p–i–n Photodiodes

Definition: photodiodes with a p–i–n semiconductor structure

A p–i–n photodiode, also called PIN photodiode, is a photodiode with an intrinsic (i) (i.e., undoped) region in between the n- and p-doped regions. Most of the photons are absorbed in the intrinsic region, and carriers generated therein can efficiently contribute to the photocurrent. In Figure 1, the electrodes are shown in black: the cathode is a flat electrode, whereas the anode has the form of a ring (of which two opposite parts are seen in the shown cross section). The positive pole of the (reverse) bias voltage is connected to the cathode. On top of the p region, there is an anti-reflection coating.

Compared with an ordinary p–n photodiode, a p–i–n photodiode has a thicker depletion region, which allows a more efficient collection of the carriers and thus a larger quantum efficiency, and also leads to a lower capacitance and thus to higher detection bandwidth.

The most common p–i–n diodes are based on silicon. They are sensitive throughout the visible spectral region and in the near infrared up to ≈ 1 μm. At longer wavelengths, the absorption efficiency and thus the responsivity drops sharply, but the parameters of this cut-off depend on the thickness of the i region.

For longer wavelengths up to ≈ 1.7 μm (or with extended spectral response up to 2.6 μm), InGaAs p–i–n diodes are available, although at significantly higher prices (particularly for large active areas). Germanium p–i–n diodes can be an alternative.

The fastest p–i–n photodiodes have bandwidths of the order of tens of gigahertz. Their active areas typically have a diameter of only a few hundred microns. Some of them are available in fiber-coupled form and can be applied e.g. in receivers for optical fiber communications.

See also: photodiodes, metal–semiconductor–metal photodetectors

Category: photonic devices