Page 98 - Photonics Essentials an introduction with experiments

Photoconductivity

92 Photonic Devices

cause the traps act like reservoirs for carriers that are excited and
move into the conduction band or the valence band. Electrons can re-
combine with holes at recombination centers, and they need to be free
to move in space to find the recombination center. A carrier in a trap
does not have this mobility, and it can recombine only by being ther-
mally excited and moving into the conduction or valence band. The re-
sponse time is associated with the time it takes for this movement
into a band plus the subsequent relaxation time. For electrons, this
can be expressed as
n t
T res = 1 + n (5.14)

n
where n t is the density of shallow electron traps, and n is the density
of excited carriers moving into the conduction band. There is an anal-
ogous expression for holes. Equation 5.14 makes it explicit that the
response time of a photoconductor will depend on the excitation level,
even though the carrier lifetime remains unchanged. In this section
we will take this idea one step further and show how the carrier life-
time can be manipulated, and, in particular, how the carrier lifetime
can be increased, thereby increasing the sensitivity of the photodetec-
tor.
The density of excited carriers is determined by the absorption rate
of photons and the carrier lifetime:

n = f cm –3
And the photocurrent is given by
n f
I = q = q
T transit T transit
We take the absorption rate of photons to be equal to the generation

Figure 5.14. Schematic energy level diagram for a photoconductor with one type of re-
combination level.

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