MHT-CET Physics · Semiconductor Devices

The p-n Junction: Depletion Layer and Biasing

Where p and n meet, carriers diffuse across and leave a thin depletion layer of fixed ions whose field opposes further flow; forward bias shrinks that barrier and lets current through, reverse bias widens it and blocks current.

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Why this matters

14 PYQs, none HARD. Two things are asked: what forms the depletion layer and which way its field points, and what forward and reverse bias do to the barrier and the layer's width — including reading from a drawing whether a diode is forward or reverse biased.

Concept 1 of 2: The Depletion Layer and Its Field

Electrons diffuse from the n-side into the p-side and holes the other way; they recombine and leave behind uncovered ions — positive on the n-side, negative on the p-side. Those fixed charges make a field pointing from n to p, which stops further diffusion. The n-side ends up at the higher potential.

Definition

The depletion layer forms by DIFFUSION of majority carriers; it holds only immobile ions — positive on the n-side, negative on the p-side.
The barrier is due to that accumulation of positive and negative ions near the junction.
Unbiased junction: field from the n-side to the p-side; the n-side is at the higher potential.
The field is strongest inside the depletion layer.

Barrier

\vec E_{\text{junction}}: \; n \to p, \qquad V_n > V_p

Worked example

In an unbiased p-n junction, which side is at the higher potential, and which way does the junction field point?

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q40Easy

Example 1 · Semiconductor Devices · p-n Junction — Depletion Layer and Biasing

In unbiased p-n junction diode

The p-side is at the higher potential

The p-side LOST holes and holds negative ions, so it sits LOWER. The field runs from n to p; the options reverse both.

Drill 4 more on the depletion layer and its field

Concept 2 of 2: Forward and Reverse Bias

Push the p-side positive and the applied field opposes the junction's own: the barrier drops, the layer thins, and current flows. Push it negative and the two fields add: the barrier rises, the layer widens, and only a tiny minority-carrier current trickles through.

Definition

Forward bias (p at the higher potential): barrier and width DECREASE; large current. Ideal diode: zero resistance.
Reverse bias (p lower): barrier and width INCREASE; almost no current. Ideal diode: infinite resistance.
Reading a drawing: the triangle's base is the p-side (anode). Forward if the voltage there is higher than at the bar, e.g. −1.0 V against −1.5 V.
A device that passes 10 mA one way and almost nothing when reversed is a p-n junction diode.

Bias rule

V_p > V_n \Rightarrow \text{forward}, \qquad V_p < V_n \Rightarrow \text{reverse}

Worked example

A diode's p-side is at 2 V and its n-side at 5 V. Biased which way, and what happens to its depletion layer?

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 12th May Shift 1 · Q43Easy

Example 2 · Semiconductor Devices · p-n Junction — Depletion Layer and Biasing

When forward bias is applied to a p-n junction, then what happens to the potential barrier

V_{B}

and the width (

X

) of the depletion region?

Comparing the sizes of negative voltages

−1.0 V is HIGHER than −1.5 V. A drawing with −1.0 V on the p-side and −1.5 V on the n-side is forward biased; the minus signs are there to trip the eye.

Drill 8 more on forward and reverse bias

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

The Depletion Layer and Its Field
Barrier
$\vec E_{\text{junction}}: \; n \to p, \qquad V_n > V_p$
Forward and Reverse Bias
Bias rule
$V_p > V_n \Rightarrow \text{forward}, \qquad V_p < V_n \Rightarrow \text{reverse}$

Watch out for (2)

The p-side is at the higher potential
→ The Depletion Layer and Its Field
Comparing the sizes of negative voltages
→ Forward and Reverse Bias

Test yourself on Semiconductor Devices

20 past MHT-CET questions from this chapter, timed at 18 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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