Page 59 - Photonics Essentials an introduction with experiments

Photodiodes

Photodiodes 53

Example 3.1
Suppose we measure I photocurrent in amperes, in joules, q in coulombs,
and P optical in watts. In our measurement = 1 eV = 1.6 × 10 –19 joules,
and we determine that the quantum efficiency is unity:
I photocurrent · (1.6 × 10 –19 ) I photocurrent
Q = = = 1 (3.26)
P optical · (1.6 × 10 –19 ) P optical
This means that 1 watt of optical power will produce 1 ampere of
photocurrent when the quantum efficiency is 100% and the photon
energy is 1 eV (optical wavelength = 1.24 m in air).
Note that if you measure I photocurrent in amperes, in eV, q = 1 elec-
tron, and P optical in watts, the result is the same!

The ratio of the photocurrent to the optical power can be thought of
as the transfer function for the photodiode. The ratio is called the re-
sponsivity. The responsivity is not the same thing as the quantum ef-
ficiency. What is more important, the two are not proportional:

Q · q
I photocurrent
Responsivity = R = = amps/watt (3.27)
P optical
at
= 1.24 m, where E = = 1 eV. R = 1 amp/watt implies that Q
= 1.0.
However, please note that at
= 0.62 m, where E = = 2 eV,
R = 0.5 amp/watt implies that Q = 1.0 (3.28)
When the photon energy is higher, it takes fewer photons to create
the same optical power. The photocurrent is proportional to the num-
ber of photons, and is not related to the energy of the photons (provid-
ed the energy is at least greater than the band gap energy).
If you make a plot of the photocurrent versus optical wavelength, you
will find that the photocurrent drops as the wavelength gets shorter,
even though the optical power and the quantum efficiency remain con-
stant (see Fig. 3.9() The responsivity is a parameter of photodiode per-
formance that is commonly found on a photodiode data sheet. It must be
cited for a particular wavelength or the number is meaningless. Some
handy reference points to remember are the following: For 100% quan-
tum efficiency, at
= 1.24 m, R = 1 amp/watt. At
= 0.62 m, R = 0.5
amp/watt.
The quantum efficiency of a well-designed photodiode is near 100%.
There are two things that can degrade the quantum efficiency:

1. Optical reflection—some photons just do not get in the diode.
2. Recombination—some photocarriers just do not make it to the
junction.

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