Negative Temperature Coefficient: Why Semiconductors Differ from Metals
Physics · Current Electricity · NEET
Semiconductors and insulators have a negative temperature coefficient of resistance: their resistance FALLS as temperature rises, while metals (positive coefficient) RISE. Reason: in a semiconductor, heat frees many more charge carriers (n increases sharply), and this jump in n beats the drop in collision time τ, so resistivity ρ = m/(ne²τ) goes down. Memory hook: "Metal Melts down (fails) when hot; Semiconductor Serves better when hot."
Left: a metal has a positive temperature coefficient, so its resistance rises almost linearly with temperature (n fixed, τ falls). Right: a semiconductor has a negative temperature coefficient, so its resistance falls steeply as temperature rises (n rises sharply and dominates the fall in τ), using ρ = m/(ne²τ).
Your doubts, answered
Why does resistance of a semiconductor DECREASE with temperature but a metal's INCREASES?
Use ρ = m/(ne²τ). Two things change with temperature: n (free carriers per unit volume) and τ (average time between collisions). Heating always makes electrons move faster, so collisions happen more often and τ drops for both. In a METAL, n is fixed (all valence electrons are already free), so only τ falls, making ρ rise (positive α). In a SEMICONDUCTOR, heat breaks covalent bonds and frees a huge number of new carriers, so n rises very steeply. This jump in n overwhelms the small fall in τ, so ρ = m/(ne²τ) drops (negative α).
Do insulators have a negative temperature coefficient, or only semiconductors?
Both do. NEET 2020 answer is 'insulators and semiconductors', not 'semiconductors only'. An insulator is really just a semiconductor with a very large energy gap. Heating it also frees extra carriers (n rises), so its resistance also falls. The difference is only in scale: insulators need much more heat to free carriers, but the SIGN of the coefficient is still negative for both.
Is τ (relaxation time) increasing or decreasing when things heat up?
τ always DECREASES with temperature for both metals and semiconductors. Hotter lattice atoms vibrate more, electrons collide more frequently, so the average time between collisions τ falls. The metal-vs-semiconductor difference is NOT about τ (it behaves the same way) — it is entirely about n. For metals n is constant so falling τ wins and ρ rises; for semiconductors rising n dominates and ρ falls.
Is carbon an NTC material? My colour-code chapter treats it like a normal resistor.
Carbon (graphite) has a negative temperature coefficient — its resistance decreases as it warms, so it behaves like a semiconductor, not a metal. This is a common NEET one-liner. Carbon resistors are still used in circuits because their overall change is small and the colour code just labels a nominal room-temperature value; the NTC behaviour does not stop you reading the bands.
What is a thermistor and how is it linked to NTC?
A thermistor is a resistor made from semiconductor material whose resistance changes strongly with temperature. An NTC thermistor has a Negative Temperature Coefficient — resistance drops as temperature rises. Because the change is large and predictable, thermistors are used as temperature sensors. For NEET, just remember: NTC thermistor = semiconductor-based = resistance down when hot.
⚠️ The NEET trap ✗ Choosing 'Semiconductors only' as the materials with a negative temperature coefficient of resistance. ✓ Both insulators AND semiconductors have a negative temperature coefficient. An insulator is a wide-gap semiconductor; heating frees carriers in both, so resistance falls in both. NEET 2020 correct option was 'Insulators and semiconductors'. 🧠 NTC is not a semiconductor-only club — insulators are members too. Read every option before ticking 'only'.
Real NEET questions
NEET 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: Use ρ = m/(ne²τ). For METALS, n is fixed and τ falls with temperature, so ρ (and R) rise → positive coefficient. For INSULATORS and SEMICONDUCTORS, heating frees many extra carriers, so n rises steeply. This rise in n beats the fall in τ, so ρ (and R) fall as temperature rises → negative temperature coefficient. Both classes qualify, so the answer is (B) Insulators and semiconductors.
NEET 2022
As the temperature increases, the electrical resistance:
A · increases for both conductors and semiconductors
B · decreases for both
C · increases for conductors but decreases for semiconductors ✓
D · decreases for conductors but increases for semiconductors
Solution: Conductors (metals): n is constant, τ decreases with heat → more scattering → R increases. Semiconductors: heat frees many carriers so n rises sharply, this dominates the fall in τ → R decreases. Therefore resistance increases for conductors but decreases for semiconductors: option (C).
ReNEET 2026
Two circuits A and B each have two resistors, one with coefficient +α and one with −α, each R₀ at the start. In A they are in series, in B in parallel, across the same battery. As temperature rises, the correct statement about currents I_A and I_B is:
A · I_A remains constant while I_B increases ✓
B · I_A decreases while I_B increases
C · I_A increases while I_B decreases
D · both I_A and I_B remain constant
Solution: Let ΔT be the temperature rise. The two resistances become R₀(1+αΔT) and R₀(1−αΔT). SERIES (A): R_eq = R₀(1+αΔT) + R₀(1−αΔT) = 2R₀, independent of temperature, so I_A = V/(2R₀) stays constant. PARALLEL (B): R_eq = [R₀(1+αΔT)·R₀(1−αΔT)] / (2R₀) = R₀(1−α²ΔT²)/2, which decreases as T rises. So I_B = V/R_eq increases. Hence I_A stays constant while I_B increases: option (A).
Solved Current Electricity NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What does 'negative temperature coefficient of resistance' mean in one line?
It means resistance decreases when temperature increases (α is negative). This is the behaviour of semiconductors, insulators and carbon, and is the opposite of metals.
Which materials are NTC and which are PTC for NEET?
NTC (negative, resistance falls when hot): semiconductors (silicon, germanium), insulators, carbon, and NTC thermistors. PTC (positive, resistance rises when hot): all metals (copper, platinum, tungsten). Alloys like nichrome, manganin and constantan have an almost-zero coefficient.
Why is n more important than τ for the sign of α?
In ρ = m/(ne²τ), τ falls with temperature for every material, so τ alone would always raise ρ. The SIGN of α is decided by n. If n is fixed (metal), τ wins and α is positive. If n rises strongly (semiconductor/insulator), n wins and α is negative.
Do alloys like nichrome have negative coefficients?
No. Nichrome, manganin and constantan have a very small, weakly positive coefficient — their resistance barely changes with temperature. That is why they are used for standard resistors and heating elements, not because they are NTC.
