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Chapter 2: Special Diodes
2
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In tunnel diode, electric current is caused by “Tunneling”
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It is also used in high-frequency oscillators and amplifiers
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Leo Esaki observed that if a semiconductor diode is heavily doped with impurities, it will exhibit negative
resistance
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The operation of tunnel diode depends on the quantum mechanics principle known as “Tunneling”
...
The germanium material is commonly used to make the tunnel diodes
...


Figure: Depletion Region in Tunnel Diode
The depletion region is a region in a p-n junction diode where mobile charge carriers (free electrons and holes)
are absent
...
The width of a depletion region depends on the number of impurities
added
...
If a small number of impurities are added to the p-n junction diode (p-type and n-type
semiconductor), a wide depletion region is formed
...

In tunnel diode, the p-type and n-type semiconductor is heavily doped which means a large number of impurities
are introduced into the p-type and n-type semiconductor
...
The concentration of impurities in tunnel diode is 1000 times greater than the normal p-n junction
diode
...
This wide
depletion layer or depletion region in normal diode opposes the flow of current
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To overcome this barrier, we need to apply sufficient voltage
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Unlike the normal p-n junction diode, the width of
a depletion layer in tunnel diode is extremely narrow
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Tunnel diodes are capable of remaining stable for a long duration of time than the ordinary
p-n junction diodes
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Prepared by: Er
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Because of these positive and negative ions, there exists a built-in-potential or electric field in the depletion region
...

Another thing we need to remember is that the valence band and conduction band energy levels in the n-type
semiconductor are slightly lower than the valence band and conduction band energy levels in the p-type
semiconductor
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Electric Current in Tunnel Diode
In tunnel diode, the valence band and conduction band energy levels in the n-type semiconductor are lower than
the valence band and conduction band energy levels in the p-type semiconductor
...
Because of this high difference in energy levels,
the conduction band of the n-type material overlaps with the valence band of the p-type material
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The depletion layer of tunnel diode is very small
...
So the
electrons can directly tunnel across the small depletion region from n-side conduction band into the p-side valence
band
...
But in tunnel diodes, a small voltage which is less than the built-in voltage of depletion region is enough
to produce electric current
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The electrons can directly tunnel from the conduction band of n-region
into the valence band of p-region
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Prepared by: Er
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In tunnel diode, the
conduction band of the n-type material overlaps with the valence band of the p-type material because of the heavy
doping
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Figure: Unbiased Tunnel Diode
When the temperature increases, some electrons tunnel from the conduction band of n-region to the valence band
of p-region
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However, the net current flow will be zero because an equal number of charge carriers (free electrons and holes)
flow in opposite directions
...
However, a small number of electrons in the conduction band of
the n-region will tunnel to the empty states of the valence band in p-region
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Thus, tunnel current starts flowing with a small application of voltage
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Because of the increase in voltage, the overlapping of the conduction band and
valence band is increased
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As a result, maximum tunnel current flows
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

Figure: Maximum Tunnel Current

Step 4: Applied Voltage is Further Increased
If the applied voltage is further increased, a slight misalign of the conduction band and valence band takes
place
...

The electrons tunnel from the conduction band of n-region to the valence band of p-region and cause a small
current flow
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Figure: Tunnel Current Starts Decreasing

Step 5: Applied Voltage is Largely Increased
If the applied voltage is largely increased, the tunneling current drops to zero
...
If this applied voltage is greater than the built-in potential of the depletion layer, the regular forward current
starts flowing through the tunnel diode
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The negative resistance region is the most important and most widely used characteristic of the
tunnel diode
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Prepared by: Er
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2 Schottky diode
Schottky diode is a metal-semiconductor junction diode that has less forward voltage drop than the P-N junction
diode and can be used in high-speed switching applications
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When a p-type semiconductor is
joined with an n-type semiconductor, a junction is formed between the P-type and N-type semiconductor
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In Schottky diode, metals such as aluminum or platinum replace the P-type
semiconductor
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Schottky
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Schottky diodes are widely used in radio frequency (RF) applications
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This junction is known as a metal-semiconductor
junction or M-S junction
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The metal-semiconductor junction
can be either non-rectifying or rectifying
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The rectifying metalsemiconductor junction is called non-Ohmic contact
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Also, the Schottky diode produces less unwanted noise than p-n
junction diode
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Figure: Schottky Diode

