Solids are grouped into three classes by their resistivity (rho): metals have very low rho of about 10^-2 to 10^-8 ohm-metre, insulators have very high rho of about 10^11 to 10^19 ohm-metre, and semiconductors sit in between at about 10^-5 to 10^6 ohm-metre. Conductivity (sigma = 1/rho) runs the opposite way, so metals conduct best and insulators worst. Memory hook: "Metal Money is Low (rho low), Insulators are Insanely High, Semiconductors In the Middle."
Solids classified by resistivity (rho): metals at the low end, insulators at the high end, semiconductors in the middle. Conductivity (sigma = 1/rho) increases toward the metal side.
Your doubts, answered
What are the exact resistivity value ranges I must remember for NEET?
From NCERT Class 12 Chapter 14: Metals have rho about 10^-2 to 10^-8 ohm-metre (conductivity sigma about 10^8 to 10^2 siemens per metre). Semiconductors have rho about 10^-5 to 10^6 ohm-metre (sigma about 10^5 to 10^-6). Insulators have rho about 10^11 to 10^19 ohm-metre (sigma about 10^-11 to 10^-19). Metals lowest, insulators highest, semiconductors in the middle.
Is conductivity the same thing as resistivity?
No, they are opposite. Conductivity sigma is just 1 divided by resistivity, so sigma = 1/rho. A material with low resistivity has high conductivity. Silver has a very small resistivity, which is exactly why it has a very high conductivity. When a question ranks materials, always convert in your head: smallest resistivity means largest conductivity.
Which has smaller resistivity, a metal or a semiconductor?
A metal. Metals have the smallest resistivity of all three classes (about 10^-8 ohm-metre at the low end). Semiconductors sit above metals. So in the standard NEET order of increasing resistivity it is: metals, then semiconductors, then insulators.
On what basis are solids classified in this chapter?
On the basis of the relative values of electrical resistivity (rho) or conductivity (sigma). NCERT states solids are broadly classified as metals, semiconductors and insulators using these value ranges. The chapter later explains the same three classes using energy bands, but the first classification is purely by measured rho and sigma.
Why do semiconductor values overlap the metal and insulator ranges a little?
Because the ranges are drawn on a logarithmic scale and cover impure and doped samples too. NCERT gives semiconductor rho as roughly 10^-5 to 10^6 ohm-metre, a very wide band, because adding small amounts of impurity (doping) can strongly lower the resistivity. For NEET, treat semiconductors as intermediate between metals and insulators.
⚠️ The NEET trap ✗ Picking the material with the largest resistivity when the question actually asks for the smallest resistivity, or mixing up which end of the scale is a metal. ✓ Read the word carefully. Smallest resistivity = best conductor = a metal (like silver). Largest resistivity = worst conductor = an insulator (like glass). Order of increasing resistivity: metal < semiconductor < insulator. 🧠 Underline the word 'smallest' or 'largest' before you look at the options. rho and sigma are opposites, so one careless flip loses the mark.
Real NEET questions
2023
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 order (increasing): metals < semiconductors < insulators. Step 2: Classify the options. Silver is a metal, Silicon and Germanium are semiconductors, Glass is an insulator. Step 3: Smallest resistivity means highest conductivity, which is the metal. Step 4: Among these, Silver (a metal) has the smallest resistivity, about 10^-8 ohm-metre. Answer: Silver (D).
2020
The solids which have the negative temperature coefficient of resistance are:
A · Semiconductors only
B · Insulators and semiconductors ✓
C · Metals
D · Insulators only
Solution: Step 1: Negative temperature coefficient means resistance (and resistivity) decreases as temperature rises. Step 2: In metals, resistivity increases with temperature, so the coefficient is positive. Rule out Metals. Step 3: In semiconductors and insulators, heating frees more charge carriers, so resistivity falls, giving a negative coefficient. Step 4: Both semiconductors and insulators show this behaviour. Answer: Insulators and semiconductors (B).
What is the resistivity range of a semiconductor as per NCERT?
About 10^-5 ohm-metre to 10^6 ohm-metre, which is intermediate between metals and insulators.
What is the SI unit of resistivity and conductivity?
Resistivity (rho) is measured in ohm-metre (ohm x m). Conductivity (sigma) is measured in siemens per metre (S/m), and sigma = 1/rho.
Which solid has the highest resistivity, metal, semiconductor or insulator?
An insulator. Its resistivity can be about 10^11 to 10^19 ohm-metre, the highest of the three classes.
Does classification by resistivity match classification by energy bands?
Yes. The three classes (metals, semiconductors, insulators) defined by resistivity values are the same three classes explained later by energy band theory and band gap.
Why is doping mentioned along with semiconductor resistivity?
Because adding a small amount of suitable impurity (doping) lowers a semiconductor's resistivity strongly, which is why its value range is so wide and why semiconductors are used in devices.
