Page 50 - Photonics Essentials an introduction with experiments

Photodiodes

44 Photonic Devices

Figure 3.3. The current–voltage characteristic of the photodiode model around V = 0.
Note that the current is 0 when the voltage is 0 only for the case when the photon flux
is also 0. This situation is indicated by the arrow.

that passes through the origin of 0 current, 0 voltage is the result for
0 photon flux. This point is indicated by the arrow in Fig. 3.3. The
presence of the I–V characteristic in the fourth quadrant of the graph
has important physical significance. It means the photodiode is gener-
ating useful power. The amount of power is given by the area of the
curve in this quadrant. This feature of all photodiodes is the basis for
the photovoltaic solar cell, which is a photodiode optimized for con-
verting photons (from the sun, for example) into electrical power.
Real photodiodes behave like this. In Fig. 3.4 we show the I–V char-
acteristic of an “off-the-shelf ” silicon photodiode subjected to illumi-
nation. The photocurrent is a linear function of light intensity over
many orders of magnitude.
In the laboratory, you can make measurements of the current ver-
sus voltage. When you compare your experimental results to the theo-
retical model, you will find that you can deduce the correct value for
the built-in voltage, but you will also find that the reverse current is
larger than your predictions, and that the forward current is smaller.
The reverse or “dark current” is increased by leakage paths that are
introduced by defects and impurities, as well as by device processing.
The forward current is limited by series resistance introduced by the
resistivity of the neutral p and n regions. Recombination of minority
carriers will also lead to a reduction in current below its expected val-

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