Q.In an unbiased p-n junction, holes diffuse from the p-region to n-region because

The key idea is that diffusion in a p-n junction is driven by a concentration gradient, not by electric fields or attraction.

1. In an unbiased junction, there is no external potential difference, so option (b) is false.
2. Free electrons in the n-region do attract holes, but this attraction is balanced by the built-in field after the depletion region forms — it is not the cause of initial diffusion. Option (a) is misleading. …

Why this question matters — and the common trap

This is a classic Class 12 Physics question on semiconductor diodes. The key is to separate diffusion (driven by concentration difference) from drift (driven by electric field). Many students mix these up, especially when they hear "potential difference" or "attraction" — those belong to the drift mechanism, not the initial diffusion.

Let’s build the picture from scratch.

Formation of an unbiased p-n junction

When a p-type semiconductor (excess holes) and an n-type semiconductor (excess electrons) are joined, there is no battery or external voltage — it is unbiased. At the instant of joining:

• The p-side has a very high concentration of holes ($\approx 10^{16} \text{–}10^{18} \text{ cm}^{-3}$).
• The n-side has a very low concentration of holes (only thermally generated, $\approx 10^{6} \text{–}10^{10} \text{ cm}^{-3}$).

Nature abhors a steep gradient. Particles always move from a region of higher concentration to a region of lower concentration — this is diffusion. So holes rush from p to n, and electrons rush from n to p.

Step-by-step reasoning

1. Identify the driving force for diffusion

   Diffusion is a statistical process. In the p-region, holes are densely packed; in the n-region, they are scarce. Random thermal motion causes more holes to cross from the high-density side to the low-density side than the reverse. The net movement is from p to n.

2. Why option (a) is wrong

   "Free electrons in the n-region attract them" — this describes an electrostatic attraction. But in an unbiased junction, before any charge movement, there is no electric field. Attraction would require opposite charges to already be separated, which hasn't happened yet. After diffusion begins, the exposed immobile ions do create a field, but that field opposes further diffusion (it's the built-in potential). So attraction is not the cause; it's a consequence that eventually stops diffusion.

3. Why option (b) is wrong …

Method: Concentration-Gradient-Driven Diffusion

The correct answer is (c) — holes diffuse from the p-region to the n-region because the hole concentration in the p-region is more than in the n-region.

Step-by-step reasoning

Step 1: Identify the driving force for diffusion

In any system, particles move from a region of higher concentration to a region of lower concentration due to random thermal motion -- this is diffusion, the same everyday process by which a drop of ink spreads through water.

Step 2: Apply to the p-n junction

In an unbiased p-n junction:

• p-side: high concentration of holes (majority carriers)
• n-side: very low concentration of holes (minority carriers)

The steep concentration gradient at the junction drives holes to diffuse from p → n.

Step 3: Eliminate the wrong options

• (a) Free electrons in the n-region do attract holes, but this is electrostatic attraction, not the primary cause of diffusion. Diffusion happens even without attraction — it's purely statistical. …

The most common mistakes here come from mixing up drift with diffusion, and from confusing the cause of diffusion with the effect of the built-in field.

Mistake 1: Picking (a) — "free electrons in the n-region attract them"

Students imagine opposite charges attracting and think that's why holes move. But in an unbiased junction, there is no external field. The holes move because of a concentration gradient, not because they are pulled by electrons. In fact, once the junction forms, the electrons near the junction actually repel holes (the built-in field opposes further diffusion). So attraction is the wrong mechanism.

Mistake 2: Picking (b) — "they move across the junction by the potential difference"

This is backwards. The potential difference (built-in voltage) does not exist until after diffusion has already happened. Diffusion creates the space-charge region, which then creates the potential barrier. Saying holes move because of the potential difference is like saying water flows downhill because of the riverbed — the riverbed forms after the flow. In an unbiased junction, there is no applied voltage; the only "potential difference" is the result, not the cause.

Mistake 3: Picking (d) — "All the above"

This is a trap. Students see two plausible-sounding options and think "well, both must be true, so all the above." But (a) and (b) are factually wrong. Only (c) is correct.

The correct reasoning:

Diffusion is driven by a difference in concentration. In the p-region, holes are majority carriers (high concentration). In the n-region, holes are minority carriers (very low concentration). Nature tends to equalise concentrations — particles move from high to low concentration. That is the sole reason for diffusion in an unbiased junction. …