Introduction

WBCHSE Unit 9, Electronic Devices, traces the subject from its historical starting point right through to the digital-logic toolkit that underlies every modern computer. It opens with the vacuum tube era (Sections 9.2-9.4): thermal (thermionic) emission of electrons from a heated metal surface, and the basic working of the two vacuum-tube devices built on that emission -- the two-electrode vacuum diode and the three-electrode triode valve, only at the level of their basic construction and working principle, per the syllabus.

The chapter then turns to the solid-state picture that eventually replaced the vacuum tube almost everywhere: the band theory of solids (Section 9.5), which explains, on a single unified picture, why some solids conduct electricity freely, some barely at all, and some -- semiconductors -- sit in between and can even be engineered to behave either way (Sections 9.6-9.8). It builds the p-n junction diode from first principles -- formation, forward bias, reverse bias, and its distinctly nonlinear I-V characteristic (Sections 9.9-9.11) -- and its most familiar application, rectification (Section 9.12).

A family of special-purpose diodes follows: the light-emitting diode (LED), the photodiode, the solar cell, and the Zener diode, the last of which is developed specifically into its most important application, a voltage regulator (Sections 9.13-9.17). The junction transistor is then developed in full -- its structure, the mechanism of transistor action, its characteristic curves, the two current-amplification factors that summarise its performance, its use as an amplifier, and its use as an electronic switch (Sections 9.18-9.23).

The chapter closes with the beginnings of digital electronics: the distinction between analogue and digital signals, the decimal and binary number systems used to represent information digitally, and the five basic logic gates -- OR, AND, NOT, NAND and NOR -- with their symbols, truth tables, Boolean equations, and a look at how gates can be combined (Sections 9.24-9.31). This last portion is the direct conceptual bridge from a single transistor switch (Section 9.23) to the billions of switches inside a real digital processor.