Page 90 - Photonics Essentials an introduction with experiments

Photoconductivity

84 Photonic Devices

The signal bandwidth will also depend on the RC product of the de-
tector. The RC product is the time required to charge or discharge the
photoconductor electrodes. In many cases, it is the RC product that
determines the bandwidth, not carrier transport properties. Once the
RC product has been determined by measurement or by calculation, it
can be compared to the photoconductor bandwidth. For example, if
the RC charging time is much longer than the transit time of the
slower charge carrier, then photoconductive gain can be exploited
with minor bandwidth degradation.

5.4 Engineering Photoconductivity
In the previous section, we considered the photoconductive effect that
occurs when an electron is excited by the absorption of a photon and
moves from the valence band to the conduction band. This is the
mechanism exploited in many photoconductive detectors, but it is not
the only one. Photoconductivity will result whenever light acts to
move a charge carrier from a relatively immobile state to one in which
the carrier can be transported by the drift of an applied electric field.
The current density in the sample can be written as
J = q (N e e + N h h ) (5.6)

For the case of photon absorption across the band gap, the number of
excess electrons created is equal to the number of photons created and
the mobilities remain constant:
J phot = q [(N e + n) e + (N h + n) h ] (5.7)
A photoconductive detector made from silicon and based on this
mechanism would have a useful sensitivity to photon wavelengths
up to 1100 nm, corresponding to the silicon band gap energy. How-
ever, we can dope silicon with an impurity element. The presence of
this impurity creates an impurity level that lies inside the band gap.
The impurity level acts like a trap and attracts electronic charges in
the same way that potholes in the road collect water after a rain-
storm.
The basic function of the semiconductor in this case is two-fold. It
acts as a host for the impurity, and it assures the transport of ionized
charge carriers into to the conduction band or valence band. The pho-
ton energy required to create a charge carrier is now a fraction of the
band gap energy. For example, the energy required to ionize an elec-
tron of gold-doped silicon is about 0.15 eV. Each impurity element has
its own particular energy level, so that doping introduces the possibil-
ity of tuning the spectral response. The maximum doping level that
can be introduced depends on the element involved, but remains

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