CBSE Class 12 › Unit IX: Electronic Devices

Semiconductor Electronics

NCERT Chapter 14 • PN Junction, Rectifiers, Zener Diode & Logic Gates

NCERT 2025–26 Unit IX • ~7 Marks JEE Main • 1 Question

1. Energy Bands in Solids

Energy band diagrams comparing metals, semiconductors, and insulators

In a solid crystal, atomic energy levels merge to form continuous bands separated by a forbidden energy gap ( $E_g$ ).

Classification by Band Gap:

Metals: Valence Band (VB) and Conduction Band (CB) overlap. $E_g \approx 0$ . Electrons are free to move.
Semiconductors: Small gap ( $E_g < 3 \text{ eV}$ ). At 0K, they act as insulators. At room temp, thermal energy excites some electrons to CB. (Si: 1.1 eV, Ge: 0.7 eV).
Insulators: Large gap ( $E_g > 3 \text{ eV}$ ). No conduction possible.

2. Extrinsic (Doped) Semiconductors

Intrinsic (pure) semiconductors have low conductivity ( $n_e = n_h$ ). We add impurities (doping) to enhance it.

$n_e n_h = n_i^2$
Intrinsic: $n_e = n_h = n_i$ . Extrinsic: Majority dominates.

Energy band diagrams showing donor and acceptor energy levels.

n-Type Semiconductor:

Dopant: Pentavalent (As, P).
Carriers: Majority Electrons, Minority Holes.
Donor Level ( $E_D$ ): A new energy level forms just below the CB.

p-Type Semiconductor:

Dopant: Trivalent (In, B).
Carriers: Majority Holes, Minority Electrons.
Acceptor Level ( $E_A$ ): A new energy level forms just above the VB.

3. P-N Junction Formation

When p-type and n-type silicon are joined, the concentration gradient causes charge movement.

Diagram illustrating diffusion, drift, and depletion region formation.

Process 1: Diffusion
Holes diffuse p $\to$ n. Electrons diffuse n $\to$ p. This constitutes Diffusion Current.

Process 2: Depletion Region Formation
As carriers diffuse, they leave behind immobile ions (negative on p-side, positive on n-side). This region, devoid of free carriers, is the Depletion Region.

Process 3: Drift & Equilibrium
The immobile ions create an electric field (n $\to$ p) and a Barrier Potential ( $V_b$ ). This field pushes minority carriers across the junction (Drift Current).
At equilibrium: Diffusion Current = Drift Current.

4. Biasing & I-V Characteristics

Diagram showing how depletion layer width changes with biasing.

Forward Bias (p to +, n to -):

External field opposes barrier field.
Depletion Width: Decreases.
Barrier Height: Decreases ( $V_b - V$ ).
Current: Large diffusion current flows (mA).

Reverse Bias (p to -, n to +):

External field aids barrier field.
Depletion Width: Increases.
Barrier Height: Increases ( $V_b + V$ ).
Current: Negligible drift current flows ( $\mu$ A) called Reverse Saturation Current.