Resistivity of Conductors, Insulators and Semiconductors Compared
Physics · Current Electricity · NEET
Resistivity (rho) tells you how strongly a material opposes current, and it splits materials into three groups. Conductors (metals) have the lowest resistivity (about 10^-8 to 10^-6 ohm-metre), insulators have the highest (10^18 times larger or more), and semiconductors sit in between. Memory hook: think "low to high = Metal, Middle, Mountain" — Metal conducts, Middle is semiconductor, Mountain-high is insulator.
Resistivity increases from conductors (metals, ~10^-8 ohm-metre) through semiconductors up to insulators (10^18 times larger or more). Smallest resistivity means the best conductor, so silver sits at the far left.
Your doubts, answered
Which material has the smallest resistivity — metal, semiconductor or insulator?
A metal (conductor) has the smallest resistivity. Small resistivity means charges move easily, so current flows well. Silver and copper are examples with resistivity near 10^-8 ohm-metre. An insulator like glass has resistivity 10^18 or more times larger, and a semiconductor like silicon sits in between. So the order from smallest to largest is: metal < semiconductor < insulator.
Is high resistivity good or bad for conduction?
High resistivity means the material conducts badly. Resistivity and conductivity are opposites: rho = 1/sigma. A high rho means low conductivity, so the material blocks current — that is why insulators (glass, rubber, plastic) have very high resistivity. A good conductor needs low resistivity.
Why does a semiconductor's resistivity fall when it gets hotter, but a metal's rises?
In a metal, the number of free electrons n stays fixed with temperature. Heating only makes collisions more frequent, so resistivity rises slightly. In a semiconductor, heating frees many more charge carriers (n increases a lot). More carriers beat the extra collisions, so resistivity falls sharply. This is why semiconductors have a negative temperature coefficient while metals have a positive one.
Where do alloys like nichrome and manganin fit?
They are conductors, but special ones. Nichrome, manganin and constantan have resistivity that barely changes with temperature (very small temperature coefficient). That is why they are used to make standard resistors and heating elements — the resistance stays almost constant, so readings are reliable.
How can adding impurity change a semiconductor's resistivity?
Adding a tiny amount of a suitable impurity (called doping) supplies extra charge carriers. More carriers means resistivity drops. This controlled tuning is why semiconductors are used in electronic devices — you can set their resistivity by choosing the impurity.
⚠️ The NEET trap ✗ Silicon or germanium has the smallest resistivity because they are famous 'semiconductors' used in chips. ✓ A metal like silver has the smallest resistivity. Silver has the highest conductivity of all, so it opposes current the least. Silicon and germanium are semiconductors (middle range); glass is an insulator (highest resistivity). 🧠 Fame is not conductivity: chips use semiconductors, but SILVER still wins the smallest-resistivity race.
Real NEET questions
NEET 2023 (Phase 2)
On the basis of electrical conductivity, which one of the following materials has the smallest resistivity?
A · Glass
B · Silicon
C · Germanium
D · Silver ✓
Solution: Step 1: Recall resistivity is the inverse of conductivity, rho = 1/sigma. The material with the highest conductivity has the smallest resistivity.
Step 2: Classify each option. Glass is an insulator (very high rho, about 10^11 ohm-metre or more). Silicon and germanium are semiconductors (middle range). Silver is a metal (conductor).
Step 3: Order the resistivities from largest to smallest: Glass (insulator) > Silicon, Germanium (semiconductors) > Silver (metal).
Step 4: Silver, a metal, has the highest conductivity and therefore the smallest resistivity. Answer: (D) Silver.
Solved Current Electricity NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The SI unit of resistivity is the ohm-metre (written ohm.m). It comes from the formula rho = R.A/L, where R is in ohm, area A in square metre and length L in metre.
What is the resistivity range of metals?
Metals have resistivity roughly 10^-8 to 10^-6 ohm-metre. Silver and copper are near the low end, which is why they are excellent conductors.
Do conductors, semiconductors and insulators differ in resistance or resistivity?
They differ in resistivity, which is a property of the material itself and does not depend on shape or size. Resistance depends on both the material and the dimensions (R = rho.L/A), so it is not used to classify materials.
Which one point about temperature is most tested for NEET?
That metals (conductors) have a positive temperature coefficient — resistivity rises with temperature — while semiconductors and insulators have a negative temperature coefficient — resistivity falls as they get hotter.
