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UNIT4
SPECIAL SEMICONDUCTOR DEVICES
SCHOTTKY BARRIER DIODE SYMBOL
• The N-type semiconductor acts as a cathode
and the metal side acts as the anode of the
diode.
CROSS SECTIONAL VIEW
Schottky diode
VI CHARACTERISTICS OF SCHOTTKY
BARRIER DIODE AND PN JUNCTION
DIODE
CONSTRUCTION
 It is a unilateral junction.
 A metal semiconductor junction is formed at one end and another metal
semiconductor contact is formed at the other end.
 It is an ideal Ohmic bidirectional contact with no potential existing
between the metal and the semiconductor.
 Schottky diode is a function of temperature dropping.
 It decreases and increasing temperature doping concentration in N type
semiconductor.
 The n+ diffusion region serves the purpose of generating ohmic contacts
Operation
 A Schottky barrier diode is also known as Schottky or hot carrier
diode.
 A Schottky barrier diode is a metal semiconductor.
 A junction is formed by bringing metal contact with a moderately
doped N type semiconductor material.
 The Schottky barrier diode is a unidirectional device conducting
current flows only in one direction.
 The contact potential between the semiconductor and metal
generates a barrier for the flow of conducting electrons from
semiconductor to metal
Continued…
• When the junction is forward biased, this barrier
is lowered and the electron flow is allowed from
the semiconductor to metal, where the electrons
are in large quantities.
• The majority carriers carry the conduction
current in schottky diode whereas in the pn
junction diode, minority carriers carry the
conduction current and it incurs an appreciable
time delay from on state to off state.
advantage
• It exhibits negligible storage time to flow the
electron from N-type silicon into aluminium
almost right at contact surface, where they
mix the free electron.
• Less forward voltage(0.4v)
Applications
• Clipping ,Clamping circuits and detection in
high frequency
• microwave integrated circuits
• Radars
• Computer gating
Varactor diode
• Varactor diode is a variable capacitor diode. It is
a reverse biased PN junction diode, which uses
the capacitance of the depletion layer.
• The depletion layer created by the reverse biased
PN junction act as a dielectric of the capacitor.
• The P type and N type regions act as the
capacitor plates.
• When the reverse bias voltage increases, the
width of the depletion region also increases.
• That is, depending on the reverse bias voltage,
the width of the depletion region varies.
operation
• We know that the capacitance of a capacitor
in terms of the distance between the plate is
given by Where A = Area of the plate d =
distance between the plates.
• Thus, if the width of the depletion region (d)
increases, the capacitance of the junction
decreases.
• The reverse biased PN junction which act as a
varactor diode is shown in figure
operation
• If the reverse bias voltage is decreased, the
depletion region width (d) also decreases and
in turn its capacitance value increases. The
variation in capacitance value
• From the figure it is clear that the value of
capacitance is maximum when the reverse
bias is equal to zero
Doping method
• The method of doping near the PN junction
makes this diodes to act as a varactor diode. In
normal PN diodes, the doping is uniform on both
side of the junction.
• But in varactor diode non-uniform doping is used.
Higher doping level is used near junction and the
doping level decreases away from the junction as
shown in the figure
• This figure shows the doping profile of a varactor
diode
Symbol and equivalent circuit
• In the equivalent circuit, R indicates the
resistance of the diode in reverse biased
condition.
• In general, the varactor diodes are used for
tuning purpose. The tuning range or
capacitance variation range is decided by the
doping profile.
Diagram
• Tuning ratio or capacitance ratio of the
varactor diode is the ratio of the diode
capacitance at a minimum reverse voltage to
the diode capacitance at a maximum reverse
voltage.
• Its value is always greater than one.
• Tuning Ratio =
• Capacitance at minimum reverse voltage
Capacitance at maximum reverse voltage
parameters
• Figure of merit (Q)
• Figure of merit is also called as the Quality factor (Q). It is defined as the ratio of energy
stored and returned by the capacitor to the energy dissipated in the resistance. High value of
Q is always desirable, because high Q indicates low loss (energy dissipation) in resistance.
