ect 292 nano electronics

1
Module 5
1
ECT 292 Nano Electronics
APJ Abdul Kalam Technological University
Department of Electronics and Communication Engineering
Musaliar College of Engineering and Technology, Pathanamthitta.

HETEROJUNCTION BIPOLAR
TRANSISTORS(HBT)
• A desirable property for junction bipolar transistors is to have a high value of the
amplification factor β up to the largest frequencies possible.
• In order to obtain a high β, both the current gain through the base, α, and the injection
efficiency factor of the emitter, γ , should be as close as possible to unity.
• This condition requires the emitter region doping level to be much higher than
that of the base region.
• However, there is a reduction in the energy gap of semiconductors when the doping
level is very high which results in a notable reduction in carrier injection from the
emitter region to the base region.
• Thus emitter could be fabricated by using a wide bandgap semiconductor.
• Bipolar transistors fabricated by using heterojuntions are called heterostructure bipolar
transistors (HBT)

• Note that in second figure the band gap of the emitter is larger than that of the base region.
• Similarly, the barrier for the injection of electrons from the emitter to the base, eVn, is lower.
• The barrier height difference has an enormous influence on carrier injection through the
emitter– base junction.
• The simultaneous reduction of the base resistance and the capacitance of the emitter–base
junction are essential for the correct performance of HBTs at high frequencies.

• Another interesting feature of heterostructures is the possibility to fabricate a
graded base HBT.
• As a consequence, an internal electric field is created which accelerates
electrons travelling through the base region, and therefore, allows HBTs to
operate at even higher frequencies.
.

• If the collector region is also fabricated from a wide bandgap semiconductor,
the breakdown voltage of the base–collector junction can be notably
increased.

• Due to the good behaviour of AlGaAs–GaAs heterojuntions, and to high
values of the mobility, HBTs are usually fabricated from III-V
semiconductors.
• In a typical HBT, the base length can be of about 50 nm and is heavily
doped.
• These transistors can be used up to frequencies of approximately 100
GHz.
• The use of InGaAs–InAlAs and InGaAs–InP based heterostructures
allows even higher operating frequencies (∼200 GHz) to be reached.
• An additional advantage of III-V semiconductor-basedHBTs is the
possibility of their integration of optoelectronic devices in the same chip.
• These optoelectronic integrated circuits (OEIC) usually include
semiconductor lasers.

• There are also research projects focused on the development of HBTs
based on silicon technology, which make use of different silicon compounds
as wide bandgap materials.
• One of these compounds is silicon carbide (SiC), whose bandgap is 2.2 eV.
• Another material widely employed is hydrogenated amorphous silicon,
whose bandgap is 1.6 eV.
• The most promising of all silicon compounds for the fabrication of HBTs are
SiGe-based alloys, from which heterojunctions can be formed.
• Commercial HBTs have cut-off frequencies over 100 GHz, while research
devices have reached values close to 400 GHz
• These HBTs have a higher power dissipation than MOSFETs, but can be
operated at higher frequencies and with lower noise.
• All these improvements make the SiGe-based HBTs very promising devices.

RESONANT TUNNEL EFFECT
• Electrons in heterojunctions and in quantum wells can respond with very
high mobility to applied electric fields.
• Here response to an electrical field perpendicular to the potential
barriers at the interfaces will be considered.
• Under certain circumstances, electrons can tunnel through these potential
barriers, constituting the so-called perpendicular transport.
• Tunnelling currents through heterostructures can show zones of negative
differential resistance (NDR), which arise when the current level decreases
for increasing voltage.
• Resonant tunnel effect (RTE) takes place when the current travels through a
structure formed by two thin barriers with a quantum well between them.

• The thickness of the quantum well is supposed to be small enough (5–10 nm).
• The well region is made from lightly doped GaAs surrounded by higher gap
AlGaAs.
• Let us suppose that an external voltage, V, is applied, starting from 0V.
• some electrons tunnel from the n+ GaAs conduction band through the potential
barrier, thus resulting in increasing current for increasing voltage
• When the voltage increases, the electron energy in n+ GaAs increases until the
value 2E1/e is reached, for which the energy of the electrons located in the
neighbourhood of the Fermi level coincides with that of level E1 of the electrons in
the well.
• In this case, resonance occurs and the coefficient of quantum transmission
through the barriers rises very sharply.
• If the voltage is further increased the resonant energy level of the well is located
below the cathode lead Fermi level and the current decreases (region 3), thus
leading to the socalled negative differential resistance (NDR) region.

