Classification of Metals, Semiconductors & Insulators ⚡
1 Based on Electrical Conductivity (σ) or Resistivity (ρ)
Remember the handy relation \(\displaystyle \rho=\frac{1}{\sigma}\). Lower ρ (or higher σ) means charges zip through easily! 🏃♂️:contentReference[oaicite:0]{index=0}
- Metals 🥇 – Super-easy path for charges
\(\rho \approx 10^{-2}\text{–}10^{-8}\ \Omega\,\text{m}\), \(\sigma \approx 10^{2}\text{–}10^{8}\ \text{S m}^{-1}\) - Semiconductors 🤖 – In-between buddies
\(\rho \approx 10^{-5}\text{–}10^{6}\ \Omega\,\text{m}\), \(\sigma \approx 10^{5}\text{–}10^{-6}\ \text{S m}^{-1}\) - Insulators 🚫 – Charges can’t sneak through
\(\rho \approx 10^{11}\text{–}10^{19}\ \Omega\,\text{m}\), \(\sigma \approx 10^{-11}\text{–}10^{-19}\ \text{S m}^{-1}\)
Quick tip: These ranges can shift a bit, but the trend stays the same – metals < semiconductors < insulators in terms of resistivity. ⚖️:contentReference[oaicite:1]{index=1}
2 Based on Energy-Band Picture 🏗️
Inside a solid, electron energies bunch up into bands separated by the energy gap (\(E_g\)). Two key bands: the valence band (usually full) and the conduction band (where mobile electrons live).:contentReference[oaicite:2]{index=2}
- Metals 🔌
• Valence band overlaps the conduction band or is partly empty.
• \(E_g \approx 0\) ⇒ loads of free electrons ⇒ excellent conduction. - Insulators 🛡️
• Large gap \(E_g > 3\ \text{eV}\).
• Electrons can’t jump the gap at room temperature ⇒ almost no current. - Semiconductors 🌟
• Small but finite gap \(E_g < 3\ \text{eV}\).
• Some electrons gain thermal energy, cross the gap, and carry current ⬆️ with temperature.
3 Types of Semiconductors 🧩
- Elemental – Silicon (Si), Germanium (Ge)
- Compound Inorganic – CdS, GaAs, CdSe, InP …
- Organic & Polymer – Anthracene, doped phthalocyanines, polypyrrole, polyaniline, polythiophene
Most everyday chips still rely on Si or Ge, but polymer-electronics is an exciting frontier! 🚀:contentReference[oaicite:3]{index=3}
4 Why Solid-State Devices Rule 💡
- No bulky vacuum tubes or heated cathodes needed.
- Tiny size, low-power, low-voltage operation. 🔋
- Long life & high reliability – perfect for modern gadgets.
- Even CRT monitors are giving way to LCDs driven by solid-state tech.
(Fun fact: early crystal radios used galena (PbS) plus a metal point contact – an ancestor of today’s diodes!)😎:contentReference[oaicite:4]{index=4}
Important Concepts for NEET 🎯
- Learn the resistivity/conductivity order and typical ranges for metals, semiconductors & insulators.
- Remember band-gap thresholds: \(E_g \approx 0\) (metal), \(E_g < 3\ \text{eV}\) (semiconductor), \(E_g > 3\ \text{eV}\) (insulator).
- Keep the key relation \(\rho = 1/\sigma\) on your fingertips. ✋
- Know common elemental (Si, Ge) and compound (GaAs, CdS…) semiconductors for device questions.
- Understand why semiconductors conduct better at higher temperatures (tiny \(E_g\) & thermal excitation).
✨ Happy studying & keep shining! ✨
