Page 60 - Photonics Essentials an introduction with experiments

Photodiodes

54 Photonic Devices

Quantum efficiency is constant
Optical power is constant

Photocurrent

Energy Gap

, Increasing wavelength

Figure 3.9. Responsivity gives the photocurrent that results from a fixed incident opti-
cal power. Since it takes more photons to produce 1 watt of optical power as the photon
wavelength increases, the responsivity will also increase as the wavelength increases,
provided, of course, that the quantum efficiency stays constant.

Optical reflection occurs at the photodiode surface because the index
of refraction of the semiconductor (n 3.4) is different from the re-
fractive index of air (n = 1). Fresnel’s equation can be used to calculat-
ed the required reflection coefficient. Frensel’s equation can be ap-
plied if the interface between the semiconductor surface and air is flat
and planar over many wavelengths distance. For light impinging on
the photodiode at normal incidence, the reflection coefficient (Fres-
nel’s law) is calculated as follows:

E R n 1 – n 2
= (3.29)
E i n 1 + n 2
where E R and E i are the amplitudes of the reflected and the inci-
dent light beams, respectively. The reflection coefficient is given by
the square of this ratio:

(n 1 – n 2 ) 2 4
R = = 25% (3.30)
(n 1 + n 2 ) 2 16
In the case of a photodiode having a planar surface, the maximum
possible quantum efficiency for any kind of semiconductor detector is
actually only 75%.
If you introduce a third layer, situated in between the semiconduc-

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