When sufficient voltage is applied to the Schottky diode, current starts flowing in the forward direction
...
This voltage loss is known as voltage drop
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6 to 0
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2 to 0
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Voltage loss or voltage drop is the amount of voltage wasted to turn on a diode
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6 to 0
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2 to 0
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Therefore,
the Schottky diode consumes less voltage to turn on
...
But germanium diodes are rarely used because the switching speed of germanium
diodes is very low as compared to the Schottky diodes
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In simple words,
Schottky barrier is the potential energy barrier formed at the metal-semiconductor junction
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The rectifying M-S junction forms a rectifying
Schottky barrier
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The nonrectifying metal-semiconductor junction forms a non-rectifying Schottky barrier
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

One of the most important characteristics of a Schottky barrier is the Schottky barrier height
...
The Schottky barrier height of Ohmic
contact (non-rectifying barrier) is very low whereas the Schottky barrier height of non-Ohmic contact (rectifying
barrier) is high
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So depletion region is negligible or absent in the Ohmic contact diode
...
So
the depletion region is present in the non-Ohmic contact diode
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The rectifying Schottky
barrier is formed when a metal is in contact with the lightly doped semiconductor, whereas the non-rectifying
barrier is formed when a metal is in contact with the heavily doped semiconductor
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The vacuum level
is defined as the energy level of electrons that are outside the material
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The work function is different
for metal and semiconductor
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Therefore, the electrons in the n-type semiconductor have high potential energy than the electrons in the metal
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The Fermi level at N-type semiconductor side lies
above the metal side
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So the electrons in the N-type semiconductor have more potential energy than the electrons in the
metal
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When the metal is joined with the n-type semiconductor, a device is created known as Schottky diode
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Prepared by: Er
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We know
that when a neutral atom loses an electron it becomes a positive ion similarly when a neutral atom gains an extra
electron it becomes a negative ion
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As a result, the atoms at the metal junction gains extra
electrons and the atoms at the n-side junction lose electrons
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Thus, positive ions are created the n-side junction and
negative ions are created at the metal junction
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Since the metal has a sea of free electrons, the width over which these electrons move into the metal is
negligibly thin as compared to the width inside the n-type semiconductor
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The built-in-voltage is the barrier seen by the
conduction band electrons of the n-type semiconductor when trying to move into the metal
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In unbiased Schottky diode, only a small
number of electrons will flow from n-type semiconductor to metal
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The transfer of free electrons from the n-type
semiconductor into metal results in energy band bending near the contact
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When a forward bias voltage is
applied to the Schottky diode, a large number of free electrons are generated in the n-type semiconductor and
metal
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2 volts
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

If the applied voltage is greater than 0
...
As a result, electric current starts flowing through the Schottky diode
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Step 3: Reverse Biased Schottky Diode
If the negative terminal of the battery is connected to the metal and the positive terminal of the battery is connected
to the n-type semiconductor, the Schottky diode is said to be reverse biased
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As a result, the electric current stops flowing
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Figure: Reverse Biased Schottky Diode
If the reverse bias voltage is continuously increased, the electric current gradually increases due to the weak
barrier
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This sudden
rise in electric current causes depletion region to break down which may permanently damage the device
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The
vertical line in the below figure represents the current
flow in the Schottky diode and the horizontal line
represents the voltage applied across the Schottky
diode
...
However,
the forward voltage drop of Schottky diode is very
low as compared to the P-N junction diode
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2 or 0
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In Schottky diode, the reverse
saturation current occurs at a very low voltage as
compared to the silicon diode
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

