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Varactor Diode
A junction diode which acts as a variable capacitor under changing reverse bias is
known as a varactor diode. When a pn junction is formed, depletion layer is created in
the junction area. Since there are no charge carriers within the depletion zone, the zone
acts as an insulator.
The p-type material with holes (considered positive) as majority carriers and n-type
material with electrons (−ve charge) as majority carriers act as charged plates. Thus the
diode may be considered as a capacitor with n-region and p-region forming oppositely
charged plates and with depletion zone between them acting as a dielectric.
This is illustrated in Fig. (i). A varactor diode is specially constructed to have high
capacitance under reverse bias. Fig. (ii) shows the symbol of varactor diode. The
values of capacitance of varactor diodes are in the picofarad (10−12
F) range.
The normal operation, a varactor diode is always reverse biased. The capacitance of
varactor diode is found as :
CT = Total capacitance of the junction
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ε = Permittivity of the semiconductor material
A = Cross-sectional area of the junction
Wd = Width of the depletion layer
When reverse voltage across a varactor diode is increased, the width Wd of the
depletion layer increases. Therefore, the total junction capacitance CT of the junction
decreases. On the other hand, if the reverse voltage across the diode is lowered, the
width Wd of the depletion layer decreases. Consequently, the total junction
capacitance CT increases.
A forward biased varactor diode would serve no useful purpose
Fig. below shows the curve between reverse bias voltage VR across varactor diode
and total junction capacitance CT. Note that CT can be changed simply by changing
the voltage VR. For this reason, a varactor diode is sometimes called voltage-
controlled capacitor.
tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very
fast operation, well into the microwave frequency region, by using quantum mechanical
effects.
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Tunnel Diodes (Esaki Diode)
Tunnel diode is the p-n junction device that exhibits negative resistance. That means
when the voltage is increased the current through it decreases.
Tunnel Diode principles Concept of Electron Tunneling
Regular p-n Diode Tunnel Diode
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For thick barrier, both Newtonian and Quantum mechanics say that the electrons
cannot cross the barrier. It can only pass the barrier if it has more energy than the
barrier height.
For thin barrier, Newtonian mechanics still says that the electrons cannot cross the
barrier. However, Quantum mechanics says that the electron wave nature will allow it
to tunnel through the barrier.
Newtonian Mechanics Quantum Mechanics
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Tunnel Diode Operation
Under Forward Bias
Step 1: At zero bias there is no current flow
Step 2: A small forward bias is applied. Potential barrier is still very high –no
noticeable injection and forward current through the junction.
Step 3: With a larger voltage the energy of the majority of electrons in the n-region is
equal to that of the empty states (holes) in the valence band of p-region; this will
produce maximum tunneling current
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Step 4: As the forward bias continues to increase, the number of electrons in the n side
that are directly opposite to the empty states in the valence band (in terms of their
energy) decrease. Therefore decrease in the tunneling current will start.
Step 5: As more forward voltage is applied, the tunneling current drops to zero. But the
regular diode forward current due to electron – hole injection increases due to lower
potential barrier.
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Step 6: With further voltage increase, the tunnel diode I-V characteristic is similar to
that of a regular p-n diode.
Under Reverse Bias
In this case the, electrons in the valence band of the p side tunnel directly towards the
empty states present in the conduction band of the n side creating large tunneling
current which increases with the application of reverse voltage. The TD reverse I-V is
similar to the Zener diode with nearly zero breakdown voltage.
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Tunnel Diode I-V
The total current I in a tunnel diode is given by
(
)
(
)
Iexcess : is an additional tunneling current related to parasitic tunneling via impurities.
This current usually determines the minimum (valley) current, Iv . Rv and Vex are the
empirical parameters; in high-quality diodes, Rv >> R0 . Vex = 1…..5 V
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Typically, m = 1….3; V0= 0.1….0.5 V R0 is the TD resistance in the ohmic region
The maximum negative differential resistance |NDR| can be found as
The peak voltage Vp
⁄
Photovoltaic cell
The reverse biased pn junction photodiode operates in the photoconductive mode. If the
illuminated diode is used without external bias, a measurable forward voltage appears
between the p and n regions. This is called photovoltaic effect.
The solar cells convert radiation from the sun directly into electrical energy. In practice,
the open-circuit voltage of the silicon photovoltaic cells is about 0.5– 0.6 V. Their
efficiency is about 15 %.
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Definition of laser
A laser is a device that generates light by a process called stimulated emission. The
acronym LASER stands for Light Amplification by Stimulated Emission of
Radiation
Semiconducting lasers are multilayer semiconductor devices that generates a
coherent beam of monochromatic light by laser action. A coherent beam resulted
which all of the photons are in phase.
