A pn junction was formed with a heavily doped (1018cm-3) p-region and lightly doped (1014 cm-3) n-region. Which of the following statement(s) is(are) correct?a)The width of the depletion layer will be more in the n-side of the junction.b)The width of the depletion layer will be more in the p-side of the junction.c)The width of the depletion layer will be same on both sides of the junction.d)If the pn junction is reverse biased, then the width of the depletion region increases.Correct answer is option 'A,D'. Can you explain this answer?

Question

Answer

The depletion region is created due to the charge carrier diffusion across the pn-junction(or movement of charge carriers). Depletion region is the region depleted of charge carriers spread across both sides of a pn junction, which penetrates farther into the less doped side. Here the depletion region is greater for less doped region because only then can we include equal number of impurity atoms on both n-side and p-side. In this question that would be the n side.
Also note that the depletion region on the n-side is made up of donor impurity atoms (negatively charged) and on the p side it is made up of acceptor impurity atoms (positively charged). And both sides have an equal number of impurity atoms.

In reverse biased condition when an external bias voltage is applied to a pn junction, positive to n-side and negative to the p-side, electrons from the n-side are attracted to the positive terminal and in p-side vice versa. Holes on p-side are attracted away from the junction on the n-side and electrons on n-side are attracted away from the junction. This causes the depletion region to be widened or in other words the region depleted of charge increases(and barrier voltage also increases).

Please ask further if you have anymore doubts on the answer written above.

Answer

Depletion Layer in a pn Junction

The pn junction is a crucial component of many electronic devices, such as diodes and transistors. It is formed by joining a p-type semiconductor (with excess holes) and an n-type semiconductor (with excess electrons). When the p-type and n-type materials are brought together, a region called the depletion layer is formed at the junction.

Depletion Layer Width

The width of the depletion layer depends on the doping concentrations of the p and n regions. Doping concentration refers to the number of impurity atoms per unit volume. In this case, the p-region is heavily doped with a concentration of 10^18 cm^-3, while the n-region is lightly doped with a concentration of 10^14 cm^-3.

Effect of Doping Concentration

The depletion layer width is primarily determined by the difference in doping concentrations between the p and n regions. Higher doping concentrations result in a narrower depletion layer, while lower doping concentrations result in a wider depletion layer.

Explanation of Correct Statements

a) The width of the depletion layer will be more on the n-side of the junction.

In this case, the n-region is lightly doped (10^14 cm^-3) compared to the heavily doped p-region (10^18 cm^-3). As a result, the depletion layer in the n-side will be wider compared to the p-side. This is because the lightly doped n-region has fewer free charge carriers (electrons) to neutralize the fixed positive charges of the acceptor impurity atoms in the p-region. Therefore, the depletion layer extends further into the n-side.

d) If the pn junction is reverse biased, then the width of the depletion region increases.

When a pn junction is reverse biased, the positive terminal of the power supply is connected to the n-region, and the negative terminal is connected to the p-region. This reverse biasing increases the width of the depletion layer. The electric field generated by the applied voltage causes the free charges to move away from the junction, widening the depletion region. Therefore, the width of the depletion layer increases under reverse bias conditions.

To summarize, the correct statements are A and D. The width of the depletion layer will be more on the n-side of the junction, and if the pn junction is reverse biased, the width of the depletion region increases.

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