Lecture%202:%20Semiconductor%20Diodes

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Lecture 2 Semiconductor Diodes

• The pn Junction Diode
• Rectifiers
• Power Supplies
• Diode Clippers
• Diode Clampers
• Diode Thermometer

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Diodes

• Diodes are semiconductor devices which might be
  described as passing current in one direction
  only. Diodes however are far more versatile
  devices than that.
• Diodes may be used as voltage regulators, tuning
  devices in RF tuned circuits, frequency
  multiplying devices in RF circuits, mixing
  devices in RF circuits, switching applications or
  can be used to make logic decisions in digital
  circuits.
• There are also diodes which emit light, of
  course these are known as light-emitting-diodes
  or LEDs.

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Figure 9.5
A pn Junction
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Figure 9.7
Forward- and Reverse-biased pn Junctions
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Figure 9.8, 9.9
Semiconductor Diode i-v Characteristic
Semiconductor Diode Circuit Symbol
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Common Diode

• The first diode in the previous Figure is a
  semiconductor diode which could be a small signal
  diode. Notice the straight bar end has the letter
  k, this denotes the cathode while the a
  denotes anode.
• Current only flows from anode to cathode and not
  in the reverse direction, hence the arrow
  appearance. This is one very important property
  of diodes.
• The anode terminal must be positive and the
  cathode terminal negative for a current to flow.
• In an ideal diode, there is no voltage drop
  across a diode when current is flowing.

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Figure 9.10
The i-v Characteristic of the Semiconductor Diode
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Figure 9.11
Large-Signal on/off Diode Model
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Physical Structure of the Junction Diode
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Forward Biasing the Junction

• Electrons in the p-type material, near the
  positive terminal of the supply, break their
  electron pair bonds and enter the supply,
  thereby producing new holes. Also electrons from
  the negative terminal of the supply enter the
  n-type material and migrate towards the junction.
• Free electrons from the n-type then flow across
  the junction and move into the holes which have
  migrated from the positive terminal. This current
  flow will continue as long as the external supply
  is connected and is called forward current flow.

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Reverse Biasing the Junction

• When the polarity of the supply is reversed, the
  potential within the device is effectively
  reinforced and the depletion layer becomes wider.
  This is because the free electrons in the n-type
  are attracted towards the positive terminal, away
  from the junction, while the electrons from the
  negative terminal of the supply enter the p-type
  and migrate towards the junction. Current flow is
  extremely small and is called reverse current.
  Note that this current is produced by minority
  carriers and the device is said to be reverse
  biased.

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Figure 9.12, 9.13, 9.14
Circuit containing ideal diode
Assuming that the ideal diode conducts
Assuming that the ideal diode does not conduct
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Zener Diode

• The second of the diodes is a zener diode which
  are fairly popular for the voltage regulation of
  low current power supplies.
• While it is possible to obtain high current zener
  diodes, most regulation today is done
  electronically with the use of dedicated
  integrated circuits and pass transistors.

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Varactor Diode

• This is actually two varactor diodes mounted back
  to back with the DC control voltage applied at
  the common junction of the cathodes. These
  cathodes have the double bar appearance of
  capacitors to indicate a varactor diode.
• When a DC control voltage is applied to the
  common junction of the cathodes, the capacitance
  exhibited by the diodes (all diodes and
  transistors exhibit some degree of capacitance)
  varies in accordance with the applied voltage.

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Vacuum Tube

• The next diode is the simplest form of vacuum
  tube or valve. It simply has the old cathode and
  anode.
• These terms were passed on to modern solid state
  devices. Vacuum tube diodes are mainly only of
  interest to restorers and tube enthusiasts.

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LED

• The last diode depicted is of course a light
  emitting diode or LED.
• A LED actually does not emit as much light as it
  first appears, a single LED has a plastic lens
  installed over it and this concentrates the
  amount of light.
• Seven LEDs can be arranged in a bar fashion
  called a seven segment LED display and when
  decoded properly can display the numbers 0 - 9 as
  well as the letters A to F.

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Diode CircuitsFrom Neamen, Electronic Circuit
Analysis and Design, McGraw Hill
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Figure 9.20, 9.21
Ideal Diode Rectifier Input and Output Voltages
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Half-Wave Rectifier (Sedra/Smith Microelectronic
Circuits, Oxford)
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Figure 9.39
Full-Wave Rectifier
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Full-Wave Rectifier (Sedra/Smith Microelectronic
Circuits, Oxford)
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Figure 9.42
Operation of Bridge Rectifier
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The Bridge Rectifier (Sedra/Smith
Microelectronic Circuits, Oxford)
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Figure 9.45
DC Power Supply
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Figure 9.49
(a) A Zener diode voltage regulator (b)
simplified circuit for Zener regulator
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Figure 9.54, 9.55
Two-sided diode clipper
Circuit model for the diode clipper
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Figure 9.58, 9.60
Two-sided (ideal diode) clipper input and output
voltages
Voltages for the diode clipper (piecewise linear
diode model)
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The Filter(Sedra/Smith Microelectronic Circuits,
Oxford)