The current through a PN junction under illumination can be written as,

Itotal = Idiode - Iphoto

where Idiode is the current given by the ideal diode equation and Iphoto is the optically generated current. When the PN junction is operated as a solar cell, which if the following statement is correct.

Question

The current through a PN junction under illumination can be written as,

Itotal = Idiode - Iphoto

where Idiode is the current given by the ideal diode equation and Iphoto  is the optically generated current. When the PN junction is operated as a solar cell, which if the following statement is correct.

BlackTom AI · Accepted Answer

The question presents the current through a PN junction under illumination as Itotal = Idiode - Iphoto, with Idiode given by the diode equation and Iphoto representing the optically generated current. We will evaluate each option in light of this relationship.

Option 1: 'Itotal is positive' — This claim asserts that the total current is always positive. However, because Itotal depends on the balance between Idiode and Iphoto via Itotal = Idiode - Iphoto, Itotal can be negative if Iphoto exceeds Idiode (for example, at certain operating voltages or illumination levels). Therefore, asserting that Itotal is always positive is not generally correct.

Option 2: 'Idiode is positive' — In the standard diode current convention, Idiode is the current flowing through the diode in the forward direction, which is taken as positive by the diode equation. The optically generated current Iphoto enters with a minus sign in Itotal, i.e., Itotal = Idiode - Iphoto. This means Idiode is positive by definition and serves as the forward component whose magnitude is reduced by the photo-generated current. This option aligns with the conventional sign convention of the diode current.

Option 3: 'The magnitude of Itotal is larger than the magnitude of Iphoto' — Since Itotal = Idiode - Iphoto, the magnitude of Itotal depends on the relative sizes of Idiode and Iphoto. There are operating points where Idiode is smaller than Iphoto, yielding |Itotal| < |Iphoto|, and other points where |Itotal| could be larger. Therefore, this statement is not universally true and cannot be taken as a general rule.

Option 4: 'The magnitude of Idiode is larger than the magnitude of Iphoto' — While it might be true in some operating regimes, it is not guaranteed across all illumination levels and voltages. The diode current and the photocurrent can vary independently with voltage and illumination, so asserting a universal magnitude relation is not justified.

In summary, analyzing the definitions and sign conventions shows that the most consistently correct statement given the formulation Itotal = Idiode - Iphoto is that Idiode is positive, since it is defined by the diode equation with forward-directed current as the positive reference. The other options either claim a universal positivity of Itotal, a universal magnitude relation, or an unconditional comparison of magnitudes that may not hold across all operating conditions.

EESM5200 (L1) Part I: Topic 9 - Optical Properties of PN Junction

The current through a PN junction under illumination can be written as, Itotal = Idiode - Iphoto where Idiode is the current given by the ideal diode equation and Iphoto is the optically generated current. When the PN junction is operated as a solar cell, which if the following statement is correct.

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