PN junction diode

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• ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY
  SCIENCES(A)Department of Electronics and
  Communication Engineering
• ECE 125 Basic Electronics Engineering

• Academic year 2022-23
• Class Section 1/4 ECE-A
• Name of the Faculty Mr.D.Anil Prasad

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UNIT-I(Semiconductor Diodes)

• Fermi level in Intrinsic Extrinsic
  semiconductors. Mass-Action law. Mobility and
  conductivity, Hall effect, Generation and
  recombination of charges, Drift and diffusion
  current, Band structure of open-circuit p-n
  junction, V-I characteristics, transition and
  diffusion capacitance, reverse recovery time,
  Avalanche and zener breakdown, zener diodes,
  Light Emitting Diodes.

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Energy bands and Fermi level

•  

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Energy bands and Fermi level

•  

• Slope of CB ,VB or intrinsic fermi level
  represents electric field .
• Electric field is directed in positive slope
  direction of CB or VB or intrinsic fermi level.

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PN junction

• Most semiconductor devices contain at least one
  junction between p-type and n-type semiconductor
  regions. Semiconductor device characteristics
  and operation are connected to these PN junction
• Applications of semiconductor devices
• Rectification
• Switching
• Amplification

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PN junction in thermal equilibrium

• Before PN junction is formed n-region has a large
  concentration of electron and few holes. P-region
  has a large concentration of holes and few
  electrons

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PN junction in thermal equilibrium

• After joining the two regions diffusion of charge
  carriers takes place at the junction because of
  large concentration gradient.
• Due to concentration gradient holes diffuse from
  p-side into n-side and electrons diffuse from
  n-side into p-side

• Holes leaving the p-region leave behind
  uncompensated acceptor ions in the p-region and
  Electrons diffusing from n-side to p-side leave
  behind uncompensated donar ions in the n-region
  as shown in fig.

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PN junction in thermal equilibrium

• Positive space charge near n-side of junction and
  negative space charge near p-side of junction is
  formed shown in fig.
• The resulting diffusion current cannot build up
  indefinitely. Because of the development of space
  charge an internal Electric field is created at
  junction

• Positive space charge near n-side of junction and
  negative space charge near p-side of junction is
  formed shown in fig.
• The resulting diffusion current cannot build up
  indefinitely. Because of the development of space
  charge an internal Electric field is created at
  junction

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PN junction in thermal equilibrium

• Electric field creates drift component of current
  from n to p opposing diffusing current.
• Since No net current can flow across the junction
  at equilibrium the current due to drift of
  carriers in the electric field must exactly
  cancel the diffusion current.

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PN junction in thermal equilibrium
In an open circuited device the drift
hole(electron) current must be equal and opposite
to the diffusion hole (electron) current so that
the net hole(electron) current is zero.
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Depletion width and charge density Under
equilibrium

• The amount of uncovered negative charge on the
  left hand side of the junction must be equal to
  the amount of positive charge on the right hand
  side of the metalurgical junction. Overall
  space-charge neutrality condition
• The higher doped side of the junction has the
  narrower depletion width

?? ?? ?? ?? ??
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Electric field and Built-in-potential
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Contd

• Electric field
• The built-in potential is
• given by area enclosed
• by the electric field curve
• i.e
• by Guass law

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area ?? ????
?? ?? ?? ?? ?? ?? ?? ??
?? ???? 1 2 ?? ?? ?? ?? ?? ?? 1 ?? ??
?? ?? ?? ?? ?? ?? ?? ??
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Contd
?? ???? 1 2 ?? ?? ?? ?? ?? ?? 1 ?? ??
?? ?? ?? ??

?? ???? 1 2 ?? ?? ?? 2 ?? ?? ?? ?? ??
?? ?? 1
?? ???? 1 2 ?? ?? ?? 2 ?? 1 ?? ??
1 ?? ??
?? ?? 2?? ?? ???? ?? 1 ?? ?? 1 ?? ??

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Contd

• Width of depletion region(Wd)
• The built-in potential is given by area enclosed
  by the electric field curve
• i.e

?? ???? 1 2 ?? ?? ( ?? ?? ?? ?? )
?? ???? 1 2 ?? ?? ?? ??
?? ?? 1 ?? ?? 2?? ?? ???? ?? ?? ?? ??
??( ?? ?? ?? ?? ) ?? ?? 1 ?? ?? 2??
?? ???? ?? ?? ?? ?? ??( ?? ?? ?? ?? )
?? ?? 2 ?? ???? ?? ??
?? ?? 2?? ?? ???? ?? 1 ?? ?? 1 ?? ??

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Energy band diagram for the p-n junction under
open circuited condition
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Energy level diagram for the Built-in voltage
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Built-in Potential(Vbi)
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?? 0 ?? ?? exp -( ?? ?? - ?? ?? ) ????
?? 0 ?? ?? exp -( ?? ?? - ?? ?? ) ????
?? 0 ?? ?? exp ( ?? ?? -????) ????
?? 0 ?? ?? exp ( ?? ?? - ?? ?? ) ????
?? ?? ?? ?? ?? - ?? ??
?? ?? ?? ?? ?? - ?? ??
?? 0 ?? ?? exp (?? ?? ?? ) ????
?? 0 ?? ?? exp (?? ?? ?? ) ????
?? ?? ???? ?? ln ?? 0 ?? ??
?? ?? ???? ?? ln ?? 0 ?? ??
?? ?? ???? ?? ln ?? ?? ?? ??
?? ?? ???? ?? ln ?? ?? ?? ??
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Built-in Potential(Vbi)
?? ???? ?? ?? ?? ??
?? ???? ???? ?? ln ?? ?? ?? ?? ????
?? ln ?? ?? ?? ??
?? ???? ???? ?? ln ?? ?? ?? ?? ?? ??
2
?? ???? ???? ln ?? ?? ?? ?? ?? ?? 2
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Forward bias(VDgt0)
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Reverse bias(VDlt0)
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Applying bias to pn junction
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Abrupt PN Junction
heavily doped p-type
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The experimental I-V characteristic of a Si diode
?? ?? 0 ?? ??/ ?? ?? -1
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References

• Robert L Boylestad, Electronic Devices And
  Circuit Theory, Prentice Hall, seventh
  edition,2021
• Jacob Millman and Christos Halkias, Electronics
  Devices and Circuits, Black edition, October,2017

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Thank you

• for listening