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OPTOELECTRONIC DEVICES


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OPTICAL EMISSION AND ABSORPTION

Review:

Electrons can change energy states moving from oneband to another (e.g. VB to CB, or CB to VB).

Electrons excited from VB to CB acquire extra energy

from vibration (lattice phonons) or optical source i.e.light (photons).

Energy of a photon related to the frequency of the lightwave (Eq. 1.70), in which light energy must be absorbed

or emitted in integer multiples of h

hE --- (1.70)Refer pg. 34 for calculation example


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The frequency of light is given by

=c/ is wavelength, is frequency, c is speed of light.

Golden Rule (Eq. 1.74), a convenient equation toconvert wavelength to energy and vice versa.

24.1)()( meVE --- (1.74)

Refer pg. 37 for calculation example


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Importantwavelengths forsemiconductoroptoelectronics.


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Example 1.3

Photon energy < Eg photon cannotbe absorbed.

Photon energy > Eg photon can be

absorbed. When the carriers relaxed atthe band edges, photon energy isreleased, the extra energy is released asheat.


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Example of optical devices photodetectors(photodiodes, solar cells), light emitting diode(LED), laser diode.

Photodetectors convert photon / optical power toelectrical power.

Photons are absorbed to generate carriers (current).

LED and laser diodes convert electrical power tooptical power.

Photons are emitted when carriers recombine.


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PHOTODETECTORS

Photodetectors a diode / transistor that is used tomeasure the amount of light energy present, i. e.

the output signal (e.g. current) is a function of lightintensity.

Optical energy is absorbed to generate excesselectron-hole pairs producing photocurrent.

Can convert optical signals to electrical signals.


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GENERIC PHOTODIODE

Photodiode is a pn junction that operates in reversebias. It is illuminated from the top.

Photons with energy greater

than the Eg create electron-hole pairs.

Electron and holes areseparated by the internal

electric field in the junctiondepletion region and flowthrough top and bottomcontacts (photocurrent isgenerated)


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GENERIC PHOTODIODE

As light penetrates into the semiconductor, it isabosorbed. The formulation relies on thecharacteristics of , called absorption coefficient.

There is partial reflection explained by the Fresnelreflection light is incident on an interface betweentwo materials with different refractive indices.

Solar cells are photodetectors that are used togenerated dc power.

Other types: p-i-n photodetectors and avalanchephotodiodes (APD) commonly used in opticalcommunications.


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GENERIC PHOTODIODE

Photocurrent is given by:

W : intrinsic layer width, : absorption coefficient,

Jph

: photon flux density

Pop : optical power density

Photon detector efficiency

R : reflection loss ( can be assumed to be = 0)

)exp(1)[0( WqAJI phL

hf

PJ

op

ph )0(

)exp(1)[1(det WR


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LIGHT-EMITTING DIODES (LEDS)

For LEDs, emission is considered instead of absorption. LEDs exhibit electroluminescence behaviour process of

generating photon emission when excitation of excess carriersare by applied electric field.

Spontaneous emission

a random event: direction of propagation, timing and polarization areall random.

LEDs operate by spontaneousemission* due to the recombination ofelectrons and holes form CB to VB

Excess energy during recombination

is emitted either as light (radiativetransition), as phonons (nonradiativetransition), or a combination of thetwo.


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For significant emission, there must be manyelectrons at elevated energy states (i.e. conductionband) and holes present in the same physical area.

One approach is to use double-heterostructure pn

junction.


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This structure has a wide band-gap p-side and wideband-gap n-side, with a narrow band-gap material inbetween.

This will form a potential well for electrons and holes.

Under forward bias, excess electrons diffuse across the

depletion region from the n-side, and holes diffuse fromp-side from the opposite direction.

They will get caught and confined in the wells,increasing probability of recombination.



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Visible LEDs were available only in green, yellow,orange, red, and blue (in late 1990s). Only afterblue LED is available, LED technology could beused for color displays.

GaN and organic polymers are the materials usedfor blue LEDs.

Quasi-white LED has been created using blue LEDand phosphor blue light absorbed by phosphorand reradiated in red. Combination of blue and redappears to be almost white to the human eye.



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Laser diodes rely on the concept of stimulatedemission (not spontaneous emission).

For stimulated emission to occur, populationinversion is required.

To achieve population inversion, large number ofelectrons in CB and large number of empty statesin VB are required.

LASER DIODES


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Incident photon interacts with an electron at ahigher energy level. When the electron makes adownward transition to valence band, it producesmore photons than the incident photon: gain.

LASER DIODES


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Lasers need gain and feedback to operate. Gain in optical systems mean photon emitted >

photon absorbed.

Optical feedback is used to increase total optical

amplification by making the photon pass throughthe gain region multiple times, typically comes fromtwo mirrors, one at each end of the laser.

LASER DIODES

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