Page 53 - Photonics Essentials an introduction with experiments

Photodiodes

Photodiodes 47

a photodiode. In fact, light-emitting diodes behave like photodiodes un-
der illumination. In the inset, we show the diode I–V as captured on a
curve tracer. The graph shows the measurement of the forward char-
acteristic on a semilog scale. There is a region near zero bias where the
log of the current is proportional to the bias voltage. The slope of the
curve in this region is used to calculate n. In this case, n = 1.38.
The photocurrent is given accurately by Eq. 3.14, and you should be
able to verify the linear dependence of the photocurrent on the inci-
dent light intensity over several orders of magnitude of incident light
intensity.

3.3 Photodiode Operation: The Photocurrent Mode
and the Photovoltage Mode

Photodiodes can be operated in two modes: as a source of photocur-
rent or as a source of photovoltage. In the photocurrent mode, the
photodiode is given a reverse bias that is large enough to put the
diode in the voltage-independent dark current regime. In Fig. 3.4 you
can see that 0.5 volts would be sufficient. The dependence of photocur-
rent on photon flux has already been given in Eq. 3.15.
In the photovoltaic mode, the output of the diode is measured with
a high-impedance voltmeter so that the photocurrent is near 0. This
mode of operation has the advantage that it is about the simplest way
to derive a signal from a photodiode. It requires, in principle, no other
circuit elements. In some real applications, this may be all that is
needed. Setting the current = 0 in Eq. 3.14,
2
qD e n i qV A /kT
qG L L e = (e – 1)
L e N A
2
N A G L L e
qV A /kT
e = + 1 (3.17)
2
D e n i
where V A is the photovoltage generated by the photodiode. Solve for
V A by taking the natural log of both sides:
kT N A G L L e 2
V A = ln + 1 (3.18)
q D e n i 2
This equation shows that the photovoltage is not a linear function of
photon flux. There are some important practical implications of this
result. When used in the photovoltage mode:

a. The photovoltage response is nonlinear.
b. The response is not easy to model for correction.

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