RESONANT TUNNELING DIODES (RTDs)
• used in microwave applications
• RTDs are based on resonant tunnel effect (RTE).
• A figure of merit used for RTDs is the peak-to-valley current
ratio (PVCR), of their I–V characteristic, given by the ratio
between the maximum current (point 2) and the minimum
current in the valley (point 3).
• The normal values of the figure of merit are about five for
AlGaAs– GaAs structures at room temperature, values up to
10 can be reached in devices fabricated from strained InAs
layers.

• It is observed that the maximum operation frequency increases as C
decreases.
• The resonant tunnel diode is fabricated from relatively low-doped
semiconductors, which results in wide depletion regions between the barriers
and the collector region, and accordingly, small equivalent capacity.
• For this reason, RTDs can operate at frequencies up to several terahertz (THz).
• RTDs are the only purely electronic devices that can operate up to frequencies
close to 1 THz.
• The power delivered from the RTDs to an external load is small and the output
impedance is also relatively small.
• The output signal is usually of low power(lower than 0.3V)
• RTDs have been used to demonstrate circuits for numerous applications
including static random access memories (SRAM), pulse generators,
multivalued memory, multivalued and self-latching logic, analogue-todigital
converters, oscillator elements, shift registers, low-noise amplification, etc..

HOT ELECTRON TRANSISTORS
• When electrons are accelerated in high electric fields, they can acquire energies
much higher than those corresponding to thermal equilibrium.
• when an external applied electric field accelerates electrons to very high velocities,
the kinetic energy, and thus Te, can reach values which are much higher than those
corresponding to the temperature of the crystal. In this case, the electrons are far
from thermodynamic equilibrium, and receive the name of hot electrons.
• Heterojunctions between different gap semiconductors allow the generation of
hot electrons, since the electrons will acquire a kinetic energy, given by the
energy discontinuity in the conduction band VEc, when travelling from a wide
bandgap semiconductor to one with smaller bandgap.
• The above effect is called electron injection by heterojunction.
• AlGaAs–GaAs heterojunction, the value of VEc, ranges from 0.2 to 0.3 eV
• One way of selecting the most energetic electrons in a given distribution consists
of making them cross a potential barrier.
• Evidently, if the barrier is not very thin, only the most energetic electrons will have
enough energy to overcome the barrier.

• Figure shows the typical structure of a hot electron transistor, consisting of a
n+ GaAs emitter, a very thin (∼50 Å) AlGaAs barrier, the GaAs base region
(∼1000 Å), another thick AlGaAs barrier of about 3000 Å, and the n+ GaAs
collector.
• When a positive voltage is applied to the collector, the injection of hot
electrons coming from the emitter takes place by tunnelling through the thin
AlGaAs barrier, since the base is positively biased with respect to the
emitter.
• It must be noted that the barrier’s energy lavel can be modulated by varying
the voltage difference between emitter and base, VBE.
• The velocity of the injected electrons is much higher than in other type of
transistors.

• The base transit time when the transistor is polarized can be of the
order of tens of femtoseconds, but that associated to the crossing of
the collector barrier is relatively higher.
• Nowadays, there are several efforts to reduce this time, although the
collector barrier cannot be reduced as desired since leakage
currents have to be prevented.
• Much effort is currently being developed to progressively reduce the
dimensions of HET devices, so that the electron transit time is as
short as possible.
• to overcome transit time problems, it was proposed to substitute the
semiconductor at the base by a material that behaves as a metal, is
non- contaminant, and does not show electromigration effects. The
resulting device is called metal base transistor (MBT).

• The two most common MBTs structures are represented in figure
• For the base region of MBTs, materials such as cobalt silicide (CoSi2) is used; this silicide shows a
conductivity almost as high as that of metals and is chemically compatible with silicon technology.
• shows the band structure of a device consisting of a metal-oxide-metal-oxide-metal heterostructure,
under forward bias between the emitter–base and base–collector electrodes.
• In this case, electrons are injected by tunnelling through a thin barrier into the emitter junction.
• Second figure shows an even simpler MBT, formed by Si–CoSi2–Si.
• CoSi2 is chosen as material for the base region due to the good lattice matching properties with silicon
that results in high quality interfaces
• In these transistors, hot electrons behave as ballistic electrons they practically do not suffer scattering
since their mean free path is larger than the thickness of the base region.
• the most significant advantages of MBTs is that they are unipolar devices and can operate at higher
frequencies.

