Photovoltaic detectors are junction-based semiconductor devices. Photovoltaic detectors can be further classified as operating in the photovoltaic mode or the photoconductive mode. When a p-n junction is formed, a carrier concentration gradient exists because of the majority carriers in the p-type and n-type regions.
Excess electrons from the n-type region diffuse into the p-type region and the excess holes diffuse in the opposite direction in order to equalize this gradient.
The movement of charges across the junction generates an electric field E within the photodiode as indicated in Fig. 5.39a, which prevents any further diffusion of carriers.
When the energy of a photon that impinges on the photodiode is equal to or greater than the energy bandgap, electron-hole pairs are generated in the regions shown in Fig. 5.39a. In the depletion region, the electrons move toward the n side and the holes toward the p side under the influence of the electric field E.
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| Figure 5.39 Photovoltaic detectors. (a) Energy-level diagram for a p-n photodiode showing the transduction mechanisms. (b) I-V characteristics of photovoltaic detectors. |
On the p side, only the electrons move toward the n side and the hole is preventednfrom moving by the field. On the n side, the situation is reversed. The accumulation of charges on either side of the junction results in an open-circuit voltage Voc that is proportional to the photon input. Figure 5.39b shows the I-V characteristics of a photovoltaic detector. Without any photon input, the I-V plot is identical to that of a rectifying diode.
As the photon input increases, the plot is successively displaced downward. For any photon input level, the open-circuit voltage Voc is proportional to that input.
If the output terminals are shorted, the short circuit current Isc is also proportional to the photon input. This corresponds to operation in the photovoltaic mode, as indicated by the fourth quadrant of the I-V plot.
When a reverse bias is applied to the photovoltaic detector, the load line is displaced to the third quadrant by an amount equal to the reverse bias. In this
quadrant, the photovoltaic detector behaves as a current source and is said to operate in the photoconductive mode.
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