1 SEMICONDUCTORS Tunnel an Varactor Diodes. 2 SEMICONDUCTORS PN diodes and zener diodes have lightly...
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1 SEMICONDUCTORS Tunnel an Varactor Diodes. 2 SEMICONDUCTORS PN diodes and zener diodes have lightly doped PN junctions and similar V-I characteristics
2 SEMICONDUCTORS PN diodes and zener diodes have lightly doped
PN junctions and similar V-I characteristics. Tunnel diodes are
heavily doped and very different from PN and zener diodes PN diodes
and zener diodes have lightly doped PN junctions and similar V-I
characteristics. Tunnel diodes are heavily doped and very different
from PN and zener diodes
Slide 3
3 SEMICONDUCTORS Tunnel diodes have a high internal barrier
voltage and a narrow depletion region. It also has an extremely low
reverse breakdown voltage (almost zero) which means it conducts
large currents when its reverse biased. At low forward bias
voltages, electrons are forced through the depletion region at
extremely high velocities. Tunnel diodes have a high internal
barrier voltage and a narrow depletion region. It also has an
extremely low reverse breakdown voltage (almost zero) which means
it conducts large currents when its reverse biased. At low forward
bias voltages, electrons are forced through the depletion region at
extremely high velocities.
Slide 4
4 SEMICONDUCTORS Under normal forward bias operation, as
voltage begins to increase, electrons at first tunnel through the
very narrow PN junction barrier because filled electron states in
the conduction band on the N-side become aligned with empty valence
band hole states on the P- side of the P-N junction. As voltage
increases further these states become more misaligned and the
current drops this is called negative resistance because current
decreases with increasing voltage. Under normal forward bias
operation, as voltage begins to increase, electrons at first tunnel
through the very narrow PN junction barrier because filled electron
states in the conduction band on the N-side become aligned with
empty valence band hole states on the P- side of the P-N junction.
As voltage increases further these states become more misaligned
and the current drops this is called negative resistance because
current decreases with increasing voltage.
Slide 5
5 SEMICONDUCTORS As voltage increases yet further, the diode
begins to operate as a normal diode, where electrons travel by
conduction across the PN junction, and no longer by tunneling
through the PN junction barrier. Thus the most important operating
region for a tunnel diode is the negative resistance region. As
voltage increases yet further, the diode begins to operate as a
normal diode, where electrons travel by conduction across the PN
junction, and no longer by tunneling through the PN junction
barrier. Thus the most important operating region for a tunnel
diode is the negative resistance region.
Slide 6
6 SEMICONDUCTORS Here is the V-I characteristics of a tunnel
diode
Slide 7
7 SEMICONDUCTORS Tunnel diodes operate as oscillators capable
of operating in the microwave frequencie range and are often used
as UHF oscillators in TV tuners Tunnel diode applications also
include trigger circuits in oscilloscopes, high speed counter
circuits, and very fast-rise time pulse generator
circuits.oscilloscopes Tunnel diodes operate as oscillators capable
of operating in the microwave frequencie range and are often used
as UHF oscillators in TV tuners Tunnel diode applications also
include trigger circuits in oscilloscopes, high speed counter
circuits, and very fast-rise time pulse generator
circuits.oscilloscopes
Slide 8
8 SEMICONDUCTORS Varactor diodes are sometimes referred to as
variable capacitor diodes (varicap) because they have a usable
amount of capacitance created at the PN junction.
Slide 9
9 SEMICONDUCTORS Varactors are operated with a reverse-bias
voltage that is less than its reverse breakdown voltage rating. As
the reverse voltage increases, the depletion region widens and acts
as a wider dielectric between the N and P sections A decrease in
reverse bias voltage will cause an increase in the diodes internal
junction capacitance. Varactors are operated with a reverse-bias
voltage that is less than its reverse breakdown voltage rating. As
the reverse voltage increases, the depletion region widens and acts
as a wider dielectric between the N and P sections A decrease in
reverse bias voltage will cause an increase in the diodes internal
junction capacitance.
Slide 10
10 SEMICONDUCTORS Here is an example of the operation of a
varicap. Here is an example of the operation of a varicap.
Slide 11
11 SEMICONDUCTORS Varicotor diodes are used to vary the
frequeny of a resonant circuit. They also find uses in high
frequency amplifiers and frequency multipliers. They are also used
as automatic frequency control (AFC) circuits found in FM radios.
Varicotor diodes are used to vary the frequeny of a resonant
circuit. They also find uses in high frequency amplifiers and
frequency multipliers. They are also used as automatic frequency
control (AFC) circuits found in FM radios.
Slide 12
12 SEMICONDUCTORS The Schottky diode also known as (hot carrier
diode) is a semiconductor diode with a low forward voltage drop and
a very fast switching action. The cat's-whisker detectors used in
the early days of wireless can be considered primitive Schottky
diodes. The Schottky diode also known as (hot carrier diode) is a
semiconductor diode with a low forward voltage drop and a very fast
switching action. The cat's-whisker detectors used in the early
days of wireless can be considered primitive Schottky diodes.
Slide 13
13 SEMICONDUCTORS When current flows through a diode there is a
small voltage drop across the diode terminals. A normal silicon
diode has a voltage drop between 0.6 1.7 volts, while a Schottky
diode voltage drop is between approximately 0.150.45 volts. This
lower voltage drop can provide higher switching speed and better
system efficiency. When current flows through a diode there is a
small voltage drop across the diode terminals. A normal silicon
diode has a voltage drop between 0.6 1.7 volts, while a Schottky
diode voltage drop is between approximately 0.150.45 volts. This
lower voltage drop can provide higher switching speed and better
system efficiency.
