AEG Electrolux ERN 2922 User Manual download pdf (Page 50)

1111111/

ELECTRON/CS NOTEBOOK

1IIII!=111

By Forrest

Miros

III

Light-

emitting diodes

can double as both

emitters and detectors of light. This

dual

capability makes

possible several unique

applications. In

this column, I

will

de-

scribe

several applications for LEDs

op-

erated as detectors. First, however, let's

review

the background

of devices that

both emit and detect light.

Detectors

that

Emit Light

A few months before

graduating

from

high school in the spring of 1962, it oc-

curred

to me that semiconductors that

detect light might also emit

light. I

decid-

ed to test this

hypothesis by passing a

cur-

rent through a bulk semiconductor, a

thin layer of cadmium- sulfide that

formed the light- sensing region of a pho-

toresistor.

When

the current

from a flashlight

cell

failed to

stimulate

the emission of pho-

tons, I decided more electromotive

force

might

do the

trick.

Since

I had been

ex-

perimenting

with

a spark coil,

connect-

ed the CdS cell to the output

leads

of the

spark coil and switched on the power.

This

time, the entire

zig -zag pattern of

CdS

emitted a greenish glow. Of much

more significance

were

tiny but bright

spots of flickering green light.

Since CdS has a peak spectral response

the green range,

was

convinced that

the green emission

was

not merely

visu-

al by- product

of the high -voltage spark

discharge but

was

what

physicists call

"recombination radiation." In other

words,

the green emission

was

the direct

result

of electrons

within

the CdS being

stimulated to a

higher-

than

-normal

ener-

gy level

by the

high -voltage discharge.

When

the electrons

resumed their normal

levels (recombined), they emitted

pho-

tons (radiation).

On November 6, 1966,

I repeated

this

experiment

with

single -crystal

platelets

of cadmium -sulfide provided

by the Uni-

versity

of Colorado.

The brittle,

yellow-

ish platelets produced green

flashes that

were

significantly brighter

than those

Using

LEDs

Detectors

SouRCE

SETE

CTO

SouRCE/

DETECTOR

Y SPLICE

L ONVEUT

low AL

SOURCE

0E1-EC-I'D

DUAL-

FUNCTION

SOURCE/ DETECTOR

SOURCE/

DETECTOR

Fig.

1. Conventional

and simplified

bidirectional

fiber links.

produced in the earlier experiment,

which

also

repeated.

Earlier,

March 14, 1966, I

conduct-

ed an experiment that eventually led

confrontation

with

Bell Laboratories. In

this experiment,

connected a surplus sil-

icon solar cell to the output

of a two

transistor audio- frequency oscillator. An

identical

cell

was

connected to the input

of an audio amplifier.

The first

cell pro-

duced

pulses of

near -infrared

radiation

that were

detected by the second cell.

This experiment demonstrated that

half -duplex optical communications

could be accomplished over a coaxial

path

with

a single semiconductor device

at either end of the

link. When

one solar

cell

was

transmitting, the other

would

function

as a receiver. Their roles

would

then

reversed for

communication

the opposite direction.

Emitters that Detect Light

Most semiconductor

junctions emit

photons

when

forward biased. This is the

principle that makes possible

visible

-light

and near -infrared emitting diodes. The

most efficient semiconductor light emit-

ters are made from single- crystal alloys

gallium

and arsenic (GaAs)

and alumi-

num, gallium and arsenic

(AIGaAs).

But

even silicon solar cells, as

noted

above,

and both

germanium

and

silicon diodes

GaAs

LED

GaAs

LED

Loo

!SO

zoo

DOWER

("W)

Fig. 2. Response

of GaAs and GaAs:Si

LEDs in detector mode.

and transistors emit near -infrared radia-

tion

when

forward biased.

The efficiency at

which

silicon and ger-

manium pn junctions transform

a cur-

rent

into

an electromagnetic

wave

is con-

siderably

lower than that of

diodes de-

signed

specifically

for this purpose.

Moreover,

many diodes

designed ex-

pressly as efficient

photon

generators

function

well

as photon

detectors.

In 1972, I

conducted a series of experi-

ments

to determine

GaAs

LEDs

and

lasers

and GaAs:Si LEDs could double as

MODERN

ELECTRONICS

/ February 1988

Say You Saw It In Modern

Electronics

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