CDS Physics · Electronics and Computer Fundamentals

Electronics, Hardware and Data

Semiconductors and the transistor, the five computer generations with quantum computing and AI, memory types and file sizes, and binary numbers with logic operators.

All Electronics and Computer Fundamentals notes

Why this matters

Nine CDS questions from 2016 (II) to 2026 (I), five of them EASY, and seven of the nine from 2025 and 2026. One asks about the transistor; the rest ask about computer generations, qubits and AI, memory and file sizes, and binary logic.

Concept 1 of 4: Semiconductors, diodes and transistors

A semiconductor such as silicon conducts better than an insulator but far worse than a metal. Adding a tiny amount of another element (doping) makes it conduct much better. Join a p-type piece to an n-type piece and you get a diode, which lets current pass one way only. Make a thin slice of one type between two pieces of the other and you get a transistor, which can amplify a weak signal or work as a switch.

Definition

What to know:

Semiconductors: silicon and germanium. Unlike a metal, a semiconductor conducts better when heated.
n-type: doped with a five-valence-electron element (phosphorus, arsenic); extra electrons carry the current.
p-type: doped with a three-valence-electron element (boron, aluminium, gallium); holes carry the current.
p-n junction diode: current flows one way only, so it rectifies AC into DC. An LED is a diode that gives out light.
Bipolar junction transistor (n-p-n or p-n-p): three regions, the emitter, the base and the collector. Used as an amplifier and a switch.

Region of a transistor	Doping	Size
Emitter	Heavy (the most impurity): it supplies the charge carriers	Medium
Base	Light (the least impurity)	Very thin, so carriers cross it without being lostQ CDS 2016 (II): in a bipolar junction transistor the base has the least impurity.
Collector	Moderate	The largest, so it can get rid of heat

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2016 · CDS (II) 2016 — General Knowledge · Q18Hard

Example 1 · Electronics and Computer Fundamentals · Electronics, Hardware and Data

In a bipolar junction transistor

The base is thin and lightly doped

In a transistor the emitter is the most heavily doped, the collector is the largest, and the base is the thinnest and the least doped.

A hot semiconductor conducts better

Heating a metal raises its resistance. Heating a semiconductor frees more charge carriers, so its resistance falls.

Concept 2 of 4: Computer generations and new kinds of computing

Computers are grouped into generations by the main part they were built from. Each new part made them smaller, faster and cheaper: vacuum tubes, then transistors, then integrated circuits, then the microprocessor, a whole processor on one chip. Two newer ideas now appear in the paper: quantum computers, which work with qubits, and artificial intelligence, machines that make decisions the way people do.

Definition

What to know:

The first microprocessor, the Intel 4004, came in 1971; it began the fourth generation.
A qubit is the unit of information in a quantum computer. A bit is either 0 or 1; a qubit can be in a mix (superposition) of 0 and 1 at the same time.
Artificial intelligence (AI): a machine doing tasks that need human intelligence, above all reasoning and making decisions. Voice assistants, face unlock and chatbots are examples.
Merely storing data is not intelligence: that is what a database does.

Generation	Main hardware	Roughly when
First	Vacuum tubes	1940s to mid 1950s
Second	Transistors	Late 1950s to mid 1960s
Third	Integrated circuits (ICs)	Mid 1960s to early 1970s
Fourth	Microprocessors (very large scale integration)	From 1971Q CDS 2026 (I): the fourth generation used microprocessors.
Fifth	Artificial intelligence and parallel processing	Still developing

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (I) 2026 — General Knowledge · Q14Easy

Example 2 · Electronics and Computer Fundamentals · Electronics, Hardware and Data

Which one among the following generation computers used microprocessors in their hardware technology ?

Integrated circuits are the third generation, microprocessors the fourth

The order is vacuum tubes, transistors, integrated circuits, then microprocessors. Do not move the microprocessor back to the third generation.

A qubit belongs to quantum computers

The qubit is the two-valued unit of a quantum computer, not of an ordinary (classical) computer or of a laser.

Drill 2 more on computer generations and new kinds of computing

Concept 3 of 4: Memory, storage and file sizes

Some memory forgets everything when the power goes off, and some keeps it. The kinds that keep data differ in how they are wiped and rewritten. File sizes come from a simple rule: a sound or video file stores a fixed number of bits every second, so the size is that rate times the playing time, divided by 8 to turn bits into bytes. Compression formats such as MP3 store far fewer bits per second than raw sound.

Definition

What to know:

RAM is volatile: it loses its data without power. ROM is non-volatile.
PROM is written once. EPROM is erased by ultraviolet light. EEPROM is erased electrically.
Flash memory is a kind of EEPROM: electrically erasable and non-volatile (pen drives, memory cards, phone storage, SSDs).
8 bits = 1 byte. Computer memory counts 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB; makers of drives often use 1000 instead.
Bit rates are in kilobits per second (kbps). MP3 at 128 kbps is about 1 MB a minute; uncompressed CD sound (1411 kbps) is about 10 MB a minute.
Multimedia mixes text, graphics, audio, video and images that the user can interact with. JPEG and PNG are image formats, MP3 is audio, MP4 is video.