Slide 14
14 SEMICONDUCTORS The most important difference between the P -
N and Schottky diode is reverse recovery time, when the diode
switches from conducting to non-conducting state. Where in a P-N
diode the reverse recovery time can be in the order of hundreds of
nanoseconds and less than 100 ns for fast diodes, Schottky diodes
do not have a recovery time, as there is nothing to recover from
(i.e. no charge carrier depletion region at the junction). The most
important difference between the P - N and Schottky diode is
reverse recovery time, when the diode switches from conducting to
non-conducting state. Where in a P-N diode the reverse recovery
time can be in the order of hundreds of nanoseconds and less than
100 ns for fast diodes, Schottky diodes do not have a recovery
time, as there is nothing to recover from (i.e. no charge carrier
depletion region at the junction).
Slide 15
15 SEMICONDUCTORS Commonly encountered Schottky diodes include
the 1N5817 series (1 ampere) rectifiers. Schottky
metalsemiconductor junctions are featured in the successors to the
7400 TTL family of logic devices, the 74S, 74LS and 74ALS series,
where they are employed as clamps in parallel with the collector-
base junctions of the bipolar transistors to prevent their
saturation, thereby greatly reducing their turn-off delays.
Commonly encountered Schottky diodes include the 1N5817 series (1
ampere) rectifiers. Schottky metalsemiconductor junctions are
featured in the successors to the 7400 TTL family of logic devices,
the 74S, 74LS and 74ALS series, where they are employed as clamps
in parallel with the collector- base junctions of the bipolar
transistors to prevent their saturation, thereby greatly reducing
their turn-off delays.
Slide 16
16 SEMICONDUCTORS A Gunn diode, also known as a transferred
electron device (TED), is a form of diode used in high- frequency
electronics.
Slide 17
17 SEMICONDUCTORS Its internal construction is unlike other
diodes in that it consists only of N-doped semiconductor material,
whereas most diodes consist of both P and N-doped regions. In the
Gunn diode, three regions exist: two of them are heavily N-doped on
each terminal, with a thin layer of lightly doped material in
between. Its internal construction is unlike other diodes in that
it consists only of N-doped semiconductor material, whereas most
diodes consist of both P and N-doped regions. In the Gunn diode,
three regions exist: two of them are heavily N-doped on each
terminal, with a thin layer of lightly doped material in
between.
Slide 18
18 SEMICONDUCTORS When a voltage is applied to the device, the
electrical gradient will be largest across the thin middle layer.
Conduction will take place as in any conductive material with
current being proportional to the applied voltage. Eventually, at
higher field values, the conductive properties of the middle layer
will be altered, increasing its resistivity, preventing further
conduction and current starts to fall. When a voltage is applied to
the device, the electrical gradient will be largest across the thin
middle layer. Conduction will take place as in any conductive
material with current being proportional to the applied voltage.
Eventually, at higher field values, the conductive properties of
the middle layer will be altered, increasing its resistivity,
preventing further conduction and current starts to fall.
Slide 19
19 SEMICONDUCTORS Because of their high frequency capability,
Gunn diodes are mainly used at microwave frequencies and above.
Their most common use is in oscillators, but they are also used in
microwave amplifiers to amplify signals. Because of their high
frequency capability, Gunn diodes are mainly used at microwave
frequencies and above. Their most common use is in oscillators, but
they are also used in microwave amplifiers to amplify signals.
Slide 20
20 SEMICONDUCTORS Gunn diode oscillators are used to generate
microwave power for: Airborne collision avoidance radar, anti-lock
brakes, car radar detectors, pedestrian safety systems, motion
detectors, traffic signal controllers, automatic door openers,
burglar alarms Gunn diode oscillators are used to generate
microwave power for: Airborne collision avoidance radar, anti-lock
brakes, car radar detectors, pedestrian safety systems, motion
detectors, traffic signal controllers, automatic door openers,
burglar alarms
Slide 21
21 SEMICONDUCTORS An IMPATT diode (IMPact ionization Avalanche
Transit- Time) is a form of high-power diode used in high-frequency
electronics and microwave devices. They are typically made with
silicon carbide owing to their high breakdown fields. An IMPATT
diode (IMPact ionization Avalanche Transit- Time) is a form of
high-power diode used in high-frequency electronics and microwave
devices. They are typically made with silicon carbide owing to
their high breakdown fields.
Slide 22
22 SEMICONDUCTORS They operate at frequencies between about 3
and 100 GHz or more. A main advantage is their high-power
capability. These diodes are used in a variety of applications from
low- power radar systems to alarms. A major drawback of using
IMPATT diodes is the high level of phase noise they generate. They
operate at frequencies between about 3 and 100 GHz or more. A main
advantage is their high-power capability. These diodes are used in
a variety of applications from low- power radar systems to alarms.
A major drawback of using IMPATT diodes is the high level of phase
noise they generate.
Slide 23
23 SEMICONDUCTORS This results from the statistical nature of
the avalanche process. Nevertheless these diodes make excellent
microwave generators for many applications. This results from the
statistical nature of the avalanche process. Nevertheless these
diodes make excellent microwave generators for many
applications.
Slide 24
24 SEMICONDUCTORS Can you identify the different diode types
below?
Slide 25
25 SEMICONDUCTORS The most commonly used diode in
electronics