# Comparison of Tunnel Diode, Schottky Diode and Varactor Diode
Advantages

Disadvantages

1
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Schottky Diode
 Low junction capacitance
 Fast reverse recovery time
 High current density
 Low forward voltage drop or low turn
on voltage
 High efficiency
 Schottky diodes operate at high
frequencies
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Applications

 cannot be fabricated in large
numbers
 Being a two terminal device,
the input and output are not
isolated from one another
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 relaxation oscillator
circuits
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 FM receivers
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 radio frequency (RF)
applications
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 detect signals
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2
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Before going
to varactor diode, let’s first take a look at the capacitor
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The basic capacitor is made up of two parallel conductive plates separated by a dielectric
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The conductive plates are good
conductors of electricity so they easily allow electric current through them
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When voltage is applied to the capacitor in such a way that the negative terminal of the battery is connected to the
right side electrode or plate and the positive terminal of the battery is connected to the left side electrode, the
capacitor starts storing electric charge
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

Because of this supply voltage, a large number of electrons start flowing from the negative terminal of the battery
through a conductive wire
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The right side plate has a larger number of electrons than protons
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The free electrons in the
right side plate or electrode will try move into the dielectric
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As a
result, a large number of electrons are built up on the right side plate
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The dielectric blocks flow of charge carriers (free electrons) but allows electric force
exerted by the negatively charged electrode
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As a result, a large number of electrons leave the
left side plate and flow towards the positive terminal of the battery
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The positive and negative charges accumulated on both plates exert attractive force on each
other
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So at both plates,
the charge is stored
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# What is Varactor Diode?
The term varactor is originated from a variable capacitor
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The varactor diode acts like a variable capacitor
under reverse bias
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The varactor diode is manufactured in such a way that it shows better transition capacitance property than the
ordinary diodes
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# How Varactor Diode Works?
Step 1: Unbiased Varactor Diode
For free electrons, n-region is the higher concentration region and p-region is the lower concentration region
...
Therefore, the
free electrons always try to move from n-region to p-region similarly holes always try to move from p-region to
n-region
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When the free electrons reach p-n junction, they experience an attractive force from
the holes in the p-region
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In the similar way, holes also cross
the p-n junction
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During this process, some neutral atoms near the junction at n-side loses electrons and become positively charged
atoms (positive ions) similarly some neutral atoms near the junction at p-side gains extra electrons and become
negatively charged atoms (negative ions)
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This depletion region prevents further current flow across the p-n junction
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

The width of depletion region depends on the number of impurities added (amount of doping)
...
We
know that an insulator or a dielectric does not allow electric current through it
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So the depletion region acts like a dielectric of a capacitor
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The p-type and n-type semiconductor
also easily allow electric current through them
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Thus, varactor diode behaves like a normal capacitor
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So the capacitance (charge storage) is very large
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Because in reverse bias, the electric current does not flow
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As a result, the depletion region
becomes negligible
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So stored charges becomes negligible which
is undesirable
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So varactor diode
should always be operated in reverse bias
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As a result, the width
of depletion region increases and the capacitance decreases
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The capacitance is
inversely proportional to the width of the depletion region and directly proportional to the surface area of the pregion and n-region
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If the reverse
bias voltage is increased, the width of depletion region further increases and the capacitance further decreases
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Thus, an increase in reverse bias voltage increases the width of the depletion region and
decreases the capacitance of a varactor diode
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So the reverse bias voltage should be kept at a minimum to achieve large storage charge
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In a fixed capacitor, the capacitance will not be varied
whereas, in variable capacitor, the capacitance is varied
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So the varactor diode is a variable capacitor
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Prepared by: Er
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4 Other Special Diodes
# Small Signal Diode
It is a small device with disproportional characteristics and whose applications are mainly involved at high
frequency and very low currents devices such as radios and televisions etc
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The appearance of signal diode is very small when compared with the power diode
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For the applications at high frequencies the
performance of the small signal diode is very effective
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The signal diode is a silicon doped semiconductor diode or a germanium doped diode but depending up
on the doping material the characteristics of the diode varies
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The silicon signal diode has high voltage
drop at the coupling about 0
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7 volts so, it has very high resistance but low forward resistance
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2 to 0
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Due to small signal the functional point is not disrupted in small signal diode
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Thus the transformation of AC to DC voltages is unbounded
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These large signals will disrupt the functional
point also
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The main applications of these diodes are
in battery charging devices like inverters
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Since it has high current and voltage performance these can be used
in electrical devices which are used to suppress high peak voltages
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First production started in 1968
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Earlier they used in inductor lamps but now in recent applications they are using in
environmental and task handling
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# Constant Current Diodes
It is also known as current-regulating diode or constant current diode or current-limiting
diode or diode-connected transistor
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It functions as a two terminal current limiter
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The constant current diode symbol is
shown in alongside
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It is also
called as PNPN diode
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As there is no trigger inputs the only way the diode can conduct is by providing forward voltage
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The diode has two operating states conducting and nonconducting
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Shockley diodes are used in trigger
switches for SCR and in relaxation oscillator
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