RESONANT TUNNELLING TRANSISTOR
• Diodes based on the resonant tunnel effect (RTE) can be
incorporated into standard bipolar transistors, field effect transistors
or into hot electron transistors.
• Such transistors are called resonant tunneling transistors (RTT)
• Let us first consider a bipolar transistor in which a RTD is added to
the emitter junction.
• Since the emitter to base polarization voltage, VEB, controls the
tunneling resonant current.
• The collector current will show the typical RTD dependence.

• The output I–V characteristics present alternate regions
of positive and negative transconductance that can be
controlled by the voltage VEB.

• Energy diagram of hot electron resonant tunnelling transistor biased in the
active
region is shown previously.
• Between the emitter and base regions of this transistor there exists a resonant
tunnelling heterostructure, capable of injecting a large current when the electron
resonant condition is reached.
• The position of the resonant level related to the emitter, is controlled by the
voltage level applied to the base region, VBE.
• This voltage can be increased until the resonant condition is reached.
• A maximum in the output current, IC, is then produced.
• If VBE is further increased, the current starts to diminish until a minimum value
at
V2 is reached, similar to the description of the I–V characteristic of RTE.
• output characteristics of this transistor also show regions of negative differential
resistance.
• The resonant tunnel structure injects electrons in a very narrow energy range

• Resonant tunnelling diodes can also be incorporated in a different manner to bipolar transistors.
• Figure shows a AlGaAs–GaAs bipolar transistor to which a RTD has been added to the base terminal.
• It consist of a quantum well between the two potential barriers in the RTD.
• The existence of several energy levels in the quantum well has been considered.
• There are several new applications of RTTs, mainly in the field of digital electronics.
• These devices allow the implementation of logic gates with a smaller number of transistors than
usually needed
• A full adder circuit can be fabricated from just one resonant tunnel bipolar transistor and two standard
ones, while the same conventional adding circuit needs about 40 transistors.

SINGLE ELECTRON TRANSISTOR
• The concept of single electron transistor (SET) is based on the behaviour of
0D nanometric structures, such as quantum dots, in which electrons are
distributed in discrete energy levels.
• One of the most interesting properties of these structures is called Coulomb
blockade effect.
• When the tiny conducting material is extremely small, the electrostatic
potential significantly increases even when only one electron is added to it.

• For the correct operation of SETs two conditions have to
be met.
• First, the change in electric energy when an electron
enters or leaves the quantum dot, i.e. the charging
energy, has to be much larger than kT.
• Secondly, the resistance RT of the tunnel junction must
be large enough compared to the quantum resistance
RQ = h/e2

• Figure (a) shows the schematic representation of a SET and Figure
(b) is its equivalent circuit as a three-terminal device.
• The quantum dot, is connected to the source and drain by two tunnel
barriers.
• The number of electrons in the Coulomb island can be
controlled by the external voltage, VG

• In order to fabricate transistors based on the Coulomb blockade
effect, three terminals are needed.
• One of these terminals can be used as a gate to control the current
flow through the quantum dot.
• Therefore, the SET basically consists of a quantum dot connected to
the source and drain electrodes through tunnel junctions.
• The gate electrode is coupled to the quantum dot by an insulating
material, in such a way that the electrons cannot tunnel through the
barrier.
• Since the source or drain current flow is controlled by the gate, the
described three-terminal device operates as a transistor.
• the quantum dot playing the role of the channel region in MOSFETs.

• The current–voltage characteristics of the SET can be determined by
applying a continuously sweeping voltage, VG, to the gate electrode.
• The applied voltage induces a charge CVG in the opposite plate of the
capacitor, which is compensated by the tunnelling of a single electron that
enters the quantum dot.
• Thus, some kind of competence between the induced charge and the
discrete one that tunnels through the barriers is established, that results in
the so-called Coulomb oscillations
• Coulomb oscillations is associated with the current flow due to the
discrete charges that tunnel through the barriers.
• Between two consecutive peaks, the number of electrons in the quantum dot
is fixed and therefore no current flows.

