PN Junction Diodes

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PN Junction Diodes

• CEC

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Contents

• PN Junction.
• Depletion Region.
• Forward Bias.
• Reverse Bias.
• Characteristic Curves.
• Zener Diodes.
• Breakdown Mechanisms.
• Zener Diode Characteristics.
• Diode Applications.

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PN Junction

• P Type Material Group IV semiconductor material
  (Si, Ge) doped with group III elements (B, In,
  Ga, etc.) trivalent impurity.
• N Type Material - Group IV semiconductor material
  (Si, Ge) doped with group V elements (P, As, Sb,
  Bi etc.) pentavalent impurity.
• P Type Material and N Type Material joined
  together at one end.

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PN Junction

• Doped regions meet together to form a PN
  Junction.
• Permit unidirectional current flow.
• Useful in the construction of diodes.

Anode
Cathode
Current flow in one direction
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Depletion Region

• Free electrons on the n side migrate/ diffuse
  across the junction to the p side.
• On the p side, free electrons are the minority
  current carriers.
• Free electrons combine with holes shortly after
  crossing over to the p side.
• A free electron leaves the n side and falls into
  a hole on the p side, creates two ions - a
  positive ion on the n side and a negative ion on
  the p side.

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Depletion Region

• Ions are immobile, electric field created.
• As the process of diffusion continues, a barrier
  potential is created, diffusion of electrons from
  the n side to the p side stops.
• Electrons diffusing from the n side sense a large
  negative potential on the p side that repels them
  back to the n side.

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Depletion Region

• Holes from the p side repelled back to the p side
  by the positive potential on the n side.
• Area where the positive and negative ions are
  located called the depletion region.
• Word depletion used because the area has been
  depleted of all charge carriers.
• Barrier potential approximately 0.7 V for Si and
  0.3 V for Ge.

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Barrier Potential

• Barrier potential stops diffusion of current
  carriers.
• Depletion region also called space charge region.
• Cannot be measured with a voltmeter.

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Depletion Region

Carriers diffuse across the junction due to
concentration gradient.
Barrier Potential VB stops carriers cross the
junction
Immobile Ions
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Biasing a PN Junction

• Application of voltage/current.
• Forward Bias and Reverse Bias.
• Forward-biasing allows current to flow easily.
• Forward Biasing reduces the width of the
  potential barrier.
• Reverse biasing impedes current flow, only
  leakage current flows.
• Reverse Biasing increases the width of the
  potential barrier.

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Forward Biasing
Depletion Region Narrows

V gt VB
Current Limiting Resistor
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Forward Bias

• n material connected to the negative terminal of
  the voltage source, V.
• p material is connected to the positive terminal
  of the voltage source, V.
• Anode positive w.r.t cathode.
• Voltage source V repels free electrons in the n
  side across the depletion zone and into the p
  side.

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Forward Bias

• On the p side, the free electron combines with a
  hole.
• Electron will then travel from hole to hole as it
  is attracted to the positive terminal of the
  voltage source.
• For every free electron entering the n side, one
  electron leaves the p side.

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Reverse Biasing
Depletion Region Widens

Negligible current flows through the device
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Reverse Bias

• Negative terminal of the voltage source connected
  to the p -type semiconductor material.
• Positive terminal of the voltage source
  connected to the n type semiconductor material.
• Charge carriers in both sections pulled away from
  the junction.

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Reverse Bias

• Free electrons on the n side pulled away from the
  junction due to attraction of the positive
  terminal of the voltage source.
• Holes in the p side pulled away from the junction
  because of the attraction by the negative
  terminal of the voltage source.
• Width of the depletion zone increases.
• Diode non-conducting, like an open switch,
  ideally with infinite resistance.

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Leakage Current

• Reverse-biased diode conducts a small amount of
  current, called leakage current.
• Leakage current mainly due to minority current
  carriers in both sides of the junction.
• Minority current carriers are holes in the n side
  and free electrons in the p side.
• Minority current carriers due to thermal energy
  producing a few electron-hole pairs.

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Leakage Current

• Increase in the temperature of the diode
  increases the leakage current in the diode.
• Minority current carriers move in opposite
  direction to the direction provided with forward
  bias.
• Also called reverse saturation current.

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V/I Characteristics
Diode Current rises sharply above cut in voltage.
Non-Linear
Avalanche Breakdown
Cut in Voltage 0.7 V for Si, 0.3 V for Ge
Very small current flows until VBR
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V/I Characteristics

• Forward current rises sharply above cut in
  voltage.
• Current that flows prior to breakdown is mainly
  due to thermally produced minority current
  carriers.
• Leakage current increases mainly with
  temperature, relatively independent of changes in
  reverse-bias voltage.

