Solids are classified into conductors, semiconductors, and insulators based on their energy band gap $E_g$, with silicon ($E_g$ = 1.1 eV) and germanium ($E_g$ = 0.67 eV) being the most important semiconductors in NEET. Pure intrinsic semiconductors have equal electron and hole concentrations ($n_e$ = $n_h$ = $n_i$), and their conductivity increases with temperature unlike metals. Doping with pentavalent atoms (P, As, Sb) creates n-type semiconductors with electron majority carriers, while trivalent atoms (B, Al, Ga) create p-type semiconductors with hole majority carriers. Both n-type and p-type semiconductors are electrically neutral, and the mass action law $n_e$ × $n_h$ = n_$i^{2}$ applies to all semiconductor types. When p-type and n-type materials are joined, carrier diffusion creates a depletion region with barrier potential (~0.7 V for Si, ~0.3 V for Ge) that blocks further diffusion at equilibrium. Forward bias narrows the depletion region and allows current above the knee voltage, while reverse bias widens it, permitting only a tiny reverse saturation current until breakdown. The four special diodes exploit the p-n junction in specific ways: Zener diode (reverse bias, voltage regulation), photodiode (reverse bias, light detection), LED (forward bias, light emission with λ = $\frac{hc}{E_g}$), and solar cell (no bias, photovoltaic EMF). Full-wave rectifiers double the output frequency ($f_{out}$ = 2$f_{in}$) while half-wave rectifiers maintain the same frequency, both converting AC to pulsating DC. The five fundamental logic gates (OR, AND, NOT, NAND, NOR) implement Boolean algebra, with NAND and NOR being universal gates capable of building any logic function independently. De Morgan's theorems — (A+B)' = A'·B' and (A·B)' = A'+B' — provide the mathematical bridge between these gates and are frequently tested in NEET.