Slide 1

Electromagnetic spectrum

insert
wavelengths of
blue to red.

6.071 Optoelectronics 1
Slide 2

Electromagnetic spectrum

E = hν = kTe
E - Energy
k - Plank’s constant
ν - frequency
k - Boltzman’s constant
Te - equivalent temperature °K
6.071 Optoelectronics 2
Slide 3

Electromagnetic Spectrum slide deleted

Slide 4

Light-emitting diodes (LEDs)

cathode anode anode cathode

p n

When electrons and holes combine in the depletion region the excess
energy is given up in the form of a photon. This energy is
characteristic of the materials from which the junction is constructed
and thus the photon always has the same wavelength. To emit
photons the junction must be forward biased, and the wavelength as
typically red, green, yellow, or in the infrared.

The LED sits in a

reflector and is
covered by a plastic
diffuser.
6.071 Optoelectronics 4
Slide 5

Light-emitting diodes (LEDs)

simple LED,red, green,

blinking LED, blinks 1-6 times
yellow, orange, or infrared.
per second. Can be used as a
trivial oscillator.

Simple means of
creating
numbers and
characters. For Tricolor LED, green one
numbers drive way, red the other.
with 74HC4511.

6.071 Optoelectronics 5
Slide 6

LEDs specs
viewing intensity forward reverse max DC max
angle voltage break- current power
drop down
LED type degrees color MCD V V mA mW
NTE3000 indicator 80 red 1.4 1.65 5 40 80
NTE3010 indicator 90 green 1 2.2 5 35 105
NTE3026 tristate 50 red/gree 1.5/0.5 1.65/2.2 70/35 200
n
NTE3130 blinker 30 yellow 3 5.25 0.4 20
NTE3017 infrared 1.28 6 100 175

Notice that the forward voltage drop is not the 0.6 V we associate
with a silicon diode, and that the reverse breakdown voltage is
quite small.

6.071 Optoelectronics 6
Slide 7

LED operation

Graphs produced with Mathematica™

6.071 Optoelectronics 7
Slide 8

LED Datasheet

6.071 Optoelectronics 8
Slide 9

LED Datasheet 2

6.071 Optoelectronics 9
Slide 10

Some LED applications

voltage level
sensor logic probe
Vcc

Vin

LED lights when the

breakdown voltage of the A logic high turns on the
zener is exceeded. The Darlington pair and the
source must provide the LED. This is a high
current for the LED. impedance measurement.

6.071 Optoelectronics 10
Slide 11

Tristate polarity indicator

Ig A tristate diode indicates the

direction of DC current flow, or if
Vin
the current is AC.
Ir R1 The diode protects for reversed
voltage breakdown and the
R2 resistors protect the tristate diode
from voltage breakdown.

positive current red

negative current green
AC current yellow

6.071 Optoelectronics 11
Slide 12

Photoresistors
insert figs and diagrams from
data sheets.

A photodiode is a light controlled resistor

that operates much like a thermistor. In an
intrinsic semiconductor, photons can
promote an electron to the conduction band
leaving a hole in the valence band. This
increase in carriers leads to a reduction in the
resistance. Photoresistors are non-linear,
slow (~100 ms) but cheap.

6.071 Optoelectronics 12
Slide 13

Photoresistor Specifications

material energy gap wavelength

(eV) (Å)
CdS 2.42 5130
CdSe 1.73 7176
PbS 0.37 33664
PdTe 0.29 42811
PdSe 0.27 45982 Graphs produced with Mathematica™

6.071 Optoelectronics 13
Slide 14

Photoresistor applications

Two simple circuits to switch a relay by the action

of a photoresistor. Which one is on in the dark?

on in the dark on in the light

6.071 Optoelectronics 14
Slide 15

Photodiodes

electric field

When a photon of sufficient energy

hits the semiconductor then an
diode current

electron/hole pair is created. The

minority carrier then diffuses to the
junction and an electric field is
created. This electric field across the
depletion zone is equivalent to a
negative voltage across the unbiased Graphs produced with Mathematica™
diode. At left shows the increased
diode voltage negative offset as a function of
6.071 Optoelectronics
increased light intensity. 15
Slide 16

Characteristics of photodiodes
Idiode
The sensitivity of photodiodes is defined as, S=
P
where P is the incident photon power per unit area. Typically S~2
µA/mW/cm2.
Silicon peaks at 900 nm in the infrared, and the sensitivity at 600
nm is down by ~60%.
Diodes are faster than photoresistors, and a reverse bias speeds up
the minority carrier diffuse and thus the response time (~ 1µs).

R Vout = RIdiode
Idiode
Vout

6.071 Optoelectronics 16
24 17-18

Go to the manufacturer’s web site to obtain a datasheet of their product. Please follow
these steps:

1. Go to the web site for Fairchild Semiconductor: http://www.fairchildsemi.com/

2. View the conditions of use for the web site by following the link on the home
page called Site Terms & Conditions, or by following this link:
http://www.fairchildsemi.com/legal/index.html
3. Return to the home page.
4. In the search box, enter the product number QSE773 into the search box, select
“Product Folders and Datasheets” and click “go”. You want the datasheet for
Plastic Silicon Pin Photodiode.
5. You will be presented with several options (download PDF, email for example).
Select how you would like to receive this datasheet.
Slide 19

Phototransistors,

emitter

A phototransistor can be an npn bipolar

transistor with a large base that does not
n
have a lead. When photons hit the base
they create electron/hole pairs, the collector
electrons are drawn to the collector and
the holes are filled with electrons from
the emitter. Thus there is a current from The dark current is
the collector to emitter. typically ~0.1 µA with a
light current of ~1 mA.

6.071 Optoelectronics 19
24 20-21

Go to the manufacturer’s web site to obtain a datasheet of their product. Please follow
these steps:

1. Go to the web site for Fairchild Semiconductor: http://www.fairchildsemi.com/

2. View the conditions of use for the web site by following the link on the home
page called Site Terms & Conditions, or by following this link:
3. http://www.fairchildsemi.com/legal/index.html
Return to the home page.
4. In the search box, enter the product number BPW36 or BPW37 into the search
box, select “Product Folders and Datasheets” and click “go”. You want the
datasheet for Hermetic Silicon Phototransistor.
5. You will be presented with several options (download PDF, email for example).
Select how you would like to receive this datasheet.
Slide 22

Uses of Phototransistors
optical receiver tachometer

Once each rotation the slot in

the disk permits light to reach
the phototransistor which then
Light reaching the photo-
shorts the output resistor. The
transistor modulates the base
output is counted for a fixed
of the bipolar transistor. The
period of time to determine the
capacitor blocks current from
rotation frequency.
the DC optical field.
6.071 Optoelectronics 22
Slide 23

Optoisolators

Optoisolators can be used to provide

isolation between components, to avoid
ground loop problems, to control
floating electronics, and to provide DC
shifts.

6.071 Optoelectronics 23
Slide 24

Optoisolators
5V 5 - 12 V 5V

4.7 k 5 - 12 V
1k 1k

Vout
Vout

4.7 k

Which acts as an inverter?

6.071 Optoelectronics 24