
95-6 Displays and Recorders
such that η = τ/τ
r
. e reason that a fast recombination time is crucial is that the longer the carrier
re
mains
in an ex
cited
st
ate,
th
e
la
rger
th
e
pr
obability
th
at
it wi
ll
gi
ve
ou
t
en
ergy
no
nradiatively
. In or
der
fo
r
th
e
in
ternal
qu
antum
e
ciency
to be hi
gh,
th
e
ra
diative
li
fetime
τ
r
needs to be small. For indirect
ba
nd-gap
se
miconductors,
τ
r
≫ τ
nr
so that very little light is generated, and for direct band-gap semi-
conductors,
τ
r
increases with temperature so that the internal quantum eciency deteriorates with the
te
mperature
.
95.4 LED Response Time
As long as the LEDs are used as display devices, it is not too i ...