Page 11 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Photodetection Basics

4 Chapter One

Anode becomes
positive
Light A
V
K

Any glass-encapsulated Similar LEDs detect
silicon diode (e.g., 1N4148) their own light
or LED
Figure 1.3 Any diode, even a silicon rectifier, can show photosensitivity if the
light can get to the junction. LEDs generate higher open circuit voltages than
the silicon diode when illuminated with light from a similar or shorter wave-
length LED.

Rather more efficient are light emitting diodes (LEDs), having been designed to let
light out of (and therefore into) the junction. Any common LED tested in this way
will show a similar positive voltage on the anode. LEDs have the advantage of a higher
open-circuit voltage over silicon diodes and photodiodes. This voltage gives an indi-
cation of the material’s bandgap energy (E g, see Table 1.1). Although with a silicon
diode (E g ª 1.1V) you might expect 0.5V under an ordinary desk lamp, a red LED (E g
ª 2.1V) might manage more than 1V, and a green LED (E g ª 3.0V) almost 2V. This
is sufficient to directly drive the input stages of low voltage logic families such as 74
LVC, 74AC, and 74HC in simple detection circuits.
This works because the desk lamp emits a wide range of energies, sufficient to gen-
erate photoelectrons in all the diode materials mentioned. However, if the photon
energy is insufficient, or the wavelength is too long, then a photocurrent will not be
detected. My 470-nm blue LEDs generate negligible junction voltage under the desk
lamp. Try illuminating different LEDs with light from a red source, such as a red-
filtered desk lamp or a helium neon laser. You should detect a large photovoltage with
the silicon diode, and perhaps the red LED, but not with the green LED. The bandgap
in the green emitter is simply too large for red photons to excite photoelectrons. You
can take this game even further if you have a good selection of LEDs. My 470-nm LED
gets 1.4V from a 660-nm red LED as detector but nothing reversing the illumination
direction. Similarly the 470nm generates 1.6V from a 525-nm emitting green LED
but nothing in return. These results were obtained by simply butting together the
molded LED lenses, so the coupling efficiency is far from optimized. The above
bandgap model suggests that LED detection is zero above the threshold wavelength
and perfect below. In reality the response at shorter wavelengths is also limited by
excessive material absorption. So they generally show a strongly peaked response only
a few tens of nanometers wide, which can be very useful to reduce sensitivity to inter-
fering optical sources. See Mims (2000) for a solar radiometer design using LEDs as
selective photodetectors. Most LEDs are reasonable detectors of their own radiation,
although the overlap of emission and detection spectra is not perfect. It can occa-
sionally be useful to make bidirectional LED–LED optocouplers, even coupled with
fat multimode fiber. Chapter 4 shows an application of an LED used simultaneously
as emitter and detector of its own radiation.

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