Ch 9Semiconductor Electronics — Class 12 Physics, concept-first.

Electronics has become part of everyday life -- it underlies mobile phones, computers, televisions, air conditioners, microwave ovens, washing machines, medical diagnostic equipment, and even the ATMs…

Section 9.1 builds the energy-band picture of a solid from scratch: in an isolated atom the electron energy levels are widely separated and discrete, but when millions of atoms are packed together in…

Defines the valence band (the band formed from the filled valence orbitals) and the conduction band (formed from the empty orbitals electrons jump into once energised), separated by the forbidden ener…

Having built the band picture, the chapter classifies every solid -- insulator, conductor, or semiconductor -- purely by the width of its forbidden energy gap and by whether the valence and conduction…

In an insulator the valence and conduction bands are separated by a very large forbidden gap, approximately 6 eV.

In a conductor the valence band and conduction band actually overlap, so there is no gap at all for an electron to cross -- a large population of electrons sits in the conduction band even with zero a…

A semiconductor sits between the two extremes: it has a narrow forbidden gap, eV, between its valence and conduction bands.

Section 9.2 splits semiconductors into two families: the intrinsic (pure) semiconductor, whose small, temperature-limited population of charge carriers is not enough for practical devices, and the ext…

A semiconductor in its pure form, without any added impurity, is an intrinsic semiconductor. Every atom in the crystal lattice -- silicon has four valence electrons -- is covalently bonded to its four…

The carrier concentration of an intrinsic semiconductor is far too small for practical electronic devices.

An n-type semiconductor is made by doping pure germanium or silicon with a pentavalent (group V) element -- phosphorus, arsenic or antimony -- each of which brings five valence electrons to a lattice…

A p-type semiconductor is made by doping germanium or silicon with a trivalent (group III) element -- boron, aluminium, gallium or indium -- each bringing only three valence electrons to a lattice sit…

Section 9.3 is the heart of the chapter's device physics: it explains how joining a p-type and an n-type crystal creates a p-n junction with a self-built depletion region and barrier potential, how th…

A p-n junction is formed simply by joining an n-type and a p-type semiconductor crystal together. What happens at that boundary immediately afterwards -- carriers diffusing across, a self-limiting dep…

The moment a p-type and n-type crystal are joined, the sharp concentration difference across the boundary drives DIFFUSION: electrons, abundant on the n-side, diffuse across into the p-side, and holes…

Recombination of diffusing carriers across the junction only proceeds up to a point, beyond which the depletion layer's own electric field acts as a barrier to further diffusion: an electron trying to…

A p-n junction diode is the single-junction device formed when one p-type region is fused to one n-type region, with external leads brought out from each side -- the p-side lead is called the anode an…

Biasing means supplying external energy (a bias voltage) to the diode's charge carriers to help them overcome -- or be blocked by -- the barrier potential.

A diode's behaviour is summarised experimentally by plotting the current through it against the voltage across it -- separately for forward bias (9.3.3.1) and reverse bias (9.3.3.2) -- since the two r…

The forward V-I characteristic is obtained by forward-biasing the diode through a current-limiting resistor R, then varying the DC supply and recording the forward voltage V (x-axis) against the resul…

The reverse V-I characteristic uses an identical circuit but with the diode connected in reverse bias: the p-region to the negative terminal and n-region to the positive terminal of the DC supply, wit…

Rectification is the process of converting alternating current into direct current, and a p-n junction diode -- which conducts in only one direction -- is the natural device for the job.

The half wave rectifier circuit consists of a transformer, a single p-n junction diode acting as the rectifying element, and a load resistor .

The full wave rectifier passes BOTH the positive and negative half cycles of the AC input through the load, in the same direction, so it is called a full wave rectifier.

If the reverse bias applied to a p-n junction is increased past a certain point, the junction 'breaks down' and the reverse current rises sharply -- destructive in an ordinary diode, but exploited del…

Zener breakdown occurs in HEAVILY doped p-n junctions, which have very narrow depletion layers, of order m.

Avalanche breakdown occurs in the opposite regime: LIGHTLY doped junctions, which have wide depletion layers.

A Zener diode is a heavily doped silicon diode, named after its inventor C. Zener, specifically designed to be operated in the (otherwise destructive) reverse breakdown region -- its doping level can…

In FORWARD bias, a Zener diode behaves just like an ordinary p-n junction diode, starting to conduct at roughly 0.7 V.

