Semiconductor laser
Introduction
Electroluminescence is the basic principle of semiconducting laser.
Among the semiconductor there are direct and indirect semiconductor. In direct band semiconductor the recombination of an electron and holes takes place directly thereby emitting photons hence used for lasers.
Based on the number of semiconducting material used for fabrication of PN junction the semiconductor is classified as homojunction laser and heterojunction laser.
Electroluminescence
It is a process in which electrons from valence band get excited to conduction band upon applying either AC or DC field.
During their dexitation the electron and hole recombine to emit light.
Semiconductor laser is also called as laser or injection laser diode
Injection laser diode : semiconductor laser is called so since the charge carrier are injected across PN junction upon applying a DC field.
Homojunction semiconductor laser
Construction
Homo junction semiconductor laser is shown below. In this type of laser the PN junction is fabricated in a single semiconductor.
The the active medium in PN junction is made up of GaAs. The p-region and n-region in the diode are made from gallium arcanoid. the P region and n-region in diode are obtained by doping germanium and tolurium respectively in gallium arcenoid.
The thickness of PN junction layer is made very thin typically of order of few micrones and the end faces of crystal are made partially reflecting to form a optical resonator. Electric current is applied to the crystal through metal electrode fixed on its upper and lower surface.
Working
The population inversion is achieved by injecting electrons across the junction from P region to n region by means of forward biasing voltage applied to it.
The recombination of electron and holes in the region result in emmision of photon. The process is spontaneous and laser radiation is random and in coherent but these photons triggers stimulated emission of photon to get generated by increasing the rate of recombination of electrons and holes.
If the current density is increased the emission becomes more and more coherent and the radiation intensity increases.
The wavelength of emmitted light radiation depends upon the concentration of donor and accepted atom in GaAs and also in energy gap of particular semiconductor.
Calculation of wavelength of emmitted radiation
Suppose the band gap of gallium arcanide is 1.44 electron volt then the band gap is
Eg=hc/λ
λ =hc/Eg
Thus GaAs laser emit infrared radiation.
Advantages
- The modulation of output is possible by controlling the junction current
- It is small in dimension but exhibit high efficiency
- The fabrication is easy
Disadvantage
- The output is usually in form of wide beam that is it has large beam divergence.
- Threshold current density is very large.
- The monochromaticity and coherence are poorer than in other type of laser.
Application
- It is used in long haul communication.
- It is used in local area network and CDs.
- It is used in laser printer.
