ENE 311

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ENE 311

• Lecture 7

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p-n Junction

• A p-n junction plays a major role in electronic
  devices.
• It is used in rectification, switching, and etc.
• It is the simplest semiconductor devices.
• Also, it is a key building block for other
  electronic, microwave, or photonic devices.

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Basic fabrication steps

• The basic fabrication steps for p-n junction
  include
• oxidation,
• lithography,
• diffusion or ion implanation,
• and metallization.

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Basic fabrication steps

• This process is to make a high-quality silicon
  dioxide (SiO2) as an insulator in various devices
  or a barrier to diffusion or implanation during
  fabrication process.
• There are two methods to grow SiO2 dry and wet
  oxidation, using dry oxygen and water vapor,
  respectively.
• Generally, dry oxidation is used to form thin
  oxides because of its good Si-SiO2 interface
  characteristics, while wet oxidation is used for
  forming thicker layers since its higher growth
  rate.

Oxidation
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Basic fabrication steps
Lithography

• This process is called photolithography used to
  delineate the pattern of the p-n junction.

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Basic fabrication steps

• (a) The wafer after the development.
• (b) The wafer after SiO2 removal.
• (c) The final result after a complete lithography
  process.

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Basic fabrication steps

• This is used to put the impurity into the
  semiconductor.
• For diffusion method, the semiconductor surface
  not protected by the oxide is exposed to a high
  concentration of impurity. The impurity moves
  into the crystal by solid-state diffusion.
• For the ion-implantation method, the impurity is
  introduced into the semiconductor by accelerating
  the impurity ions to a high-energy level and then
  implanting the ions in the semiconductor.

Diffusion Ion Implantation
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Basic fabrication steps
Metallization

• This process is used to form ohmic contacts and
  interconnections.
• After this process is done, the p-n junction is
  ready to use.

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Thermal equilibrium condition

• The most important characteristic of p-n junction
  is rectification.
• The forward biased voltage is normally less than
  1 V and the current increases rapidly as the
  biased voltage increases.
• As the reverse bias increases, the current is
  still small until a breakdown voltage is reached,
  where the current suddenly increases.

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Thermal equilibrium condition

• Assume that both p- and n-type semiconductors are
  uniformly doped.
• The Fermi level EF is near the valence band edge
  in the p-type material and near the conduction
  band edge in the n-type material.

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Thermal equilibrium condition

• Electrons diffuse from n-side toward p-side and
  holes diffuse from p-side toward n-side.
• As electrons leave the n-side, they leave behind
  the positive donor ions (ND) near the junction.
• In the same way, some of negative acceptor ions
  (NA-) are left near the junction as holes move to
  the n-side.

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Thermal equilibrium condition
Space-charge region

• This forms 2 regions called