Summary

WBCHSE Unit 9, Electronic Devices, moved from the vacuum-tube era through to the modern solid-state and digital-logic picture. Thermionic emission (Section 9.2) -- electrons boiled off a heated metal surface once its work function is overcome -- powers the vacuum diode (one-way conduction, Section 9.3) and the triode valve (a small grid-voltage change producing a large plate-current change, the first practical amplifier, Section 9.4).

Band theory (Section 9.5) explains conductors, insulators and semiconductors purely by the size of the forbidden gap $E_g$ between the valence and conduction bands (Section 9.6). An intrinsic semiconductor generates electron-hole pairs, always in equal numbers ($n_e=n_h=n_i$), by thermal bond-breaking (Section 9.7); doping with a pentavalent (donor) or trivalent (acceptor) impurity produces, respectively, n-type or p-type extrinsic semiconductors (Section 9.8). A p-n junction forms a carrier-free depletion region and a built-in potential barrier (Section 9.9); FORWARD bias narrows this barrier and lets a steeply rising, nonlinear current flow (Section 9.10), while REVERSE bias widens it, permitting only a tiny, temperature-dependent leakage current until breakdown (Section 9.11) -- the basis of rectification (Section 9.12).

Special-purpose diodes follow the same junction physics to different ends: the LED converts forward-bias recombination energy directly into light (Section 9.13); the photodiode, reverse biased, turns incident light into a proportional current (Section 9.14); the solar cell uses the same photogeneration, with NO external bias, to generate its own EMF via the photovoltaic effect (Section 9.15); and the Zener diode, heavily doped for a thin depletion region, undergoes controlled Zener breakdown at a low, precise reverse voltage (distinct from ordinary avalanche breakdown, Section 9.16) and is used directly as a voltage regulator, absorbing surplus current to hold $V_O=V_Z$ steady (Section 9.17).

The junction transistor sandwiches a thin base between two wider, oppositely doped regions (Section 9.18); with the emitter-base junction forward biased and the base-collector junction reverse biased, nearly all emitter-injected carriers cross the thin base to become collector current -- transistor action (Section 9.19), captured by input/output characteristic curves (Section 9.20) and by the current-gain factors $\alpha=I_C/I_E$, $\beta=I_C/I_B$, related by $\beta=\alpha/(1-\alpha)$ (Section 9.21). Biased into the flat active region, a common-emitter amplifier delivers a larger, phase-inverted output (Section 9.22); driven instead to the two extremes, cut-off and saturation, the same transistor behaves as an electronic switch (Section 9.23) -- the direct physical basis of digital logic. …