• logic circuits based on SETs can be implemented.
• For exapmle, an inverter with SET.
• each tunnel barrier is represented by a resistance in parallel with a
capacitor.
• The periodicity of the output voltage is e/CG and its amplitude is given by
e/(CS + CD + CG).
• The above inverters have been already used in the design of logical circuits
and as unit memory cells.

comparison of SETs and MOSFETs
• It is well known that until recently MOSFETs have been the basic devices of
semiconductor chips, but we are already approaching their limiting feature
size.
• SETs would present the advantage of fabricating devices with smaller sizes.
• They would also dissipate less power,
• One significant disadvantage of SETs is given by their high output impedance,
due to the resistance associated to tunnel barriers.
• Another disadvantage is related to the size of the quantum dot, since for room
temperature operation its capacitance should be as a small as possible.

Stimulated emission
• Suppose a simple electron system of just two energy levels E1 and E2 (E2 > E1).
• Electrons in the ground state E1 can jump to the excited state E2 if they absorb
photons of energy E2−E1.
• On the contrary, photons of energy E2−E1 are emitted when the electron drops
from E2 to E1.
• In general, the emission of light by a transition from the excited state E2 to the
ground state E1, is proportional to the population of electrons n2 at the level E2. This
is called spontaneous emission.
• Electrons can also drop from E2 to E1 if they are stimulated by photons of energy
hν = E2 − E1.
• This process is therefore called stimulated emission and is proportional to the density
ρ(ν) of photons.
• One very interesting aspect of stimulated emission is that the emitted photons are
in phase with the stimulating ones.
• In order to dominate over absorption (proportional to n1) we should have n2 > n1.
This condition is known as population inversion

HETEROSTRUCTURE
SEMICONDUCTOR LASERS
• Lasers based on p–n junctions of the same
semiconductor, as for example GaAs, have
several drawbacks:
– Bad definition of the light emitting active region,
with a size of about the diffusion length LD.
– The threshold current, i.e. the minimum current
necessary for laser action, is quite large.

• In the 1970s that double heterostructure (DH) lasers,
which provide both carrier and optical confinement,
could be much more efficient than homojunction lasers
and show threshold density currents (∼1000Acm−2) at
least one order of magnitude lower.
• Figures (a) and (b) show the basic structures of
homojunction and DH lasers.

• In the double heterostructure, stimulated emission occurs only within a thin
active layer of GaAs, which is sandwiched between p- and n- doped AlGaAs
layers(cladding layers) that have a wider band gap.
• The surrounding cladding layers provide an energy barrier to confine
carriers to the active region.
• When a bias voltage is applied in the forward direction, electrons and holes
are injected into the active layer.
• Since the band gap energy is larger in the cladding layers than in the active
layer, the injected electrons and holes are prevented from diffusing across
the junction by the potential barriers formed between the active layer and
cladding layers.
• The electrons and holes confined to the active layer create a state of
population inversion, allowing the amplification of light by stimulated
emission.

• The cladding layers serve two functions. First, inject charge carriers.
Second, light confinement.
• The actual operation wavelengths may range from 750 - 880 nm
due to the effects of dopants, the size of the active region, and the
compositions of the active and cladding layers.
• When a certain parameter is fixed, the wavelength can vary in
several nanometers due to other variables.
• refractive index of active region is slightly larger than that of the
surrounding layers and this helps to confine the light.

• The heterojunctions allow the formation of potential wells for
electrons and holes, as shown in Figure.
• Which increases the carrier concentration and, more importantly, the
degree of inversion of the population of electrons and holes.
• In optics, the refractive index or index of refraction of a material is a
dimensionless number that describes how fast light propagates
through the material.
• One interesting additional aspect of DH lasers is that larger value
(∼5%) of the GaAs refractive index in comparison with the AlGaAs
surrounding material. This difference is enough to provide an
excellent optical confinement.
• The optical confinement factor , which indicates what fraction of the
photon density is located within the active laser region and can
approach unity.

• In order to make the DH laser more efficient, the transverse stripe
geometry configuration has been adopted almost universally.
• In this geometry, the transverse or horizontal dimension of the active
region, and consequently the threshold current, is greatly reduced.
• Because of the shape of the active region, stripe geometry lasers
are much easier to couple with fibres, waveguides, etc.
• These lasers are called index guided lasers or buried DH lasers.

