#diode

p-n Junction, Depletion Region and Diode Characteristics

The thickness of the depletion region in a p-n junction is typically:

**p-n junction formation** diffusion of holes from p to n and electrons from n to p leaves ionized donors positive on n-side and acceptors negative on p-side, forming space-charge depletion region with electric field directed from n to p (positive to negative), barrier potential V_b ≈0.3 V Ge, 0.7 V Si, opposes further diffusion, drift current due to minority carriers swept by field balances diffusion at equilibrium net current zero. The depletion region, formed by diffusion and drift, is very thin, on the order of one-tenth of a micrometer ( 0.1 μ m ), due to the sharp transition at the junct

Ref: NCERT > Physics Book > Electronic Devices > p-n Junction, Depletion Region and Diode Characteristics

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p-n Junction, Depletion Region and Diode Characteristics

The barrier potential in a p-n junction opposes the flow of:

**Diode characteristics** forward bias reduces barrier, width of depletion region decreases, resistance low ~10-100 Ω, current primarily due to majority diffusion, exponential I = I_s(e^{eV/kT}-1), reverse bias widens depletion region, resistance high ~MΩ, dominant current drift due to minority carriers, reverse saturation current small μA-nA, typically 10⁻⁶ A, increases sharply at breakdown Zener/avalanche. The barrier potential arises due to the space-charge region and opposes the diffusion of majority carriers (electrons from n-side to p-side and holes from p-side to n-side) at equilibrium.

Ref: NCERT > Physics Book > Electronic Devices > p-n Junction, Depletion Region and Diode Characteristics

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p-n Junction, Depletion Region and Diode Characteristics

In a rectifier circuit, the diode conducts when it is:

**Diode characteristics** forward bias reduces barrier, width of depletion region decreases, resistance low ~10-100 Ω, current primarily due to majority diffusion, exponential I = I_s(e^{eV/kT}-1), reverse bias widens depletion region, resistance high ~MΩ, dominant current drift due to minority carriers, reverse saturation current small μA-nA, typically 10⁻⁶ A, increases sharply at breakdown Zener/avalanche. A diode conducts in forward bias (p-side positive, n-side negative), allowing current to flow through the load, which is the basis of rectification in both half-wave and full-wave circuits

Ref: NCERT > Physics Book > Electronic Devices > p-n Junction, Depletion Region and Diode Characteristics

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p-n Junction, Depletion Region and Diode Characteristics

In reverse bias, the width of the depletion region:

**Diode characteristics** forward bias reduces barrier, width of depletion region decreases, resistance low ~10-100 Ω, current primarily due to majority diffusion, exponential I = I_s(e^{eV/kT}-1), reverse bias widens depletion region, resistance high ~MΩ, dominant current drift due to minority carriers, reverse saturation current small μA-nA, typically 10⁻⁶ A, increases sharply at breakdown Zener/avalanche. In reverse bias, the applied voltage increases the barrier height and electric field, widening the depletion region as more immobile charges are exposed due to reduced carrier diffusion. S

Ref: NCERT > Physics Book > Electronic Devices > p-n Junction, Depletion Region and Diode Characteristics

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Resistance, Resistivity and Ohm's Law

Why does a non-ohmic device, like a diode, exhibit a non-linear current-voltage relationship?

**Resistance** R = ρ l/A, ρ resistivity (Ω·m), l length (m), A area (m²), ρ = m/(n e² τ) from Drude model, τ average collision time. Ohm's law V = I R holds when ρ constant, independent of V. Volume constant stretching l→2l implies A→A/2, so R' = ρ·2l/(A/2)=4R, resistance quadruples when length doubles at constant volume. In a diode, resistance depends on the direction and magnitude of the applied voltage due to its semiconductor junction properties. Forward bias reduces resistance, while reverse bias increases it, leading to a non-linear I -versus- V curve. Applying I = n e A v_d, R = ρ l/A,

Ref: NCERT > Physics Book > Current Electricity > Resistance, Resistivity and Ohm's Law

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PHYSICS

The forward bias current in a diode is typically measured in:

In forward bias, the current due to majority carrier diffusion is large, typically in the milliampere (mA) range, requiring a milliammeter for measurement.

Ref: NCERT Physics Textbook - Latest Edition for Academic Session 2026-27 (Rationalized Textbook for Class XI and XII, continuing as per NCERT advisory for 2026-27), Chapter: Thermal Properties of Matter (Latest NCERT 2026-27), Topic: Latent heat Q = mL_f, ice melting, L_f = 3.35 × 10⁵ J kg⁻¹ and mass ca

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PHYSICS

The primary function of a p-n junction diode is to:

A p-n junction diode allows current in forward bias (low resistance) and blocks it in reverse bias (high resistance), making it ideal for rectification of AC to DC.

Ref: NCERT Physics Textbook for Class XI and XII, Chapter: Current Electricity, Topic: Resistivity, temperature dependence and circuit analysis.

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