Ch 14Electronic Devices — Class 12 Physics, concept-first.

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.

What thermionic emission is. Every free electron inside a metal is held back from escaping into the surrounding space by an energy barrier at the metal's surface, called the work function of that meta…

Construction. A vacuum diode consists of two electrodes sealed inside an evacuated glass (or metal) envelope: a heated CATHODE, which supplies electrons by thermionic emission (Section 9.2), and a col…

Construction. A triode valve adds a THIRD electrode to the basic vacuum-diode structure of Section 9.3: a fine, open wire mesh or spiral called the CONTROL GRID, placed in the vacuum envelope between…

From discrete atomic levels to continuous bands. In a single, isolated atom, each electron occupies one of a set of sharply defined, discrete energy levels.

Conductors (, bands overlapping). In a typical metal (copper, silver, aluminium), the valence and conduction bands OVERLAP -- there is effectively no forbidden gap between them at all.

Definition. An intrinsic semiconductor is a semiconductor crystal (silicon or germanium) in a chemically PURE state, with no deliberately added impurity atoms at all.

Why dope at all. An intrinsic semiconductor's conductivity (Section 9.7) is small and strongly temperature-dependent, making it, on its own, of limited practical use for building a reliable device.

Bringing p-type and n-type together. A p-n junction is formed when a p-type region and an n-type region exist within a SINGLE, continuous semiconductor crystal (produced, in practice, by changing the…

Connecting the bias. In forward bias, the external battery's positive terminal is connected to the p-side of the junction and its negative terminal to the n-side -- i.e.

Connecting the bias. In reverse bias, the external battery's positive terminal is connected to the n-side of the junction and its negative terminal to the p-side -- the applied voltage connected in th…

The rectifying idea. Because a p-n junction diode conducts easily when forward biased but blocks current almost completely when reverse biased (Sections 9.10-9.11), connecting a diode in series with a…

Structure. A light-emitting diode (LED) is a p-n junction diode, heavily doped on both sides and constructed from a semiconductor material specifically chosen (commonly gallium arsenide, gallium phosp…

Structure. A photodiode is a p-n junction diode built with a thin, transparent window positioned directly over its junction region, so that light from outside the package can reach the junction itself…

Structure. A solar cell (or photovoltaic cell) is, physically, a p-n junction very similar to a photodiode -- a large-area junction exposed to light -- but built with NO external bias battery anywhere…

Structure. A Zener diode is, physically, a p-n junction diode -- but doped much more heavily than an ordinary rectifier diode, which makes its depletion region considerably THINNER for the same applie…

The regulator circuit. A basic Zener voltage regulator connects an unregulated DC supply voltage (one that may fluctuate somewhat) through a series resistor to a Zener diode held in REVERSE bias, with…

Structure. A junction transistor is a three-layer semiconductor sandwich, formed from a single crystal, in which a very THIN, lightly doped layer of ONE type is sandwiched between two much WIDER, more…

The biasing arrangement. In its normal, or 'active', mode of operation, a transistor's emitter-base junction is FORWARD biased (so carriers are readily injected from the emitter into the base, exactly…

Two families of curves. A transistor's practical, measurable behaviour in the common-emitter (CE) configuration -- the emitter terminal common to both the input (base) circuit and the output (collecto…

Common-base current amplification factor, . The common-base current amplification factor is defined as the ratio of the collector current to the emitter current:

The common-emitter configuration. In a common-emitter (CE) amplifier, the emitter terminal is connected to a common reference point (ground) shared by BOTH the input signal loop (applied between base…

Amplifier versus switch: two different uses of the same device. Section 9.22's amplifier deliberately biases a transistor to sit in the FLAT, LINEAR active region of its output characteristic (Section…

Two broad categories of electrical signal. Every electrical signal used to carry information -- whether a voltage, a current, or some other measurable quantity that varies with time -- falls into one…

The decimal (base-10) system. The number system used in everyday arithmetic is the DECIMAL system, built on TEN distinct digit symbols ( through ) and place values that are successive powers of ten: r…

Definition. An OR gate is a logic circuit with two (or more) inputs and a single output, whose output is HIGH () whenever ANY ONE, OR MORE, of its inputs is HIGH -- the electronic realisation of the e…

Definition. An AND gate is a logic circuit with two (or more) inputs and a single output, whose output is HIGH () only when ALL of its inputs are simultaneously HIGH -- the electronic realisation of t…

Definition. A NOT gate, also called an INVERTER, is a logic circuit with a SINGLE input and a single output, whose output is always the exact OPPOSITE (complement) of its input: HIGH when the input is…

Definition, as a combination. A NAND gate (short for 'NOT-AND') is functionally an AND gate (Section 9.27) whose single output is immediately passed through a NOT gate (Section 9.28) -- its output is,…

Definition, as a combination. A NOR gate (short for 'NOT-OR') is functionally an OR gate (Section 9.26) whose single output is immediately passed through a NOT gate (Section 9.28) -- its output is, at…

The idea behind universality. Because a NAND gate (Section 9.29) and a NOR gate (Section 9.30) are each, individually, capable of reproducing every one of the other basic gate functions, a real digita…

WBCHSE Unit 9, Electronic Devices, moved from the vacuum-tube era through to the modern solid-state and digital-logic picture.

Sample papers and previous-year board questions for this subject.