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A G R E AT E R M E A S U R E O F C O N F I D E N C E built-in potential or equilibrium potential)
across an area called the depletion region (or
layer). Reverse biasing the photodiode causes
this potential to increase, and the depletion
region to expand. Photons of sufficient en-
ergy impinging on this region will create
electron-hole pairs, which will separate due
to the combined equilibrium potential and
externally applied reverse bias. These charg-
es will quickly drift away from the junction
and be collected by the electrodes. Drift cur-
Photodetectors--Choose rent generated by photon absorption in the
depletion region is the main component of
and Use Wisely for Best
photocurrent. It also is the component with
the fastest response time.
Another component of photocurrent is
Results in Pulsed Laser called diffusion current. This component
originates from charge carriers created by
absorption of photons outside the depletion
Diode Test Systems layer. While the majority of these charge
carriers will recombine in the neutral region,
some of them may manage to diffuse slowly
to the junction and make a contribution to
the photocurrent. This contribution is not
Helga Alexander, Keithley Instruments, Inc. welcome when a fast photodiode response is
desired.
Designing for Fast Response. For fast
response time, a photodiode has to be de-
Testing laser diodes for fiberoptic communi- of these photodetectors involves a number of signed such that most of the photon absorp-
cation systems requires a photodetector with compromises, but a fast response time often tion takes place in the depletion layer. Since
a fast response. However, a 10