Semiconductors form one of the most practically important chapters in modern physics, and NEET 2026 typically draws 2–3 questions from this topic. The conceptual framework begins with energy band theory and extends through doping, p-n junctions, special diodes, and finally logic gates.

Energy Band Theory

When atoms condense into a solid, their discrete atomic energy levels broaden into continuous energy bands due to quantum mechanical overlap of electron wave functions. The two most important bands are the valence band (highest occupied band, formed by valence electrons) and the conduction band (lowest unoccupied band, where electrons can move freely). Separating them is the forbidden energy gap or band gap ($E_g$), within which no electron states exist. Materials are classified by $E_g$: conductors have $E_g$ = 0 (bands overlap; examples: Cu, Ag, Al), semiconductors have small $E_g$ (Si: 1.1 eV; Ge: 0.67 eV), and insulators have large $E_g$ (> 3 eV; diamond: 5.4 eV). A critical distinction from metals: semiconductor conductivity increases with temperature because thermal energy excites electrons across the gap — the opposite of metals.

Doping and Extrinsic Semiconductors

Pure (intrinsic) semiconductors have equal electron and hole concentrations: $n_e$ = $n_h$ = $n_i$, where $n_i$ is the intrinsic carrier concentration. Conductivity is increased by doping — adding controlled impurities. Adding a pentavalent dopant (P, As, Sb — group 15, five valence electrons) donates an extra conduction electron, creating n-type silicon where electrons are the majority carriers and holes the minority. Adding a trivalent dopant (B, Al, Ga — group 13, three valence electrons) creates an electron deficiency (hole), giving p-type silicon where holes are the majority carriers.

A critical but frequently misunderstood point: both n-type and p-type semiconductors are electrically neutral. The dopant atom contributes both an extra carrier AND an equal and opposite ionic charge fixed in the lattice. The mass action law $n_e$ × $n_h$ = n_$i^{2}$ holds universally for both intrinsic and extrinsic semiconductors, meaning that increasing one carrier type proportionally decreases the other.

p-n Junction Physics

Joining p-type and n-type materials creates a p-n junction. Majority carrier diffusion across the interface causes electrons and holes to recombine near the boundary, leaving behind immobile ionized donor atoms (positive) on the n-side and ionized acceptor atoms (negative) on the p-side. This region, depleted of free carriers, is the depletion region. The immobile ions create an internal electric field pointing from n to p, establishing a barrier potential (~0.7 V for Si, ~0.3 V for Ge) that prevents further net diffusion.

Forward bias (positive terminal to p-side) opposes the internal field, narrows the depletion region, and allows substantial current once the applied voltage exceeds the knee voltage. Reverse bias (positive terminal to n-side) reinforces the barrier, widens the depletion region, and permits only a tiny reverse saturation current from minority carriers — until breakdown occurs at high reverse voltage.

Special Purpose Diodes

Four special diodes exploit p-n junction physics in distinct ways. The Zener diode, heavily doped to create a well-defined breakdown voltage $V_Z$, operates in reverse bias as a voltage regulator — the voltage across it stays constant at $V_Z$ despite current variations. The photodiode also operates in reverse bias: incident photons create electron-hole pairs in the depletion region, generating a photocurrent proportional to light intensity — used in optical fiber receivers, barcode scanners, and medical devices. The LED (Light Emitting Diode) operates in forward bias: electron-hole recombination at the junction releases energy as photons with wavelength λ = $\frac{hc}{E_g}$. The band gap of the semiconductor material determines the emitted color (GaAs → IR; GaAsP → red/yellow; GaP → green; GaN → blue). The solar cell operates with no external bias: the photovoltaic effect uses the junction's built-in field to separate photogenerated carriers, producing an EMF of ~0.5–1 V per cell.

Rectifiers

Rectifiers convert AC to pulsating DC. The half-wave rectifier (single diode) conducts only during positive half-cycles, producing an output frequency equal to the input frequency f. The full-wave rectifier (center-tapped with 2 diodes, or bridge with 4 diodes) rectifies both half-cycles, producing an output frequency of 2f. Full-wave rectifiers are more efficient and produce a higher average output voltage.

Logic Gates

Logic gates are electronic switches implementing Boolean algebra. The five fundamental gates are: OR (Y = A+B; output 1 if any input is 1), AND (Y = A·B; output 1 only if all inputs are 1), NOT (Y = A'; inverts single input), NAND (Y = (A·B)'; output 0 only when all inputs are 1), and NOR (Y = (A+B)'; output 1 only when all inputs are 0). Both NAND and NOR are universal gates — any Boolean function can be constructed from either alone, which is why early computer chips were built entirely from NAND gates. De Morgan's theorems provide the bridge between these gates: (A+B)' = A'·B' and (A·B)' = A'+B'. NEET frequently tests truth table identification and De Morgan simplification.