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Figure 14.21 Circuit model of a real photodiode.

Figure 14.21 Circuit model of a real photodiode.

The open-circuit voltage of the photodiode is found by setting IL = 0 in Equation 14.45:

The voltage depends on the Iv/I0 ratio. Both currents are proportional to the cross section of the junction; their ratio, therefore, does not depend on the cross section. It depends on the photon flux (the light power density) and on the characteristics of the diode. Typically, this ratio is of the order of 107 at one sun (1000 W/m2) in silicon. At 300K, this corresponds to an open-circuit voltage of 0.42 V.

Owing to its logarithmic dependence, the open-circuit voltage varies slowly with changes in light power density and is not a convenient photometric measure.

The V-I characteristic of a photodiode can be obtained by plotting Equation 14.45 as shown in Figure 14.22. The currents were normalized by dividing by I0. Three different illumination levels are displayed, corresponding to Iv/I0 of 106, 5 x 106, and 107.

For the highest level of illumination in the figure, in addition to the characteristics for diodes with no series resistance, we have also plotted the curve for a diode with normalized resistance of Rs = 7.5 x 10~9/I0 ohms.

The power a photodiode delivers to a load depends on, among other things, the value of the load resistance. The optimum operating point occurs at a specific voltage, Vm, and a corresponding current, Im (see derivation further on). These points are indicated by the small black circles in Figure 14.22.

The area under the curves has the dimension of power. The large darker gray rectangle represents the Vm x Im product (the maximum power the diode can deliver when an ideally matched load is used), whereas the lighter gray rectangle represents the Voc x Iv product. The ratio between these two areas (or the two powers) is called the fill factor, Ff. Both gray areas are for the case of Iv/I0 = 107, which is the highest level of

Figure 14.22 V-I characteristic of a typical photodiode.

illumination appearing in the figure. Vm and Im are fo series resistance.

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