Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits | Summary

Silicon and germanium have diamond-like crystal structures where each atom forms four covalent bonds with its nearest neighbours by sharing valence electrons. At absolute zero, all bonds are intact and no free carriers exist. As temperature rises, some electrons gain enough thermal energy to break free from covalent bonds, leaving behind positively charged vacancies called holes.

In an intrinsic semiconductor, the number of free electrons (ne) equals the number of holes (nh), both denoted as the intrinsic carrier concentration ni. Holes behave as apparent positive charge carriers and move in the direction opposite to electrons under an applied field. The total current is the sum of electron current and hole current. Generation and recombination of carriers occur simultaneously, reaching equilibrium at a given temperature.

The conductivity of intrinsic semiconductors is very low at room temperature, making them unsuitable for practical devices. Doping — the deliberate addition of trace impurities (a few ppm) — increases conductivity manifold. Dopants must have atomic sizes similar to Si or Ge to occupy lattice sites without distorting the crystal. Two types of dopants are used: pentavalent donor atoms (As, Sb, P) and trivalent acceptor atoms (In, B, Al).

In n-type semiconductors formed by pentavalent doping, the extra fifth electron is very weakly bound (ionisation energy ~0.01 eV for Ge, ~0.05 eV for Si) and becomes a free conduction electron at room temperature. Electrons are the majority carriers and holes are minority carriers (ne >> nh). In p-type semiconductors formed by trivalent doping, the dopant creates a hole in the fourth bond. Holes are the majority carriers and electrons are minority carriers (nh >> ne). In both cases, nenh = ni^2 and overall charge neutrality is maintained.

A p-n junction is formed when part of a semiconductor wafer is doped p-type and the adjacent part n-type. Due to the concentration gradient, holes diffuse from the p-side to the n-side and electrons diffuse from the n-side to the p-side, creating a diffusion current. This migration leaves behind immobile ionised cores — positive on the n-side and negative on the p-side — forming the depletion region. The depletion region is about 0.1 μm thick.

The space-charge region creates an internal electric field directed from the n-side to the p-side, which opposes further diffusion and causes a drift current in the opposite direction. Equilibrium is reached when diffusion current equals drift current. The resulting built-in potential (barrier potential) prevents further net flow of carriers. A p-n junction cannot be formed by simply pressing two separately doped slabs together because surface roughness prevents atomic-level contact.

A semiconductor diode is a p-n junction with metallic contacts at both ends, forming a two-terminal device. Under forward bias (p-side to positive terminal), the barrier height decreases to (V0 - V), the depletion region narrows, and majority carriers are injected across the junction causing a significant current (in mA). The threshold or cut-in voltage is ~0.2 V for Ge and ~0.7 V for Si.

Under reverse bias (n-side to positive terminal), the barrier height increases to (V0 + V), the depletion region widens, and only a tiny drift current (in μA) flows due to minority carrier sweep. This reverse saturation current remains nearly constant with increasing reverse voltage until the breakdown voltage (Vbr) is reached, at which point current increases sharply. Dynamic resistance is defined as rd = ΔV/ΔI and is much lower under forward bias than reverse bias.

A rectifier converts alternating current (ac) into unidirectional (dc) current using the one-way conduction property of a diode. In a half-wave rectifier, a single diode conducts only during the positive half-cycle of the ac input, producing a pulsating output at the same frequency as the input (50 Hz for 50 Hz input). The output is present for only half the input cycle.

A full-wave rectifier uses two diodes and a centre-tap transformer. Each diode conducts for alternate half-cycles, so the output appears for both the positive and negative half-cycles of the input, giving an output frequency of 100 Hz for a 50 Hz input. To smoothen the pulsating dc output, a filter capacitor is connected in parallel with the load. The capacitor charges to the peak voltage and discharges slowly through the load, reducing ripple and producing an output closer to steady dc.

Chapter 14 establishes the foundation of modern semiconductor electronics by explaining the physics of charge carriers in intrinsic and extrinsic semiconductors, the formation and behaviour of the p-n junction, and its application as a rectifier. The energy band model clearly distinguishes metals, insulators, and semiconductors, while the concepts of doping, depletion region, forward and reverse bias form the core of understanding all semiconductor devices.

From a board examination perspective, this chapter is highly important as questions frequently appear on energy bands, types of semiconductors, the V-I characteristics of a diode, the working of half-wave and full-wave rectifiers, and numerical problems involving carrier concentrations using nenh = ni^2. Students should focus on diagrams of the rectifier circuits, the role of the filter capacitor, and the distinction between majority and minority carriers in n-type and p-type semiconductors.