Laser Operating Principles
Two things are required to operate a laser: (1) gain medium that can amplify the
electromagnetic radiation propagating inside it and provide the spontaneous emission
noise input and (2) a feedback mechanism that can confine the electromagnetic field
through the well-defined optical modes.
the gain medium for a semiconductor laser consists of a semiconductor material.
A simple, practical and most commonly used method employs current injection through
the use of a forward-biased p-n junction. Such semiconductor lasers are referred to as
injection lasers or laser diodes.
The excited atoms eventually return to their normal “ground” state and emit light in the
process. Light emission can occur through two fundamental processes known as
spontaneous emission and stimulated emission.
1. Spontaneous emission represents the case of an electron in the conduction band
recombining spontaneously with a hole (missing electron) in the valence band to
generate a photon.
2. Absorption stimulates the generation of an electron in the conduction band while
leaving a hole in the valence band.
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3. Stimulated emission is stimulating the recombination of an electron a hole and
simultaneously generating a new photon. This is the all-important positive gain
mechanism that is necessary for lasers to operate.
Producing intense laser beam or amplification of light through stimulated emission
requires
higher rate of stimulated emission than spontaneous emission and self-absorption,
which is only possible f or N2 > N1 even though E2 >E1 (opposite to the Boltzmann
statistics). It means that one will have to create the condition of population inversion
In a semiconductor laser, population inversion is accomplished by injecting
electrons into the material to fill the lower energy states of the conduction band.
Background Physics
Consider the „stimulated emission‟ as shown previously.
Stimulated emission is the basis of the laser action.
The two photons that have been produced can then generate more photons, and the 4
generated can generate 16 etc… etc… which could result in a cascade of intense
monochromatic radiation.
Population Inversion
Therefore we must have a mechanism where N2 > N1
This is called population inversion
Population inversion can be created by introducing a so call metastable centre where
electrons can piled up to achieve a situation where more N2 than N1
The process of attaining a population inversion is called pumping and the objective
is to obtain a non-thermal equilibrium.
It is not possible to achieve population inversion with a 2-state system.
If the radiation flux is made very large the probability of stimulated emission and
absorption can be made far exceed the rate of spontaneous emission.
But in 2-state system, the best we can get is N1 = N2.
To create population inversion, a 3-state system is required.
The system is pumped with radiation of energy E31 then atoms in state 3 relax to
state 2 non radiatively.
The electrons from E2 will now jump to E1 to give out radiation.
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Laser diode
Forward electrical bias across the laser diode causes holes and electrons to be
"injected" from opposite sides of the p-n junction into the depletion region.
When an electron and a hole are present in the same region, they may recombine or
"annihilate" with the result being spontaneous emission
The difference between the photon-emitting semiconductor laser and conventional
phonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use
of a different type of semiconductor, one whose physical and atomic structure
confers the possibility for photon emission. These are the "direct bandgap"
semiconductors.
Laser Diode Principle
Consider a p-n junction
In order to design a laser diode, the p-n junction must be heavily doped.
In other word, the p and n materials must be degenerately doped
By degenerated doping, the Fermi level of the n-side will lies in the conduction
band whereas the Fermi level in the p-region will lie in the valance band.
Diode Laser Operation
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Population Inversion in Diode Laser
P-n junction must be degenerately doped.
Fermi level in valance band (p) and
conduction band (n).
No bias, built n potential; eVo barrier to
stop electron and holes movement
Forward bias, eV> Eg
Built in potential diminished to zero
Electrons and holes can diffuse to the
space charge layer
EFn-EfP = eV
eV > Eg
eV = forward bias voltage
Fwd Diode current pumping
injection pumping
There is therefore a population inversion
between energies near EC and near EV
around the junction.
This only achieved when degenerately
doped p-n junction is forward bias with
energy > Egap
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The Lasing Action
The population inversion region is a layer along the junction also call inversion
layer or active region
Now consider a photon with E = Eg
Obviously this photon cannot excite electrons from EV since there is NO electrons
there
However the photon can stimulate electron to fall down from CB to VB.
Therefore, the incoming photon stimulates emission than absorption
The active region is then said to have „optical gain‟ since the incoming photon has
the ability to cause emission rather than being absorbed.
Pumping Mechanism in Laser Diode
It is obvious that the population inversion between energies near EC and those near
EV occurs by injection of large charge carrier across the junction by forward
biasing the junction.
Therefore the pumping mechanism is forward diode current Injection pumping
Laser Diode Characteristics
Nanosecond & even picosecond response time (GHz BW)
Spectral width of the order of nm or less
High output power (tens of mW)
Narrow beam (good coupling to single mode fibers)
Laser diodes have three distinct radiation modes namely, longitudinal, lateral and
transverse modes.
In laser diodes, end mirrors provide strong optical feedback in longitudinal
direction, so by roughening the edges and cleaving the facets, the radiation can be
achieved in longitudinal direction rather than lateral direction.