Transcript semiconductor laser

CHAPTER 9: PHOTONIC DEVICES
Part 2
Semiconductor lasers

Semiconductor lasers are diodes
that
emit
coherent
light
by
stimulated emission. They consist of
a p-n junction inside a slab of
semiconductor that is typically less
than a millimeter in any dimension.
Excitation is provided by current
flow through the device, and the
cleaved ends of the diode provide
the feedback mirrors.
Semiconductor laser structures
Homojunction 40,000
A/cm2
Single heterojunction
10,000
Double heterojunction
1,300
Double heterojunction,
large optical cavity 600
Top:
Homojunction.
Middle: Single
heterojunction
. Bottom:
Double
heterojunction
SEMICONDUCTOR LASER
Semiconductor Laser Materials:
• Have direct Eg
• Laser emission : range from 0.3 to 30m
• 3 most important III-V compound:
•GaxIn1-xAsyP1-y
•GaxIn1-xAsySb1-y
•AlxGa1-xAsySb1-y
Figure 9.18.
Energy band gap and lattice constant for three III-V compound solid alloy system.
SEMICONDUCTOR LASER
LASER OPERATION
• To enhance stimulated emission –need population
inversion – consider p-n junc. Or a heterojunction
formed between degenerate semicond.
• Both sides of the junc. is high – EFV below EV edge on
the p-side – EFC is above the EC edge on the n-side
• When large bias is applied – high concen. of
electrons & holes are injected into the transition region
– region d contains large conc. of electrons in EC &
large conc. of holes in the EV
Condition necessary for population inversion:
(EFC-EFV)>Eg
Figure 9.20. Comparison of some characteristics of
(a) homojunction laser and (b) double-heterojunction (DH) laser. Second from the top
row shows energy band diagrams under forward bias. The refractive index change for
a homojunction laser is less than 1%. The refractive index change for DH laser is
about 5%. The confinement of light is shown in the bottom row.
SEMICONDUCTOR LASER
CARRIER & OPTICAL CONFINEMENT
• Double-hetero (DH) laser
• carriers are confined on both sides of the active region by the
heterojunction barriers
• Optical field is also confined in the active region
•(see fig. 9.21)
• Refractive indices: n1 , n2 & n3
• n2  n1  n3 ray angle 12 at the layer 1/layer 2 interface – exceed the
critical angle by:
n
sin  c  1
n2
• Similar situation occurs for 23 at the layer 2/layer 3 interface
• When refractive index (in active layer) > index of its surrounding layers –
propagation of the optical radiation is confined in a direction parallel to the
layer interfaces
SEMICONDUCTOR LASER
Figure 9.21. (a) Representation of a three-layer dielectric waveguide.
(b) Ray trajectories of the guided wave.
n
SEMICONDUCTOR LASER

Confinement factor  (ratio of the light intensity within the active
layer to the sum of light intensity – within & outside the active layer)

  1  exp  C nd

c: constant
n : difference in the refractive index
d: thickness of the active layer
Larger n & d  higher 
SEMICONDUCTOR LASER
OPTICAL CAVITY & FEEDBACK




Condition necessary to produce laser action: population inversion
Photons released by stimulated emission – cause further stimulations
as long as there is population inversion  phenomenon of optical gain
To increase gain – multiple passes of a wave must occur – achieved
using mirrors placed at either end of the cavity (reflection planes at
left side & right side)
Reflectivity R at each mirror:
 n 1

R  

n

1


2
n
: refractive index in semicond.
corresponding to the wavelength
n
Generally a function of 
SEMICONDUCTOR LASER


If an integral no. of ½  fit between the 2 planes –
reinforced & coherent light will be reflected back & forth
within the cavity
For stimulated emission – length L of cavity must satisfy
the condition
 
m   L
 2n 
m  2nL
m: integral number
Figure 9.22. (a) Resonant modes of a
laser cavity. (b) Spontaneous emission
spectrum. (c) Optical-gain wavelengths.
• Many values of  can satisfy the condition in fig. 9.22(a)
•Only those within the spontaneous emission spectrum will be produces – fig. (b)
•Optical losses in the path traveled by the wave mean that only strongest lines
will survive – leading to a set of lasing modes – fig. (c)
2 m
 
2nL[1  ( / n)( dn / d )]
Separation  between allowed modes in
the longitudinal direction – is the difference
in the  corresponding to m & m+1
SEMICONDUCTOR LASER
Fig.9.23(a) – basic p-n junction laser (homojunction laser)
• A pair of parallel planes are cleaved/polished perpendicular to the 110 axis – laser
light will be emitted from these planes
• To remaining sides of the diode are roughened to eliminate lasing in other directions
• Structure called a Fabry-Perot cavity
• Fig.9.23(b) – Double heterostructure (DH) laser
• thin layer is sandwiched between layers of different semicond.
• Broad area laser – entire area along the junction plane can emit radiation
• Fig.9.23(c) - DH laser with a stripe geometry
• Oxide layer isolates all but stripe contact
•The lasing area is restricted to a narrow region under the contact
• Advantages of stripe geometry – reduced operating current, elimination of multiple
emission areas along the junction & improved reliability
SEMICONDUCTOR LASER
Figure 9.23.
Semiconductor laser structure
in the Fabry-Perot-cavity
configura-tion. (a)
Homojunction laser.
(b) Double-heterojunction (DH)
laser. (c) Stripe-geometry DH
laser.
Photo detectors
Semicond. device that can convert optical signal into electrical signal
If light of the proper wavelength is incident on the depletion
region of a diode while a reverse voltage is applied, the absorbed
photons can produce additional electron-hole pairs.
Photon detectors may be further subdivided into the
following groups:



