Control of the decay time
Now we continue with time dependend conditions: The figure shows the
simulation of the temporal evolution of the total voltage drop at different constant currents. A laser pulse is
simulated by a numerical switching to a higher generation rate for a well defined duration (shaded area). If the
generation rates are choosen not too high and at low sample currents (small signal regime) the decays of the
photoconductive signals are exponential. But even in this situation the relative photoconductive response can be in
the order of 50% and more. The huge signals are not only because of the additional carriers in the n-layer but
mainly because of the gradual switching of the p-layers in parallel to the n-layer due to the excess carrier induced
change of the potential modulation. Increasing the external current first tends to increase the decay times because of
the shrinking recombination area as already discussed above . At the same time the
potential modulation near the negative contact is reduced, which overcomes the shrinking effect at higher currents
providing an effective recombination channel. This finally leads to a drastical reduction of the decay times. Looking
at the shape of the transients, we see a strongly non exponential behaviour: Immediately after the excitation the bias
voltage is low and the sample behaves like in the small signal regime (long decay time). As the bias voltage
recovers with time the effective recombination near the negative contact starts to work resulting in a fast final
decay. The efficiency of the recombination at the negative contact can be controlled by an additional current
supplied directly into the p-layers, which in our case is provided by a break-through of the positive selective contact.
This current has to be redistributed to the n-layer in order to leave the sample at the negative contact and thus
causes a further reduction of the potential modulation near this contact. The whole process can be interpreted as a
kind of self accelerating mechanism which starts with the usual nipi behaviour but finally turns to a decay which is
very fast as compared to the small signal decay timeconstant.
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