Semiconductor ElectronicsNEET Physics · Class 12 · NCERT Chapter 14

You can expect 1 to 2 questions in NEET 2027. Common asks: identifying n-type vs p-type, knee voltage / barrier potential of Si and Ge, identifying half-wave vs full-wave rectifiers (output frequency), Zener diode operation, special diodes, and logic gate truth tables.

Conductors have overlapping conduction and valence bands (no band gap), so electrons flow freely. Insulators have a large band gap (Eg > ~3 eV), so very few electrons reach the conduction band. Semiconductors have a small band gap (Eg ~1 eV; Si: 1.1 eV, Ge: 0.7 eV), so heat or light easily promotes electrons.

Intrinsic: pure Si or Ge. n_e = n_h = n_i, set entirely by temperature. Extrinsic: doped semiconductor. Adding a tiny amount of impurity (1 part in 10^6 or so) drastically changes carrier counts. n-type uses pentavalent dopants (P, As, Sb): extra electrons. p-type uses trivalent dopants (Al, B, Ga): extra holes.

n_e × n_h = n_i^2, where n_i is the intrinsic concentration. This holds at thermal equilibrium even after doping. So if doping makes n_e huge, n_h drops below n_i.

When p and n materials are joined, electrons from the n-side diffuse into the p-side and holes diffuse the other way. They recombine near the junction, leaving immobile ions on both sides. This narrow region without free carriers is the depletion region. The trapped ions create a built-in barrier potential (~0.7 V for Si, ~0.3 V for Ge).

Forward: positive terminal of battery on p-side, negative on n-side. The applied voltage opposes the barrier; once it exceeds the knee voltage, current flows easily. Reverse: opposite polarity widens the depletion region; only a tiny saturation current flows from minority carriers, until reverse breakdown.

Half-wave uses 1 diode and conducts only on the positive half of the AC cycle. Output frequency = input frequency. Full-wave (centre-tapped or bridge) uses 2 or 4 diodes and conducts in both halves of the cycle. Output frequency = 2 × input frequency. Full-wave gives smoother DC and higher efficiency.

A heavily doped p-n junction designed to operate in reverse breakdown. Beyond a sharp Zener voltage V_Z, the voltage across it stays nearly constant for a wide range of currents. Used as a voltage regulator: load gets a steady voltage even when input fluctuates.

LED: forward-biased p-n junction emits light when electrons recombine with holes. Energy of photon ≈ E_g. Photodiode: reverse-biased p-n junction. Light incident on the depletion region creates electron-hole pairs that increase the reverse current. Solar cell: large-area p-n junction with no external bias. Photons drive minority carriers across the junction, generating an EMF.

Because every other logic gate (NOT, AND, OR, XOR, etc.) can be built using only NAND or only NOR. NOT is just NAND with both inputs tied together. AND is NOT(NAND). OR can be built from De Morgan with NAND chains, etc. So you can build any digital circuit from one type of gate.