• Photoconductive. The electrical conductivity of the material
changes as a function of the intensity of the incident light.
Photoconductive detectors are semiconductor materials. They
have an external electrical bias voltage.
• Photovoltaic. These detectors contain a p-n semiconductor
junction and are often called photodiodes. A voltage is self
generated as radiant energy strikes the device. The photovoltaic
detector operate without external bias voltage. A good example
is the solar cell used on spacecraft and satellites to convert the
sun’s light into useful electrical power.
• Photoemissive. These detectors use the photoelectric effect, in
which incident photons free electrons from the surface of the
detector material. These devices include vacuum photodiodes,
bipolar phototubes, and photomultiplier tubes.
PHOTODETECTOR



Operations:
 Carrier generation by incident light, carrier transport
&/or multiplication by whatever current gain
mechanism
 Interaction of current with the external circuit to
provide output signal
Applications:
 Infrared sensors in opto-isolators & detectors for
optical-fiber communications
Consist of:
 Photoconductor
 Photodiode
 Avalanche Photodiode (APD)
PHOTOCONDUCTOR
When incident light falls on the surface of photoconductor – electronhole pairs are generated – conductivity increased
• For intrinsic photoconductor - Increasing of conductivity under
illumination – due to the increase in the no. of carriers
• For extrinsic photoconductor – photoexcitation may occur between
the band edge & energy level in the Eg
•Photocurrent between electrodes:
Primary photocurrent:
 Popt 

I ph  q
 hv 
 Popt    nE 
  
I p  q

 hv   L 
: quantum efficiency
Photocurrent gain:
Gain 
Ip
I ph

 nE
L


tr
Popt:incident optical power
hv: photon energy
: carrier lifetime
E: electric field
= carrier
transit
time
PHOTOCONDUCTOR (Cont.)
Figure 9.30. Schematic diagram of a photoconductor that consists
of a slab of semiconductor and two contacts at the ends.
PHOTODIODE
basically a p-n junction or a metal-semicond. contact operated under
reverse bias.
• When optical impinges the photodiode – depletion region separate the
photogenerated electron-hole pairs
• For high freq. operation – depletion region must be kept thin – to reduce
transit time
• Quantum efficiency (no. of electron-hole pairs generated for each
incident photon):
Ip
  
 q
  Popt
  
  hv



1
•Response speed is limited by 3 factors:
•Diffusion of carriers
• Drift time in the depletion region
•Capacitance of the depletion region
p-i-n photodiode

A p-i-n photodiode (also called PIN
photodiode) is a Photodiode with an
intrinsic (i) (i.e., undoped) region in
between the n- and p-doped regions.
Compared with an ordinary p-n
photodiode, a p-i-n photodiode has
a thicker depletion region, which
allows a more efficient collection of
the carriers and thus a larger
quantum efficiency, and also
leads to a lower capacitance and
thus to higher detection
bandwidth.
P-i-n Photodiode (Cont.)
P-i-n Photodiode
• Its depletion region thickness can
be tailored to optimize the quantum
efficiency & freq. response
• Light absorption in the semicond.
Produces electron-hole pairs
• Pairs produced in the depletion
region or within a diffusion length –
separated by the electric field
• current flows in the external circuit
as carriers drift across the depletion
layer
Figure 9.32.
Operation of a p-i-n photodiode. (a) Cross-section view of a p-i-n photodiode. (b)
Energy band diagram under reverse bias. (c) Carrier absorption characteristics.
Metal-semiconductor photodiode
Figure 9.33. Metal-semiconductor photodiode.
avalanche photodiode
A photodiode that exhibits internal amplification of
photocurrent through avalanche multiplication of carriers
in the junction region.
Figure 9.34.
A typical silicon avalanche
photodiode: (a) device structure
and (b) quantum efficiency.
The avalanche photodiode (APD), is also
reverse-biased. The difference with the
PIN diode is that the absorption of a
photon of incoming light may set off an
electron-hole pair avalanche
breakdown, creating up to 100 more
electron-hole pairs. "This feature gives
the APD high sensitivity" (much greater
than the PIN diode).
SOLAR CELL
•Advantages:
•Can convert
sunlight
directly to
electricity with
good
conversion
efficiency
•Provide
nearly
permanent
power at low
operating cost
•Non polluting
A solar cell consists of two
layers of semiconductor, p-type
and
n-type, sandwiched
together to form a 'pn junction'.
This pn interface induces an
electric field across the junction.
When 'photons are absorbed by
the semiconductor, they transfer
their energy to some of the
semiconductor's
electrons,
which are then able to move
about through the material. For
each such negatively charged
electron,
a
corresponding
mobile positive charge, called a
'hole', is created. In an ordinary
semiconductor, these electrons
and holes recombine after a
short time and their energy is
wasted as heat.
P-N JUNCTION SOLAR CELL
Figure 9.36.
Schematic representation of a
silicon p-n junction solar cell.
P-N JUNCTION SOLAR CELL
(Cont.)
•Constant-current source – parallel
with the junction
• Source IL: results from the
excitation of excess carriers by
solar radiation
•Is: diode saturation current
•RL: load resistance
Figure 9.37. (a) Energy band diagram of a p-n junction solar cell under solar
irradiation. (b) Idealized equivalent circuit of a solar cell.
P-N JUNCTION SOLAR CELL
(Cont.)
Figure 9.38. (a) Current voltage characteristics of a solar cell under
illumination. (b) Inversion of (a) about the voltage axis.
Test 2
will be tomorrow on Thursday
16th Oct. 2008 @ K. Perlis (DKP1)
8.30pm – 9.30pm
The presentation of Miniprojects (with
Assignments) will be on Wednesday
22/10/2008 @ K. Perlis (DKP1)
8-10am
The attendance is Compulsory