• Quality factor (Q) =
• Energy stored and returned by the capacitor
• Energy dissipated in resistance
• Temperature coefficient
• Varactor capacitance has a positive temperature coefficient. That is, the value of
• capacitance increases with the increase in temperature.
• Applications
• 1. It is used as a electronic tuning device.
• 2. It is used in parametric amplifiers.
• 3. It is used in FM modulation circuits.
• 4. It is used in Automatic Frequency Control (AFC) circuit and adjustable bandpass
• filter circuits.
Tunnel Diode
• Tunnel diodes are PN diodes with high impurity concentration (high doping)
compared to normal diode. The conduction mechanism in which the charge
carriers bore through the barrier directly, instead of climbing over it is called as
tunneling. So, the highly doped PN junction diodes are called as Tunnel diodes.
• The characteristics of a PN junction diode are completely changed if the
concentration of the impurity atom are greatly increased. The depletion layer of
PN junction diode becomes extremely thin if it is heavily doped than the zener
diode.
• Effects of High Doping
• The effects of high doping are :
• i) The depletion layer becomes very thin.
• ii) It will widen the level of donor in N material and level of acceptor in P material.
• iii) Fermi level moves towards the conduction band in N-type material.
• iv) Fermi level moves towards the valance band in P-type material
SYMBOL
• It was invented by Dr. Esaki and also called as
Esaki diode.
OPERATION
• The forward current in tunnel diode is due to
the sum of the following two components
• i) The current due to external bias voltage,
which overcomes the depletion region
• (majority carrier flow).
• ii) The current due to tunneling effect, which
passes through the junction without over
coming the barrier potential.
• The mechanism of tunneling is purely a
quantum mechanical phenomenon
CONTINUED…
• Due to heavy doping, the depletion region is very
thin and narrow. The filled level on the P side are
exactly opposite to that of filled level on N side.
• Therefore, there is no empty lower energy level
available for the electron movement from either
side and the tunneling operation will not take
place.
• The conduction and valence band on the P side
are higher than the N side. This happens because
of the electrons crossing from N side to the P side
during junction formation
FORWARD BIAS
• The energy level of the N side is raised under
forward biased condition.
• Since the barrier region is very thin, the
electrons tunneling occurs from N side to P
side.
IF WE INCREASED SLIGHTLY
• If we increase the forward bias, the energy level
of the N side is further raised.
• Maximum empty level on the P side is formed
which is opposite to the filled level on the N side.
• If the forward bias is further increased, the filled
energy level on the N side is raised to the level of
forbidden energy gap on the P side.
• Now the empty level on the P side is reduced and
the tunnelling operation decreases
Increased voltage higher
• On continuous increase of forward bias, the
energy level on the N side is raised such that
the electrons on the N side find no empty
level on the P side f or tunnel ling operation to
take place.
Reverse bias
• If reverse bias is applied, the height of the barrier
is increased than the unbiased barrier height
some energy state in the valance band of P side
are opposite to the empty level on the
conduction band of N side.
• This result in tunneling of electron through the
barrier from P side to N side region.
• Even though the junction is reverse biased,
current flows through it due to tunneling.
• Under increased reverse bias condition, more
current flows through the diode.
Reverse bias
VI characteristics
• The following points are observed from the characteristic curve.
• i) At low values of forward bias, the tunnel effect controls the flow
of current. That is, the tunneling current is larger than the normal
injection current.
• ii) The tunnelling current reaches a maximum value of Ip at a
forward bias of Vp.
• iii) For bias voltage greater than Vp, suppression of tunnelling
operation takes place. So the tunnelling current reduces and
reaches the minimum value of 1v. This current is known as valley
current.
• iv) Between the peak current and valley current it is found that the
current decreases with the increase in voltage. This is due to
negative resistance effect.
• v) The voltage at which value of current is Iv is called as
valley voltage Vv. At this point the tunnel current
becomes zero.
• vi) Beyond the valley voltage, the current flow is only
due to the applied bias voltage and it is similar to the
normal PN junction diode current.
• vii) Under reverse bias condition, the tunnel diode acts
like a good conductor. The reverse current increases
with increase in reverse bias.
