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Photodiodes
56 Photonic Devices
Antireflection
coated photodiode
Responsivity (amps/watt)
Uncoated photodiode
, Increasing wavelength
Figure 3.10. Quantum efficiency can be improved by reducing or eliminating reflec-
tions at the wavelength of interest. This will also result in increasing the reflection
losses at other wavelengths.
emissivity surface. This means that the reflection coefficient is rela-
tively elevated. The emissivity of the surface can be raised by roughen-
ing the surface so that it looks more matte, eventually appearing like
black velvet. Such a surface has very low reflectance. If the roughening
is done carefully, the minority carriers that are generated when the
light is absorbed will be collected at the junction, creating a photocur-
rent. Achieving such surface roughening for a photodetector is not a
simple task. The inverse property, the case of a light-emitting diode, is
easier to implement and is discussed in Chapter 6.
3.5 Summary
A photon can be absorbed by a semiconductor if the energy of the pho-
ton exceeds the band gap energy. The absorption of a photon creates
an electron and a hole and increases the nonequilibrium concentra-
tion of minority carriers. If this absorption takes place in a p-n junc-
tion structure, the minority carriers will diffuse to the junction, creat-
ing a photocurrent and a photovoltage at the contacts.
The photocurrent can be calculated using the same approch as that
used to calculate the current–voltage characteristic in a p-n junction.
The results show that the photocurrent varies linearly with the flux of
photons, and is independent of the photon energy as long as the pho-
ton energy exceeds the band gap. The photovoltage generated in open
circuit operation does not vary linearly with the flux of photons.
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