Lasers and Semiconductors

Spontaneous emission: A process whereby a photon is emitted when an electron in an excited atom falls naturally to a lower energy level, i.e. without requiring an external event to trigger it.

Stimulated emission: A process whereby an incoming photon causes/induces another photon of the same frequency & phase (& direction) to be emitted from an excited atom.

Laser: A monochromatic, coherent, parallel beam of high intensity light.

Meta stable state: An excited state whose lifetime is much longer than the typical (10-8s) lifetime of excited states.

Population inversion: A condition whereby there are more atoms in an excited state than in the ground state.

{A meta stable state is essential for laser production because it is required for population inversion to be achieved, which, in turn, increases the probability of stimulated emissions.}

Conditions to achieve Laser action:

  1. Atoms of the laser medium must have a meta-stable state.
  2. The medium must be in a state of population inversion.
  3. The emitted photons must be confined in the system long enough to allow them to cause a chain reaction of stimulated emissions from other excited atoms.

Formation of Energy Bands in a Solid/Band theory for solids:

Valence Band: The highest energy band that is completely filled with electrons.

Conduction Band: The next higher band; For some metals/ good conductors, it is partially-filled; For other metals, the VB & CB overlap {hence it is also partially-filled}

Energy Gap {Forbidden Band}: A region where no energy state can exist; It is the energy difference between the CB & VB

Properties of Conductors, Insulators and Semi-conductors at 0 K {“low temp”}:

  Conductors Insulators Semi-conductors
Conduction Band Partially filled Empty
Valence Band Completely Occupied
Energy gap between the bands NA Large (≈10 eV) Small (≈1 eV)
Charge Carriers free electrons - free electrons & holes

How band theory explains the relative conducting ability of a metal, intrinsic semiconductor & insulator:


Explain why electrical resistance of an intrinsic semiconductor material decreases as its temperature rises:

Based on the band theory, a semiconductor has a completely filled valence band and an empty conduction band with a small energy gap in between. Hence there are no charge carriers and the electrical resistance is high.

  1. When temperature is low, electrons in the valence band do not have sufficient energy to jump across the energy gap to get into the conduction band.
  2. When temperature rises, electrons in the valence band receive thermal energy to enter into the conduction band leaving holes in the valence band.
  3. Electrons in the conduction band & holes in the valence band are mobile charge carriers and can contribute to current.
  4. Increasing the number of charge carriers means lower resistance.

2 Differences between p-type silicon & n-type silicon:

  1. In n-type Si, the majority charge carrier is the electron, its minority charge carrier is the hole.
    For p-type Si, the situation is reversed.
  2. In n-type Si, the dopants are typically pentavalent atoms (having 5 valence electrons);
    In p-type Si, the dopants are typically trivalent atoms (valency = 3)
Origin of Depletion Region

How a p-n junction can act as a rectifier

{Thus a p-n junction {diode} allows current to flow in one direction only {when the p-n junction is in forward bias} and so, it can be used as a rectifier to rectify an ac to dc}