Physics of Semiconductor Devices

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Physics of Semiconductor Devices
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• Formation of PN - Junction
• When a P-type Semiconductor is joined together
  with an N-type Semiconductor a PN junction is
  formed. And it is also known as a Semiconductor
  Diode.
• Semiconductor diodes are widely used in
  Rectifiers which converts input AC signal into
  DC output signal.

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Formation of a PN-junction
P
N
Space charge region
(OR)
Depletion region
Potential barrier height(V0)
Potential barrier width (W)
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• Depletion Region Space Charge
• The diffusing majority carriers from the two
  regions recombine near the junction and
  disappear.
• The uncompensated Acceptor and Donor ions set up
  an Electric field which halts majority carrier
  Diffusion and causes minority carrier Drift.
• The two kinds of majority carriers diffusing
  across the junction meet each other near the
  junction and undergo recombination's, leaving
  negative ions on the P-side and positive ions on
  the N-side of the junction.
• This distribution of Positive and Negative
  Charges is called Space charge.

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Diode Symbol
N
P
Cathode
Anode
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Energy level diagram
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• V - I Characteristics of PN Junction
• The diode can be operated in two different ways,
  as Forward and Reverse bias.
• When positive terminal of the battery is
  connected to the P-type negative terminal is to
  the N-type of the PN-junction diode, known the
  diode is kept in forward bias.
• When negative terminal of the battery is
  connected to the P-type positive terminal is to
  the N-type of the PN-junction diode, known the
  diode is kept in reverse bias.

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Open circuit PN -junction
P
N
Space charge region
P
N
Forward bias
VF
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Open circuit PN -junction
P
N
Space charge region
P
N
Reverse bias
VR
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From the graph the following points are noted.

• The region between knee voltage breakdown
• voltage is known as non-ohmic region.

• Above the knee breakdown voltage the current
• increases.

• Breakdown voltage is due to thermally broken
• covalent bonds.

• Diode is conducting in forward bias
• non-conducting in reverse bias.

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• Rectifiers
• A Rectifier is a device which converts
  alternating current to direct or unidirectional
  current.
• Rectifiers are mainly three types
• 1.Half wave Rectifiers
• 2.Full wave Rectifiers
• 3.Bridge Rectifiers

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Full Wave Rectifier
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Efficiency of a Rectifier The efficiency of a
Rectifier is defined as the ratio of D.C. out
put power to the A.C. input power supplied to
the Rectifier.
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• Light emitting diodes
• LED s are the most visible type of diode, that
  emits a fairly visible colored light, invisible
  infra-red or laser type light when a forward
  current is passed through them.
• Principle
• Basically LED are made from a very thin layer
  of fairly heavily doped semiconductor material.
• When the diode is forward biased, electrons
  from conduction band combine with holes from in
  valence band, releasing sufficient energy to
  produce photons of light.

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Semiconductor material Wave length Color Vf _at_ 20mA
GaAs 850-940nm Infra-Red 1.2V
GaAsP 630-660nm Red 1.8v
GaAsP 605-620nm Amber 2.0v
GaAsPN 585-595nm yellow 2.2v
GaP 550-570nm Green 3.5v
SiC 430-505nm Blue 3.6v
GAInN 450nm white 4.0v
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Current I(MA)
V
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• Photo diode
• A photodiode is a diode optimized to produce an
  electron current flow in response to irradiation
  by ultraviolet, visible, or infrared light.
• Silicon, Ge gallium arsenide is most often
  used to fabricate photodiodes.
• Few electron-hole pairs in the N and Pregions,
  and most in the depletion region contribute to
  photo current.

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