Optical communications

Chapter 3.1Chapter 3.1

Pham Quang Thai

Pqthai.hcmut@gmail.com

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Content

• Semiconductor fundamentals

• Light emitting diode (LED)

• Laser diode

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Types of material

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Light absorption: atom absorbs photon (E=hv) and

moves from ground state (E1) to excited state (E2)

E2-E1=E

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Light emission: excited atom (unstable) returns to

ground state and may emit photon in the process

(video)

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PN Junction: n type and p type semiconductor are

combined (video)

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Non-radiative recombination: excited atom

returns to ground state but does not emit light

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•Radiative recombination lifetime: excited duration of atoms that will

emit photons when recombine

•Non-radiative recombination lifetime: excited duration of atoms that

will not emit photons when recombine

Direct band gap: excited atom=photon

Indirect band gap: excited atom=photon + phonon

• Direct band gap semiconductor (GaAs, InP, AlAs, etc.) is used

for better efficiency

E E

Photon absorption in an indirect bandgap semiconductor (Si, Ge)

Photon absorption in a direct bandgap semiconductor (GaAs, InAs,

InP, GaSb, InGaAs, GaAsSb).

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CB

VB

k–k

Direct Bandgap Eg

k–k

VB

CB

Indirect Bandgap

Photon

Phonons

EgEC

EV

EC

EV

Photon

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