# Step Recovery Diodes
It is also called as snap-off diode or charge-storage diode
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The
rise time of the current pulse is equal to the snap time
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The applications of these diodes are in higher order multipliers and in pulse shaper circuits
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As multiplier this diode has the cut-off frequency
range of 200 to 300 GHz
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The efficiency is high for lower order multipliers
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Electrically laser diode is p-i-n diode in which the active region is in
intrinsic region
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Laser Diode Types
1
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2
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3
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4
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5
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# Transient Voltage Suppression Diode
In semiconductor devices due to the sudden change in the state voltage transients
will occur
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To overcome this problem
voltage suppression diode diodes are used
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The operation of these diodes is normal as
p-n junction diodes but at the time of transient voltage its operation changes
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When any transient voltage occurs in the circuit the diode
enters in to the avalanche breakdown region in which the low impedance is provided
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Transient voltage suppression diode will clamp
the voltage to the fixed levels, mostly its clamping voltage is in minimum range
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It responds to over voltages faster
than varistors or gas discharge tubes
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These diodes are faster than other diodes
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Even at the higher voltage drop it allows the diode to operate in signal
frequencies
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# Super Barrier Diodes
It is a rectifier diode having low forward voltage drop as Schottky diode with surge handling capability and low
reverse leakage current as p-n junction diode
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Super barrier rectifiers are the next generation rectifiers with low forward voltage than Schottky
diode
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS

# Crystal Diode
This is also known as Cat’s whisker which is a type of point contact diode
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In this a metal wire is present which is pressed against the
semiconductor crystal
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These diodes
are obsolete in nature
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They are used in rectifier, detector
circuits and in radio receiver
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It works in reverse
bias condition
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These diodes are specially designed to undergo breakdown at specific
reverse voltage to prevent the damage
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Used in radio equipment and also in hardware random number generators
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Single Photon Avalanche Detector: These are high gain photon detectors used in light level applications
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Cathode is made up of tungsten which emits the electrons in the
direction of anode
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So, it acts like a switch
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Anode is a bit long in size and in some
cases their surface is rough to reduce the temperatures developing in the diode
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# PIN Diode
The improved version of the normal P-N junction diode gives the PIN diode
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The intrinsic material means the material which has no
charge carriers is inserted between the P and N regions which increase the area of
depletion layer
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At some point
due to this high injection level the electric field will conduct through the intrinsic material also
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PIN diodes are used in:
RF Switches: for both signal and component selection e
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in low phase noise oscillators
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Photo Detectors: to detects x-ray and gamma ray photons
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The depletion
region of two N-type materials is very thin
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After certain level of voltage the current will
exponentially decrease thus this exhibits the negative differential resistance
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It is also termed as transferred electron device
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It can also use as an amplifier
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Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
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