What is the primary difference between a p-type and an n-type semiconductor?

A p-type semiconductor is created by doping an intrinsic semiconductor with trivalent impurities, resulting in an excess of 'holes' which act as majority charge carriers. It has immobile negative acceptor ions. An n-type semiconductor is formed by doping with pentavalent impurities, leading to an excess of 'free electrons' as majority charge carriers. It has immobile positive donor ions. Both types are electrically neutral overall, as the charge of the mobile carriers is balanced by the charge of the immobile ions.

Why is the depletion region called 'depletion' region?

It's called the depletion region because it becomes 'depleted' of mobile charge carriers (free electrons and holes). When the p-n junction forms, electrons from the n-side and holes from the p-side diffuse across the junction and recombine. This leaves behind immobile positive donor ions on the n-side and immobile negative acceptor ions on the p-side, creating a region with a net charge but no free carriers to conduct current.

What is barrier potential and what factors affect its value?

Barrier potential is the potential difference developed across the depletion region due to the electric field created by the immobile positive and negative ions. It acts as a barrier to the further diffusion of majority carriers. Its value depends on the type of semiconductor material (e.g., $0.7, ext{V}$ for silicon, $0.3, ext{V}$ for germanium), the doping concentration (higher doping leads to a slightly higher barrier), and temperature (it decreases with increasing temperature).

How does forward biasing affect the p-n junction?

In forward bias, an external voltage is applied such that the positive terminal connects to the p-side and the negative to the n-side. This external voltage opposes the internal barrier potential. As a result, the effective potential barrier is reduced, the width of the depletion region decreases, and majority carriers can easily cross the junction, leading to a significant current flow. The current increases exponentially with the applied voltage after the knee voltage.

What is the reverse saturation current and why is it so small?

The reverse saturation current is the very small, almost constant current that flows through a p-n junction when it is reverse biased. It is primarily due to the drift of minority charge carriers (electrons from the p-side and holes from the n-side) that are swept across the junction by the strong electric field in the widened depletion region. It's small because the concentration of minority carriers is very low, but it is highly temperature-dependent as temperature increases minority carrier generation.

Differentiate between Zener breakdown and Avalanche breakdown.

Zener breakdown occurs in heavily doped p-n junctions, leading to a narrow depletion region. The strong electric field directly pulls electrons out of covalent bonds, creating electron-hole pairs. It's a reversible process. Avalanche breakdown occurs in lightly doped junctions, with a wider depletion region. Minority carriers gain enough energy from the electric field to collide with lattice atoms, knocking out more electrons, leading to a cascade effect. This can be destructive if not current-limited. Zener breakdown typically occurs at lower voltages than avalanche breakdown.

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Version 1Updated 23 Mar 2026

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A p-n junction is a fundamental semiconductor device formed by joining a p-type semiconductor with an n-type semiconductor. This interface creates a region known as the depletion layer, characterized by an absence of free charge carriers and the presence of an internal electric field, which establishes a potential barrier. This barrier dictates the unidirectional flow of current, allowing signific…

Quick Summary

A p-n junction is formed by joining p-type and n-type semiconductors. At the interface, electrons from the n-side and holes from the p-side diffuse and recombine, creating a 'depletion region' devoid of mobile charge carriers but containing immobile ions.

These ions establish an internal electric field and a 'barrier potential' (e.g., $0.7,\text{V}$ for Si, $0.3,\text{V}$ for Ge) that opposes further majority carrier diffusion. When forward biased (p-side positive, n-side negative), the external voltage reduces the barrier and depletion width, allowing significant majority carrier current.

When reverse biased (p-side negative, n-side positive), the external voltage increases the barrier and depletion width, allowing only a tiny 'reverse saturation current' due to minority carriers. Beyond a certain reverse voltage, breakdown occurs (Zener or Avalanche), leading to a sharp increase in current.

This unidirectional conduction makes the p-n junction a fundamental component in diodes and other semiconductor devices.

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Key Concepts

Depletion Region Formation

When p-type and n-type materials are joined, majority carriers (electrons from n-side, holes from p-side)…

Barrier Potential and its Significance

The electric field within the depletion region, pointing from n-side to p-side, establishes a potential…

Forward Bias Operation

In forward bias, the external voltage source is connected such that its polarity opposes the internal barrier…

Reverse Bias Operation

In reverse bias, the external voltage source is connected such that its polarity adds to the internal barrier…

p-n Junction: — Interface of p-type and n-type semiconductors.

Depletion Region: — Region near junction devoid of mobile carriers, contains immobile ions.

Barrier Potential ($V_B$): — Internal potential difference across depletion region.

- Si: $V_B approx 0.7,\text{V}$ - Ge: $V_B approx 0.3,\text{V}$

Forward Bias: — p-side to positive, n-side to negative.

- $V_{applied}$ opposes $V_B$ . - Depletion width decreases. - $V_{eff} = V_B - V_{applied}$ . - Large current due to majority carriers.

Reverse Bias: — p-side to negative, n-side to positive.

- $V_{applied}$ adds to $V_B$ . - Depletion width increases. - $V_{eff} = V_B + V_{applied}$ . - Small reverse saturation current ( $I_0$ ) due to minority carriers.

Breakdown: — Sudden current increase in reverse bias.

- Zener: Heavily doped, narrow depletion, direct bond breaking, reversible. - Avalanche: Lightly doped, wide depletion, collision ionization, potentially destructive.

Temperature Effect: —

V_B

decreases with

T

;

I_0

increases with

T

(doubles every

10^circ C

Positive Negative, Forward Bias, Decreased Width, Large Current. Positive Negative, Reverse Bias, Increased Width, Small Current. (PN-FB-DW-LC, PN-RB-IW-SC) - Helps remember the effects of biasing on depletion width and current.

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