19-Lasers and Semiconductors

Lasers and Semiconductors

Definitions

Spontaneous emission – Photons are emitted when the excited atoms are de-excited to a lower energy state and it happens naturally and randomly without requiring an event to trigger that transition. Photons of energy equal to the energy difference between the two energy levels are emitted.

Stimulated emission – Photons incident on a matter trigger excited atoms to transit from a higher energy level to a lower one and emit photons. The incident photons and their emitted counterparts have the same frequency and phase; this frequency corresponds to the energy difference between the two energy levels.

Population inversion – A condition in which a higher energy state (in an atomic system) has much more electrons/atoms than a lower energy (or ground) state of the same system.


Metastable state – It is an energy state where atoms stay much longer (10-3 s) before transiting to lower energy states as compared with other excited states (10-7 s).

Characteristics of lasers
Unidirectional, Monochromatic, Coherent



Intrinsic semiconductor – made up of valency 4 atoms only (e.g. Si or Ge). Hole-electron pairs are formed due to thermal energy to improve its conductivity. Energy band gap is about 1 eV. Energy band gap decreases with increasing temperature.

Extrinsic semiconductor – made up of valency 4 atoms doped with small amount of impurity


p-type semiconductor

– an intrinsic semiconductor doped with trivalent impurity (e.g. boron, gallium, indium)
– an additional discrete energy level corresponding to the impurity lies slightly above the VB so that electrons from the VB can easily jump to this level with little energy (0.01eV) given to them. Hence holes are produced in the VB and conductivity increases. – no net charge
n-type semiconductor
– an intrinsic semiconductor doped with pentavalent impurity (e.g. antimony, phosphorus, arsenic)
– an additional discrete energy level corresponding to the impurity lies slightly below the CB so that electrons from this level can easily jump to the CB with little energy (0.01eV) given to them. Hence more electrons in the CB and conductivity increases.
– no net charge
Depletion region at p-n junction
– formed when p-type and n-type are joined (no external emf is applied)
– holes from p-side cross over the junction to the n-side to neutralize the electrons
– electrons from n-side cross over the junction to the p-side to neutralize the hole
s – a potential barrier is formed to stop these movements


p-n junction acts as a rectifier (external emf is applied)
forward bias – positive terminal of battery connected to p-side and negative to n-side

– potential barrier would be lowered, hence charge carriers can move across the junction and results in a current

– width of depletion layer will decrease

reverse bias – positive terminal of battery connected to n-side and negative to p-side
– potential barrier would be increased, hence charge carriers cannot move across the junction and results in no current

– width of depletion layer will increase

– a small current may arise due to minority charge carriers crossing the junction