QUANTUM WELL SEMICONDUCTOR LASERS
• Regular double heterostructure (DH) semiconductor lasers have an active region of
0.1 to 0.2um thick.
• Since the 1980s, lasers with very thin active regions, quantum well lasers, were being
developed in many research laboratories.
• A quantum well laser is a laser diode in which the active region of the device is so
narrow that quantum confinement occurs.
• This results in a set of discrete energy levels and the density of states is modified to a
two-dimensional-like density of states.
• The wavelength of the light emitted by a quantum well laser is determined by the
thickness of the active region rather than just the bandgap of the material from which
it is constructed.
• This means that much shorter wavelengths can be obtained from quantum well lasers
than from conventional laser diodes using a particular semiconductor material.
• The efficiency of a quantum well laser is also greater than a conventional laser diode
due to the stepwise form of its density of states function.

• improved characteristics of quantum well lasers are mainly due to the
properties of the 2D density of states function and characteristic of quantum
wells.
• Figures (a) and (b) show two separate structures frequently used.

• Quantum well lasers require fewer electrons and holes to reach
threshold than conventional double heterostructure lasers.
• A well-designed quantum well laser can have an exceedingly low
threshold current.
• To compensate for the reduction in active layer thickness, a small
number of identical quantum wells are often used. This is called a
multi- quantum well laser.

• These structures are obtained for instance by grading the
composition of AlxGa1−xAs compounds with values of x between
zero and about 0.30, the value of the gap increasing from 1.41 eV to
about 2.0 eV.
• The waveguiding effect can be further improved by grading the
refractive index, as shown in the lower part of Figure (b), in the so
called graded index separate confinement heterostructures
(GRINSCH).
• Very often, in order to enlarge the emitted laser signal, a structure
with multiple quantum wells is implemented (Figure (c)) instead of
just one single quantum well.

• The gain can be related to the current density J by assuming a value
of τ for the recombination time, since J = endτ−1, where d is the
thickness of the active region; alternatively, τ could be obtained from
the rate of radiative recombination.
• For the case of MQW structures with nw quantum wells, each with a
gain gw, the total gain nwgw as a function of the total injection current
density nJ is plotted in Figure (b) for nw = 1, 2, 3, and 4.
• In order to obtain a high differential gain one should use a MQW
structure.
• QW lasers also show a higher efficiency and smaller internal losses
than DH lasers.
• For the high-speedoperation of QW lasers, a proper design of the
separate confinement well heterostructure is important.

Strained Quantum-Well Lasers
• Quantum well lasers have also been fabricated using an active layer whose
lattice constant differs slightly from that of the substrate and cladding layers.
Such lasers are known as strained quantum-well lasers.
• They show many desirable properties such as low-threshold current
density and lower linewidth than regular Multi-Quantum-Well (MQW)
• strain introduces a new variable to extend wavelength tunability, in addition
to controlling the width and barrier height of the quantum wells.
• The origin of the improved device performance lies in the band-structure
changes induced by the mismatch-induced strain.
• One of the most investigated strained quantum wells for lasers is the GaAs-
InGaAs– GaAs. In this case, the inner InGaAs layer is under compressive
strain. This has important consequences in the band structure.
• it changes considerably the values of the hole effective masses and
increases the value of the energy gap.

VERTICAL CAVITY SURFACE EMITTING
QUANTUM WELL LASERS (VCSELS)
• Light is emitted perpendicularly to the
heterojunctions.
• There are several obvious advantages related to this geometry
– Ease of testing at the wafer scale before packaging,
– Construction of large arrays of light sources (more than one
million on a single chip),
– Easy fibre coupling.
– Possibility of using chip-to-chip optical interconnects.

The geometry consists of a vertical cavity along the direction of
current flow.
• Light is extracted from the surface of the cavity rather than from the
sides.
• Two very efficient reflectors are located at the top and bottom of the
active layer.
• The reflectors are usually dielectric mirrors made of multiple
quarter-wave thick layers of alternating high and low refractive
indexes.

