photoresistors • photodiodes • photovoltaic cells ... slides/other elements.pdf · a...

ELEKTRONIKOS ĮTAISAI 2009

VGTU EF ESK [email protected]

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• Photoresistors

• Photodiodes

• Photovoltaic cells

• Phototransistors

• Optical photo detectors

• Optocouplers

• Optical repeaters

PHOTOELECTRICAL SEMICONDUCTOR DEVICES

Operation of photoelectrical semiconductor devices are based on the internal

photoelectric effect.

Photons with energy greater than the width of the forbidden band can cause

generation of charge carriers.



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Photoresistors

Voltampere characteristics and luxampere characteristic of a photodiode

The photoresistor is a two-terminal semiconductor device which

resistance varies with the intensity of the incident light.

Io – dark

current



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Photoresistors

Photoresistors are used in DC and AC circuits.

Disadvantage: low operation speed.



4

Photodiodes

The main part of a photodiode is a reverse-biased pn junction that can be

illuminated. In the dark the reverse current through the device is small. When

the diode is illuminated an additional reverse current dependent on the

incident optical power appears.

The structure of a photodiode and its I-U characteristics



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Photovoltaic cell

The reverse biased pn junction photodiode operates in the photoconductive mode.

If the illuminated diode is used without external bias, a measurable forward

voltage appears between the p and n regions. This is called photovoltaic effect.

The solar cells convert radiation from the sun directly into electrical

energy. In practice, the open-circuit voltage of the silicon photovoltaic cells

is about 0.5– 0.6 V. Their efficiency is about 15 %.



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Photodiodes

Schematic drawing of a p-i-n

photodiode



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Solar batteries



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Solar batteries



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Phototransistors

The phototransistor consists of a transistor structure with a window to allow

illumination of the collector-base region.

Due to amplification, the photocurrent is very much stronger than for the

corresponding photodiode. A disadvantage is that phototransistors are sufficiently

slower than photodiodes due to the transit time of carriers across the base.



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Photodetectors for telecommunications

Quantum efficiency:

λγ

λη

q

hc

q

hc

/

q/ f

f

f ===P

I

WP

I

hc

qf ληγ ==

P

I

24,1

ληγ =

)exp()0()( xPxP α−=

Responsitivity (jautris):

( ) ( )( )( )[ ]21

10

exp1

exp1/

xx

xRPP

−−−×

×−−=

α

α

Due to absorption power decreases:

Power absorbed in the active layer:

pn junction must be near surface. The

depleted layer must be thick, but charge

carriers must transit it without

recombination (the transit time must be

less than life-time).



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1. Germanium photodiodes.

2. InGaAs-InP heterojunction

photodiodes.

3. Schottky photodiodes.

4. Avalanche photodiodes.

Material for a photodetector must be selected taking into account light

wavelength.

Avalanche photodiodes: photocurrent amplification, but less operation

speed and higher noise.

Photodetectors for telecommunications

Optical fiber

Transparent

InP substrate

Contact

layer



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Optocouplers

The opto-isolator, or optocoupler is a package that contains an infrared LED

and a photodetector such as a silicon diode, transistor, Darlington pair or

SCR.

Optocouplers are used to couple signals between the two circuits where these

circuits require total isolation.



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ELEMENTS OF OPTICAL ELECTRONICS.

Optical repeaters

1. Optical repeaters can be realized using photodetectors, electrical

amplifiers, pulse forming circuits, modulators and light sources.

2. At small dispersion, optical amplifiers are used.

Optical amplifier can be realized using optical fiber doped by

erbium.

At pumping power of 100 mW and fiber length of 10–20 m, the

amplification factor is about 20 dB.

Pumping

Signal

Absorber

Fiber doped with

erbium

Coupler

Amplified

signal



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1. GaAs is used for a laser diode. The length of the optical

resonator is 0.5 mm. The approximate width of the optical

line is 1.5 nm. Characterize the light spectrum of the diode.

The refraction coefficient of GaAs is 3.7.

Problem



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1. Puslaidininkinio lazerio aktyviosios srities ilgis l = 0,3 mm, storis – 2 µm, lūžio

rodiklis n =3,6, spinduliuojamos šviesos bangos ilgis – 0,84 µm, slopinimo

koeficientas α = 3,5⋅103 m-1. Raskime šviesos stiprinimo koeficientą, kuris būtinas,

kad susižadintų virpesiai. Kaip pasikeistų gmin, jeigu vienas veidrodis pilnai atspindėtų

šviesą?

2. GaAs lazerinio diodo optinio rezonatoriaus ilgis – 0,5 mm, n = 3,7. Spinduliuojamų

virpesių spektro linijos plotis – ~1,5 nm. Galimai išsamiau apibūdinkime diodo

spinduliuojamos šviesos spektrą.

3. Puslaidininkinio lazerio λ = 1,3 µm. Jo rezonatorius yra stačiakampio gretasienio

formos, l = 150 µm, w = 20 µm, 2d = 1 µm. Medžiagos lūžio rodiklis n ≅ 4.

Apskaičiuokime išilginės modos numerį ir nuotolį tarp artimiausių generuojamos

šviesos bangos ilgių.

4. Pakomentuokite 2.16 ir 2.17 paveikslus.

5. Pagal 2.17 paveikslą raskime srovės tankį ir stiprumą, kurie užtikrina g > 0. Raskime

charakteristikos gmax(Jef) statumą. Lazerinio diodo aktyviosios srities storis – 0,5 µm,

plotis – 20 µm, ilgis – 0,4 mm. Vidinis kvantinis našumas ηvid = 0,5.

Užduotys



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6. Raskime GaAlAs-GaAs lazerinio diodo aktyviuoju sluoksniu sklindančios šviesos

galios dalį, jeigu galio arsenido n = 3,6, ∆n = 0,4, 2d = 0,5 µm.

7. Lazeriniam diodui panaudota GaAlAs-GaAs heterostruktūra. ηvid = 0,8, Γ = 0,8,

2d = 0,5 µm, l = 0,4 mm, b = 10 µm, R1 = 1, R2 = 0,33, α = 1 mm-1,

(Jef)0 = 4,3⋅1013 A⋅m-3, S = 4,8⋅10-10 m2/A. Raskime slenkstinį srovės tankį ir stiprį.

8. Laikydami, kad lazerinio diodo aktyviajam sluoksniui panaudotas GaAs, pagal

2.19 paveikslą raskime diodo naudingumo koeficientą, kai I = 0,5 ir 0,6 A.

9. InGaAsP lazerinio diodo spinduliuojamos šviesos bangos ilgis yra ~1550 nm. Kaip

galima rasti vienmodžio lazerinio diodo, sudaryto pagal 2.22 paveikslą, a, gardelės

periodą?

10. Silicio fotodiodo n srityje priemaišų koncentracija yra Nd = 5⋅1021 m-3. Diodo atbulinė

įtampa – 100 V. Kai bangos ilgis – 0,8 µm, šviesos absorbcijos koeficientas yra ~105

m-1, n = 3,5. Koks gali būti maksimalus diodo kvantinis našumas?

Užduotys



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