Electric Circuits & Electron Devices

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Electric Circuits Electron Devices

• Unit V
• Special Semiconductor Devices

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FIGURE - Zener diode symbol.
1. ZENER DIODE
Zener Diode- is a silicon pn junction device
that differ from rectifier diodes
because it is designed for operation
in the reverse- breakdown region.
- if Zener diode is forward-biased, it
operates the same as a rectifier
diode. Function- to provide a stable
reference voltage for use in power
supplies, voltmeter other
instruments, voltage regulators.
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FIGURE - General diode V-I characteristic.
Zener breakdown- occurs in a Zener diode at low
reverse voltages. - Zener diode is heavily
doped to reduce the breakdown voltage. - This
causes a very thin depletion region.
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FIGURE - Tunnel diode symbols.
2. 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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Forward bias operation 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 pn 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. 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.
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FIGURE - Tunnel diode characteristic curve.
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FIGURE - Parallel resonant circuit.
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FIGURE - Basic tunnel diode oscillator.
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3. VARACTOR DIODE
The reverse-biased varactor diode acts as a
variable capacitor.
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FIGURE - The reverse-biased varactor diode
acts as a variable capacitor.
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FIGURE - Varactor diode capacitance varies
with reverse voltage.
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FIGURE 6 - A Resonant band-pass filter using a
varactor diode for adjusting the
resonant frequency over a specified range.
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FIGURE - Symbol for an LED. When forward-biased,
it emits light.
4. LED
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FIGURE - Electroluminescence in a
forward-biased LED.
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FIGURE - Basic operation of an LED.
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FIGURE - Examples of typical spectral output
curves for LEDs.
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FIGURE - Typical LEDs.
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FIGURE - The 7-segment LED display.
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5. LASER DIODE

• A Laser diode, also known as an injection laser
  or diode laser, is a semiconductor device that
  produces coherent radiation (in which the waves
  are all at the same frequency and phase) in the
  visible or infrared (IR) spectrum when current
  passes through it.
• Laser diodes are used in
• optical fiber systems,
• compact disc (CD) players,
• laser printers,
• remote-control devices,
• and intrusion detection systems.

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Figure Structure of DH LASER Diode
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FIGURE - Basic laser diode construction and
operation.
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FIGURE - Photodiode.
6. PHOTODIODE
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FIGURE - Typical photodiode characteristics.
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FIGURE - Operation of a photodiode.
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FIGURE - PIN diode.
7. PIN DIODE
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• A PiN diode is a diode with a wide, lightly doped
  'near' intrinsic semiconductor region between a
  p-type semiconductor and an n-type semiconductor
  regions.
• The p-type and n-type regions are typically
  heavily doped because they are used for ohmic
  contacts.
• The wide intrinsic region is in contrast to an
  ordinary PN diode. The wide intrinsic region
  makes the PIN diode an inferior rectifier (the
  normal function of a diode),
• but it makes the PIN diode suitable for
• attenuators,
• fast switches,
• photo detectors, and
• high voltage power electronics applications.

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FIGURE - PIN diode characteristics.
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FIGURE - Diode symbols.
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8. SILICON CONTROLLED RECTIFIER
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Two Transistor model of SCR
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The switching action of gate takes place only
when (i)     SCR is forward biased i.e. anode is
positive with respect to cathode. (ii)    Suitabl
e positive voltage is applied between the gate
and the cathode. Once the SCR has been switched
on, it has no control on the amount of current
flowing through it. The current through the SCR
is entirely controlled by the external impedance
connected in the circuit and the applied voltage.
The forward current through the SCR can be
reduced by reducing the applied voltage or by
increasing the circuit impedance.
A minimum forward current must be maintained to
keep the SCR in conducting state. This is called
the holding current rating of SCR. If the current
through the SCR is reduced below the level of
holding current, the device returns to off-state
or blocking state.
Note The gate can only trigger or switch-on the
SCR, it cannot switch off.
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Terminology
Firing Angle The angle (in the input AC) at
which the gate is triggered is known as 'firing
angle'.
Holding Current It is the minimum anode current
(with gate being open) required to keep the SCR
in ON condition.
Break Over voltage It is the minimum forward
voltage with gate being open, at which an SCR
starts conducting heavily (i.e., the SCR is
turned ON) .
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9. UNIPOLAR JUNCTION TRANSISTOR
A unijunction transistor (UJT) is an electronic
semiconductor device that has only one junction.
The UJT has three terminals an emitter (E) and
two bases (B1 and B2). The base is formed by
lightly doped n-type bar of silicon. Two ohmic
contacts B1 and B2 are attached at its ends.
The emitter is of p-type and it is heavily
doped.
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Intrinsic Standoff Ratio
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Unijunction transistor (a) emitter
characteristic curve, (b) model for VP .
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Application of UJT RELAXATION OSCILLATOR
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• REVIEW
• A unijunction transistor consists of two bases
  (B1, B2) attached to a resistive bar of silicon,
  and an emitter in the center.
• The E-B1 junction has negative resistance
  properties it can switch between high and low
  resistance.
• The intrinsic standoff ratio is ? RB1 /(RB1
  RB2), for a unijunction transistor. The trigger
  voltage is determined by ?.
• Unijunction transistors and programmable
  unijunction transistors are applied to
  oscillators, timing circuits, and Thyristor
  triggering.