#FocusOnExams : Physics GCE ‘A’ L – 28th April 2020

PHYSICS

ADVANCED LEVEL

Review of previous lesson

  • Define a semiconductor
  • Differentiate between electrical conductors, insulators and semiconductors using the Band theory.
  • Explain why some materials conduct electricity and why others do not.
  • Explain why the conductivity of a semiconductor increases with an increase in temperature.

Content

1.3 Types of semiconductors

1.4 Doping

2.0 P-N JUNCTION AND THE P-N JUNCTION DIODE

2.1 P-N junction

1.3 TYPES OF SEMICONDUCTORS

 

  • An Intrinsic Semiconductor is one which does not contain any amount of impurity atoms.
  • g silicon (Si), germanium (Ge), arsenic (As),antimony (Sb) etc.
  • An extrinsic semiconductor is one where a small amount of impurity atoms has been added to a pure semiconductor.
  • The impurity atoms here can be a trivalent atom or a pentavalent

The process of adding impurity atoms to a pure semiconductor in order to increase conductivity is called doping.

1.4 DOPING

  • DOPING is the addition of a small amount of impurity atoms (trivalent or pentavalent) to a pure semiconductor in order to increase its conductivity.
  • The conductivity of a pure semiconductor can be increased by;
  1. Increasing the temperature.
  2. Doping   
  3.  

The type of extrinsic semiconductor formed depends on the type of doping

 

Trivalent atoms

  • Boron (B)
  • Aluminum (Al)
  • Gallium (Ga)
  • Indium (In)
  • Thallium (Tl)
  • Pentavalent atoms
  • Nitrogen (N)
  • Phosphorous (P)
  • Arsenic (As)
  • Antimony (Sb)
  • Bismuth (Bi)

 

 Silicon  for example:

  • Group IV
  • Has 4 electrons in outermost shell
  • Tetravalent
  • Forms covalent bond with 4 other Si atoms

 

 

  1. N-type semiconductor is one in which a small number of pentavalent atoms has been added to a pure semiconductor. The impurity atom here bonds with four valence electrons leaving the fifth electron unbonded (free electron). This fifth electron is free to move in the lattice and serves as a charge carrier. Majority charge carriers here are electrons which are negatively charged, thus, the name N-type.
  2. P-type semiconductor is one in which a small amount of trivalent atoms has been added to a pure semiconductor. The impurity atom here only brings three electrons for bonding instead of four. The absence of the fourth electron creates an electron deficiency called a hole. Majority charge carriers here are hole. Conduction here is by jumping of electrons from hole to hole. This hole carries are positive charged, thus, the name P-type.

 

2.0 THE P-N JUNCTION AND THE P-N JUNCTION DIODE

2.1 THE P-N JUNCTION

  • It is formed when a P-type and an N-type semiconductor are fused together.
  • When this happens, some of the excess electrons in the N-type migrate to fill (occupy) the holes in the P-type.

 

  • This will create a difference in potential (p.d) across the two layers at their boundary.
  • This layer between them is called depletion layer.
  • The depletion layer is a thin layer created between an N-type and a P-type semiconductor when brought together.
  • Across the depletion layer, a small potential difference is created called barrier potential.
  • Barrier potential is the potential that must be overcome across a P-N junction for electric current to flow across it.

 

 

 

 

 

 

 

 

 

 

 

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