QUANTUM DOT LASERS
• A quantum dot laser is a semiconductor laser that uses quantum
dots as the active laser medium in its light emitting region.
• Quantum dot and quantum wire lasers would exhibit higher and
narrower gain spectrum, low threshold currents, better stability with
temperature, lower diffusion of carriers to the device surfaces, and a
narrower emission line than double heterostructure or quantum well
lasers.
• the experimental values obtained for quantum wire lasers are still far
from
the theoretical predictions and hence we refer only to quantum dot
lasers
• The growth technologies for quantum wire structures will have to
improve, especially with respect to the quality of interfaces,
uniformity of the wires.

• Let us assume that the quantum dots are small enough so that the
separation between the first two electron energy levels for both
electrons and holes is much larger than the thermal energy kT.
• Then for an undoped system, injected electrons and holes will
occupy only the lowest level.
• Therefore, all injected electrons will contribute to the lasing transition
from the E1 to the E2 levels, reducing the threshold current with
respect to other systems.
• The lowest threshold currents have already been reached for
quantum dot lasers.
• an ideal quantum dot laser is very sharp and does not depend on
temperature. Therefore, quantum dot lasers should have a better
stability with temperature without the need for cooling.

• Quantum dots should have the narrowest spectrum and the
highest gain.
• Also they should have a symmetrical spectrum, which would
produce no jitter.
•

quantum dots fabrication
• Traditional methods for fabricating quantum dots include
semiconductor precipitates in a glass matrix or etching away a
previously grown epitaxial layer.
• None of these methods can produce large densities of dots, and the
control of size and shape is difficult.
• Moreover they introduce large defects into the dots and create
many surface states that lead to non-radiative recombination
methods
• The growth techniques of quantum dots not yet matured.

• The most successful method to date has been the growth of self-
assembled quantum dots at the interface of two lattice-mismatched
materials.
• In this method a material such as InAs is grown by chemical vapour
deposition, metalorganic vapour phase epitaxy or molecular beam
epitaxy on a substrate with a larger lattice parameter and a larger
bandgap such as GaAs.
• The first few monolayers grow in a planar mode with a large tensile
strain But beyond a critical thickness, it is more energetically
favourable to form islands (the so-called Stranski–Krastanow regime)
as shown in Figure
• Subsequently, a layer is overgrown epitaxially on top of the dots,
creating an excellent heterostructure between two single-crystal
materials

• Previous figure shows schematically an edge-emitting laser based on self- assembled
quantum dots.
• The device consists of several layers forming a pin diode structure.
• The layers are, from bottom to top, the n-GaAs substrate, a n-AlGaAs layer, an intrinsic
GaAs layer with the dots, a p-AlGaAs layer, and a p-GaAs cap layer.
• Metallic contacts on the substrate and the cap layer connect the device to an
external circuit.
• Under a forward bias voltage, electrons and holes are injected into the middle intrinsic
GaAs layer or active layer, where they fall into the quantum dots, which have a smaller
bandgap, and recombine there.
• The emission wavelength corresponds to the interband transition of the InAs quantum
dots.
• The GaAs layer, which is sandwiched between AlGaAs layers with a lower refractive
index, confines the light and increases the interaction with the carriers.
• It is useful for telecommunication amplifiers and tunable lasers.
• Also dot present a better stability with operation temperature

PHOTODETECTORS
• Two types
• (a) Quantum well subband photodetectors
or
Quantum Well Infrared Photodetector (QWIP)
• (b) Superlattice avalanche photodetectors

Quantum well subband
photodetectors
• Quantum wells can be used for the detection of light.
• It is commonly used in the IR region between 2 and 20μm.
• Quantum well photodetectors are preferably used for applications of night
vision and thermal imaging.
• Normal photodiodes are based on band to band transitions across the
semiconductor gap Eg.
• This require materials with very low values of Eg, which makes it necessary
to work at cryogenic temperatures.
• The materials used for IR detection are normally quite soft, difficult to
process, and have large dark currents. This makes quantum wells very
appropriate for use in IR detection.

• Figure shows the absorption transitions suitable for IR detection for a
single quantum well under the action of an applied electric field.
• Practical devices are made with MQWs.
• The separation between levels should be in the range 0.1–0.2 eV,
which for III-V compounds implies a width of the wells of about 10
nm.
• Polarization of the incident radiation should be parallel to the
confinement direction.
• Under light irradiation, this type of photodetectors generates a
current by tunnelling of the carriers outside the wells.

