Lecture 6: Light Emitting and Detecting Devices
MSE 6001, Semiconductor Materials Lectures
Fall 2006
While silicon dominates most electronic applications because of the electrical properties of
the silicon-silicon dioxide interface, silicon emits light too poorly for making devices because of
its “indirect-gap” band structure. Many of the III-V and other compound semiconductors have
“direct” bandgaps that are used for light emitting devices. Most light emitting devices rely on
electrons and holes recombining at the boundary between p and n doped materials, a pn junction.
6.1
pn Junctions and Light Emission
The local concentration of electrons and holes in a semiconductor can be represented by the Fermi
level in a band diagram. (Fig. 1) The Fermi level is the local chemical potential for electrons
in the crystal. A large electron concentration, for example due to doping with donors, pulls the
conduction band edge EC down to near the Fermi level EF . A large hole concentration lifts the
valence band edge EV to near the Fermi level. At a pn junction, with no current flowing across
it, the bands bend to align the bulk Fermi levels, giving a single, flat Fermi level. The material
has a depletion layer near the junction with essentially no electrons or holes, where the immobile
acceptor and donor ions left behind are depleted of free carriers. The resulting electric field due to
the depletion layer ions provides a barrier keeping the electrons in the n type material and the holes
in the p type material. (Fig. 2) Such junctions are at the heart of pn diodes, LEDs, diode lasers,
and solar cells.
When the junction is forward biased with a positive voltage applied to the p side with respect
to the n side, large currents flow. The holes and electrons that carry this current flow in opposite
directions because of their opposite charges, passing through the depletion region. The electrons
and holes can recombine in the depletion region, emitting photons with energies hν ≥ Eg for
direct band gap materials. Carriers injected across the depletion region can also recombine on the
other side to give light.
p-type
(NA)
n-type
(ND)
EC
electrons
EF
EC
EV
EF
holes
EV
F IGURE 1: In p type semiconductors the Fermi level is close to the valence band edge and in n type
material the Fermi level is close to the conduction band edge.
6-1
depletion
W
p-type
(NA)
n-type
(ND)
EC
electrons
EF
EV
holes
F IGURE 2: A depletion region forms at a pn junction. When forward biased, electron and holes
recombine in the depletion region, emitting light.
6.2
Light Emitting Diodes
Figure 3 plots badgap energies versus the crystal lattice constants for semiconductors used for
LEDs. The horizontal band of colors corresponds to the energies of the visible light spectrum.
Light emitting diodes have been developed for light emission from infrared to ultraviolet wavelengths.
Over the coming decade or two, it is thought that LEDs will replace many other lighting technologies because of their much greater efficiencies. Already, LEDs are being used in traffic lights
GaN, InGaN, and AlGaN materials
provide longer wavelength emission
But nitrides hav
AlN
6.0
0.2
•No lattice-matche
(dislocations)
G@N, AlG@N: UV
5.5
3.5
InG@N: blue,
4.0
0.3
green, @mber
G@N
0.4
3.0
AlP
G@P
2.5
InN
2.0
0.5
AlAs
AlSb
1.5
1.0
G@As InP
1.0
G@Sb
0.5
0.0
4.2
W@velength ( ! m)
(eV)
4.5
Energy G@p
5.0
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
•Mg p-type doping
(poor activation, h
•Highly non-linear
(poor control)
2.0
InSb5.0
InAs
•AlGaN not lattice
(dislocations and c
6.4
6.6
L@ttice Const@nt ( )
F IGURE 3: Materials for LEDs. (J. Simmons, Sandia National Labs)
•High T, high pres
(poor stability)
Optical & electrical properties
6-2dependent on defect concentrations
F IGURE 4: Heterostructure laser band diagrams. (B. Spears, http://britneyspears.ac/lasers.htm )
because of their much longer lifetimes. For context, an average incandescent light bulb converts
electrical power to visible light with an efficiency of about 4%. This means that a 100 W bulb is
only putting out 4 W of light. LEDs typically have efficiencies in the range of 20 - 40%, though
with device engineering these efficiencies can be above 50%.
