
44-1
44.1 Introduction
Photojunction sensors (photodiodes and phototransistors) are semiconductor devices that convert the
el
ectrons
ge
nerated
by th
e
ph
otoelectric
e
ect
in
to
a de
tectable
el
ectronic
sig
nal
. e ph
otoelectric eect
is a ph
enomenon
in wh
ich
ph
otons
lo
se
en
ergy
to el
ectrons
in a ma
terial
. In th
e
ca
se
of a se
miconductor,
wh
en
th
e
en
ergy
of an in
teracting
ph
oton
(hv) ex
ceeds
th
e
en
ergy
of th
e
se
miconductor
ba
ndgap
(E
g
),
th
e
en
ergy
ab
sorbed
ca
n
pr
omote
an el
ectron
fr
om
th
e
va
lence
ba
nd
to th
e
co
nduction
ba
nd
of th
e
ma
te-
rial
. i
s
ca
uses
th
e
fo
rmation
of an el
ectron–hole
pa
ir
. In th
e
pr
esence
of an el
ectric
e
ld,
th
ese
ch
arges
dr
i