Concerning drift and diffusion as the driving forces for carrier motion, ,which of the following statement is correct

Question

BlackTom AI · Accepted Answer

In this question, we are evaluating how drift and diffusion contribute to carrier motion in a semiconductor, distinguishing between majority and minority carriers.
Option 1: 'Drift is the dominant mechanism for both majority and minority carrier currents.' This is incorrect because while drift from the electric field is indeed crucial for majority carriers, minority carriers often move primarily due to diffusion created by concentration gradients, especially under low-field or steady-state conditions. The statement overgeneralizes drift as dominant for both types of carriers.
Option 2: 'Diffusion is the dominant mechanism for both majority and minority carrier currents.' This is misleading because majority carriers experience significant drift under applied fields, which tends to dominate the current for those carriers. Diffusion is important for minorities and under certain conditions, but not the sole or dominant mechanism for majority carriers, making this option inaccurate.
Option 3: 'Drift is the dominant mechanism for the majority carrier current and diffusion is the dominant mechanism for the minority carrier current.' This aligns with the standard understanding in semiconductor physics: majority carriers respond strongly to the electric field, producing a drift current that is typically larger for them, while minority carriers largely contribute through diffusion due to their concentration gradients, especially when the field is weak or when injection creates gradients.
Option 4: 'Diffusion is the dominant mechanism for the majority carrier current and drift is the dominant mechanism for the minority carrier current.' This flips the conventional roles and is incorrect because the majority carrier current is not typically diffusion-dominated; it is the drift component that dominates due to the presence of the electric field acting on a large concentration of majority carriers, whereas minority-carrier diffusion arises from their concentration gradients.

Putting these pieces together, the scenario described in option 3 correctly captures the established behavior: majority-carrier currents are mainly drift-driven because of the applied field acting on a large population, while minority-carrier currents are mainly diffusion-driven due to concentration gradients when drift effects are smaller for these less abundant carriers. The other options misattribute dominance to diffusion for majority carriers, or to drift for minority carriers, which contradicts the typical carrier transport picture in semiconductors.

EESM5200 (L1) Part I: Topic 6 - Carrier Actions in a PN Junction

Concerning drift and diffusion as the driving forces for carrier motion, ,which of the following statement is correct

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