Electronic Devices - Semiconductor Diodes and Rectifiers

Energy Bands in Solids: In semiconductors, the valence band is completely filled and the conduction band is empty at $0 \text{ K}$. The forbidden energy gap $E_g$ is small (e.g., $E_g \approx 1.1 \text{ eV}$ for $Si$ and $E_g \approx 0.7 \text{ eV}$ for $Ge$).

Intrinsic and Extrinsic Semiconductors: Pure semiconductors are intrinsic ($n_e = n_h = n_i$). Extrinsic semiconductors are formed by doping: $n$-type (pentavalent impurities like $As$, $P$) where $n_e \gg n_h$, and $p$-type (trivalent impurities like $B$, $Al$) where $n_h \gg n_e$.

Mass Action Law: In thermal equilibrium, the product of the concentrations of electrons and holes is constant: $n_e n_h = n_i^2$.

$p$-$n$ Junction Formation: When $p$ and $n$ type materials are joined, diffusion of charge carriers creates a depletion region (devoid of mobile carriers) and a barrier potential $V_B$ (approx. $0.7 \text{ V}$ for $Si$).

Biasing: Forward bias (p to positive, n to negative) decreases the depletion layer width and allows current flow. Reverse bias (p to negative, n to positive) increases the depletion layer width and prevents current flow, except for a tiny leakage current $I_s$.

Half-Wave Rectifier: Uses a single diode to conduct during only one half-cycle of the $AC$ input. The output frequency is equal to the input frequency ($f_{out} = f_{in}$).

Full-Wave Rectifier: Uses two diodes (center-tapped) or four diodes (bridge) to conduct during both half-cycles. The output frequency is twice the input frequency ($f_{out} = 2f_{in}$).

Zener Diode: A heavily doped diode designed to operate in the reverse breakdown region (Zener breakdown) to act as a voltage regulator.