A Zener diode operating in its breakdown region maintains a constant output voltage across a load even when the input voltage or the load current varies -- this is the classic Zener shunt voltage regu…

Optoelectronics is the branch of device physics dealing with components that convert electrical energy into light or light into electrical energy through a semiconductor junction.

An LED is a p-n junction diode that emits visible or invisible light when it is FORWARD biased -- since electrical energy is converted directly into light energy, this process is called electrolumines…

A photodiode is a p-n junction diode that converts an optical signal INTO an electrical signal -- the operating principle is essentially the inverse of the LED's, and (unlike the LED) a photodiode is…

A solar cell, also known as a photovoltaic cell, converts light energy directly into electrical energy (an emf) via the photovoltaic effect -- it is essentially a large-area p-n junction designed to g…

William Shockley invented the modern bipolar junction transistor in 1951; being a semiconductor device with very low heat loss, it triggered a technological revolution that eventually let thousands of…

A transistor circuit has only three terminals but needs both an input and an output loop, so one terminal must necessarily be shared (common) between the two loops -- this gives exactly three possible…

In the common-base configuration, the BASE terminal is common to both the input and the output circuit loop.

In the common-emitter configuration, the EMITTER terminal is common to both the input and output loops.

In the common-collector configuration, the COLLECTOR terminal is common to both the input and output loops.

In the forward active mode, the emitter-base junction is forward biased by (shrinking that junction's depletion region) while the collector-base junction is simultaneously reverse biased by (widening…

Knowing a transistor's input resistance, output resistance and current gain is essential for using it effectively in real circuits, and all three are read off its STATIC characteristic curves, measure…

The input characteristic plots the relationship between base current and base-to-emitter voltage , measured at several fixed values of collector-to-emitter voltage : is first set to a chosen value (ab…

The output characteristic plots the collector current against the collector-to-emitter voltage , at several fixed values of base current : is first set to a chosen value, then is stepped up and the re…

The current transfer characteristic plots the variation of collector current against base current , at a fixed collector-emitter voltage .

The two current-gain figures defined for the two configurations -- in common base, in common emitter -- are not independent; they are related by , equivalently .

Operated in its saturation and cut-off regions, a transistor behaves like an electronic switch that a small control signal at the base can turn fully ON or fully OFF. When a HIGH input voltage (e.g.

The operating point (also called the Q-point or quiescent point) is the specific point on a transistor's output characteristics at which it is set up to operate for a given application.

A transistor biased in its active region can amplify a weak input signal -- amplification means increasing a signal's amplitude/strength.

An electronic oscillator converts DC energy into AC energy of a chosen (usually high) frequency, ranging from a few Hz to several MHz -- unlike an amplifier, an oscillator needs NO external AC input s…

Digital electronics is the sub-branch of electronics dealing specifically with digital signals -- signals restricted to a small number of discrete states rather than varying continuously -- and is use…

There are two fundamentally different kinds of electrical signal. An ANALOG signal is a continuously varying voltage or current with respect to time -- rectifier outputs (9.3.4) and transistor amplifi…

A logic gate is an electronic circuit that operates on digital signals -- it is the basic building block of essentially every digital system, taking one or more binary inputs and producing a single bi…

Boolean algebra, formulated by George Boole in 1854, is a system of algebra built entirely around a choice between exactly two options -- yes/no, or high/low -- represented by the binary digits 0 and…

De Morgan's theorems are two complementary identities linking the AND, OR and NOT operations -- proved directly using truth tables in the two sub-sections below -- and they carry an important circuit-…

The first theorem states: the complement of the SUM of two logical inputs equals the PRODUCT of their individual complements, .

The second theorem states: the complement of the PRODUCT of two logical inputs equals the SUM of their individual complements, .

An integrated circuit -- also called an IC, a chip, or a microchip -- packs thousands to millions of transistors, resistors and capacitors onto a single small flat piece of semiconductor material, nor…

The chapter's own bullet-point summary restates every major definition and law in condensed one-line form: energy bands classify solids into metals/insulators/semiconductors; electrons are majority ca…

A branching diagram roots the whole unit at two parallel top-level strands born from the band-theory classification -- Metals, Semiconductors and Insulators under 'Energy Bands' -- with the Semiconduc…

The chapter's own end-of-unit assessment, printed in four labelled parts exactly as the book divides them: I.

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