Transistor Fabrication

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Transistor Fabrication
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Most Employed Methods

• Alloy Junction
• Grown Junction
• Diffusion

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Alloy Junction

• An extension of the alloy method of manufacturing
  semiconductor diodes
• Two dots of the same impurity are deposited on
  each side of a semi conductor wafer
• The entire structure is then heated until melting
  occurs and each dot is alloyed to the base wafer
  resulting in the p - n junctions

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Alloy Junction
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Grown Junction

• Czochralski technique is used to form the two p
  - n junction s of a grown junction transistor
• The process requires that the impurity control
  and the withdrawal rate be such as to ensure the
  proper base width and doping levels of the n and
  p type materials.
• Transistors of this type are limited to less than
  i w rating

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Grown Junction
Crystal pulling rod
Seed
P
P
Melt
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Diffusions

• The most frequently employed method of
  manufacturing transistors today
• The basic process was introduced in the
  discussion or semiconductor diode fabrication
• The diffusion technique is employed in the
  production of mesa and planar transistors, each
  of which can be of diffused or epitaxial type

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Diffusions

• In the pnp, diffusion-type mesa transistor the
  first process is an n-tpe diffusion into a p-type
  wafer to form the base region

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Diffusions

• Next the p-type emitter is diffused or alloyed to
  the n-type base

p
n
p
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Diffusions

• Etching is done to reduce the capacitance of the
  collector junction.

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Diffusion

• The major difference between the epitaxial mesa
  transistor and the mesa transistor is the
  addition of an epitaxial layer on the original
  collector substrate.
• The original p-type substrate is placed in a
  closed container having vapor of the same
  impurity. Through proper temperature control, the
  atoms of the vapor will fall upon and arrange
  themselves on the original p-type substrate
  resulting in the epitaxial layer. Once this
  layer is established the process continues.
• The original p-type substrate will have a higher
  doping level and correspondingly less resistance
  than the epitaxial layer
• The result is a low resistance connection to the
  collector lead that will reduce the dissipation
  losses of the transistor

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Epitaxial Mesa Transistor
Epitaxial Layer
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Diffusion

• The planar and epitaxial transistor are
  fabricated using two diffusion processes to form
  the base and emitter regions.
• The plabar transistor has a flat surface, which
  accounts for the term planar
• An oxide layer is added to eliminate exposed
  junctions, which will reduce substanstially the
  sufrface leakage loss

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Planar Transistor
Oxide layer