ELEKTRONIKOS ĮTAISAI
1
2009
PHOTOELECTRICAL SEMICONDUCTOR DEVICES
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Photoresistors
Photodiodes
Photovoltaic cells
Phototransistors
Optical photo detectors
Optocouplers
Optical repeaters
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.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
2
2009
Photoresistors
The photoresistor is a two-terminal semiconductor device which
resistance varies with the intensity of the incident light.
Io – dark
current
Voltampere characteristics and luxampere characteristic of a photodiode
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
3
2009
Photoresistors
Photoresistors are used in DC and AC circuits.
Disadvantage: low operation speed.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
4
2009
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
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
5
2009
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 %.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
6
2009
Photodiodes
Schematic drawing of a p-i-n
photodiode
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
7
2009
Solar batteries
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
8
2009
Solar batteries
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
9
2009
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.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
10
2009
Photodetectors for telecommunications
Quantum efficiency:
I /q
I hc
hc
η= f
= f
=γ
P / Wf
P qλ
qλ
Responsitivity (jautris):
γ=
If
qλ
=η
P
hc
γ =η
λ
1,24
Due to absorption power decreases:
P ( x) = P(0) exp(−αx)
Power absorbed in the active layer:
P / P0 = (1 − R ) exp(− αx1 )×
× [1 − exp(− α (x1 − x2 ))]
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).
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
11
2009
Photodetectors for telecommunications
Material for a photodetector must be selected taking into account light
wavelength.
1. Germanium photodiodes.
2. InGaAs-InP heterojunction
photodiodes.
3. Schottky photodiodes.
4. Avalanche photodiodes.
Transparent
InP substrate
Optical fiber
Contact
layer
Avalanche photodiodes: photocurrent amplification, but less operation
speed and higher noise.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
12
2009
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.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
13
2009
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.
Pumping
Coupler
Absorber
Signal
Fiber doped with
erbium
Amplified
signal
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.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
14
2009
Problem
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.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
15
2009
Užduotys
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.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
16
2009
Užduotys
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?
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
VGTU EF ESK
17
2009
stanislovas.staras@el.vgtu.lt