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V/I Characteristics

• Slight increase in reverse current with increases
  in the reverse voltage due to surface leakage
  current.
• Surface leakage current exists since there are
  many holes on the edges of a silicon crystal due
  to unfilled covalent bonds.
• Holes on the crystal edges provide a path for a
  few electrons along the surfaces of the crystal.

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Diode Current Equation

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Silicon Diode vs Germanium Diode

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Avalanche Action

• Avalanche occurs when the reverse-bias becomes
  excessive.
• Thermally produced free electrons on the p side
  accelerated by the voltage source to very high
  speeds as they move through the diode.
• Electrons collide with valence electrons in other
  orbits, sets them free.

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Avalanche Action

• Free valence electrons accelerated to very high
  speeds, dislodges more valence electrons.
• Process is cumulative called avalanche effect.
• When breakdown voltage, VBR , reached, reverse
  current, IR , increases sharply.
• Diodes not to be operated in breakdown region.
• For rectifier diodes VBR gt 50 V.

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Diode Parameters

• DC Resistance of a forward biased diode (VF -
  forward voltage drop and IF - the forward
  current).
• Bulk resistance of a forward biased diode
• (?V - change in diode voltage produced by the
  change in diode current, ?I).

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Diode Approximations

• First Approximation
• - Ideal Diode Approximation.
• - Forward-biased diode as a closed switch
• with a voltage drop of zero volts.
• - Reverse-biased diode as an open
• switch with zero current.

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First Approximation

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Diode Approximations

• Second Approximation
• - forward-biased diode as an ideal diode
• in series with a battery.
• - accounts for cut in voltage.
• - reverse-biased diode as an open
• switch.

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Second Approximation

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Third Approximation

• Includes the bulk resistance, the resistance of
  the p and n materials.
• Bulk resistance dependent on the doping level and
  the size of the p and n materials.
• Bulk resistance causes the forward voltage across
  a diode to increase slightly with increases in
  the diode current.
• Resistance across the open switch is a high
  leakage resistance for the reverse-bias condition.

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Third Approximation

Slope due to rB
Piecewise Linear Model
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Diode Ratings

• Breakdown Voltage voltage at which avalanche
  occurs.
• Average Forward Current - maximum allowable
  average current that the diode can handle safely.
• Maximum Forward Surge Current - maximum
  instantaneous current the diode can handle safely
  from a single pulse (eg capacitor current).

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Diode Ratings

• Maximum Reverse Current -
• Chance of diode failure if ratings exceeded.
• Current limiting resistor in series to limit
  diode current to safe values.

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Diode Applications

• Rectifiers.
• Clippers.
• Clampers.
• Voltage Multipliers.
• For Unidirectional Current Flow.
• Surge Suppression.

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Zener Diode

• A special diode optimized for operation in the
  breakdown region.
• Connected in parallel with the load of the power
  supply.
• Zener voltage remains nearly constant despite
  load current variations.
• Under forward bias, zener diode acts like an
  ordinary silicon diode.

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Zener Diode

• Under reverse-bias region, a small reverse
  leakage current flows until breakdown voltage is
  reached.
• After breakdown voltage, reverse current through
  the zener increases sharply, reverse current
  called zener current.
• Breakdown voltage remains nearly constant as the
  zener current increases.
• Zener diodes used as voltage regulators.

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Zener Power Rating

• Power dissipated by the zener diode
• VZ - Zener Voltage, IZ - Zener Current.
• Both zener and avalanche breakdown occur in zener
  diodes.

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Zener Breakdown

• Reverse Voltage 6 V applied across zener diode,
  narrow depletion region.
• Intense electric field of the order of 3 x 105
  V/cm across the narrow depletion region.
• Electric field strong enough, to pull electrons
  from the valence band to the conduction band
  (free electrons) Field Ionisation.
• Large number of free electrons constitute a large
  reverse current zener effect.
• Occurs in heavily doped diodes.

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Avalanche Breakdown

• In zener diodes with breakdown voltage gt 6V,
  wider depletion region.
• Minority carriers accelerate as reverse bias
  increases, their kinetic energy increases.
• Accelerated carriers collide with stationary
  atoms, impart energy to valence electrons.
• Valence electrons jump into conduction band
  free electrons and get accelerated.

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Avalanche Breakdown

• Free electrons collide with and knock out more
  valence electrons avalanche multiplication.
• Large reverse current flows due to avalanche
  effect impact ionisation.
• Occurs in lightly doped diodes.
• V/I characteristics not sharp in breakdown
  region.

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Zener Breakdown vs Avalanche Breakdown
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Zener Diode Characteristics
Sharp if zener breakdown, more slope if avalanche
breakdown.
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Zener Diode Applications

• Voltage Regulators.
• Clippers.
• Biased Clampers.
• Voltage Limiting.
• Voltage Overshoot Protection.

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Zener Diode Voltage Regulators
IS
IZ
Unloaded
Loaded
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Zener Diode vs PN Junction Diode

Silicon/Germanium Diode
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