Page 49 - Photonics Essentials an introduction with experiments

Photodiodes

Photodiodes 43

2
qD e n i
J = – – qG L L e (3.15)
L e

Dark Photocurrent
current
The dark current is just the reverse bias leakage current that re-
mains when the level of light is reduced to 0. The photocurrent can
easily exceed the dark current for modest photon fluxes. In this situa-
tion, it is much easier to measure the photocurrent.
In Figs. 3.2 and 3.3, we show the result of evaluating Eq. 3.14 for sev-
eral values of photon flux G L . The photocurrent is easily resolved for all
reverse bias greater than –1 volt. Near 0 volts, the photocurrent is still
resolvable, but the distinction between the various curves disappears
rapidly as the diode becomes forward biased because of the dominance
of the diffusion current imposed by the forward bias voltage.
In Fig. 3.3, we expand these data around the origin. The values of
photon flux are the same as those in Fig. 3.2. Note that the only curve

Figure 3.2. The current–voltage characteristic according to the photodiode model of Eq.
3.14. Note the presence of a dark current equal to –1 × 10 –12 amps. The photocurrent in-
creases the negative current in proportion to the photon flux. The photocurrent does
not depend on the reverse bias voltage.

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