138 CHAPTER 4. OPTICAL RECEIVERS
Figure 4.4: Response of ap–nphotodiode to a rectangular optical pulse when both drift and
diffusion contribute to the detector current.
written as
τRC=(RL+Rs)Cp, (4.2.2)
whereRLis the external load resistance,Rsis the internal series resistance, andCpis
the parasitic capacitance. Typically,τRC∼100 ps, although lower values are possible
with a proper design. Indeed, modernp–nphotodiodes are capable of operating at bit
rates of up to 40 Gb/s.
The limiting factor for the bandwidth ofp–nphotodiodes is the presence of a dif-
fusive component in the photocurrent. The physical origin of the diffusive component
is related to the absorption of incident light outside the depletion region. Electrons
generated in thep-region have to diffuse to the depletion-region boundary before they
can drift to then-side; similarly, holes generated in then-region must diffuse to the
depletion-region boundary. Diffusion is an inherently slow process; carriers take a
nanosecond or longer to diffuse over a distance of about 1μm. Figure 4.4 shows how
the presence of a diffusive component can distort the temporal response of a photodi-
ode. The diffusion contribution can be reduced by decreasing the widths of thep- and
n-regions and increasing the depletion-region width so that most of the incident opti-
cal power is absorbed inside it. This is the approach adopted forp–i–nphotodiodes,
discussed next.
4.2.2 p–i–nPhotodiodes
A simple way to increase the depletion-region width is to insert a layer of undoped
(or lightly doped) semiconductor material between thep–njunction. Since the middle