• A Quantum Well Infrared Photodetector (QWIP) is an
infrared photodetector , which uses electronic intersubband
transitions in quantum wells to absorb photons.
• When a bias voltage is applied to the QWIP, the entire
conduction band is tilted.
• Without light the electrons in the quantum wells just sit in
the ground state.
• When the QWIP is illuminated with light of the same or
higher energy as the intersubband transition energy, an
electron is excited.
• Once the electron is in an excited state, it can escape
into the continuum and be measured as photocurrent.

Superlattice avalanche photodetectors
• As the name implies, it uses the avalanche process.
• It operates under a high reverse bias condition.
• This enables avalanche multiplication of the holes and electrons created by the
photon / light impact.
• charge carriers will be pulled by the very high electric field away from one another.
• Their velocity will increase to such an extent that when they collide with the lattice,
they will create further hole electron pairs and the process will repeat.
• avalanche photodetectors (APD) based on semiconductors can present a high level
of noise if precautions are not taken.
• The noise can be gradually reduced if the avalanche multiplication coefficient, α, is
much larger for one of the carriers, for instance electrons, in comparison to the other
carrier (hole) multiplication coefficient.
• In this sense, silicon is a very appropriate semiconductor for APDs, since the ratio
αe/αh has a value of about 30.
For a given semiconductor, the ratio αe/αh is practically fixed by the semiconductor
band structure

• Quantum wells, on the other hand, allow a design control of αe/αh.
• a superlattice or MQW structure can be designed such that the conduction
band discontinuities VEc are much larger than the VEv ones corresponding to
the valence band.
• In this way, the electrons gain much more kinetic energy than the holes
when they cross the band discontinuity.
• The same objective can be achieved by the design of a staircase profile
superlattice for which the bandgap is graded in each well.
• In this case, the electrons have an extra kinetic energy Ec when they enter the
next quantum well.
• This extra energy makes the impact ionization phenomenon very efficient so
that electron avalanches are easily generated under the action of an electric
field F.
• However, it should be mentioned that staircase superlattices are difficult to
fabricate since their production requires strict control of the deposition
parameters

.
QUANTUM WELL MODULATORS
• Quantum wells can be conveniently used for the direct modulation of light,
since they show much larger electro-optic effects than bulk semiconductors.
• Electro-optic effects are rather weak in bulk semiconductors and, for this
reason conventional modulators make use of materials such as lithium
niobate.
• due to the quantum confined Stark effect (QCSE), large changes in the
optical absorption spectrum of quantum wells could be induced by the
application of electric fields.
• Electroabsorption modulators are based on the change of the optical
absorption coefficient in a quantum well under effect of an electric field.
• An electro-absorption modulator (EAM) is a semiconductor device which
can be used for modulating the intensity of a laser beam via an electric
voltage

• Previous Figure shows a mesa-etched modulator.
• To make the effect more significant, one uses a set of multiple
quantum wells (MQW).
• The MQWs tructure consists generally of an array of several
quantum wells (5 to 10 nm in thickness each) of the type AlGaAs–
GaAs–AlGaAs.
• The structure is placed between the p+ and n+ sides of a reverse
biased junction.
• Since the whole MQW structure has a thickness of about 0.5μm,
small reverse voltages can produce electric fields in the 104 to 105
Vcm−1 range.
• These fields induce changes in the excitonic absorption edge in the
energy range 0.01–0.05 eV

• Electroabsorption modulators, such as the one described, allow high speed
modulation with a large contrast ratio of transmitted light through the device.
• The contrast ratio can be as high as 100 by working in the reflection mode
instead of the transmission one.
• This is done by depositing a metal layer substrate and forcing light to
make two passes.
• The modulation factor can also be improved by working at low
temperatures.
• Electroabsorption modulators can operate up to frequencies of several tens
of GHz and if high electric fields are applied, the maximum frequency can
approach 100 GHz.
• For low fields, the generated electron–hole pairs during absorption are
unable to escape from the quantum well.
• However, if the fields are high enough, the electrons and holes can escape
from the wells by tunnelling with a characteristic time of a few picoseconds.

• The incoming signal from an optical waveguide is split in two beams of the
same intensity which travel through different channels in the material of the
same length before they recombine again.
• An electric field is applied to one of the branches causing differences in phase
between the two beams and causing interference patterns at the meeting
point.

ect 292 nano electronics

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