Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
| Resistivity (ohm-metre) | Very low: 10^-2 to 10^-8 | Very high: 10^11 to 10^19 |
| Band gap (Eg) | Zero (bands overlap) | Large, greater than 3 eV |
| Free electrons | Very many | Almost none |
| Temperature coefficient | Positive (R rises on heating) | Negative (R falls on heating) |
| Example | Silver, copper | Glass, diamond, wood |
A conductor has very LOW resistivity, about 10^-2 to 10^-8 ohm-metre. An insulator has very HIGH resistivity, about 10^11 to 10^19 ohm-metre. So a conductor lets current flow easily, while an insulator blocks it. Semiconductors are in the middle, about 10^-5 to 10^6 ohm-metre. Low resistivity means high conductivity.
An insulator has a much larger band gap. For an insulator Eg is greater than about 3 eV (for diamond it is about 6 eV). For a semiconductor the band gap is small, about 1 eV (silicon 1.1 eV, germanium 0.7 eV). For a conductor there is no gap because the valence and conduction bands overlap. The larger the gap, the harder it is for electrons to reach the conduction band.
In a semiconductor, heating gives electrons enough energy to jump the small band gap, so more charge carriers appear and resistance drops. This is a negative temperature coefficient. In a conductor the number of free electrons is already fixed, but heating makes atoms vibrate more, which blocks electron flow, so resistance rises. This is a positive temperature coefficient.
Silicon is a semiconductor. At absolute zero its conduction band is empty, so it behaves like an insulator. But its band gap is only about 1.1 eV, so at room temperature some electrons cross it and silicon conducts a little. This is why silicon and germanium are the two main semiconductors in NEET.
Smallest resistivity (best conductor) is metals, then semiconductors, then insulators (largest resistivity). Example order: silver (metal) less than silicon or germanium (semiconductor) less than glass (insulator). Conductivity follows the reverse order: metals highest, insulators lowest.
On the basis of electrical conductivity, which one of the following materials has the smallest resistivity?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the band gap and resistivity. A conductor has no band gap and very low resistivity, a semiconductor has a small gap (about 1 eV) and medium resistivity, and an insulator has a large gap (more than 3 eV) and very high resistivity.
No. Conductors have a POSITIVE temperature coefficient, meaning resistance rises with temperature. Only semiconductors (and insulators to a degree) have a negative coefficient, where resistance falls as temperature rises.
Conductor: about 10^-2 to 10^-8 ohm-metre. Semiconductor: about 10^-5 to 10^6 ohm-metre. Insulator: about 10^11 to 10^19 ohm-metre. These NCERT ranges are important for NEET.
At 0 kelvin no electron has energy to cross even the small band gap, so the conduction band is empty and the semiconductor cannot conduct. As temperature rises, electrons cross the gap and it starts conducting.
