The Material of the Digital Age
Conductors, Insulators, and Semiconductors
The electrical properties of materials are determined by how tightly their electrons are bound.
Conductors (e.g., metals): Have a sea of delocalized electrons that are free to move, allowing for easy current flow. Low resistance.
Insulators (e.g., glass, plastic): Valence electrons are tightly bound to their atoms and cannot move freely. High resistance.
Semiconductors (e.g., silicon, germanium): Have electrical properties intermediate between conductors and insulators. Their conductivity can be precisely controlled.
Doping
The key to making semiconductors useful is doping: intentionally introducing impurities into an intrinsic (pure) semiconductor crystal to change its electrical properties.
1.n-type Semiconductor: A pure silicon crystal (which has 4 valence electrons) is doped with an element with 5 valence electrons (e.g., phosphorus). The fifth electron is not needed for bonding and is free to move, becoming a negative charge carrier. 'n' stands for negative.
2.p-type Semiconductor: A pure silicon crystal is doped with an element with 3 valence electrons (e.g., boron). This creates a 'hole'—a vacant spot in the crystal lattice where an electron should be. This hole acts as a positive charge carrier, as nearby electrons can move into it. 'p' stands for positive.
Semiconductor Devices
The Diode:
Structure: Created by joining a p-type and an n-type semiconductor, forming a p-n junction.
Function: It allows electric current to flow easily in one direction (from p to n, called forward bias) but severely restricts current flow in the opposite direction (reverse bias). It acts as a one-way valve for electricity. Diodes are used as rectifiers (to convert AC to DC).
The Transistor (BJT or MOSFET):
Structure: A three-terminal device, typically made by sandwiching semiconductor layers (e.g., n-p-n). The three terminals are the base, collector, and emitter.
Function: The transistor is the fundamental building block of all modern electronics. It has two main functions:
1.As a Switch: A small current applied to the base terminal can control a much larger current flowing between the collector and emitter, allowing it to be turned 'on' or 'off'. This is the basis of all digital logic and computer processors.
2.As an Amplifier: A small, varying signal applied to the base can produce a much larger, identical copy of the signal at the collector. This is the basis of amplifiers for audio and radio signals.