• viii) The current I between peak value (Ip) and valley
value (Iv) can be obtained using three different values
of bias voltage V1, V3 and V5.
VI characteristics curve
LDR

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UNIT4.pptx

  • 2. SCHOTTKY BARRIER DIODE SYMBOL • The N-type semiconductor acts as a cathode and the metal side acts as the anode of the diode.
  • 5. VI CHARACTERISTICS OF SCHOTTKY BARRIER DIODE AND PN JUNCTION DIODE
  • 6. CONSTRUCTION  It is a unilateral junction.  A metal semiconductor junction is formed at one end and another metal semiconductor contact is formed at the other end.  It is an ideal Ohmic bidirectional contact with no potential existing between the metal and the semiconductor.  Schottky diode is a function of temperature dropping.  It decreases and increasing temperature doping concentration in N type semiconductor.  The n+ diffusion region serves the purpose of generating ohmic contacts
  • 7. Operation  A Schottky barrier diode is also known as Schottky or hot carrier diode.  A Schottky barrier diode is a metal semiconductor.  A junction is formed by bringing metal contact with a moderately doped N type semiconductor material.  The Schottky barrier diode is a unidirectional device conducting current flows only in one direction.  The contact potential between the semiconductor and metal generates a barrier for the flow of conducting electrons from semiconductor to metal
  • 8. Continued… • When the junction is forward biased, this barrier is lowered and the electron flow is allowed from the semiconductor to metal, where the electrons are in large quantities. • The majority carriers carry the conduction current in schottky diode whereas in the pn junction diode, minority carriers carry the conduction current and it incurs an appreciable time delay from on state to off state.
  • 9. advantage • It exhibits negligible storage time to flow the electron from N-type silicon into aluminium almost right at contact surface, where they mix the free electron. • Less forward voltage(0.4v)
  • 10. Applications • Clipping ,Clamping circuits and detection in high frequency • microwave integrated circuits • Radars • Computer gating
  • 11. Varactor diode • Varactor diode is a variable capacitor diode. It is a reverse biased PN junction diode, which uses the capacitance of the depletion layer. • The depletion layer created by the reverse biased PN junction act as a dielectric of the capacitor. • The P type and N type regions act as the capacitor plates. • When the reverse bias voltage increases, the width of the depletion region also increases. • That is, depending on the reverse bias voltage, the width of the depletion region varies.
  • 12. operation • We know that the capacitance of a capacitor in terms of the distance between the plate is given by Where A = Area of the plate d = distance between the plates. • Thus, if the width of the depletion region (d) increases, the capacitance of the junction decreases. • The reverse biased PN junction which act as a varactor diode is shown in figure
  • 13. operation • If the reverse bias voltage is decreased, the depletion region width (d) also decreases and in turn its capacitance value increases. The variation in capacitance value • From the figure it is clear that the value of capacitance is maximum when the reverse bias is equal to zero
  • 14.
  • 15. Doping method • The method of doping near the PN junction makes this diodes to act as a varactor diode. In normal PN diodes, the doping is uniform on both side of the junction. • But in varactor diode non-uniform doping is used. Higher doping level is used near junction and the doping level decreases away from the junction as shown in the figure • This figure shows the doping profile of a varactor diode
  • 16. Symbol and equivalent circuit • In the equivalent circuit, R indicates the resistance of the diode in reverse biased condition. • In general, the varactor diodes are used for tuning purpose. The tuning range or capacitance variation range is decided by the doping profile.
  • 17. Diagram • Tuning ratio or capacitance ratio of the varactor diode is the ratio of the diode capacitance at a minimum reverse voltage to the diode capacitance at a maximum reverse voltage. • Its value is always greater than one. • Tuning Ratio = • Capacitance at minimum reverse voltage Capacitance at maximum reverse voltage
  • 18. parameters • Figure of merit (Q) • Figure of merit is also called as the Quality factor (Q). It is defined as the ratio of energy stored and returned by the capacitor to the energy dissipated in the resistance. High value of Q is always desirable, because high Q indicates low loss (energy dissipation) in resistance. • Quality factor (Q) = • Energy stored and returned by the capacitor • Energy dissipated in resistance • Temperature coefficient • Varactor capacitance has a positive temperature coefficient. That is, the value of • capacitance increases with the increase in temperature. • Applications • 1. It is used as a electronic tuning device. • 2. It is used in parametric amplifiers. • 3. It is used in FM modulation circuits. • 4. It is used in Automatic Frequency Control (AFC) circuit and adjustable bandpass • filter circuits.
