
hω
e
h
h2k2
2m*
• Electrons-holes recombination time is a function of carrier density.At high carrier densities the e-h recombination time approaches a nanosecond in most direct gap semiconductors.
10–5
10–6
10–7
10–8
10–9
10–10
1014 1015 1016 1017 1018
Nd (for holes injected into an n-type semiconductor)
n = p (for excess electron hole pairs injected into a region)
Rad
iativ
e L
ifet
ime
(τ r
)(s)
Typical carrier densities for laser operation Low
Injection Regime
τ o
Carrier occupation is degenerate fe = fh = 1
Semiconductor GaAs Temperature is 300 K
1019cm–3
~
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
_ _
Eg hω
CONDUCTION BAND
VALENCE BAND
hω
No Photons Spontaneous Emission
+ +
__
+
Photons
CONDUCTION BAND
VALENCE BAND
Coherent Emission
Stimulated Emission
+
hωhω
hω
STIMULATED EMISSION AND SPONTANEOUS EMISSION
SPONTANEOUS EMISSION: Responsible for light emission in LEDs• Electron-hole recombination in the absence of photons• Outcoming photons are incoherent, i.e., have random phases• Electron-hole recombination lifetime is limited by ~1 ns
STIMULATED EMISSION: Responsible for light emission in laser diodes• Electron-hole recombination in the presence of other photons• Photons produced are coherent, i.e., have the same phase• Electron-hole recombination lifetime is
1τstim
nphτspon
=
nph: photon number
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
THE LASER STRUCTURE: FORWARD BIASED P-N DIODE AND OPTICA CAVITY
Output light
Roughened surfaces
Optically flat and polished parallel faces
L
n-type
p-type
I
Cladding region
Active region
Cladding region
z y
x
Optical cavity, produced by cleaving the crystal causes photons to be reflected back into the cavity. The photon build-up starts the stimulated emission responsible for lasing
Polished face
Polished face
Optical modes in the cavity
Active region
p- region n- region
DIE
LE
CT
RIC
CO
NST
AN
T
DISTANCE PERPENDICULAR TO THE CAVITY (z)
Confined optical wave
The light wave is confined in the cavity by the waveguide of the laser structure
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
Stimulated emission
Spontaneous emission
Below threshold Above threshold
Jth J
INJECTED CURRENT DENSITY
LIG
HT O
UT
PUT
LASER OPERATION: GAIN AND LIGHT OUTPUT
• Gain = emission coefficient – absorption coefficient• As more and more electrons and holes are injected into the active region of the laser the gain increases.• When the gain overcomes the laser in the cavity, photon build-up occurs and lasing starts.
Light output in the lasing mode is very small below threshold. It increases rapidly once the laser is in the above threshold state.
1.42 1.46 1.50
160
120
80
40
0
-40
-80
1.5 x 10 18 cm-3
2.5 x 10 18 cm-3
Photon Energy (eV)
Gai
n (c
m-1
)
© Prof. Jasprit Singh www.eecs.umich.edu/~singh
hω
hω
hω
____
e-h in bands
++++++
n = nthJ > Jth
Cavity resonant modes
cavity loss
hω
GA
IN
0
GA
IN
0
GA
IN
0
Gain spectrum Light emission
PHO
TO
N IN
TE
NSI
TY
PHO
TO
N IN
TE
NSI
TY
kBT
PHO
TO
N IN
TE
NSI
TY
Dominant mode
++++++
_____
_
_____
_
n < nthJ < Jth
++++
hω
hω
SPECTRAL OUTPUT OF A SEMICONDUCTOR LASER
Below threshold the laser acts like a light emitting diode. There is no coherence in the light output.
At threshold, photon number starts to increase in the cavity. The photon output in the lasing mode starts to increase.
Above threshold most of the current injected results in photon emission in the lasing mode. Photon spectral output is very sharp and light coming out is coherent.