Size of a sound or video file

\text{size in bytes} = \dfrac{\text{bit rate (bits per second)} \times \text{time (seconds)}}{8}

bit ratebits stored for each second of playing time
8bits in one byte

Worked example

A 4-minute song is stored as MP3 at 192 kbps. About how many megabytes does it need? How much would the same song need as uncompressed CD sound at 1411 kbps? (Take 1 MB = 1,000,000 bytes.)

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (I) 2026 — General Knowledge · Q34Moderate

Example 3 · Electronics and Computer Fundamentals · Electronics, Hardware and Data

In MP3 compression scheme, 60 minutes of music can be stored in about :

Flash is erased electrically, not by ultraviolet light

EPROM is wiped by ultraviolet light through a window in the chip. Flash memory is a kind of EEPROM: it is wiped electrically and keeps its data without power.

Bits are not bytes

A bit rate in kbps counts bits. Divide by 8 to get bytes before you compare with a size in MB.

Two sizes of megabyte

Memory counts 1 MB = 1024 × 1024 bytes; drive makers and bit-rate sums often use 1,000,000 bytes. The answers differ by about 5%, so pick the option nearest to yours.

Drill 2 more on memory, storage and file sizes

Concept 4 of 4: Binary numbers and logic operators

A computer stores everything as 0s and 1s. Each place in a binary number is worth twice the place to its right: 1, 2, 4, 8 and so on. Logic operators work on true (1) and false (0): AND needs both inputs true, OR needs at least one, and NOT flips the input. Arithmetic signs such as plus and division work on numbers, not on true and false. To subtract, a computer adds the 2's complement of the number instead.

Definition

The rules:

Binary place values from the right: 1, 2, 4, 8, 16, ... So $1011_2 = 8 + 0 + 2 + 1 = 11$ .
AND: 1 only if both inputs are 1. OR: 1 if at least one input is 1. NOT: turns 1 into 0 and 0 into 1. NAND, NOR and XOR are also logic operators.
Arithmetic operators (+, −, ×, ÷) are not logic operators.
2's complement of a number: flip every bit, then add 1.
To find $A - B$ : add $A$ and the 2's complement of $B$ . On an $n$ -bit machine only $n$ bits are kept, so the carry out of the left end falls away.

Subtraction by 2's complement

A - B = A + (\overline{B} + 1) \quad \text{(keep only } n \text{ bits)}

$\overline{B}$ B with every bit flipped (1's complement)
nnumber of bits the machine holds

Worked example

On a 4-bit machine, work out 6 − 2 using the 2's complement.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2025 · CDS (II) 2025 — General Knowledge · Q84Easy

Example 4 · Electronics and Computer Fundamentals · Electronics, Hardware and Data

Which one of the following is not a logical operator ?

AND needs both, OR needs one

AND gives 1 only when both inputs are 1. OR gives 1 when at least one is 1. Mixing them up reverses a truth table.

Flip, then add 1

The 2's complement is the flipped number plus 1. Flipping alone gives the 1's complement, which is one less.

Drill 1 more on binary numbers and logic operators

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Memory, storage and file sizes
Size of a sound or video file
$\text{size in bytes} = \dfrac{\text{bit rate (bits per second)} \times \text{time (seconds)}}{8}$
Binary numbers and logic operators
Subtraction by 2's complement
$A - B = A + (\overline{B} + 1) \quad \text{(keep only } n \text{ bits)}$

Reference tables (2)

Semiconductors, diodes and transistors3 rows

Region of a transistor	Doping	Size
Emitter	Heavy (the most impurity): it supplies the charge carriers	Medium
Base	Light (the least impurity)	Very thin, so carriers cross it without being lostQ CDS 2016 (II): in a bipolar junction transistor the base has the least impurity.
Collector	Moderate	The largest, so it can get rid of heat

Computer generations and new kinds of computing5 rows

Generation	Main hardware	Roughly when
First	Vacuum tubes	1940s to mid 1950s
Second	Transistors	Late 1950s to mid 1960s
Third	Integrated circuits (ICs)	Mid 1960s to early 1970s
Fourth	Microprocessors (very large scale integration)	From 1971Q CDS 2026 (I): the fourth generation used microprocessors.
Fifth	Artificial intelligence and parallel processing	Still developing

Watch out for (9)

The base is thin and lightly doped
→ Semiconductors, diodes and transistors
A hot semiconductor conducts better
→ Semiconductors, diodes and transistors
Integrated circuits are the third generation, microprocessors the fourth
→ Computer generations and new kinds of computing
A qubit belongs to quantum computers
→ Computer generations and new kinds of computing
Flash is erased electrically, not by ultraviolet light
→ Memory, storage and file sizes
Bits are not bytes
→ Memory, storage and file sizes
Two sizes of megabyte
→ Memory, storage and file sizes
AND needs both, OR needs one
→ Binary numbers and logic operators
Flip, then add 1
→ Binary numbers and logic operators

Test yourself on Electronics and Computer Fundamentals

10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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