Part 1: Energy Bands and Material Classification

The band theory of solids explains why materials conduct, resist, or block electricity. Valence electrons in solids occupy a valence band; the next higher band is the conduction band. The energy gap $E_g$ between them determines material type. Conductors ($E_g$ = 0) allow free electron movement at all temperatures. Semiconductors (small $E_g$: Si = 1.1 eV, Ge = 0.67 eV) conduct moderately, with conductivity increasing with temperature. Insulators ($E_g$ > 3 eV: diamond = 5.4 eV) barely conduct under normal conditions.

Part 2: Intrinsic Semiconductors

Pure Si or Ge at room temperature have equal numbers of electrons and holes generated by thermal excitation: $n_e$ = $n_h$ = $n_i$. The intrinsic carrier concentration $n_i$ increases with temperature. The conductivity is low at room temperature but increases significantly as temperature rises, distinguishing semiconductors from metals. The negative temperature coefficient of resistance is the defining characteristic.

Part 3: Extrinsic Semiconductors (Doping)

n-type (pentavalent dopant — P, As, Sb): Each dopant atom donates one extra electron to the conduction band. Donor energy level lies just below the conduction band. Majority carriers: electrons; minority: holes. p-type (trivalent dopant — B, Al, Ga, In): Each dopant atom accepts one electron from the valence band, creating a hole. Acceptor energy level lies just above the valence band. Majority carriers: holes; minority: electrons. Both types remain electrically neutral. Mass action law $n_e$ × $n_h$ = n_$i^{2}$ governs all carrier concentrations.

Part 4: p-n Junction and Biasing

The p-n junction is formed by metallurgical contact of p-type and n-type regions. Carrier diffusion creates the depletion region and establishes barrier potential. Forward bias: reduces barrier → current flows (> knee voltage). Reverse bias: increases barrier → only tiny reverse saturation current. The I-V characteristic is the key experimental signature of p-n junction behavior.

Part 5: Special Diodes — Four Types

Zener (reverse, voltage regulation at $V_Z$), Photodiode (reverse, light detection), LED (forward, light emission — wavelength = $\frac{hc}{E_g}$), Solar cell (no bias, photovoltaic EMF generation). Each has a unique bias condition and application — the most common NEET question type from this section.

Part 6: Rectifiers

Half-wave rectifier: one diode, positive half-cycle only, $f_{out}$ = $f_{in}$. Full-wave rectifier: two (center-tapped) or four (bridge) diodes, both half-cycles, $f_{out}$ = 2$f_{in}$. Key NEET number: full-wave output frequency doubles the input frequency.

Part 7: Logic Gates and Boolean Algebra

Five basic gates (OR, AND, NOT, NAND, NOR) and the truth tables. NAND and NOR are universal gates (build any circuit from either alone). De Morgan's theorems: (A+B)' = A'·B' and (A·B)' = A'+B'. Gate identification from truth tables is the most frequent NEET question pattern in this section.