  • 19. Tunnel Diode • Tunnel diodes are PN diodes with high impurity concentration (high doping) compared to normal diode. The conduction mechanism in which the charge carriers bore through the barrier directly, instead of climbing over it is called as tunneling. So, the highly doped PN junction diodes are called as Tunnel diodes. • The characteristics of a PN junction diode are completely changed if the concentration of the impurity atom are greatly increased. The depletion layer of PN junction diode becomes extremely thin if it is heavily doped than the zener diode. • Effects of High Doping • The effects of high doping are : • i) The depletion layer becomes very thin. • ii) It will widen the level of donor in N material and level of acceptor in P material. • iii) Fermi level moves towards the conduction band in N-type material. • iv) Fermi level moves towards the valance band in P-type material
  • 20. SYMBOL • It was invented by Dr. Esaki and also called as Esaki diode.
  • 21. OPERATION • The forward current in tunnel diode is due to the sum of the following two components • i) The current due to external bias voltage, which overcomes the depletion region • (majority carrier flow). • ii) The current due to tunneling effect, which passes through the junction without over coming the barrier potential.
  • 22. • The mechanism of tunneling is purely a quantum mechanical phenomenon
  • 23. CONTINUED… • Due to heavy doping, the depletion region is very thin and narrow. The filled level on the P side are exactly opposite to that of filled level on N side. • Therefore, there is no empty lower energy level available for the electron movement from either side and the tunneling operation will not take place. • The conduction and valence band on the P side are higher than the N side. This happens because of the electrons crossing from N side to the P side during junction formation
  • 24. FORWARD BIAS • The energy level of the N side is raised under forward biased condition. • Since the barrier region is very thin, the electrons tunneling occurs from N side to P side.
  • 25. IF WE INCREASED SLIGHTLY • If we increase the forward bias, the energy level of the N side is further raised. • Maximum empty level on the P side is formed which is opposite to the filled level on the N side. • If the forward bias is further increased, the filled energy level on the N side is raised to the level of forbidden energy gap on the P side. • Now the empty level on the P side is reduced and the tunnelling operation decreases
  • 26.
  • 27. Increased voltage higher • On continuous increase of forward bias, the energy level on the N side is raised such that the electrons on the N side find no empty level on the P side f or tunnel ling operation to take place.
  • 28. Reverse bias • If reverse bias is applied, the height of the barrier is increased than the unbiased barrier height some energy state in the valance band of P side are opposite to the empty level on the conduction band of N side. • This result in tunneling of electron through the barrier from P side to N side region. • Even though the junction is reverse biased, current flows through it due to tunneling. • Under increased reverse bias condition, more current flows through the diode.
  • 30. VI characteristics • The following points are observed from the characteristic curve. • i) At low values of forward bias, the tunnel effect controls the flow of current. That is, the tunneling current is larger than the normal injection current. • ii) The tunnelling current reaches a maximum value of Ip at a forward bias of Vp. • iii) For bias voltage greater than Vp, suppression of tunnelling operation takes place. So the tunnelling current reduces and reaches the minimum value of 1v. This current is known as valley current. • iv) Between the peak current and valley current it is found that the current decreases with the increase in voltage. This is due to negative resistance effect.
  • 31. • v) The voltage at which value of current is Iv is called as valley voltage Vv. At this point the tunnel current becomes zero. • vi) Beyond the valley voltage, the current flow is only due to the applied bias voltage and it is similar to the normal PN junction diode current. • vii) Under reverse bias condition, the tunnel diode acts like a good conductor. The reverse current increases with increase in reverse bias. • viii) The current I between peak value (Ip) and valley value (Iv) can be obtained using three different values of bias voltage V1, V3 and V5.
  • 33. LDR