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Electronics

13 min read

Conductors, insulators and semiconductors

Electronics deals with circuits that control the flow of electrons in devices such as diodes and transistors. Materials differ in how well they conduct. Semiconductors such as silicon and germanium are between conductors and insulators. Their conductivity increases as they warm, or when impurity atoms are added (doping). n-type silicon is doped with an element such as phosphorus that adds free electrons (negative charge carriers); p-type is doped with an element such as boron that creates holes (positive charge carriers).

The diode and rectification

A p–n junction diode conducts current in only one direction (forward bias) and blocks it the other way (reverse bias). A rectifier turns alternating current into direct current. A single diode gives half-wave rectification. A bridge of four diodes gives full-wave rectification. A capacitor across the output smooths the pulses into an almost steady voltage.

Half-wave rectification with a diodeAn alternating voltage is applied to a diode, which only lets current flow one way. On the left the input is a sine wave going above and below zero. On the right the output has only the positive half cycles, so the current always flows in the same direction, a pulsating d.c.input (a.c.)output (pulsating d.c.)diode: current one way onlya smoothing capacitor makes it steadier

A light-emitting diode (LED) gives out light when current flows forwards. LEDs use little power and last long: they are used in torches, displays and street lights, and run from solar-charged batteries in rural areas.

The transistor

A transistor has three terminals: the base, the collector and the emitter. A small current into the base controls a much larger current from the collector to the emitter. It can be used as an amplifier (a small signal makes a large one) and as an electronic switch (no base current: off; enough base current: on). Transistors, millions to a chip, make up the integrated circuits in phones and computers.

A small base current turns on a large current.

Logic gates

Digital circuits work with two states: 0 (low, off) and 1 (high, on). Logic gates combine them. The NOT gate inverts the input. The AND gate gives 1 only if both inputs are 1. The OR gate gives 1 if either input is 1. NAND is AND followed by NOT, and NOR is OR followed by NOT.

Logic gatesSymbols and truth tables for three logic gates. An AND gate gives output 1 only when both inputs are 1. An OR gate gives output 1 when either or both inputs are 1. A NOT gate gives the opposite of its single input.ANDA B Q0 0 00 1 01 0 01 1 1ORA B Q0 0 00 1 11 0 11 1 1NOTA Q0 11 0
Try itLogic gate outputs

Choose the two inputs A and B (0 or 1). See the output of each gate.

ApplicationGate idea
A burglar alarm that sounds when a door OR a window opensOR
A machine that starts only when the guard is closed AND the button is pressedAND
A night-light that comes on when it is NOT lightNOT
A fire alarm that sounds if smoke OR heat is detectedOR
Check yourself A diode allows current to flow…
Check yourself The output of an AND gate is 1 when…
Check yourself In a transistor, a small current into the…

Key terms

Semiconductor
A material whose conductivity is between that of a conductor and an insulator.
Doping
Adding impurity atoms to a semiconductor to change its conductivity.
Rectifier
A circuit that changes a.c. into d.c.
Logic gate
A circuit that gives a high or low output depending on its inputs.

Summary

  • Semiconductors can be doped to make n-type and p-type materials.
  • A diode conducts one way and is used for rectification and as an LED.
  • A transistor amplifies or switches with a small base current.
  • AND, OR, NOT, NAND and NOR gates form digital circuits.
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