6.3
Heterostructure Diode Lasers
A laser may be made by placing a light-emitting material in a cavity formed with partiallyreflecting mirrors and then electrically or optically exciting, or “pumping”, the material. The
light field in the cavity forms a standing wave pattern that coherently stimulates the emission of
more photons with the same frequency and and same phase, a process that feeds back on itself,
building up the strength of the field until the losses from the mirrors balances the power input into
the emitting material. The light that leaks out of a mirror forms the coherent laser beam.
If a simple pn junction is used for a laser, the electrons and holes that are injected into the
depletion region at the very high concentrations needed for lasing can diffuse away, reducing the
numbers of carriers available for emitting light into the cavity. This problem is avoided by using a
“double heterostructure”, which clads the depletion region with wider-bandgap material to confine
the electrons and holes to the junction. Figure 4 shows the equilibrium band diagram and the
band diagram for a double heterostructure laser under forward bias. In a semiconductor laser, the
pn junction is placed between mirrors that are formed by cleaving the crystal to give partiallyreflecting surfaces. (Fig. 5)
Semiconductor lasers are used in fiber optic communication systems, where the laser beam is
coupled into a silicon dioxide fiber. Higher-power semiconductor lasers have also been developed
for welding and cutting applications.
6-3
F IGURE 5: Heterostructure laser. (B. Spears, http://britneyspears.ac/lasers.htm )
6.4
Quantum Cascade Lasers
Quantum cascade lasers (QCLs) have been recently developed that do not rely on pn junctions.
Figure 6 plots the conduction band edge of the active region of a QCL. The superlattice has been
designed such that as electrons flow through the structure, they make quantum transitions that emit
light, emitting one photon per period.
QCLs are being developed for the near infrared to far infrared wavelength region. The applications for these devices are in chemical and biochemical detection and infrared imaging. For the
very long wavelengths, QCL-like devices may be the active devices in electrical circuits above 1
THz, at frequencies where
will
work. 2001
Appl. transistors
Phys. Lett., Vol.
79, no
No. longer
24, 10 December
TABLE I. Population of the individual levels in t
at 80 K with only carrier–phonon "c–p# scatterin
carrier–carrier "c–c# scattering. Letters refer to th
ing the first miniband in Fig. 1. g represents the in
capital letters label the other levels in ascending o
Subband
index
FIG. 1. Conduction band energy diagram of the chirped superlattice under
an average electric field of 3.5 kV/cm. The layer thickness "in nm# are, from
left to right, starting from the injection barrier 4.3/ 18.8/ 0.8/ 15.8/ 0.6/ 11.7/
2.5/ 10.3/ 2.9/ 10.2/ 3.0/ 10.8/ 3.3/ 9.9, where Al0.15Ga0.85As layers are in
bold face and the 10.2 nm wide GaAs well is doped 4"1016 cm#3. Also
shown are the moduli squared of the wave functions; the optical transition
occurs between states 2 and 1.
3
2
1
F
E
D
C
B
g
Energy
"meV#
Pop. "c–p only#
(109 cm#2)
51.7
36.1
17.9
14.1
11.0
7.7
4.5
3.2
0
0.02
0.94
4.72
7.13
8.08
7.34
5.14
1.99
6.19
the absence of c–c interaction, the car
high in the upper states of the injector m
larly in subband 1. On the other hand, in
scattering, electrons efficiently relax into
F IGURE 6: Quantum
cascade
laser. Conduction
band electrons
through the superlattice,
pute
a significantly
longer lifetime
! 1 !14.5cascade
ps resulting
the injector (g,A), from where they ar
emitting one photon each
period.c–c
(Kohler,
et al.,
Applied Physics
from superlattice
almost comparable
and c–p
contributions.
In real-Letters,
into2001)
the upper laser state of the following
ity, the electron dynamics is dominated by very fast c–c
case a population inversion of 'n!1.
scattering with a quasiresonant level "1 ! in Ref. 7#. However,
between subbands 2 and 1: compared t
such a process acts in both directions "1→1 ! and 1 ! →1#
the number of electrons of subband 1 i
with nearly equal rates, thus6-4
giving only a marginal contriwhereas that of state 2 increases by a fa
bution to the electron depletion of state 1. From the simulalated lifetimes are ! 2 !0.8, ! 21!8.3, !
tion we also obtain the value of the operational current denwith ! 2 and ! 21 dominated by c–p scatte
sity, 50 A/cm2, in very good agreement with the measured
that ! 21 is much greater than ! 2 due to th